FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
NORTHWEST REGION PACIFIC NORTHWEST WATER LABORATORY
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FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
NORTHWEST REGION, PORTLAND, OREGON
James L. Agee, Regional Director
PACIFIC NORTHWEST WATER LABORATORY
CORVALLIS, OREGON
A. F. Bartsch, Director
NATIONAL THERMAL
POLLUTION RESEARCH
Frank H. Rainwater
NATIONAL EUTROPHICATION
RESEARCH
A. F. Bartsch
NATIONAL COASTAL
POLLUTION RESEARCH
D. J. Baumgartner
BIOLOGICAL EFFECTS
Gerald R. Bouck
TRAINING
Lyman J. Nielson
WASTE TREATMENT RESEARCH
AND TECHNOLOGY: Pulp,
Paper, Wood Products;
Food Processing;
Special Studies
James R. Boydston
CONSOLIDATED LABORATORY
SERVICES
Daniel F. Krawczyk
POLLUTION SURVEILLANCE
Barry H. Reid
TECHNICAL ASSISTANCE
AND INVESTIGATIONS
Danforth G. Bodien
NATIONAL THERMAL POLLUTION
RESEARCH STAFF
Frank H. Rainwater, Chief
Bruce A. Tichenor
Alden G. Christiansen
Ronald R. Garton
Philip W. Schneider
Lawrence D. Winiarski
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INDUSTRIAL WASTE GUIDE
ON
THERMAL POLLUTION
Revised
Revisions consist mainly
of changes in the Example Problem,
pages 93 - 104.
U.S. Department of the Interior
Federal Water Pollution Control Administration
Northwest Region
Pacific Northwest Water Laboratory
Corvallis, Oregon
September 1968
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FOREWORD
Industry's accelerating emission of heated wastes to the en-
vironment poses hazards that have triggered an acute awareness and
concern on the part of the public, the policy makers and industry
itself. Establishment of State-Federal water quality standards
brings the concern into sharp focus. The waters of the nation can-
not accept heated wastewaters without quality degradation and human
sacrifice of beneficial uses. Adding to the problem is the increas-
ing lead time required for delivery of heavy power generating equip-
ment and other industrial machinery, causing hurried management and
public policy decisions on plant siting, design and equipment that
will have long-range impact on the environment.
This Industrial Waste Guide on Thermal Pollution seeks to as-
sist State, Federal and local regulatory personnel, community and
regional planners and industrialists in making sound decisions.
The literature contains much information on thermal pollution. Yet,
the operating engineer is hard pressed for the time required to
search the widespread reference material in sufficient depth to ob-
tain a working understanding of the subject. This Guide provides
such orientation and directs the investigator to sources of more
detailed information.
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IV
The principal authors, Alden G. Christiansen and Bruce A.
Tichenor, have drawn material from many sources as well as on their
own experiences for this document. Nevertheless, the Guide is less
than totally adequate. Deficiencies are recognized in today's know-
ledge of heat sources and effects and of technology for water tem-
perature prediction and control. Even as this treatise is published,
a revision is underway to include new facts and knowledge emitting
from governmental and privately sponsored research.
We wish to express appreciation to our numerous colleagues in
State pollution control agencies who cooperated in developing the
content of the Guide. Such cooperation foretells a vigorous nation-
wide attack on thermal pollution of our water resources.
Frank H. Rainwater, Chief
National Thermal Pollution Research Program
Federal Water Pollution Control Administration
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CONTENTS
Page
FOREWORD iii
GENERAL PROBLEM 1
INDUSTRIAL WASTE - HEAT LOADS 5
GENERAL 5
ELECTRIC POWER INDUSTRY 6
Power Requirements and Sources 6
Plant Evaluation 7
MANUFACTURING INDUSTRIES 10
EFFECTS 13
PHYSICAL 13
CHEMICAL 18
BIOLOGICAL 21
Water Temperature Criteria 21
General 26
Fish Life 27
General 28
Metabolism 28
Reproduction 29
Development 29
Distribution 29
Synergistic Action 30
Dissolved Oxygen 31
Acclimation 32
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VI
Page
Freshwater Fishes 33
Maximum Temperatures 33
Preferred Temperatures 35
Marine, Estuarine, and Anadromous Fishes 36
Spawning 36
Eggs 38
Young Fish 38
Adult Fish 41
Shellfish 43
Algae and Other Aquatic Plants 45
Benthos 46
Bacteria 46
CONTROL 51
PROCESS CHANGES 51
Increased Efficiencies 51
New Methods 56
ENERGY UTILIZATION 58
COOLING DEVICES 60
Types 60
Cooling Ponds 61
Cooling Towers 62
Natural Draft Towers 69
Mechanical Draft Towers 74
Side Effects 77
Costs 79
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vii
TRANSPORT AND BEHAVIOR MANIPULATION 81
Dispersion and Dilution Techniques 81
Water Quality Management 83
Temperature Prediction 83
Micro Models 87
Macro Models 88
Thermal Regulation 89
EXAMPLE PROBLEM 90
REFERENCES 105
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GENERAL PROBLEM
Waste heat is a pollutant equally as dangerous to our waters
as the more tangible forms of wastes which enter our nation's rivers,
lakes, and estuaries. Thermal pollution has been defined as man-
caused deleterious changes in the normal temperature of water (75).
Human activity can change the normal temperature of water in
many ways. Temperature changes may be induced by altering the en-
vironment of a watercourse, e.g., through road building or logging,
by creating impoundments, or by diverting flows for irrigation. Or,
water temperatures may be changed directly by adding or taking away
heat.
Present and future demands indicate that industrial cooling
water, when viewed on the national scale, is a first-order source
of waste heat. The electric power generating industry alone ac-
counts for about 80% of the cooling water used. Therefore, the best
single index of the thermal pollution potential lies in projecting
future electric power production (75). Power generation has ap-
proximately doubled each 10 years during this century, and estimated
future demands indicate a shortening of the time span for similar
increases (60,74). Waste heat output has not multiplied as fast as
power generation because of continued improvements in thermal plant
efficiency and development of hydropower. However, fossil-fueled
plants are reaching a limit of efficiency because of metallurgical
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2
restrictions, and nuclear plants, planned or built, necessarily
waste an even higher proportion of heat than fossil-fueled plants.
Waste heat increase can be expected to more closely parallel power
production increase in the foreseeable future.
With these considerations in mind, heat rejection from the
predicted mixture of nuclear and fossil power plants is expected to
increase almost ninefold by the year 2000 (44,60). Waste heat out-
put from the manufacturing industries will also increase. However,
the demand for electricity is expected to continue to increase at
a more pronounced rate than the demand for manufactured goods (75).
This indicates a somewhat smaller rate of increase in heat rejection
from manufactur-ing as compared with the power industry.
The problem is one of managing tremendous amounts of waste heat
in a manner that will maintain, or enhance if necessary, the physi-
cal, chemical, and biological nature of our water resources. Water
quality standards are being implemented to protect these resources,
but thermal pollution control measures are costly and complicated.
An estimate by the Federal Water Pollution Control Administration
(FWPCA) places the cost of cooling facilities needed over the five
years 1968 through 1972 at $1.8 billion (74). By comparison, in
one year alone (1965), $3 billion was spent on sport fishing (3).
Thus, the potential effect of thermal pollution on this and other
beneficial water uses must be considered. We must develop a thorough
understanding of the causes, effects, and control of thermal pollu-
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3
tion in order to apply equitable, reasonable, and effective ap-
proaches toward its solution.
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INDUSTRIAL WASTE-HEAT LOADS
GENERAL
Tremendous amounts of water are used specifically for cooling
purposes. Almost one-half of all water used in the United States
is utilized for cooling and condensing by the power and manufactur-
ing industries (74). In 1964 this amounted to about 50 trillion
gallons, nearly 90% of the intake water for these industries (79).
Cooling water usage by industry is cited in Table 1.
TABLE 1 (73)
USE OF COOLING WATER BY U.S. INDUSTRY, 1964
Cooling Water Intake
Industry (Billions of Gallons)
Electric Power
Primary Metal s
Chemical and Allied Products....
Petroleum and Coal Products
Paper and Allied Products
Food and Kindred Products
Machinery. ...
Rubber and Plastics
Transportation Equipment
All Other
TOTAL
40,680
3,387
3,120
1,212
607
392
164
128
102
273
50,065
% of Total
81.3
6.8
6.2
2.4
1.2
0.8
0.3
0.3
0.2
0.5
100.0
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ELECTRIC POWER INDUSTRY
Power Requirements and Sources
Power production has steadily increased throughout our nation's
history. Table 2 indicates the amounts of electrical power used in
certain past years as well as projected national requirements through
1985.
TABLE 2 (4)
U.S. ELECTRIC POWER -- PAST USE, FUTURE REQUIREMENTS
Year Billion KWH
1912 11.6
1960 753
1965 1,060
1970 1,503
1975 2,022
1980 2,754
1985 3,639
For almost a half-century power generation has increased at a
rate of 7.2% annually, virtually doubling every 10 years (74). This
trend is expected to continue, and possibly accelerate, in future
years (60).
Power is usually generated by hydro- or steam-electric plants,
with the latter process requiring cooling water for waste heat re-
moval. The remaining sites which are suitable for hydroelectric
stations are limited, so the power industry must depend more and
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7
more on thermal power generation to meet future needs. As shown in
Table 2, total power consumption In 1960 was over 65 times that for
1912. During this same period, power production from fossil-fueled,
steam-electric plants multiplied by a factor of 85-plus (47). At
the present time thermal plants produce approximately 81% of the
electricity generated in the United States; predictions indicate
this figure may reach 92% by the year 2000, when two-thirds of the
plants may operate on nuclear fuel (44,60). Thus, thermal plants
will continue to gain a larger portion of a steadily increasing
market for power. Waste heat production will increase proportion-
ately, which in turn reflects the increase in waste heat disposal
that we must accomodate.
Plant Evaluation
The overall thermal efficiency (ri ) of a steam-electric plant
is calculated as:
TVm - Electrical Output 1nn
t(/o) ~ Th^^TTT^F x 10°
For one kilowatt-hour:
ntm - 3413 BTU/KWH
L{/°> 3413 BTU/KWH + Waste Heat (BTU/KWF)X
The denominator of the efficiency equation above represents the
"heat rate" of a plant, which is defined as the average amount of
heat required to produce one kilowatt-hour of electricity. This
measure is used throughout the power industry for comparisons. It
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is actually a measure of thermal efficiency due to the inverse pro-
portionality of the two.
Thermal plant heat rates on a national average basis have de-
creased continually as more efficient power production techniques
were employed (See Table 3).
TABLE 3 (30)
STEAM-ELECTRIC PLANT STATISTICSNATIONAL AVERAGE BASIS
Year
1930
1940
1950
1960
1962
1964
1966
Heat Rate
(BTU/KWH)
19,800
16,400
14,030
10,760
10,558
10,462
10,415
Thermal Efficiency
(%)
17.24
20.81
24.33
31.72
32.33
32.62
32.77
Waste Heat To
Cooling Water
(BTU/KWH)
13,420
10,530
8,510
5,730
5,560
5,480
5,440
Sizable gains in efficiency, however, are a thing of the past.
The most efficient fossil-fueled plant presently operates at about
40% efficiency. This figure is generally assumed to be a ceiling
which the national average for fossil-fueled plants may ultimately
approach (74). Efficiency of nuclear plants of the pressurized or
boiling water reactor types now being built or planned to 1975 will
not exceed about 33% (72,74).
Table 3 includes estimates of the amounts of heat wasted to
cooling water in BTU's per kilowatt-hour of electricity produced.
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9
These values are less than those representing the "Waste Heat" por-
tion of the efficiency equation because of in-plant and stack losses,
Plant design and operating data indicate that 15% of the thermal in-
put is a reasonable approximation of these losses for coal -fired
plants; for calculations on a kilowatt-hour basis this equals 15%
of the plant heat rate. Therefore, the following equation applies
to fossil-fueled plants:
Heat to cooling water (BTU/KWH) = 0.85 x Heat Rate - 3413
The amount of heat rejected to cooling water from nuclear plants
may be computed in the same manner, except that heat losses up a
stack are not involved. Typical in-plant losses are about 5% of
the heat rate, so that the resulting equation which applies to
nuclear-fueled plants is:
Heat to cooling water (BTU/KWH) = 0.95 x Heat Rate - 3413
As an example of the quantities of waste heat which can be ex-
pected in the future, the two types of plants are compared. For a
fossil-fueled plant at 40% efficiency:
Heat Rate = ~: ^ = - = 8533 BTU/KWH
Heat to cooling water = 0.85(8533) - 3413 - 3800 BTU/KWH
For a nuclear-fueled plant at 33% efficiency:
heat Rate = ^J = 10,342 BTU/KWH
Heat to cooling water = 0.95(10,342) - 3413 = 6400 BTU/KWH
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We may infer from these calculations that the advent of nuclear
power plants will not lessen, but rather will increase, by over 60%
(32), the amount of heat rejected to cooling water per KWH of elec-
tricity they produce. This factor is taken into account in the es-
timated ninefold increase in heat rejection by the year 2000 as pre-
viously mentioned.
A final observation is noteworthy in regard to waste heat re-
jection: on a percentage basis, the incremental decrease in waste
heat to cooling water per KWH is much greater than the increase in
efficiency which is responsible. For example, Table 3 indicates
that efficiency increased by about 15.5% from 1930 to 1966; the
amount of waste heat to cooling water decreased almost 60% in the
same period. Hence, any gain in efficiency is multiplied in terms
of decreased waste heat rejection.
MANUFACTURING INDUSTRIES
As indicated in Table 1, about 80% of all industrial cooling
water is used by the power industry, and the remaining 20% by manu-
facturing industries. Manufacturers' cooling facility needs are
expected to increase at a 4.5% annual rate, compared to the 7.2%
annual increase expected by the power industry (74).
To assess the impact of an industry or an industrial complex
on the thermal properties of a stream, one must determine the quan-
tity of waste heat, i.e., total BTU's, discharged from each plant.
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Specific values of heat wasted by various processes are generally
not applicable to individual plants, due to the wide range of pro-
duction processes, equipment combinations, and efficiencies employed.
Therefore, no attempt to formulate quantitative guidelines will be
made here. Rather, it is suggested that the situation be approached
through studying each manufacturing plant's discharge. In this
manner, incorrect assumptions will not be used and a more valid heat
load determination will be assured.
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EFFECTS
PHYSICAL
Temoerature affects many physical prooerties of water. Of
these, the most significant to water quality are density, viscos-
ity, vapor pressure and solubility of dissolved gases. Table 4
shows the effect of incremental changes in temperature on these
properties for fresh water.
TABLE 4
WATER PROPERTIES
Temperature Density Abs. Viscosity
T°Cj (°F) (gm/OTH) (centi poises)
0
4
5
10
15
20
25
30
35
40
32
39.
41
50
59
68
77
86
95
104
0.99987
,2 1.00000
0.99999
0.99973
0.99913
0.99823
0.99707
0.99567
0.99406
0.99224
1.7921
1.5188
1.3077
1.1404
1.0050
0.8937
0.8007
0.7225
0.6560
Dissolved Oxygen
Pressure Saturation
(mm Hg) (nig/1 )
4.58
6.54
9.21
12.8
17.5
23.8
31.8
42.2
55.3
14.6
12.8
11.3
10.2
9.2
8.4
7.6
7.1
6.6
Stokes' law describes the velocity of settling particles in
a non-turbulent medium according to the following equation.
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_,?.
(PS-PW)
where V = settling velocity, cm/sec
ps = density of settling particle, gm/cm
pw - density of water, gm/cm^
y = viscosity of liquid, poises
d = diameter of particle, cm
g = acceleration of gravity = 980 cm/sec
As indicated, settling velocity is inversely proportional to
the water viscosity and density. Thus, both properties contribute
to increased settling rates at elevated temoeratures. These in-
creased settling rates may promote better water treatment plant
operation, though probably to no measurable degree. A difference
in settling velocities can have a significant effect on the loca-
tion and amount of sediment and sludge deposition in sluggish
rivers, reservoirs and estuaries. For a synopsis of the field of
sedimentation, see the following publications: Fluvial Sedi-
ments, A Summary of Source, Transportation, Deposition, and
Measurement of Sediment Discharge, Colby, B. R., 1963, U.S.
Geological Survey Bulletin 1181-A; Determination of Fluvial Sedi-
ment Discharge, Report No. 14, Water Resources Council, Interagency
Subcommittee on Sedimentation, 1963.
Very slight differences in density are sufficient to cause
thermal stratification in quiescent water bodies, but stratifica-
tion stability also depends on water movement and depth. Stable
stratification is common in lakes and reservoirs where there is
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a specific gravity difference of about .001 or .002 between
waters of the upper layer (epilimnion) and lower layer (hypo-
limnion) (22,80). Such stratification inhibits vertical mix-
ing and oxygen transfer to lower waters.
In stratified reservoirs, cool, incoming waters may travel
almost directly to the dam outlet in a density current at a depth
of compatible specific gravity. This reservoir characteristic
is important in predetermining release temperatures and in select-
ing optimum discharge elevation.
Evaporation rate increases as water temperature rises elevate
water vapor pressure. Evaporation is caused by the wind and the
difference in water vapor pressure between the air and the water.
This is expressed mathematically as:
F - ^ (es-ea)
where F = evaooration flux (Ib ft"^
L = latent heat of vaporization (BTU Ib )
C = empirical evaporation coefficient
W = wind speed (mph)
es = water vapor pressure of saturated air at the
temoerature of the water surface (mm Hg)
ea = water vapor pressure of overlying air (mm Hg)
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The effect of changing water temperatures on evaporation
rates can be shown by holding all terms constant except es:
L = 1050 BTl) Ib"1
C = 11.4 (Lake Hefner coefficient (27,p.35))
W = 10 mph
ea = 7.5 mm Hg (for an air temperature of 70°F with a
relative humidity of 40%
I , W I I I V* C / W / rj
(1050) s ft/ day
Solving this equation for various water temperatures, i.e.,
various es values, gives the following table:
Water Temperature
50
60
70
80
90
es
(mm Hg)
9.2
13.3
18.8
26.2
36.1
F
(Ib ff2 day-
0.19
0.64
1.24
2.06
3.15
1)
The values shown for evaporation flux (F) are specific for
the given values of L, C, W and ea. However, similar relative
changes in F with respect to water temperature will occur for
other values of the independent variables.
Most living organisms depend on oxygen in one form or
another to maintain their life and reproductive processes; thus
an adequate supply of oxygen must be available for any healthy
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aquatic environment. Hence, the relation of water temperature to
gas solubility is a very important aspect of thermal pollution.
Oxygen does not react chemically with water. Therefore, its solu-
bility is directly proportional to its partial pressure at any
given temperature under equilibrium conditions with the atmosphere.
The effect of temperature on the solubility of oxygen in fresh
water under one atmosphere of pressure is shown in Table 4.
Temperature changes cause complicated adjustments in the
dynamic oxygen balance in waters and compound the difficulty of
relating dissolved oxygen and other factors to oxygen demand, atmo-
spheric reaeration, photosynthetic production, diffusion, mixing,
etc. However, Phelps (57) states that "very few waters in nature
carry a full quota of dissolved oxygen." Thus, general tempera-
ture rises, which decrease the oxygen holding capacity, may limit
oxygen quantities which are already less than optimum.
Atmospheric nitrogen, with a solubility about one-half that
of oxygen, is usually not considered an important control para-
meter for water quality. However, recent evidence on the Columbia
River indicates that fish may be seriously affected in waters
which have become supersaturated with nitrogen through raoid warm-
ing or pressure reduction after dam discharge.
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CHEMICAL
Many factors affect chemical reactions, including the nature
and concentration of reacting substances, catalytic influence,
and temperature. The last named is important because chemical
changes speed up as the temperature rises. In general, the speed
of a chemical change is approximately doubled for each 10°C (18°F)
rise in temperature (42,71).
In an irreversible reaction, higher temperatures will de-
crease the time required to produce the final products. In a
reversible reaction, the process is complete when the reactants
reach a point of dynamic stability, i.e., when the rate of forward
reaction equals the rate of reverse reaction. In this case,
temperature influences both the length of time required to reach
equilibrium and the proportion of reactants and products at
equilibrium conditions.
Most of the chemical effects on water quality, which are in-
fluenced by temperature, center around microbial activity. Any
chemical reaction or change that results from a life process is
properly termed a biochemical reaction. The majority of chemical
reactions that organisms bring about occur through catalytic
action at far lower temperatures than would be needed in the
absence of catalysts. Such catalysts are known as enzymes, and
are themselves temperature-sensitive. The rate of microbial
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activity increases, to a point, with the rates of chemical reac-
tions. The majority of organisms affecting chemical water
quality are in the mesophylic classification and thrive in a
temperature range of 10 to 40°C (18 to 104°F). For this group,
activity usually reaches a maximum between 30 and 37°C (86 and
99°F), then falls off as enzymes become less active.
Taste and odor problems induced by temperature-accelerated
chemical or biochemical action are accentuated when oxygen is
depleted. Substances which may accumulate include hydrogen sul-
fide, sulfur dioxide, methane, partially oxidized organic matter,
iron compounds, carbonates, sulfates, and phenols, to list a few.
In addition to the greater amounts of such substances, tastes
and odors are usually more noticeable in warmer water due to
decreased solubility of gases.
Biodegradable organic material entering water exerts a bio-
chemical oxygen demand (BOD) which must be satisfied before
assimilation of the material is completed. When the temperature
of such a receiving water rises, the intensified action of micro-
organisms causes the BOD to be satisfied in a shorter distance
from the discharge than would be accomplished at a lower tempera-
ture. Figure 1 depicts oxygen sag curves for a stream in which
all conditions, i.e., streamflow, wasteflow, BOD of the waste,
and initial percent DO saturation, were held constant, except
temperature. It is apparent from the curves that the deoxygenation
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effect caused by waste assimilation is exerted over a shorter
stream distance at higher temperatures; also that oxygen deple-
tion occurs to a greater extent, since the sag point is lower
at elevated temperature. Hence, it is possible that the dis-
charge of an organic waste that previously had not caused
excessive oxygen depletion could pose problems at an elevated
temperature (26).
FIGURE I (26)
RELATION BETWEEN TEMPERATURE AND OXYGEN PROFILE
(After La Berge)
10
\
o> 8
E
7
UJ
^6
x
°s
IU
o
^ 3
en °
Q
0
WASTE OUTLET
40" C
0 10 20 30
Miles from Outlet Discharging Organic Waste
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Chemical effects of slightly increased temperatures may have
minor beneficial influences on water treatment. Disinfectant
action is generally more rapid at higher temperatures. For ex-
ample, for a given dose of free chlorine, the period required to
disinfect water at 46°F is more than nine times greater than at
104°F (18). Reports on coagulant dosages are contradictory,
although a report from the Commonwealth of Pennsylvania indicated
a savings of 30 to 50<£ per million gallons per each 10°F rise in
temperature (56). In any case, the potential beneficial effects
must be weighed against the undesirable effects such as induced
slime or algae growth, taste and odor problems, or unpalatable
drinking water temperatures (7,39).
BIOLOGICAL
Hater Temperature Criteria
On April 1, 1968, the National Technical Advisory Committee
on Water Quality Criteria to the Federal Water Pollution Control
Administration published its Repart_p_n Water Quality Criteria (76).
Among the water quality parameters considered by this distinguished
panel was temperature. The Committee produced recommendations
with regard to water temperature for both freshwater organisms
and marine and estuarine organisms. These recommendations, based
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on the best available information, indicate the temperature
levels which will ensure the future growth and development of
aquatic organisms. The Committee's recommendations are as
follow:
Fresh Water Organisms (76, pp. 42,43)
"Recommendation for Warm Waters: To maintain a well-
rounded population of warm-water fishes, the following restric-
tions on temperature extremes and temperature increases are
recommended:
"(1) During any month of the year, heat should not be added
to a stream in excess of the amount that will raise the tempera-
ture of the water (at the expected minimum daily flow for that
month) more than 5 F. In lakes and reservoirs, the temperatures
of the epilimnion, in those areas where important organisms are
most likely to be adversely affected, should not be raised more
than 3 F above that which existed before the addition of heat of
artificial origin. The increase should be based on the monthly
average of the maximum daily temperature. Unless a special study
shows that a discharge of a heated effluent into the hypolimnion
or pumping water from the hypolimnion (for discharging back into
the same water body) will be desirable, such practice is not
recommended.
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"(2) The normal daily and seasonal temperature variations
that were present before the addition of heat, due to other than
natural causes, should be maintained.
"(3) The recommended maximum temperatures that are not to
be exceeded for various species of warm water fish are given in
table III-l.
"Recommendations for Cold Waters: Because of the large num-
ber of trout and salmon waters which have been destroyed, or made
marginal or nonproductive, the remaining trout and salmon waters
must be protected if this resource is to be preserved:
"(1) Inland trout streams, headwaters of salmon streams,
trout and salmon lakes and reservoirs, and the hypolimnion of
lakes and reservoirs containing salmonids should not be warmed.
No heated effluents should be discharged in the vicinity of
spawning areas.
"For other types and reaches of cold-water streams, reser-
voirs, and lakes, the following restrictions are recommended:
"(2) During any month of the year, heat should not be added
to a stream in excess of the amount that will raise the tempera-
ture of the water more than 5 F (based on the minimum expected
flow for that month). In lakes and reservoirs, the temperature
of the epilimnion should not be raised more than 3 F by the addi-
tion of heat of artificial origin.
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24
TABLE III-l
(Provisional maximum temperatures recommended as compatible with
the well-being of various species of fish and their associated
biota.)
93 F: Growth of catfish, gar, white or yellow bass,
spotted bass, buffalo, carpsucker, threadfin
shad, and gizzard shad.
90 F: Growth of largemouth bass, drum, bluegill, and
crappie.
84 F: Growth of pike, perch, walleye, smallmouth bass,
and sauger.
80 F: Spawning and egg development of catfish, buffalo,
threadfin shad and gizzard shad.
75 F: Spawning and egg development of largemouth bass,
white, yellow, and spotted bass.
68 F: Growth or migration routes of salmonids and for
egg development of perch and smallmouth bass.
55 F: Spawning and egg development of salmon and trout
(other than lake trout).
48 F: Spawning and egg development of lake trout, walleye,
northern pike, sauger, and Atlantic salmon.
NOTE: Recommended temperatures for other species, not listed
above, may be established if and when necessary information
becomes available.
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25
"(3) The normal daily and seasonal temperature fluctuations
that existed before the addition of heat due to other than natural
causes should be maintained.
"(4) The recommended maximum temperatures that are not to
be exceeded for various species of cold-water fish are given in
table III-l.
"NOTE: For streams, total added heat (in BTU's) might be
specified as an allowable increase in temperature of the minimum
daily flow expected for the month or period in question. This
would allow addition of a constant amount of heat throughout the
period. Approached in this way for all periods of the year,
seasonal variation would be maintained. For lakes the situation
is more complex and cannot be specified in simple terms.
Marine and Estuarine Organisms (76, pp. 69-70)
"Recommendation: In view of the requirements for the well-
being and production of marine organisms, it is concluded that the
discharge of any heated waste into any coastal or estuarine waters
should be closely managed. Monthly means of the maximum daily
temperatures recorded at the site in question and before the addi-
tion of any heat of artificial origin should not be raised by
more than 4 F during the fall, winter, and spring (September
through May), or by more than 1.5 F during the summer (June
through August). North of Long Island and in the waters of the
-------
26
Pacific Northwest (north of California), summer limits apply July
through September, and fall, winter, and spring limits apply
October through June. The rate of temperature change should not
exceed 1 F per hour except when due to natural phenomena.
"Suggested temperatures are to prevail outside of established
mixing zones as discussed in the section on zones of passage."
General
Water temperature plays a major role in the ability of any
water-based ecological system to maintain optimum characteristics
throughout all biological stages. Temperature effects on all
organisms in an aquatic community are important because of the
interdependence of species. For example, temperatures which are
not lethal to fish or shellfish may affect metabolism, reproduc-
tion and growth, as well as reduce important food organisms,
thereby inducing a change in the balance of the entire system.
All natural biological systems are highly complex; hence it is
very difficult and potentially misleading to generalize on the
effect temperature changes have on the aquatic biota. A more
realistic approach is to direct investigations to locally impor-
tant species.
The scientific literature contains a large volume of informa-
tion on the effect of temperature on all levels of the aquatic
biota. Data are given which indicate maximum temperatures,
optimum temperature ranges, maximum permissible temperature
-------
27
changes, acclimation temperatures, etc., for a wide variety of
organisms. These data are based upon both laboratory and field
investigations of various degrees of depth. A report on
Temperature and Aquatic Life (71) by FWPCA's Technical Advisory
and Investigations Branch, Cincinnati, Ohio, covers the important
known effects which temperature has on aquatic biota.
The information presented on the biological effects of
thermal pollution is not complete. Hopefully, however, it will be
useful in detecting potential biological problems associated with
water temperature changes. Where such problems are anticipated,
a complete analysis of the situation, tailored to the specific
site and problem, is needed.
Fish Life
The physiology of fishes is directly affected by temperature.
Fishes are classed as poikilothermic animals, i.e., their body
temperatures follow changes in environmental temperatures rapidly
and precisely. In such animals, the factors favoring heat loss
tend to equal the factors producing body heat, and thus the body
approaches environmental temperature (59). In a majority of fishes,
the body temperature differs by only 0.5 to 1.0°C (0.9 to 1.8°F)
from that of the surrounding water (52). Therefore, a fundamental
requirement of fishes is that the external temperature be well
suited to internal tissue functionality (8).
-------
28
The single most important point in analyzing or predicting
the effects of temperature change on a fishery is to look at the
individual species important to the specific water body under
study. The following discussions are presented primarily to
give insight to how and why changes in thermal regimen affect
fish. To analyze specific cases, the worker is referred to the
literature on specific species.
General
Metabolism.--Rates of metabolism and activity increase with
increasing temperature. For example, according to van't Hoff's
law, metabolic activity can double or even triple over a 10°C
(18°F) rise in temperature. This increase in metabolic rate and
activity will occur over most of the tolerated temperature range
and then often cease suddenly near the upper lethal temperature.
The rates of increased activity vary with different species, meta-
bolic processes, and temperature ranges or levels. The rates may
also be modified by salinity and oxygen factors (43). Changes in
metabolic rates caused by temperature changes may be signaling
factors for spawning or migration (52). Chemical reactions within
the fish's body cells may be accelerated by temperature increases.
Temperature-induced changes in cell chemistry are associated with
four possible death mechanisms: 1) enzyme inactivity caused by
the acceleration of the enzyme reaction to such a state that the
-------
29
enzyme is no longer effective, 2) coagulation of cell proteins,
3) melting of cell fats, and 4) reduction in the permeability of
cell membranes (58). Cells may also be killed by the toxic
action of the products of metabolism (28).
Reproduction.--The temperature range within which many fishes
reproduce is narrower than that required by the majority of func-
tions. Fishes generally spawn when a certain temperature level
is reached. Of course, this level varies from species to species.
Some fish spawn on a drop in temperature, while others respond to
a rise in temperature (36). Even though the temperature require-
ments for breeding are narrow, fishes may populate a heated area
by continued migration from the outside (51).
Development.--Temperature changes affect fish development in
several ways. Abnormal temperatures can affect embryonic develop-
ment. Low temperatures may slow down development, but in some
cases fish attain a larger final size because of their slow, long-
continued growth rather than the raoid growth experienced at
higher temperatures (43).
Distribution.--Temperature is one of the more important factors
governing the occurrence and behavior of fish life; it affects the
general location of a given species and may also modify the species
composition of a community or an ecosystem (43,78,36). Tropical
and subtropical fishes are more stenothermal (tolerant of a narrow
-------
30
temperature range) than fishes of higher latitudes, and marine
forms are more stenothermal than those found in fresh water (52).
Some cold-water stenothermal forms may be eliminated by heated
discharges, while the effect on some eurythermal (tolerant of a
wide temperature range) species may be to increase the popula-
tion (51). In tropical areas, species live close to their upper
thermal limits, thus the effect of a thermal discharge can be
quite severe. However, in northern areas, species may live in
temperatures as much as 16°C (28.8°F) below their upper lethal
temperature and not be as severely affected. Laboratory tests (8)
have shown that a slow rate of decrease in environmental tempera-
ture is more important for maintaining life than a slow rate of
increase. Lethal cold can be more important than lethal heat as
a factor limiting the distribution of marine fish and as a hazard
to some in their native habitats (25).
Synergistic Action.--Synergism is defined as the simultaneous
action of separate agents which, together, have a greater total
effect than the sum of their individual effects. In reference to
water temperatures, synergistic action refers to the fact that
temperature rises increase the lethal effect of toxic substances
to fish and may also increase the susceptibility of the fish to
many diseases. For example, a 10°C (18°F) rise in temperature
doubles the toxic effect of potassium cyanide, and an 8°C (14.4°F)
-------
31
rise triples the toxic effect of 0-xylene (45). It must be
realized, however, that since the temperature effect on toxicity
varies with each substance and with concentrations of any specific
material, no hard and quick rule may be formulated to determine
this temperature effect (45). Since domestic and industrial wastes
are numerous in our nation's waters, the synergistic action between
temperature and toxicity is a relatively common occurrence. Fish
kills have accompanied small temperature rises which might have
been relatively harmless in an unpolluted stream free of toxic
substances (45). Thus, the concentration of a substance may be
harmless at one temperature, but may contribute to fish mortalities
when combined with the stress imposed by higher temperatures. Also,
the virulence of fish pathogens may be increased by higher temoera-
tures. For example, the myxobacteria Chondrococcus columnaris,
which can cause death through tissue destruction, becomes more
virulent as temperature is increased (68).
Dissolved Oxygen.--Two factors associated with rising water
temperatures are decreases in available oxygen and increases in
metabolic rates. These factors combine to render the aquatic
environment less compatible to fish life at high water temperatures.
At low water temperatures in the range of 0 to 4°C (32 to 39.2°F),
a dissolved oxygen level of one to two mg/1 is sufficient for
survival of many freshwater fish species. When the temperature
-------
32
reaches 15 to 20°C (59 to 68°F), less than three mg/1 of dis-
solved oxygen is often lethal. At these temoeratures, oxygen
levels as high as five mg/1 are sometimes required to support
minimum activity. Safe or acceptable levels may be even higher
in order to support normal activity beyond that of merely
staying alive.
Acclimation.--The temperature to which fish become adjusted
over an extended period of time, i.e., the acclimation temperature,
is important because of its influence on lethal temperature levels.
The capacity to acclimate depends on the genetic background, en-
vironmental history, physiological conditions, and age of the
organism involved (43). For example, the resistance of animals
to cold is much more variable than resistance to heat, and resis-
tance to cold varies with size, smaller fish resisting best (36).
Acclimation to different temperatures may involve changes in
orientation, migration, and other behavioral aspects such as
territorialism and biological rhythms (43). Gradual temperature
changes are tolerated much better than sharp changes. Brief or
intermittent exposure to high temperature can result in markedly
increased resistance to heat which is not readily lost on subse-
quent exposure to low temperature (25). However, it is the rapid
onset of low temperatures that probably causes death, outstripping
the ability of fish to acclimate and resulting in greater mortal-
ities (9). Deaths resulting from the inability of fish to raoidly
-------
33
acclimate to lowering temperatures have been reported (34,37).
Acclimation to low temperature usually tends to shift the lower
thermal limit downward, and acclimation to high temperatures
tends to shift the upper limts upward (43). Thus, the ability
to acclimate affects the temperature range that a fish can
tolerate. Fish acclimated to cold winter temperatures are often
subjected to lethal temperatures in the spring as warmer water
is encountered (71).
Freshwater Fishes
Maximum Temperatures.--Maximum temperatures have been deter-
mined for numerous species of freshwater fish. These temperatures
indicate the highest temperature at which a fish can survive, but
they are often higher than the maximum temperature at which a
species can survive for long periods. Thus, maintaining water
temperature at these maximums does not insure the maintenance of
a fish population.
Table 5 shows maximum and minimum temperatures for various
species and acclimation temperatures (71,56). Values shown are
LDgQ temperatures, i.e., temperatures survived by 50 percent of
the test animals. These figures are based on specific test con-
ditions, so care must be taken in interpreting the data, since
temperature limits for a given species will vary slightly depending
on the fish's rate of heating, size, and physiological condition.
-------
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A temperature need not kill a fish directly for it to be
lethal. Brook trout were found to be comparatively slow in
catching food minnows at 17.2°C (63°F) and virtually incapable
of catching minnows at 21°C (70°F). Thus, even though their
lethal limit is 23 to 25°C .(73.4 to 77°F) (Table 5), the fish
could not survive in this temperature due to a lack of food (56).
Preferred Temperatures.--Most biologists agree that fish
can live for short periods in waters of abnormally high tempera-
tures, but at these high temperatures the fish cannot perpetuate
their population. Thus, fish seek out the temperature that is
best suited for their survival. This "preferred temperature" is
given in Table 6 for several species of yearling fish based upon
laboratory experiments (71,31). Table 7 shows the temperature at
which fish in the natural environment seem to congregate, thus
indicating their "preferred temperature" (71,31).
The level of thermal acclimation influences the range of
temperatures preferred. In general, the difference between the
acclimation temperature and the preferred temperature decreases
as the acclimation temperature increases (31).
Competition between species is also important to distribution
and survival since different species have different preferred
temperatures. For example, temperatures higher than optimum may
-------
36
not kill trout, but they may produce environmental conditions
favorable for the production of coarse fish and thus reduce the
trout's food supply (70).
TABLE 6
THE FINAL PREFERRED TEMPERATURE FOR VARIOUS SPECIES
OF FISH AS DETERMINED BY LABORATORY EXPERIMENTS
Species
Bass, largemouth
Bass, smallmouth
Bluegill
Carp
Muskel lunge
Perch, yellow
n ii '
Trout, brook
Trout, brown
Trout, lake
Trout, rainbow
Marine, Estuarine
Fi
Preferred
°C
30.0-32.0
28.0
32.3
32.0
24.0
24.2
21.0
14.0-16.0
12.4-17.6
12.0
13.6
nal
Temperature
OF
86.0-89.6
82.4
90.1
89.6
75.2
75.6
69.8
57.2-60.8
54.3-63.7
53.6
56.5
Authority
Fry, 1950
Fry, 1950
Fry & Pearson, 1952
Pitt, Garside & Hepburn,
1956
Jackson & Price, 1949
Ferguson, 1958
McCracken & Starkma,
1948
Graham, 19-48
Tait, 1958
McCauley & Tait, 1956
Garside & Tait, 1958
, and Anadromous Fishes
Spawning.--Limits of temperature requirements for spawning are
usually much more stringent than for adult fish survival. For ex-
ample, the normal spawning temperature for sockeye salmon is between
7.2 and 12.8°C (45 and 55°F) (63); lower and upper lethal limits
are 0°C (32°F) and about 25°C (77°F), respectively. Pink salmon
spawn best near 10°C (50°F) (65).
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During migration salmon do not feed, so high water tempera-
tures which increase their metabolic rate may result in fuel
depletion before spawning can occur (33). Fish migration is
hampered by unfavorable temperature conditions. A thermal block
of 21.1°C (70°F) at the mouth of the Okanogan River, Washington,
prevented migration into the stream from the Columbia River (46).
Generally for salmon, upstream migration and reproduction occur
best at temperatures between 7.2 and 15.6°C (45 and 60°F) (12).
Eggs.--The incubation of eggs and development of fry generally
have more critical temperature requirements than either fingerling
or adult fish. Table 8 shows the minimum and maximum temperatures
reported for the successful hatching of various species of marine,
estuarine, and anadromous fish eggs. Care must be taken not to
equate successful hatching with fry survival; Chinook salmon eggs
incubated at 16.1°C (61°F) hatched successfully, but suffered
severe mortality in the late fry stage (54).
Young Fish.--Table 9 shows the temperature limits for the
survival of various species of young marine, estuarine, and anad-
romous fishes at several acclimation temperatures. This informa-
tion is based on laboratory tests which often produce data not
directly transferable to the natural environment. For example,
laboratory tests on striped bass fingerlings showed an upper lethal
-------
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temperature of 35°C (95°F), but studies in the Atlantic Ocean
indicated striped bass fish kills occurring at temperatures of
25 to 27°C (77.0 to 80.6°F) (69).
Fish in the estuarine environment are more susceptible to
temperature changes than those in fresh water. However, wider
ranges of tolerance between species exists in estuarine waters.
Decreases in temperature seem to have more of an effect on
estuarine fishes than on freshwater fishes. In studies of young
greenfish to determine the resistance and acclimation of marine
fishes to temperature changes, Doudoroff (24) found that heat
resistance was gained rapidly and lost slowly. Acclimation to
decreasing temperatures was slower than acclimation to increasing
temperatures. Transposed to the practical case, this fact implies
that the shutdown of a power generating plant may be more detri-
mental than its normal discharge of heat.
Anadromous fingerlings have maximum growth in the temperature
range of 10 to 15.6°C (50 to 60°F) (12). Research on the effects
of temperature on swimming speed indicates that for young sockeye
salmon optimum cruising speed occurred at 15°C (59°F), and for
young silver salmon at 20°C (68°F) (10); thus, the fish's mobility
for protection and feeding is affected by temperature changes.
Adult Fish.--Table 10 shows the lethal temperature limits for
several species of adult marine, estuarine, and anadromous fishes.
-------
42
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Adult fish are usually able to select their preferred temperatures,
unless trapped in shallow water or forced to migrate through
thermal blocks. Adult anadromous fishes spend most of their life
cycle in the ocean and enter fresh water only to migrate up a
stream to spawn, where previously mentioned thermal conditions may
affect them.
Shellfish
Most shellfish, such as clams, oysters, crabs and lobsters,
which are directly beneficial to man as a food source, are marine,
stenothermal organisms. Some species are stenothermal for one
developmental stage and eurythermal for another. Generally, how-
ever, breeding and spawning requirements are stenothermal. The
time of mollusc, e.g., clams, oysters, etc., spawning is tempera-
ture dependent. Most molluscs with specific temperature-breeding
relationships spawn in the spring and summer, and many do not
spawn until a certain temperature is reached (1). The American
oyster Crassostrea virginica spawns at temperatures between 15 and
34 C (59 and 93.2°F) depending on its condition, and spawning is
usually triggered by a rise in temperature (35).
Many species tolerate temperatures in excess of those at which
breeding occurs. For example, the shore crab Carcinus maenas
thrives, but does not breed, at temperatures of 14 to 28°C (57.2
to 82.4°F) (51). In this case, temperature limits the population,
but migration of organisms can occur from outside the heated area.
-------
44
Physiology, metabolism and development are all affected by
temperature. The American oyster C. virginica ceases feeding at
temperatures below 7°C (44.6°F). Above 32°C (89.6°F) ciliary
activity, which is responsible for water movement, is decreased,
and at 42.0°C (107.6°F) almost all body functions cease, or are
reduced to a minimum. The European oyster Ostrea lurida tends to
close its shell as temperatures drop. At 4 to 6°C (39.2 to 42.8°F)
the oyster's shell remains closed most of the time. At 6 to 8°C
(42.8 to 46.4°F) the shell opens for about six hours per day, and
at 15°C (59°F) the shell stays open for 23 hours a day (35). Very
little is known about the prolonged effects of temperatures above
32 to 34°C (90 to 94°F) on oysters; however, long exposure to such
temperatures may impede the oyster's normal rate of water circula-
tion. When either low or high temperatures cause shells to close
or ciliary action to cease, oysters cannot feed and subsequently
lose weight. Thus, temperature changes can produce an effect
similar to chronic toxicity.
The distribution of benthic organisms is temperature dependent.
The American oyster C. virginica is present in Gulf Coast waters
that may vary between 4 and 34°C (39.2 and 93.2°F), but the European
oyster 0. edulis is restricted to water temperatures of 0 to 20°C
(32 to 68°F) (36). The opossum shrimp Neomysis americana is not
often found at temperatures above 31°C (87.8°F) in the Cheasapeake
estuary (49).
-------
45
Algae and Other Aquatic Plants
Successive changes from continual temperature increase may
result in the elimination of desirable species along with the
establishment of undesirable or nuisance organisms. Some species
of algae and other aquatic plants are nuisances because they are
unsightly or produce undesirable tastes and odors. In addition,
the biological oxygen demand of the dead cells depletes dissolved
oxygen resources. Blue-green algae are especially undesirable
from the standpoint of taste and odor problems in municipal water
supplies.
There appear to be particular temperature ranges that are
tolerated by each algal species and by closely related species or
groups of species. For example, in an unpolluted stream, diatoms
grow best at 18 to 20°C (64.4 to 68°F); green algae at 30 to 35°C
(86 to 95°F); and blue-green algae at 35 to 40°C (95 to 104°F) (15).
If temperatures are increased from 10 to 38°C (50 to 100.4°F), the
predominant species groups change correspondingly from diatoms to
green algae to blue-green algae (77). Some of the more high-
temperature-tolerant species belonging to algal groups other than
the blue-green may persist with the predominant blue-greens at
increased temperatures, and several less tolerant species of the
blue-green algae may succumb with the diatoms and green algae as
the temperature rises.
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46
Benthos
Studies of particular species of benthic macro-invertebrates
have indicated that lethal temperatures vary considerably with the
type of organism. Laboratory investigations on the freshwater
snail Lymnaea stagnalis showed a lethal temperature of 30.5°C
(89.6°F) (53), while the species Viviparus malleatus did not suc-
cumb until the temperature reached 37.5°C (99.5°F) (40).
Field work on rivers has indicated that benthic organisms
decrease in number when water temperature exceeds 30°C (86°F) (67).
The macro-invertebrate riffle fauna of the Delaware River has
decreased due to heated water discharges. At 35°C (95°F) many
caddisfly, Hydropsyche, were dead, and those which remained alive
were extremely sluggish. This study suggests that there is an
upper tolerance level near 32.2°C (90°F) for a variety of different
benthic forms with extensive losses in numbers and diversity
accompanying a further increase in temperature (20).
Bacteria
In discussing the effect of temperature changes on bacteria,
it is important to distinguish between rapid changes, which may
induce thermal shock, and slow, gradual changes. Organisms can
adapt to gradual changes in the environment; however, such adapta-
tion may take several life cycles, i.e., each successive generation
is better adapted to its environment. Since bacteria have very
-------
47
short life cycles, many generations may occur within a relatively
short time. If a gradual temperature increase occurs over
several life cycles, then each successive generation is subjected
to only a small portion of this total temperature increase. Thus,
bacteria can adapt to slow temperature changes more easily than
higher forms such as fish.
The relationships of temperature to microorganism growth and
survival are very complex. Bacteria can be grouped according to
their temperature requirements for growth:
<20°C (68°F) - psychrophiles
20 to 55-65°C (68 to 131-149°F) - mesophiles
>55-65°C (131-149°F) - thermophiles
The majority of bacteria are mesophilic. Many found in natural
waters are saprophytes, i.e., organisms that live on preformed
organic matter, which have optimum temperatures of 22 to 28°C
(70 to 82°F). Parasitic bacteria have optimum temperatures near
37°C (98.6°F) and include those microorganisms pathogenic to man.
Changes in temperature have a large effect on these organisms' rate
of activity.
The effect of temperature on bacteria cannot always be con-
sidered separately from other environmental factors. Some species
are more abundant in winter, while others abound in the summer
when different environmental conditions are encountered (13).
-------
48
Rising stream temperatures can be favorable for those
bacteria which multiply in water by inducing the recurring cycles
of life and death more rapidly (61). Higher temperatures in an
organically polluted stream generally result in increased bac-
terial numbers, and low temperatures are not conducive to rapid
growth. Temperatures of 1 to 8°C (33.8 to 46.4°F) may suppress
growth and multiplication, but bacteria persist longer at these
cool temperatures.
Increases in bacterial populations are not necessarily harm-
ful. For example, those bacteria which play an active role in
stream self-purification do perform a useful function. These
include the bacteria which aerobically oxidize organic material,
as well as those responsible for nitrification and anaerobic
decomposition of bottom sediments. However, increases in patho-
genic bacteria should always be avoided.
Bacterial slimes cause unsightly scums and foul fishing nets.
Rivers carrying a high organic load often develop such deleterious
slime growths at low temperatures. A study on the Columbia River
indicated that Sphaerotilus slime grows best at 10 to 15°C (50 to
59°F). Growth ceases below 4°C (39.2°F). Infestation of
Sphaerotilus may occur below 10°C (50°F), given sufficient time (2).
Beds of Sphaeroti1 us slime extend farther downstream from a waste
-------
49
outfall during the winter than in the summer, because warmer
temperatures may inhibit the organism's food conversion effi-
ciency (23), or because of competition for food from other
microorganisms.
A recent study on the survival of bacterial indicators of
pollution showed that, the lower the temperature, the longer the
survival (17). Table 11 shows the average time for 99% reduc-
tion in original titers of microorganisms from three sources (17).
In summary, the temperature of natural waters, even during
the summer, is usually below the optimum for pollution-associated
bacteria. Increasing the water temperature increases the bacterial
multiplication rate when the environment is favorable and the food
supply is abundant. Increasing temperature within the growth range
causes a more rapid die-off when the food supply is limited.
-------
50
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CONTROL
PROCESS CHANGES
Increased Efficiencies
The adage about prevention and cure applies as well to waste
heat as to anything else. Reduction of waste heat production is
desirable for two reasons: 1) less waste of expensive energy, and
2) fewer problems with or after disposal. Upgrading the efficiency
of operations which produce heat as a by-product is, therefore, a
logical starting point when considering control of thermal pollu-
tion.
Efficiency gains in fossil-fueled, steam-electric stations
have reduced their waste heat discharge rate by approximately one-
half over the past 30 years (Table 3). Such gains have been accom-
plished through a number of refinements in the plant itself and in
operating conditions. Figure 2 presents a schematic diagram of the
basic components of a fossil-fueled steam-electric plant which may
be referred to for a better understanding of plant operation and
efficiency.
The basic steam plant cycle is as follows: the steam drum
furnace combination turns water into high-pressure steam, which
is carried to the turbines at a speed of about 200 miles per hour.
Within l/30th of a second the steam rushes through the turbines,
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-------
53
traveling through a series of stationary nozzles and revolving
buckets which spin the turbine rotor and connected generator shaft
at either 1800 or 3600 revolutions per minute.
Drastic changes occur in the steam as it releases its energy
to the rotating buckets. Steam may enter the turbines at tempera-
tures of over 1000°F and leave at less than 100°F. It enters at a
pressure of 2000 or more psi, expands to a thousand times its en-
trance volume, and leaves at a pressure less than atmospheric.
From the turbine exhaust the expanded, low-pressure steam goes
to the condenser, where it is cooled until it condenses to water.
The process reduces the volume of the steam by a factor of 27,000
in a near-perfect vacuum, thereby returning it to a state in which
it can be easily handled. The water is returned to the boiler
through the feedwater heaters to be used over and over again as the
cycle continues endlessly.
As the cycle repeats, it is necessary to continuously remove
from the condenser an amount of heat equal to that given up in
converting exhaust steam into water. Plant and operational refine-
ments reduce the amount of heat entrained in exhaust steam which,
in effect, increases the overall plant thermal efficiency. Reduc-
tion of exhaust heat is attained through several methods (44):
1) Increasing steam pressureElevating the steam pressure at
the turbine entrance reduces exhaust heat by varying increments,
but in general a 100 psi pressure increase will reduce exhaust heat
-------
54
per unit of electricity by 0.4%.
2) Superheating steamSteam generated in the boiler is heat-
ed even more in a superheat section of the furnace. Each 50°F ad-
ditional temperature rise reduces exhaust heat per unit of electri-
city by about 1.4%.
3) Reheating steam--After the steam has passed through the
high-pressure turbine section it is returned to the furnace reheat
section to absorb additional heat energy. Again, each 50°F increase
here reduces exhaust heat per unit of electricity by about 1.4%.
4) Boiler feedwater heating--A portion of the steam is with-
drawn before it reaches the final turbine exhaust, thereby eliminat-
ing its passage through the condenser. This steam is utilized to
increase the temperature of water entering the boiler. Feedwater
heating can reduce exhaust heat up to 37% per unit of electricity,
depending on the number of heaters used.
5) Reducing exhaust pressureThe pressure in the condenser is
transmitted to the turbine exhaust, i.e., turbine backpressure.
This pressure influences heat rejection to the extent that each 1
psi reduction in pressure reduces exhaust heat per unit of electri-
city by 2.5%.
Modern power plants are designed to make use of these effi-
ciency refinements as much as possible. Through such techniques,
new fossil-fueled plants attain efficiencies near 40%; nuclear-fueled
plants about 33%.
-------
55
Nuclear-fueled plants are inherently less efficient than
fossil-fueled plants even though they too utilize the basic steam
cycle to spin turbines and generate power. The major factor in
the efficiency limitation is that imposed through reduced operat-
ing temperatures. Technological difficulties make it impractical,
uneconomical, or unsafe to produce high-pressure, superheated steam
in a water-cooled reactor system. Whereas modern fossil-fueled
plants utilize steam at temperatures near 1050 F and pressures over
3000 psi, nuclear-fueled reactors of the boiling water or pressur-
ized water types produce steam at about 600 F and about 1000 psi
or 2000 psi, respectively.
Lower temperatures and pressures imply less energy content per
unit volume of steam. Hence, nuclear-fueled plants, compared to
fossil-fueled plants, require larger turbines and condensers for a
given generating capacity in order to compensate for the lower
quality of steam used.
The principal advantage of nuclear fuel is its tremendous energy
density. One ton of uranium has the energy potential of three mil-
lion tons of coal. Present-day reactors convert only about 0.5%
of this energy to usable heat, i.e., combustion efficiency is 0.5%.
Advanced reactor design, i.e., breeder types, will convert much
more energy into a useful form (66). However, while advanced de-
signs will improve fuel consumption, they will not necessarily in-
crease the thermal efficiency to reduce waste heat in steam-electric
-------
56
plant operation.
Reduction of waste heat output from nuclear plants will de-
pend on development of advanced converters which will allow higher
operating temperatures. Such converters will employ a reactor
coolant other than water in the primary flow loop, which circulates
fluid through the reactor core for heat absorbtion. The heat is
then transferred to steam through a heat exchange process in a steam
generator. Systems of this type, still in developmental stages,
are using fluids such as helium, liquid sodium or liquid potassium
for coolants. Reactor outlet temperatures of over 1000°F are pos-
sible.
Because of the vast energy stores in nuclear fuel, its utili-
zation for power production is inevitable. For the next 15 to 20
years, the number of plants in the 30% efficiency class will in-
crease rapidly, which indicates that more efficient systems will
surely lag behind our demands for power. This being the case, our
immediate efforts must focus on waste heat utilization and dissi-
pation while technology is being developed for more efficient power
production methods.
N_ew_Methp_ds_
Gas turbines offer a means of power production without cool-
ing water. Air is taken from the atmosphere, compressed, and sub-
sequently burned with a liquid or gaseous fuel. The resulting
-------
57
high-temperature, high-pressure gases expand through a power tur-
bine and then exit to the atmosphere.
Today's gas turbine efficiencies of less than 25% are not
competitive for power production on a large scale, although some
relatively small turbines are being used for standby and peaking
operation. Future development may achieve higher operating tem-
peratures and increased air flow, which in turn would increase
efficiency to a level near that of fossil-fueled steam plants.
Heat exhausted from gas turbines might possibly be put to use
in a conventional steam-electric plant. Such a combination would
reduce the waste heat discharge rate, although cooling water would
still be used.
Other systems currently under development may offer promise
for future bulk power generation without the need for cooling water.
Two systems receiving much attention lately are fuel cells and
magnetohydrodynamics (MHD), which would convert heat energy directly
into electrical energy (38).
Fuel cells are somewhat similar to conventional storage bat-
teries in that they consist of two electrodes separated by an
electrolyte. The fuel cell does not contain a store of energy; it
generates current as long as fuel and oxidant supply chemical energy
for conversion to electricity.
Individual fuel cells produce very small quantities of power.
Hence, thousands of cells would have to be connected in groups
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58
to increase power output to a level which would permit large-
scale production. Predicted eventual efficiencies of 50 to 85%
is a further attribute of the fuel cell system. A prototype and
larger scale central station fuel cell plant are being developed
by Westinghouse under a contract from the Office of Coal Research,
Department of the Interior. Target dates for construction of the
two plants are 1969 and 1973, respectively (38).
MHD generators utilize the principle of passing a conductor
through a magnetic field to produce current. In this system the
moving conductor is an ionized gas. Very high temperatures and
gas velocities must be maintained, which at the present time pre-
sents some major technical difficulties. In theory, the applica-
tion of a MHD generator can be visualized, possibly in combined
operation with a conventional steam plant, but major advances in
materials must be achieved before the future of MHD power generation
can be predicted (38).
ENERGY UTILIZATION
The optimum solution to problems associated with waste heat
disposal would be the use of rejected heat for beneficial purposes.
When dealing with power plant discharges, however, one is con-
fronted with immense quantities of water which are of low quality
when considered as a heat source. Utilization of waste heat in
this form is therefore restricted to only a very few possible
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59
applications.
One potential for such utilization is aquiculture, the farm-
ing of plants or animals in fresh or salt water. Aquiculture has
been successfully practiced in Japan and elsewhere. Heated dis-
charges could be used to enhance the environment and increase pro-
duction of commercially valuable species such as pompano, catfish,
shrimp, oysters, and scallops. Research, development, and pilot
studies are in progress to determine the feasibility of such cul-
tivation in American waters (50).
Some nuclear plants could possibly be located to provide bene-
ficial use of heated discharges in keeping shipping lanes free from
ice for extended seasons. A recent journal article (5) contained
specific recommendations for such application to the St. Lawrence
Seaway, proposing extension of the shipping season to the end of
December or even January.
Heated discharges may have application in irrigating and creat-
ing controlled environments for agricultural crops. In this manner,
growing seasons could be lengthened in certain areas for common
crops, and subtropical or tropical varieties might be produced where
they are not normally adaptable.
Cooling water may provide heat to warm swimming areas. Such
use should be guided by the National Technical Advisory Committee
on Water Quality Criteria (76) which recommends: "In primary con-
tact recreation waters, except where caused by natural conditions,
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60
maximum water temperature should not exceed 30°C (85°F)."
Thermal discharges may also provide heat for desalination
or other processes, some of which might require higher temperature
water than that normally discharged from steam plants. In such
cases, it may be possible to implement a "trade-off" whereby it
would be economical to alter the usual process for other benefits.
For example, steam from the final turbine exhaust may be 80°F and
0.5 psia. If the steam were tapped off in front of the final tur-
bine stage it may be 250°F and at 10 psig, which might be worth more
for some use other than its final turbine passage. Marketable
steam for use by other industries, for home heating, or for any
other purpose may warrant "trade-off" situations which would help
to reduce waste heat loads to streams and other water bodies.
Creation and application of improved technology for electri-
cal energy conversion is an indirect, but effective, control mea-
sure. The advent of the fluorescent light is a classic example of
getting more useful work from a watt of electricity. Other fertile
areas are the heating, refrigeration, air-conditioning and motor
industries.
COOLING DEVICES
Types
Waste heat rejection to the aquatic environment can be re-
duced through the use of cooling ponds or cooling towers. The most
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61
popular devices now in use in this country are various types of
evaporative cooling towers. In the future, large reservoirs used
as cooling ponds may find increased application in some areas.
Any method that provides air-water contact for cooling removes
about 75% of the heat through evaporation and the remainder through
conduction-convection. As water is vaporized, heat is consumed .at
the rate of about 1000 BTU per pound of water evaporated. Almost
all of this heat is taken from the water that remains, thereby
lowering its temperature.
Cooling Ponds
A cooling pond or reservoir is the simplest method of cooling
thermal discharges, although it is the least efficient in terms of
air-water contact. In a flow-through system, warm water is intro-
duced at one end of the pond, cooled through heat dissipation to
the atmosphere, and eventually withdrawn as cold water from the op-
posite end of the impoundment. Since the natural cooling process
is relatively slow without induced air and water movement, surface
area requirements may average about two acres per megawatt output,
depending on local climatic conditions.
Advantages of cooling ponds or reservoirs include:
1. Low construction cost.
2. Serves as a large settling basin.
3. May be beneficial for other purposes, e.g., recreation.
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62
Disadvantages are:
1. Low heat transfer rate, necessitating large land area.
2. Possible fogging, icing of nearby roads, etc.
Cooling can be accelerated in a pond by introducing the warm
water through a spray system located 6 to 8 feet above the water
surface. Such a system may reduce the required pond surface area
by a factor of 20 through increased cooling efficiency. This ad-
vantageous savings in land area may be negated through spray system
cost, pumping costs, and increased water loss with its associated
problems.
Cooling Towers
There are many versions of cooling towers. Terminology applied
to towers stems from basic differences in design or operation which
serve to categorize the types.
A tower may be either "wet" or "dry," depending on whether
water is exposed directly to the air; "natural draft" or "mechani-
cal draft," depending on whether fans are employed for air movement;
"cross-flow" or "counter-flow," depending on horizontal or vertical
air flow through the heat transfer section of the tower. In mechani-
cal draft towers, air flow can be either "forced," i.e., pushed
through by fan on bottom, or "induced," i.e., pulled through by fan
on top. See Figures 3, 4, 5 and 6 for schematics (29,41) illustrat-
ing these types.
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FIGURE 3
NATURAL DRAFT TOWER
DRIFT
HOT WATER /ELIMINATOR
DISTRIBUTION
COLD WATER
BASIN
WET (Evaporative) COUNTERFLOW
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FIGURE 4
NATURAL DRAFT TOWER
HOT WATER
BASIN
FILL I
DRIFT
ELIMINATOR
SHROUD
AIR
INLET
^COLD WATER
BASIN
WET (Evaporative) CROSSFLOW
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FIGURE 5
MECHANICAL DRAFT TOWER
AIR OUT
WATER IN
\ ' /' ' ,
.'/ ','', \'!/
!'. 0:/',^/V,
VII, Ml ' / "
<*,/ '"//
WATER OUT
DRIFT
ELIMINATOR
AIR IN
FAN
WET,(Evaporative) FORCED AIR FLOW
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FIGURE 6
MECHANICAL DRAFT TOWER
AIR OUT
FAN
WATER IN
WATER OUT
AIR IN
-COOLER
SECTION
AIR IN
DRY, INDUCED AIR FLOW
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67
Because it is important to understand the working "language
when discussing cooling towers, the following discussion of signi-
ficant terms is presented.
Dry-Bulb Temperature: ( F, C) Temperature of air read on
an ordinary thermometer.
O ""I
Wet-Bulb Temperature: ( F, C) Obtained by covering the
bulb of an ordinary thermometer with wetted gauze and reading
in moving air. It depends on the dryness and initial tempera-
ture of the air, but is lower than dry-bulb temperature because
some water evaporates from the gauze, removing heat. The wet-
bulb temperature is the theoretical limit to which water can
be cooled through evaporation.
Relative Humidity: (%) The ratio of the amount of water vapor
actually present in the air to the greatest amount it could
hold if saturated at that temperature and pressure. When rela-
tive humidity is 100%, wet-bulb temperature equals dry-bulb
temperature; therefore, the lower the relative humidity, the
greater the difference between wet-bulb and dry-bulb tempera-
tures .
Evaporation Rate: The rate, e.g., gallons per minute, at which
water is being evaporated to cool the circulating water. It
depends on the area of air-water surface contact, length of
time of contact, and difference between wet-bulb temperature
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68
of the air and water temperature.
Cooling Range: ( F, C) The temperature difference between
hot water entering a tower and cold water leaving.
Approach: (°F, C) The temperature difference between cold
water leaving a tower and wet-bulb temperature of the surround-
ing air.
Heat Load: (BTU/hr, BTU/min) The amount of heat dissipated
in a cooling tower per unit time. It equals water circulation
rate multiplied by cooling range, i.e.,
) - BTU/mln
Drift, Carry-Over, or Windage Loss: Water carried out of a
tower in mist or small droplet form. It is usually expressed
as a percentage of the circulating water rate. Caused by high
air velocities, it can be almost entirely eliminated through
good design and operation.
t3as_i_n_: The depressed bottom portion of a tower used for col-
lecting and storing cold water.
Slowdown : The continuous or intermittent discharge to waste
of a small portion of circulating water from the tower basin.
It is usually expressed as a percentage of the circulating
water rate. It prevents build-up of dissolved solids left
behind during evaporation.
Makeup: (gpm,cfs) Water required to replace normal system
losses from evaporation, drift, blowdown, and small leaks.
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69
Packing or Fill: Material placed in a tower over which water
flows. It increases the air-water surface area and time of
contact and maintains uniform air and water flow distribution.
Packing design dictates the type of flow. Counter-flow towers
usually use film-type packing; cross-flow towers usually use
splash- or droplet-type packing.
Water Distribution System: A network of pipes (usually-) which
spreads incoming hot water uniformly over the packing in a
tower.
Drift Eliminators: Baffles located above the water distribu-
tion system in a tower. As air flows through the baffles in
a curved path, water particles are thrown from the airstream
by centrifugal force.
Natural Draft Towers.--The cooling tower finding frequent ap-
plication to large power plant discharges at the present time is
the wet, natural draft, hyperbolic type. Over 20 of these towers
are now in various stages of operation or construction in this
country.
Hyperbolic towers derive their name from the hyperbolic profile
of the reinforced concrete shell. The largest are almost 450 feet
high and over 300 feet in diameter at the base. The function of
the shell is that of a stack or chimney in providing an enclosure
for air flowing through the packing which is located near the ground
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either inside or outside of the shell. Air flow through a tower
is created by the difference in density between the internal and
external air. Warm water passing through the packing heats the
air and saturates it with water vapor during evaporative heat trans-
fer. Both processes decrease the air density and cause it to rise
up the tower. More air is drawn in, resulting in the establishment
of a continuous air flow through the tower. External winds moving
across the top of the tower also contribute somewhat to the air
flow by reducing the internal pressure, thereby creating a drawing
effect. However, tower operation is not dependent on external wind-
induced air movement.
Internal packing is located in the bottom 10 to 20 feet of the
tower shell. Warm water distributed above the packing flows down-
ward through it in thin layers, contacting air rising vertically
through the tower. Such opposed flow denotes the counter-flow clas-
sification. (See Figure 3)
Packing may also be located on the outside periphery of a shell.
In this system the water is broken up into droplets by splash-type
packing and allowed to fall downward. Incoming air flows horizon-
tally through the falling water and then travels upward through
the empty tower shell. This case describes the cross-flow classi-
fication. (See Figure 4) Water loading over the packing is from
2 to 4 gpm per square foot of horizontal area (29).
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71
Advantages of the evaporative, natural draft type of tower
are (48):
1. Does not have mechanical or electrical components, yet
for a given load it has the same cooling capability as
the mechanical draft type.
2. Low maintenance cost, ijoth in time and money.
3. Independent of wind velocity.
4. Large water loading capacity.
5. For counter-flow design, air and water flows are opposite
in direction, with the driest air meeting the colder
water first, ensuring maximum efficiency.
Disadvantages are (48):
1. Internal resistance to air flow must be kept minimal.
2. Great shell heights are required to produce a draft.
3. Inlet hot water temperature must be hotter than air dry-
bulb temperature to induce air movement.
4. Exact control of outlet water temperature is difficult
to achieve.
Evaporative loss of water through the system amounts to about
1.5% of the flow circulated. Slowdown may require an additional
2 or 3%, depending on the quality of the circulating and makeup
water. Drift is negligible in a well designed tower, not exceed-
ing 0.2% of the circulating flow (41).
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72
A fossil-fueled power plant in the 600 MW class or a nuclear-
fueled plant of about 400 MW would discharge about 2500 X 106 BTU's
per hour. Typical design conditions for a hyperbolic, evaporative
tower to handle this heat load may be as follows (41):
Waterflow 450 cfs
Hot Water Temperature 115°F
Cold Water Temperature 90 F
Design Wet-Bulb Temperature 72°F
Design Relative Humidity 50%
Such conditions would require a tower about 400 feet high and 285
feet in diameter for counter-flow design, or about 370 feet high
and 380 feet in diameter for cross-flow design (41).
Care must be exercised when comparing natural draft evaporative
tower requirements at different sites. Each design is necessarily
based on specific conditionstemperature, humidity, etc.--for a
given geographical location. Hence, the size and performance cited
for a tower at one plant site may not apply to another, even though
the plants to be served are quite similar.
Other kinds of natural draft towers depend on air movement
without the use of a chimney or shell.
Atmospheric spray-filled towers, which are really more like
narrow spray ponds, have nozzles located from 6 to 15 feet high
surrounded by a louvered fence which permits air passage but inhi-
bits water loss. Air circulation is dependent mainly on wind velo-
city. Such cooling systems are suited for small refrigeration and
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engine-water jacket installations, where variations in a relative-
ly small cooling range will not seriously hamper operations. Where
locations permit prevailing winds to flow unobstructed, the system
works fairly well with a minimum of maintenance. Water loading
capacity may be up to 1.5 gpm per sq ft of active horizontal area
with wind blowing at 5 mph (29). Disadvantages may include clog-
ging nozzles, nigh pumping pressures required to atomize water, and
high windage losses.
Atmospheric packed towers are similar to the spray-filled type
except that they are usually larger and contain packing for water
breakup and additional wetted surface exposure. Such systems are
rarely built any more because of their high capital and pumping
costs, and of their dependence on wind for air circulation. Small
mechanical draft towers which handle comparable loads at similar
costs are now preferred.
A final consideration in the natural draft category is the
hyperbolic dry tower. This type also employs a shell for the draft
effect, but utilizes indirect heat transfer instead of evaporation
for cooling. Completely closed cooler sections give up heat through
conduction-convection, hence there is no evaporative loss of heat
or water. The indirect transfer of heat is much less efficient than
the evaporative process so that much larger volumes of air must be
circulated than in a wet tower. Therefore, the shell size must be
larger, which adds to capital cost.
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74
Dry towers are also at a disadvantage compared to wet types
because of the lower limit to which water may be cooled. The theo-
retical lower limit in a dry process is the dry-bulb air tempera-
ture; the corresponding limit in a wet process is the wet-bulb
temperature, which is usually appreciably lower (64).
Natural draft dry towers have not been generally used for
large-scale cooling because the capital cost is estimated to be
about three times as much as its wet counterpart (64). However,
the elimination of water loss through the use of such systems may
prove to be a deciding factor in their acceptance in some loca-
tions in the future.
Mechanical, Draft Towers.--Until 1963 when the first hyperbolic,
natural draft evaporative tower was built at the Big Sandy power
plant at Louisa, Kentucky, only mechanical draft towers had been
used in the United States.
Advantages of evaporative, mechanical draft cooling towers
are (48):
1. Close control of cold water temperature.
2. Generally low pumping head.
3. Location is not critical.
4. More packing per unit volume of tower.
5. A smaller approach and greater cooling range are possible.
6. Small land area requirements.
7. Capital cost is less than for a natural draft chimney.
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75
Disadvantages include the following (48):
1. High operating costs, including power.
2. High maintenance costs, both in time and money.
3. Subject to recirculation of hot, humid exhaust into
the air intakes.
4. Climatic variation can affect performance because fans
move a fixed volume of air regardless of its density and
related heat transfer properties. Also, efficiency de-
creases as wind speed increases, up to a critical velocity;
at velocities above the critical value the opposite is
true because of less recirculation.
Mechanical draft towers are used over a broad range of heat
loadings because they can be designed specifically for almost any
capacity. Maximum fan sizes limit the capacity of any single cell,
but additional cells may be built to form a bank or unit capable
of meeting the total requirement.
The two air flow designs commonly used in mechanical draft
towers, forced draft or induced draft, were referred to previously
and depicted schematically in Figures 5 and 6.
Forced draft towers, with one or more fans located in the air
intake, are slightly more efficient than induced draft types because
some of the pressure of air velocity is converted to static pres-
sure in the tower and recovered in the form of useful work. Fan
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76
location close to solid foundation lessens vibration. Also, locat-
ing fans at the air inlets reduces the possibility of moisture con-
densation on the equipment since most of the air moved is dry.
Recirculation of hot air is common in forced draft towers be-
cause of the proximity of the exhaust to the low-pressure air-intake
region. Hence, efficiency may vary and winter operation often causes
frost and ice formation on equipment or nearby buildings due to the
presence of moisture-laden air near ground level.
Forced draft type fan diameters are limited to 12 feet or less,
compared to nearly 60 feet for the induced draft type, which neces-
sitates more cells for a given capacity (29).
While forced draft towers employ only counter-flow air movement
through the packing, induced draft towers may use either counter-
flow or cross-flow design. The basic differences in packing design
and arrangement, as described previously, apply here also. For
either type of flow, recirculation is generally not a problem because
the top-mounted fan discharges heated air upward, directly away from
the air intake below. Because hot, humid exhaust air moves through
the fan, corrosion of mechanical parts is more pronounced than in a
forced draft system.
Operational pros and cons of cross-flow design versus counter-
flow design are fairly well balanced for operational characteristics.
Physically, cross-flow types can be built lower and provide easier
access to the water distribution system.
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77
Mechanical draft systems also may provide air movement in dry
towers, thereby fulfilling the purpose of the hyperbolic shell as
previously discussed. As noted before, large volumes of air are
required because of less efficient heat transfer through conduction
and convection than through the wet evaporative process (64).
Therefore, large fans are required which continuously demand an
appreciable amount of powerabout 3% of total electric power pro-
duction (6). Economics related to mechanical draft dry systems
discourage large-scale application at this time.
Side Effects
Concern over possible side effects from cooling devices has
risen along with the projected use of such systems. Primary con-
cern is with potential fogging conditions caused by cooling towers
(14,62), but other possible adverse side effects should be considered.
Because of the large amount of water vapor expelled from eva-
porative towers, extreme climatic conditions may cause condensation,
resulting in ground level fog or drizzle. However, such conditions
are not often encountered in practice (48). A recent investigation
of fogging problems from natural and mechanical draft towers pres-
ently operating in the eastern U.S. supports this conclusion (11).
Reports indicate that natural draft towers did not produce ground-
level fog or drizzle under any weather conditions. Plumes rarely
dropped below the top of the tower for an extended distance, and
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78
generally dissipated within a few hundred feet of a tower. Mechani-
cal, induced draft towers reportedly produced substantial amounts
of ground level fog, especially during the winter. The area affect-
ed by the fog was very small, however, extending a maximum of about
one-fourth mile from the towers. Carry-over of some water droplets
was also noted from the mechanical draft type. Resulting precipita-
tion occurred in the immediate vicinity of the towers, causing some
minor icing problems up to 300 feet away.
In general, undesirable meteorologic effects from towers can
be prevented or controlled to a large degree through modern design--
effective drift eliminators, air-flow control, etc. In situations
where problems arise, the area affected is limited to that immediate
to the tower installation (16).
Water circulating through a condenser-cooling tower system may
be treated chemically to prevent corrosion and inhibit biological
growth (21). hence, water extracted for blowdown purposes may con-
tain constituents which would be detrimental to aquatic life in
receiving waters. Such effects may be prevented by treating blow-
down waste separately or, more efficiently, by integrating blowdown
disposal with other plant waste disposal facilities.
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Cqst^s
An article (41) discussing natural draft, evaporative cool-
ing towers states the following in regard to tower costs:
"Single towers, regardless of design, will cost in the area
of $2.5 to $3.5 million each for the size of towers which have been
utilized to date in the United States.
"The physical size and cost of a tower installation for a
given size generating unit, however, can vary widely with location,
fuel costs, load factors, and with individual practices regarding
capability penalties and capitalization rates. It is possible that
the optimum cooling tower installations for two plants with identi-
cal rated outputs could differ in cost by a factor of two. For this
reason, it is difficult and, indeed, dangerous to generalize when
discussing tower reouirements and costs.'1
The following outlines cooling tower costs as related to each
other, to total plant or production costs, and to consumer costs.
Natural draft evaporative tower capital costs in the eastern
U.S. have been at least 5% of the total capital cost of a power
plant. For a given plant heat load, mechanical draft, evaporative
towers cost approximately two-thirds as much as natural draft;
either natural draft or mechanical draft dry types would probably
cost three to four times more than their wet counterparts.
The indicated savings in initial cost for the mechanical draft,
wet tower is somewhat negated because of power costs for fans, which
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consume about 0.8% of the power generated by the unit served (44).
For the proposed 540 MW nuclear plant at Vernon, Vermont, the an-
nual cost for production of power to operate fans is estimated to
be about $130,000--about 2% of the initial capital tower cost of
$6 million. Other maintenance costs for the mechanical draft tower
installation are estimated at $60,000 annually--l% of initial tower
costs. For this plant, the initial cost for mechanical draft
towers amounts to 5.1% of the total plant cost of $118 million (19).
In a detailed study for siting 1000 MW nuclear power plants in
the Pacific Northwest (55), natural draft cooling towers were recom-
mended at a number of locations. At all sites the estimated initial
cost of such towers and related appurtenances was about $7.5 mil-
lion, or approximately 5.5% of a total plant capital cost of about
$140 million. Induced draft towers were recommended in one case,
also at a capital cost of about 5.5% of the total plant cost. The
cost of a 2600-acre pond at a location conducive to that type of
cooling system would be about 2.3% of the total plant cost. It
should be noted, however, that the pond location was in an area of
relatively low land value.
These examples indicate that evaporative cooling systems will
increase the capital cost of a power plant by about 5%. An assess-
ment of all fixed and variable costs indicates that power produc-
tion costs are also increased by about 5% (19). It should be noted,
however, that such increases are not carried over to consumer costs
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in the same proportion because of intermediate fixed costs in-
curred through transmission and administration. According to
Converse (19), production cost is only about one-fifth of consu-
mer cost. In that case, the cost of electricity to the consumer,
because of the use of evaporative cooling towers, would increase
approximately 1%.
TRANSPORT AND BEHAVIOR MANIPULATION
Dispersion and Dilution Techniques
The design of discharge outfalls is, at the present time,
more art than science. Hydraulic model studies, along with other
empirical information, are new the basis for design because theo-
retical equations of fluid motion have not yet reached the stage
of engineering aoplication. Future developments in the area of
mathematical models, hopefully, will place the design of outfall
structures more on a theoretical, rather than empirical, basis.
The physical and hydraulic characteristics of the effluent
and receiving water largely determine the rate of dispersion and
areal extent of dilution. Rate of dispersion is a function of the
existing turbulence of the receiving water, turbulence induced by
the kinetic energy of the effluent, and the density gradient be-
tween the effluent and receiving water. The areal extent of dilu-
tion is a function of the dispersion rate plus the number and
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82
orientation of ports in the discharge manifold. Outfalls should
be designed to accomplish specific objectives.
If the objective is to minimize temperature gradients within
the water body, a diffuser designed to maximize induced turbulence
and to distribute the discharge over a broad area is recommended.
The diffuser pipe could be placed in any of the three spacial dimen-
sions of the water body, but common practice is to use a horizontal
diffuser pipe placed across the stream.
A discharge technique which is quite common with TVA is the
use of a discharge channel which floats the warm water on the sur-
face of the receiving stream or reservoir. The objective of this
method is to maximize dissipation of heat to the atmosphere and
minimize the water areas and volume affected by the effluent; mix-
ing is not desired. By forming a warm water wedge over the cool
receiving water, rapid heat loss to the atmosphere is encouraged
by increasing the energy transfer due to evaporation, convection,
and back radiation. Thus, the warm water wedge acts in much the
same manner as a cooling pond. However, this technique results in
sharp temperature gradients near the surface and serious problems
can occur if temperature sensitive organisms are subjected to such
gradients. For example, this technique would not be desirable on
a stream inhabited by a cold-water fishery.
A third alternative discharge technique is to develop a
thermal plume. A single-point discharge below the surface of the
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83
receiving water will result in a plume extending in the direction
of the water flow and vertical density gradient. Mixing is oro-
tnoted by both the energy of the receiving water's current and by
the kinetic energy of the discharge. In some streams where plumes
originate near the bed, vertical mixing is rather rapid, but
lateral mixing is slower. For example, plumes from AEC's nuclear
reactors at Hanford, Washington, on the Columbia River exhibit
such characteristics.
A term which has recently been appearing in the literature
regarding thermal pollution is "thermal block." A thermal block
occurs when a warm water mass interferes with fish migration (76,
p. 31). Outfalls should be designed to prevent such a block from
occurring.
A final point must be made concerning the control of heated
discharges by dispersion and dilution. Such techniques cannot be
expected to solve the problems associated with the discharge of
large heat loads into moderate volumes of receiving water. Any
heated discharge which will promote excessive temperature rises
will require the use of cooling devices.
Hater Quality Management
Temperature Prediction
Mathematical and physical models of varying degrees of com-
plexity have been developed to determine the fate and persistence
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84
of heat in quiescent waters, flowing streams, estuaries, and the
ocean. The ability to predict water temperatures accurately is
necessary in order to determine the thermal impact of:
1. Proposed waste heat discharges.
2. Changes in the hydraulic characteristics of a water
body or streamfor example, due to the construction
of a dam with its resulting flow regulation.
3. Releases of water from stratified reservoirs with
multilevel outlets.
4. Unusual meteorological conditions.
The following discussion presents the basic approach which
is used to solve temperature prediction problems. However, the
mathematical formulation of the physical heat transfer processes
which occur is not a simple matter. The scope of this guide pre-
vents a presentation of the mathematical derivations leading to
temperature prediction models for all situations. However, a
simplified case is presented in the example problem of the next
section. For information on more sophisticated models, the reader
is urged to consult the technical literature. An excellent basic
reference is the Edison Electric Institute's Publication No. 65-902,
Heat Exchange in the Environment, by J. E. Ldinger and J. C. Geyer,
Department of Sanitary Engineering and Water Resources, The Johns
Hopkins University, June 1, 1965 (27).
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85
It is necessary to consider heat transfer mechanisms in
water and between water and the atmosphere in order to describe
temperature regimes mathematically. For localized problems,
e.g., outfalls or plumes, the mechanisms acting in the water
are most important; analysis of conditions throughout larger
systems (rivers, reservoirs, etc.) requires emphasis on the air-
water heat transfer mechanisms as well as those in water. In
either case, specific predictions are desirable so that effec-
tive control or management techniques can be applied.
There are two heat transport mechanisms which occur in
water--advection and dispersion or turbulent mixing. Advection
is the transport of heat by the motion of a mass of water and is
accomplished through ordinary streamflow, utilization of a dis-
charge stream's kinetic energy, or water movement due to density
gradients. Mathematical terms for advection express the rate of
heat energy transfer in terms of the water mass temperature and
velocity along longitudinal, lateral, and vertical axes.
Turbulent mixing or dispersion cause heat interchange through
eddy diffusion or molecular diffusion. Eddy diffusion occurs under
turbulent flow, which depends on fluid velocity and channel
characteristics. Mixing results from the action of small fluid
masses known as eddies, which are random both in size and orienta-
tion. Molecular diffusion is that resulting from random motion
of molecules. Its influence is much less than that from turbulent
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86
mixing. Diffusion effects are combined in mathematical expres-
sions, and may be defined along longitudinal, lateral, and
vertical axes.
Heat exchange which takes place between the water surface
and the atmosphere is made up of seven mechanisms.
The mechanisms which are independent of water temperature
are (27):
Qs = Incoming short-wave solar radiation (400 to 2800
BTU/ft2/day).
Q, = Incoming long-wave atmospheric radiation (2400 to
a
3200 BTU/ft2/day).
Qsr Qar = Portions of both short-wave and long-wave
radiation which are reflected or scattered by
the water surface (40 to 200 and 70 to 120
BTU/ft2/day, respectively).
The mechanisms of heat transfer which are dependent on the
surface water temperature include the following (27):
Qbr = Long-wave back radiation from the water to the
atmosphere (2400 to 3600 BTU/ft2/day). It is pro-
portional to the fourth power of the absolute water
surface temperature (AT$), i.e., 0.^ <* (ATS) .
Qc = Heat exchange due to conduction-convection (-300 to
+400 BTU/ft2/day), which is proportional to the wind
speed (W) and the difference between water temperature
-------
87
(T_) and air temoerature (T J, i.e., (1 cc W(TC-T ).
b a c b a
A positive Qc indicates an energy loss.
Qe = Heat loss due to evaporation (2000 to 8000 BTU/ft2/
day), which is proportional to the product of wind
speed (W) and the difference between the water vapor
pressure in saturated air at the water temperature
(es) and the water vapor pressure in the overlying
air (ea), i.e., Qe <* W(es - ea). If ea > es, conden-
sation takes place and the water body gains energy.
The algebraic sum of these surface heat exchange parameters
is equal to the net rate of surface heat exchange. Equilibrium
temperature is reached when this sum is zero.
Micro Models.--In temperature prediction terminology, a micro
model is one which describes or predicts the distribution of heat
in the immediate vicinity of a thermal discharge. The micro models
available at the present time are based on many simplifying assump-
tions which render them less than totally adequate for predicting
local temperature distributions. However, they are useful for
approximating heated discharge dilution rates by receiving waters.
Theoretical work is progressing in areas such as mixing and dis-
persion and will aid in development of generalized micro models
for jets and plumes.
Another factor which adds to difficulties with micro models
is the unique character of many situations. For example, the nature
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88
of a stream channel at any location will influence mixing charac-
teristics. Curvature or depth variation may direct discharges into
certain portions of a channel, thereby nullifying the theoretical
description of the length or degree of mixing. Each situation will
therefore require careful description in terms of discharge pumping
rate, river flow, and channel characteristics.
Until generalized micro models can accurately predict local
temperature distributions under various hydraulic conditions, esti-
mates based on simplified models and past experience will be required.
Macro Models.--Macro models are those which describe or pre-
dict temperature regimes in a complete river or river system, lake
or reservoir, estuary, or coastal area. Such models combine heat
transfer mechanisms and water movement on a continual basis with
respect to time. With the current state of the art, technology for
macro-prediction is more reliable than for micro-prediction. This
is possible because of the compensating and averaging effects of
spacial differences in environmental variables.
A macro model maintains an energy budget for the water body
under consideration, i.e., it maintains a heat balance of both the
internal heat exchange and the heat transfer at the water surface.
This heat budget may be expressed as follows:
(Rate of Heat In) - (Rate of Heat Out) + (Rate of Heat
Storage) + (Rate of Heat Exchange at Water Surface) - 0
The rate at which heat flows into and out of the water body is
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89
determined from the flow rates and temperatures of inflowing and
outflowing water. These rates require evaluation of the heat
transfer mechanisms in water to define the motion of heat entrained
water masses. The rate of heat storage is determined from the
temperature and volume of the water body in consecutive time
periods. The rate of heat exchange at the water surface is the
algebraic sum of all the water-atmosphere heat exchange rates.
Macro models evaluate terms in the heat budget over specific
time periods, e.g., hourly, daily, weekly, etc. The rate-time
evaluation results in quantitative values for each portion of the
heat budget for each period through the study time span. By super-
imposing flow rates on the time scale, the location of a specific
water mass may be determined at any desired time. For temperature
prediction, a model routes hypothetical masses of water along a de-
scribed course in order to simulate their movement and changes in
thermal properties; temperatures are thereby predicted at specific
locations and points in time.
Thermal Regulation
The need to manage our environment has only recently been rec-
ognized, but we now realize that in order to continue the effective
use of our natural resources, environmental management is a neces-
sity. In terms of thermal pollution control, we can perform water
temperature management through plant siting, coupled with effective
use of regulated river systems. Effective water temperature predic-
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90
tion models will enable water resources managers to predict tempera-
tures which will result from combined dam releases and thermal in-
puts to a system. Conditions can then be selected to minimize un-
desirable thermal effects. Such temperature-discharge requirements
can be met through the use of multilevel dam outlets which permit
waters of various temperatures to be selectively released from
stratified reservoirs.
EXAMPLE PROBLEM
This section presents a problem concerning temperature predic-
tion on a well-mixed stream and the sizing of flow-through cooling
ponds. A complete explanation of the methodology is beyond the scope
of this guide, and the reader is urged to consult the literature for
an in-depth review of the many available computational techniques.
The problem solution uses basic methods, all of which can be found in
the publication by Edinger and Geyer (27). As an aid in analyzing the
problem, references to appropriate pages in this reference are given.
The Situation
A 1000 MW electrical output nuclear power plant of 33% effi-
ciency is to be located on a medium-sized river in the temperate
region of the nation. Using applicable hydrologic and meteorologic
data, we wish to compute:
A. Downstream temperature, assuming once-through cooling
and complete mixing in the river.
B. The area of a flow-through cooling pond necessary to
prevent violation of water temperature standards.
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91
Part A
Compute the heat energy entering the cooling water as des-
cribed in the "Plant Evaluation" section.
1. For 33% thermal efficiency (r\.}:
541-3
Heat Rate = ° . 1An - - = 10,340 BTU/KWH
r). v i uu .00
2. Assuming a 5% in-plant heat loss:
Heat to cooling water - (0.95 x Heat Rate - 3413) BTU/KWH
Heat to cooling water - [0.95(10,340) - 3413] BTU/KWH
Heat to cooling water = 6410 BTU/KWH
Total heat to cooling water for the 1000 MW (106 KW) plant
106 KW x 6410 BTU/KWH = 6.41 x IP9 BTU/hr
Compute the temperature rise in the stream, assuming once
through cooling and complete mixing.
Given a design flow in the stream of 3500 cfs, which in terms
of Ib/hr is:
Q = (3500 cfs) (62.4 lb/ft3) (3600 sec/hr)
Q = 7.86 x 108 Ib/hr
Since 1 BTU will raise the temperature of 1 Ib of water 1°F,
AT = AT in river = <6'41 x 10' BTU/h^
r (7.86 x 10B Ib/hr) (1 BTU/lb °F)
AT = 8.2°F
i
-------
92
Equation for computing downstream temperatures.
Downstream temperatures are computed by assuming exponential
temperature decay. This concept is presented mathematically
as:
^= -K(T - E) (27, p. 43)
where -rr = net rate of water surface heat exchange (BTU ft day )
K = energy exchange coefficient (BTU ft day' °F )
T = water surface temperature (°F)
E = equilibrium temperature (°F)
For a well-mixed stream, this equation can be written as:
pCpyu|I = -K(Tx - E) (27, p. 129)
_3
where p = water density (62.4 Ib ft )
C = specific heat of water (1 BTU Ib"1 °F"1)
y = mean stream depth (ft)
U = mean stream velocity (ft day )
TT~ = longitudinal temperature gradient (°F ft" )
x = downstream distance (ft)
Define T = temperature at x = o; then
-Kx )
Pcpyu
Tx = (T - E)e
/\ \J
Kx
By defining a = ^ y ; then
T = (T - E)ea + E
x v o
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93
Meteorologic Data
The following data are used in determining K and E:
Time Period
(6 hr intervals)
Midnight - 6 am
6 am - Noon
Noon - 6 pm
6 pm - Midnight
DAILY AVERAGE
For K
1 1
Wind ,u.
Speed w
(mph)
4.0
12.0
12.0
6.0
8.5
1
Net Radiation ,u ,
Input (tV
(BTU ft-* hr-1)
120
290
320
130
215
For E
Air (T ,
Temp 1V
(°F)
65
75
85
70
74
Relative
Humidity
{%}
40
30
20
35
--
Water Vapor '
Pressure of (e )
Ambient Air
(mm Hg)
6.3
6.7
6.2
6.6
6.5
Determination of K
The energy exchange coefficient is computed using a
variation of the equation given on page 48 (27):
K = [15.7 + (0.26 + 3)(bW)]
where W = wind speed (mph)
b = experimental evaporation coefficient (a value of
15 is used in this example)
(3 = proportionality coefficient [See following table]
Range of E g
(OF)(mm Hg op-I)
50 to 60 0.405
60 to 70 0.555
70 to 80 0.744
80 to 90 0.990
-------
94
Thus, for an average daily value of K, using W = 8.5 mph:
K = (15.7 + [0.26 + 3] [(15) (8.5)])
Using appropriate values of 3 for two ranges of E:
i (°F) K(BTU ft"2 day"1 °F"])
60 to 70 120
70 to 80 144
Determination of E
The equilibrium temperature is reached when the rate of change
of energy at the water surface equals zero. Edinger and Geyer
(27) present a method for computing E (pp. 55-59). The method
involves assuming a likely 10°F temperature range for E and by
using the appropriate value for K and the given meteorological
data, computing a value for E. If the computed value of E falls
within the assumed range, the process is complete. However, if
the computed value of E falls outside the assumed range,
another range must be assumed and the process repeated until E
falls within the proper limits. Thus, E is computed by a trial
and error method.
For the stated meteorological conditions and computed values
of K, we can determine a daily average E by the following seven
steps (27):
Step 1. Assumed range of E = 70 to 80°F
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95
Step 2. Compute F(K) for use in step 6:
K - 15.7
F(K) =
K
As computed for an E range of 70 to 80°F, K = 144 BTU ft"2 day"1 °F
.'. F(K) - ^ 144 - = °-891
Step 3. Compute E-, for use in step 6:
HD - 1801
E =
-1 K
7 1
From the meteorologic data table, HR = 215 BTU ft hr
or in terms of days, HR = 5160 BTU ft"2 day"1
- 5160 - 1801 _
Step 4. Compute E? for use in step 6:
(0.26) (Ta)
E2 ~ "(0.26 + 3)
From the meteorologic data table, T = 74°F, and from
a
the table of E range vs. 6, 3 = 0.744
. F _ (0.26) (74) _ ,q
' ' L2 (0.26 + 0.7447 Iy^
Step 5. Compute E~ for use in step 6:
e, - C(3)
a
3 (0.26 + 3)
From the meteorologic data table, e = 6.5 mm Hg. C(3)
a
is related to ranges of E as follows:
Range of E C(3)
(OF) " "(mm Hg)
50 to 60 -11.22
60 to 70 -20.15
70 to 80 -33.30
80 to 90 -53.33
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96
Thus for an E range of 70 to 80°F, C(6) = -33.3
. E _ 6.5 - 1-33.3) _ -q fi
' ' 3 (0.26 + 0.744) Jy'D
Step 6. Compute M for use in step 7:
M = E1 + F(K) (E2 + E3)
M = 23.3 + (0.891) (19.2 + 39.6) = 75.7
Step 7. Compute E using the following relationship:
M=E+0.051E2
K
Inserting M and K and setting up a quadratic equation gives:
2 °°
E - 75.7 - 0
.'. 0.000354E2 + E - 75.7 - 0
Solving this equation using the quadratic formula gives:
F = -1 ± [1 - (4) (0.000354) (-75.7)]1/2
t ~ 2 (0.000354)
F = -1 ± (1.10719)172 = -1 ± (1.05223)
0.000708 0.000708
Rejecting the negative value gives:
= n'nnrwna = 73.8°F (This value is acceptable because it
falls within
70 to 80°F.)
falls within the assumed range of
Compute Average Stream Velocity
Q = 3500 cfs
Given an average cross section 800 feet wide and 5 feet deep:
U = S ft) = °'875 ft/sec = ZS^OO ft/day
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97
Evaluation of a
-Kx
a =
pCpyU
For x' in miles: a = ( (1) (5) (75,600)
a = -0.0322x'
Solve for T , for x1 = 10, 20, 50 miles
X
Assume unheated river temperature = 74 F
.-. TQ = 74°F + ATR = 74°F + 8.2°F = 82.2°F
TX, = (TO - E)e-°-0322x' + E
For x' = 10 miles
T , = (82.2-73.8)e-(°-0322>(10)+73.8
X
Tx, = (8.4)e'°-322 + 73.8
T , = (8.4) (0.725) + 73.8 = 79.9°F
x
For x' =20 miles
Use same value of a and replace T by T , for x' = 10 miles:
0 X
T , = (79.9 - 73.8) (0.725) + 73.8 = 78.2°F
A
fojlA' = 3°» 40' 50 miles
Following the same procedure:
30 miles, T , - (78.2 - 73.8) (0.725) + 73.8 - 77.0°F
X
40 miles, T , - 76.1°F
A
50 miles, T , = 75.5°F
X
-------
98
These values represent the exponential temperature decay
which is graphically shown on the following plot:
Q 70
Plant
10 20 30
Distance Downstream from Plant
(miles)
50
This graph presents an idealized picture of the downstream
temperatures, since the computations were based on average daily
conditions, and thus no diurnal effect is evident. It also assumes
that the weather data on which K and E are based are indicative
of conditions along the 50-mile stretch of the river. In addition,
no tributary inflows or heated discharges are accounted for in the
50 miles.
-------
99
The diurnal effect may be evaluated by using the six-hour
average meteorologic conditions given previously. Following the
methods described, values of K and E were computed as:
Time Period
(6 hr intervals)
Midnight to 6 am
6 am to Noon
Noon to 6 pm
6 pm to Midnight
K
(BTU ft"2 day"1)
56
196
241
76
E
(°F)
53.8
79.2
84.0
58.1
0
(°F)
71
72
78
76
The values of water temperature (T ) just upstream from the
plant reflect natural diurnal fluctuations.
Using the exponential temperature decay relationship presented
previously and assuming slug flow in the stream, i.e., no longitudi-
nal mixing, the variation in temperature was computed for a parcel
of water which left the plant location at 6 pm. The following
graph demonstrates the effect of diurnal variations in meteorological
conditions on the temperature of the water parcel for a distance of
50 miles downstream. Note that the initial temperature of the
parcel is equal to the natural stream temperature (T ) plus the
temperature increase of 8.2°F caused by the plant discharge.
-------
100
TEMPERATURE OF A WATER PARCEL
85
Q)
3
O
v_
Q)
a
I
E
o
«
+-
to
80
75
I
I
I
10 2O 30 40
Miles Downstream from Plant
50
Time (Military)
0
o
00
o
o
^j-
CM
O
O
(Q
O
O
O
CM
O
O
5j-
CVI
O
O
CM
" *
O
o
^-
CM
6 '
o
CM
O
O
CM
-------
101
Part B
Assuming a maximum allowable daily average stream temperature
of 80°F, what flow-through cooling pond area would be required at
the site? The following sketch describes the plant-river-pond lay-
out:
Q! = 1500 cfs
I, = 74° F
RIVER
Q3 = 1500 cfs
T3 = 93°F
COOLING
POND
Q0=3500cfs^)
T0=74°F '
,02= 2000 cfs
1T2 = 74°F
/Q4= 1500 cfs
T4= ?
;= 3500 cfs
-------
102
Temperature Rise Through Plant
Heat to cooling water = 6.41 x 109 BTU/hr
Condenser flow = 1500 cfs = 3.37 x 108 Ib/hr
AT = AT through condenser = - = 19.QoF
C (3.37 x 10b lb/hr)(l BTU/lb °F)
. T = 74°F + 19°F = 93F
Temperature Drop Through Pond
A flow-through cooling pond is assumed to be well mixed
in each cross section, but as in a stream, there is a longitu-
dinal temperature decay. Thus, the equation for predicting the
temperature drop through the pond is equivalent to the exponen-
tial temperature decay equation used on we'll -mixed streams.
Using the temperature subscripts given on the sketch, the
temperature from the pond can be computed by:
T4 = (T3 - E)e'a' + E (27, p. 113)
, , KA
where a = ~r~nr~
pCPQ3
3 -1
Q3 = plant discharge (ft day )
2
A = pond area (ft )
Using an experimental evaporation coefficient (b) of 12, K =
118 and E = 76.9 F. These values are used in the subsequent
cooling pond calculations.
-------
103
Case I - Pond area required for discharge from pond = 80°F.
.'. T = 80°F
Solving the prediction equation for a'
80 = (93 - 76.9)e"a' + 76.9
e~a= (80 - 76.9) / (93 - 76.9)
e"a= 0.193
.-. a' - 1.65
Solving the a' equation for A:
a< = T6274T7lTTi500jT24 hr/dayj73600 sec/hrj
a1 = (1.46 x 10~°)A - 1.65
.'.A = (1.65)7(1.46 x 10~8) - 11.3 x 107 ft2
In acres: A =
(4.36 x 1(T ftVacre)
Ca,se__n_ - Pond area required for mixed river temperature = 80 F.
If a mixing zone is allowed in the stream such that the
mixed river temperature below this zone is equal to or less
than 80°F, a much smaller pond could be used.
-------
104
Referring to the sketch:
Solving for T. :
T T5Q5 - T2Q2 = (80) (3500) - (74) (2000)
4 Q4 1500
. M = 88.0°F
By using the same computational techniques as for Case I
a1 = 0.373
.'. A = (0.373)/(1.46 x 10"8) = 2.55 x 107 ft2
in acres: A = ^55 x 10' ft*> - = 585 acres
(4.36 x 10^ ft /acre) -
-------
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