United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-109 Aug. 1981
Project Summary
Performance Evaluation of the
Braintree Electric Light
Department Dry Cooling Tower
M. D. Henderson, C. H. Armstrong, and D. H. Newton
The performance of a dry cooling
tower for the 20-MW steam-electric
generation portion of an 85-MW com-
bined-cycle power plant was evaluated
in a 5-year project. Under a grant to
the Braintree Electric Light Depart-
ment, the objectives of the demon-
stration were to demonstrate dry
cooling tower technology at a Mas-
sachusetts seacoast site, document
and optimize heat rejection perform-
ance, evaluate the effect of dry cooling
tower operation on the environment,
and define the effect of environmental
conditions on dry cooling tower per-
formance.
Since startup of the plant in 1977.
the unit has been on-line for only
about 2100 hours due to several
equipment failures associated with
the gas turbine which caused 17
months of forced outage. Another
major reason for low utilization has
been the escalating cost of fuel oil.
Originally conceived as an intermediate
load unit, high fuel costs have shifted
it to peaking service. This decreased
operation reduced the scope of the
originally planned demonstration.
During 1979 and 1980 the perform-
ance of the dry cooling tower was
close to design. The combined-cycle
heat rate has always exceeded its
design value, being about 10 percent
higher in 1980.Data collected were
inadequate to demonstrate the freezing
and corrosion resistance of the tower's
finned tubes or the noise generated by
the dry cooling tower apart from the
entire combined-cycle unit.
This Project Summary was devel-
oped by EPA's Industrial Environmen-
tal Research Laboratory, Research
Triangle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
The purpose of this report is to present
the results of a performance evaluation
of the dry cooling tower serving Braintree
Electric Light Department's (BELD's)
combined-cycle power plant. This pro-
ject was initiated with an EPA-sponsored
. demonstration grant in December 1975.
Several areas of investigation were
planned, the most important being
optimization of plant performance. The
plant became commercial in April 1977.
Performance monitoring equipment
was leased and installed between Sep-
tember 1976 and June 1978. During
1978 two separate failures of gas
turbine equipment occurred. Another
gas turbine equipment failure occurred
early in 1980. The total forced outage
time to date is approximately 17 months.
Because of increases in the price of gas
turbine fuels, the plant is currently used
onlyfor peaking service, rather than for
intermediate load service as anticipated.
At the time of preparation of this final
report only about 2000 hours of operation
have been accumulated. Thus the pro-
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4
gram envisioned at the start of this
project has been greatly curtailed even
though the original grant period of 4
years was extended for 1 year.
The objectives of the program were to
demonstrate dry cooling tower technol-
ogy and to document and optimize heat
rejection performance. The effect of
environmental conditions on perform-
ance and the effect of operation on the
environment were to be defined. Pro-
visions were made in plant design to
utilize residual oil for fuel resulting in a
performance penalty. However, only
distillate oil has ever been used.
Conclusions
BELD's combined-cycle unit has not
operated nearly as frequently as planned
due to the substantial increase in fuel oil
cost subsequent to its design and con-
struction. There have also been sub-
stantial forced outages due to major
failures of gas turbine equipment.
However, with rebuilding and increased
operating experience the performance
of the unit has improved.
While it has not been possible to
optimize the performance of the dry
cooling tower, monitoring equipment is
available to detect deviations in steam-
turbine operation, dry cooling tower
heat rejection, and combined-cycle heat
rate. Ambient temperature is the most
important environmental condition
affecting dry tower performance and
unit output. Available data are inade-
quate to demonstrate the freezing and
corrosion resistance of thefintubesorto
address the noise generated by the dry
cooling tower apart from the entire
combined-cycle unit. There are no data
that would indicate passage of the dry
tower plume in the downwind environ-
ment.
Data from approximately 350 hours of
operation in 1979 and 1980 have been
analyzed. The performance of the dry
cooling tower is reasonably close to
design when condensate temperatures
are analyzed. It appears that pressure
measurements are not as accurate as
temperatures in this regard. However,
at ambient temperatures above approxi-
mately 60°F*, a deterioration in per-
formance is noticeable. It is possible
that seasonal adjustments to the pitch
angle of the fan blades (to increase air
flow in the summer) is warranted to
overcome this problem.
The combined-cycle heat rate has
always exceeded the design value, even
though it has decreased since initial
operation. The problem is thought to be
with the gas turbine combustor since
steam cycle parameters and gas turbine
generator output are reasonably close
to design values. Adjustments to com-
pressor air flow and combustion tem-
perature may allow reduction of the
heat rate.
Recirculation of hot exhaust air from
the dry cooling tower to the fan inlet
happens only to the bank of cells closest
to the combined-cycle plant building.
The causes for this recirculation are
unknown but seem to be related to the
diminished heat transfer of the lower
portion of south-side fintubes on Cell
2A-4. Otherwise, heat transfer seems
to be uniform across individual cells and
the entire dry cooling tower.
Data Collection Analysis
All the performance monitoring equip-
ment was installed by June 1978.
However, minimal additional operation
occurred that year. From April 1979
(when Unit 2 was returned to service
following an August 14, 1978, forced
outage) through October 1980 about
1200 hours of operation occurred. For
that period, data have been obtained
and analyzed for some 350 hours. The
magnetic tape unit was found to be
malfunctioning in January 1980 and
only data manually obtained by plant
personnel (on a schedule of approxi-
mately once per hour when the unit is
running) are available.
Data on steam parameters, fuel con-
sumption, fan operation, power produc-
tion, and temperatures of air and con-
densate have been nearly complete.
However, both air quality and meteoro-
logical data (with the exception of
temperature profiles) have been sporadic
and of little value.
A Fortran computer program has
been written to read and analyze the
performance data and to echo the input.
The program computes an average
condensate temperature and corre-
sponding backpressure (based on the
GEA expectation of 4°F subcooling),
temperature corresponding to the mea-
sured backpressure, steam-side duty
(based on measured steam flow and an
assumed heat of condensation of 1000
Btu/lb), average fan inlet and fintube
exhaust temperatures, localized tem-
*Nonmetric units are used in this report because
they remain the standard in the utility industry.
Metric equivalents appear at the end of this
summary for readers more familiar with that
system.
perature differentials on one cell, aver-
age fan power, air-side duty, theoretica
backpressure (based on the GEA per-
formance curves), heat rate, and theo-
retical heat rate (based on the manufac-
turer's design values of gas turbine
output and fuel consumption vs. tem-
perature, steam turbine output vs
steam flow, and backpressure). Tests
are also performed to determine varia-
tions in local heat transfer on one cell,
recirculation (with possible correlation
to low-level winds), same operating
power on all fans (only situation foi
which GEA backpressure is available)
and backpressure limits (limited range
of steam flow and air temperature).
Results
More than 90 percent of the data have
net power values greater than 75 MW
with steam flow in excess of 180,000
Ib/hr and fans running at full speed.
Variations in duty between steam-side
cooling and air-side heating are within
the range of ± 10 percent. Therefore,
the primary influence on steam turbine
backpressure is ambient temperature.
(The average fan inlet temperature frollj
all 10 cells is considered to be ambient
temperature in the following discussion.)
The gas turbine flow is essentially a
constant volumetric rate, so that as
ambient temperature decreases gas
turbine power generation increases.
Steam production in the heat recovery
boiler also increases with decreasing
ambient temperature and the condens-
ing duty decreases in difficulty. Conse-
quently backpressure goes down and
steam turbine power generation in-
creases. While generation is increasing
so is fuel consumption, with the net
result that heat rate changes little with
ambient temperature. Both design con-
ditions and actual data are presented in
Figures 1 through 5 and are discussed
below.
Backpressure
Recorded backpressure measure-
ments (converted from the vacuum
readout) at nominal full-load conditions
range from 2 to 13 in. Hg absolute. The
high end of this range exceeds the
expected range for the dry cooling
tower. The data which have been anal-
yzed in this study are presented in
Figure 1. Measured backpressure ex-
ceeds the GEA estimate by approxi-
mately 2 in. Hg in 75 percent of the
comparisons. It is also apparent that the
performance of the dry cooling tower
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deteriorates appreciably at ambient
temperatures above 60°F. Only limited
data were available with the fans run at
half-speed and no comparisons were
made for the condition.
Data from the plant operator's log
indicate that the temperature of steam
exhausting the turbine is very close (an
average difference of approximately
2°F) to the condensate temperature.
This would indicate that GEA's estimate
of only 4°F subcooling is correct. There-
fore, it was decided that the thermo-
couple measurement of condensate
temperature would be a more accurate
means of comparing actual and design
backpressure. Figure 2 shows values
approximately 1 in. Hg higher than the
GEA estimate in 75 percent of the
comparisons. The deviation from design
values at ambient temperatures above
60°F is also less than that for the
measured backpressure data.
While the average fan power is close
to design, the average temperature
differential across the fintubes is 58°F,
compared with the design value of 53°F.
This would indicate that air flow is less
than design. At present the pitch angle
of the fan blades is less than the design
value due to the observation by BELD
personnel that power consumption by
the fan motors was excessive when the
pitch angle was higher. However, it was
also observed that backpressure was
closer to design when the pitch angle
was higher. It is estimated that a reduc-
tion of 1 in. Hg in backpressure would
result in approximately a 1 percent
increase insteam turbine generator
output, or approximately 200 kW. The
current fan power consumption is 300
to 400 kW. Therefore, the tradeoff
would have to be analyzed with operating
data to see if it is worthwhile to optimize
the performance of the dry tower by
increasing the fan blade angles.
Heat Rate
Of even greater concern to BELD
management is the fact that the unit
heat rate exceeds the design value.
Operation in 1980 has been at a higher
load and at a lower heat rate than 1979.
Figures 3 and 4 compare actual and
design values in the 2 years. As can be
seen from Figures 3 and 4, the design
heat rate changes little with ambient
temperature. However, it is expected
that heat rate increases with elapsed
time since major overhaul and with
decreased load. The gas turbine com-
bustion temperature also influences
heat rate (higher temperatures result in
lower heat rates and less time between
overhauls) and is monitored by opera-
tions personnel. Due to the expected
lower efficiency for 1979 operation, it
was decided to continue the heat rate
analysis only for the 1980 data.
The data in Figure 4 show an average
heat rate approximately 1200 Btu/kWh
above the design value. The variation at
a given ambient temperature is approxi-
mately 1000 Btu/kWh. No attempt has
been made in this study to explain this
variation. In an attempt to explain why
the actual heat rate is so far above the
design value, a comparison was made of
actual and design power generation. As
shown in Figure 5, the relationship
between the two and with ambient
temperature is excellent. It should be
noted that data at temperatures below
approximately 32°F were taken in Jan-
uary 1980, prior to a major equipment
failure. This may account for the reduced
power output at this time.
Based on the good agreement be-
tween design and actual steam flows
mentioned earlier, it is thought that the
steam turbine generator is performing
properly. Since the combined-cycle
output is also up to par, operation of the
gas turbine generator is acceptable.
However, the gas turbine requires more
fuel than anticipated. Verification that
fuel metering is correct has been ob-
tained from periodic checks on levels in
the fuel-oil storage tank. Further invest-
igation of compressor air flow and
temperatures, at various points in the
flow path is planned. It may also be that
the machine, which is one of the first of
its type, is not as efficient as the manu-
facturer expected.
It should also be mentioned that
recent (subsequent to the data analyzed)
modifications to the gas turbine com-
bustor have reduced the heat rate by
approximately 200 Btu/kWh and in-
creased the gas turbine power output by
2 MW. This modification is only one in a
series of fine-tuning steps which may
enable the combined-cycle unit to lower
its heat rate.
Recirculation
With regard to the question of recircu-
lation of exhaust air from the dry cooling
tower to the fan inlet, analysis of the
data indicates that it happens approxi-
mately one-third of the time. The recir-
culation test requires that the tempera-
ture at one cell exceed the average of all
10 by more than 5°F. Typically this
requirement is met when the tempera-
ture at one cell exceeds those adjacent
to it by at least 10°F. In all cases the
recirculated air is experienced by theA-
bank of cells closest to the combined-
cycle building. In most cases, the affected
cell is in the middle and least accessible
to ambient air.
The exhaust steam header passes
beneath the middle cell and could
increase the inlet air temperature to this
cell. However, most of the time this is
not the case. Downwash is normally
expected to occur when hot exhaust air
is forced down by high-velocity winds.
The intent of the wind screen around
the dry cooling tower is to eliminate this
problem by not allowing interaction
until the hot plume is consolidated near
the top of the fintube bundles. Judging
by the lack of recirculation along the
outer bank of cells, the wind screen is
effective.
Low-level winds (at the 30-ft height
on the meteorological tower) were
investigated to determine whether any
correlation with recirculation exists.
The average wind direction is bivariate
with azimuth angles of approximately
50 degrees and 210 degrees, approxn'
mately perpendicular to the long axis of
the dry cooling tower. The range of
values in each case is approximately
100 degrees. Thus the wind direction
values correspond to a downwash inter-
pretation of the recirculation phenome-
non. The average wind speed is approxi-
mately 8 mph with more than that
amount of variation. By contrast, the
expected velocity of hot air exhausting
from the fintubes is approximately 5
mph. While the average speed is con-
sistent with expected downwash occur-
rence, the wide variation discredits this
interpretation.
The recirculation is related to the
adjacent, higher combined-cycle build-
ing. However, a wind-related cause
cannot be convincinglydemonstrated.
Therefore, it is planned to investigate
further the flow path of hot air exhausted
from the inner bank of cells.
Localized Heat Transfer
Although not originally designed for
that purpose, the localized heat transfer
data from one of the A-cells seem to
offer some insight to recirculation
occurrences. The temperature differen-
tial across the fintubes is almost.always
greater on the north side than on the
south side. Most of the time there is also
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30
40
50 60
Inlet Temperature (°FJ
70
80
90
Figure 2. Variation of condensate temperature-derived backpressure with ambient temperature.
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10,500
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1 9.500
9.000
8,500 -
8.000
20
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Inlet Temperature f°F)
Figure 3. Variation of heat rate with ambient temperature-1979.
6
70
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Inlet Temperature (°F)
Figure 5. Variation of power generation with ambient temperature-1980.
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local variation on the south side. Closer
examination has revealed the cause.
The plenum air temperature of this cell
is highest at the bottom (closest portion
of the fintube bundle to the combined-
cycle building) and lowest at the apex.
Exhaust temperatures are relatively
constant leading to the variation in
temperature differential. As further
evidence of this phenomenon, the
shroud temperatures (measured below
the fan) are also highest on the building
side.
The data suggest that, at least on the
building side of the a A-bank of cells,
heat transfer across the lower portion of
the f intubes is impeded by recirculation.
The exact path of this recirculation has
not been determined. Otherwise, heat
transfer across the fintubes appears to
be uniform and in accordance with
design.
Metric Conversions
Although EPA's policy is to use metric
units in its publications, this document
uses certain nonmetric units that remain
the standard in the utility industry.
Readers more familiar with metric units
should use the following conversion
factors:
1 in. = 2.54 cm
1 ft = 0.305 m
1 gal. = 0.0038 m3
1 ft3 = 0.028 m3
1 Ib = 0.45 kg
1 in. Hg = 0.033 atm
1 psi = 0.068 atm
1 mph = 0.45 m/sec
1 Btu = 252 cal
1 kWh = 860 kcal
1 hp = 0.75 kW
°C = 5/9(°F-32)
1°F (change) = 0.556°C
M. D. Henderson and C. H. Armstrong are with R. W. Beck and Associates,
Denver, CO 80204; D. H. Newton is with the Braintree Electric Light Depart-
ment, East Braintree, MA 02189.
Theodore G. Brna is the EPA Project Officer (see below).
The complete report, entitled "Performance Evaluation of the Braintree Electric
Light Department Dry Cooling Tower," (Order No. PB8J -222 242; Cost: $8.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
» U.8 GOVERNMENT PMNTINO OFFICE: 1M1 -757-012/7308
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