Warm-Liquid Defrost for Commercial Food Display Cases:
Experimental Investigation at 32.2°C Condensing
Cynthia L. Gage
U.S. EPA, NRMRL
RTP,NC 27711, USA
Georgi S. Kazachki
ARCADIS Geraghty & Miller, Inc.
RTP, NC 27711 USA
Abstract
A refrigeration test, rig with two open cases and two reach-in cases was tested using warm-liquid defrost
(WLD) at -34.4°C evaporating, 32.2°C condensing, and 4.4 K subcooling below the condensing temperature.
Results were compared to electric defrost (ED) at the same conditions. For all cases, WLD at 32.2°C condensing
performed as well as ED (i.e., coils reached comparable temperatures at the end of defrost). For the open cases
where defrost times were comparable, WLD causes a 1.7 K smaller rise in product temperature than is observed
with ED. For the reach-in cases, defrost time for the WLD was 1 hour compared to 35 minutes for ED. As a result
of the longer defrost time, the product temperature was higher by 1.7 K with WLD than with ED. Increasing the
flow rate of the liquid would shorten the defrost time and improve the product conditions.
1.	Introduction
A major energy demand in refrigeration systems comes from the display cases; therefore, there is interest in
investigating energy saving opportunities for this equipment. One of the energy components in the display cases is
the defrost energy. In fact, the requirement for defrosting of the coils in the cases has a dual impact on energy
consumption. The first impact is the energy required for the defrosting process. The second impact is the additional
refrigeration energy that is required to recool the coil and the product from the effects of excessive defrost energy.
Common defrost methods for commercial refrigeration systems include electric defrost (ED), hot-gas defrost
(HGD), and cool-vapor defrost (CVD). ED, the most common method, requires significantly more energy than the
heat that is actually used to melt the ice. Much of the excess energy ends up warming the product in the case. In
HGD, additional piping is required, and the wide temperature swings between hot defrost and normal operation can
stress system components and eventually result in leaks. CVD also requires additional piping; however, it subjects
the system components to less stress from the defrost-to-normal-operation temperature swings than HGD.
Recently the warm-liquid defrost (WLD) concept has been proposed for direct-expansion (DX)
refrigeration systems (Mei et al., 2001a, 2001b). In this concept, high-pressure liquid from the receiver is used to
defrost the coil. The pressure of the cold liquid is then reduced after the coil by an expansion device. This method
has several anticipated advantages which should reduce both the energy penalty of defrost and the temperature
swing of the products during defrost, and shorten the post-defrost pull-down time for the product. This concept has
an added benefit of using existing liquid and suction piping.
In this work, WLD is implemented on four cases connected to a common suction manifold of a three-
compressor rack. Initial investigations were performed on various aspects of the concept including effectiveness of
the defrost at lower condensing temperatures, control strategy options, viability of a thermostatic expansion valve
(TXV) as the after-coil expansion device, and observations for the expected benefits. In this paper, results at 32.2°C
condensing are presented and compared to tests performed with ED at the same operating conditions.
2.	Test Equipment and Procedure
The commercial refrigeration research facilities include an instrumented supermarket refrigeration test rig,
chambers for environment control around the cases, two synchronized data acquisition systems, and a chiller for
condensing temperature control. The refrigeration test rig includes: two low-temperature single-deck display
refrigerators; two two-door reach-in cases; and a condensing unit with three unequal compressors, a water-cooled
condenser, a water-cooled subcooler, an oil management system, and a programmable controller. The system uses
R-404A refrigerant. The primary factors of interest are the product temperatures and the energy consumed by the
various components, including the total energy. Figure 1 is a test-rig schematic. Table 1 is a nomenclature list.
In order to simulate operation in a store, the cases are located in an environmental chamber where ambient
temperature and relative humidity are maintained at 23.9QC and 55% RH (17.8°C wet bulb), respectively. Doors on
the reach-in cases are pneumatically opened to simulate customer usage (six 10-second openings per door per hour
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for 8 consecutive hours). To monitor product temperatures in the cases, test and dummy packages as prescribed in
ASHRAE 72-1998 and ASHRAE 117-1992 are used. Test packages (i.e., product temperature sensors) are located
in all four cases. Each reach-in case has 36 test packages, and each open case has 12. Energy usages of each case
(ED heaters, fans, antisweat heaters, and lights) are also monitored. Energy usage of the individual compressors is
measured, as well as the total energy usage of the refrigeration system.
Test conditions are specified by setting evaporating temperature (suction pressure at the compressor
manifold), vapor superheat leaving the coils, condensing temperature, and liquid subcooling. For all tests, liquid
subcooling was set to 4.4 K below the condensing temperature and superheats were set to 4.4-5.6 K at each case.
The refrigeration test rig is capable of operating the cases under pressure control or under temperature
control. Under pressure control, suction pressure at the manifold is specified, and the solenoid valves at the cases
remain open constantly. The compressors cycle to maintain the set suction pressure. All tests reported here were
conducted with the system operating under pressure control.
For each case, ED control involved setting defrost start time, defrost termination temperature, and default
duration. Default duration sets the maximum length of defrost on time, if the termination temperature is not
reached. All cases were set for temperature termination at 8.9°C with default duration of 45 minutes for the reach-
in cases and 60 minutes for the open cases. Defrost start times for the cases were staggered by 2 hours each after the
first case initiated defrost. Each case defrosts once per 24 hours.
2.1	Warm-liquid defrost
For WLD, case piping was modified by the installation of shut-off valves and a second thermostatic
expansion valve (TXV2). Figure 2 is a schematic of the modified system. TXV2 was selected with the same
capacity as TXVt but with a 10-foot capillary tube for the sensing bulb so that it could be attached at the common
suction line for the cases. The valve was initially set at the full-open position. At the initiation of defrost, SVL
closes. After a 2-minute delay to allow for pump-down, SVs closes and SVb opens to allow the flow of the warm
liquid to the coil. At the end of defrost, SVB closes and the openings of SVs and SVL are delayed for 10 minutes to
allow the warm liquid remaining in the coil to flow out through TXV2. Mixing of the cold and warm streams would
have reduced performance efficiency of the individual streams. Heat was provided to vaporize any remaining liquid
before reaching the suction manifold. Defrost control was set for time-termination at 1 hour, the maximum of the
electric-defrost durations recommended by the manufacturer for the reach-in and open cases. At higher condensing
temperatures (>32.2°C), TXV2 was adjusted from the full-open position to achieve complete defrost at the end of
time-termination. As with ED, defrost start times were staggered at 2-hour intervals.
2.2	Data collection
An automated data acquisition system collects and logs 300-plus parameters once a minute. A running log
of 36 hours of data is maintained and downloaded every 24 hours. These instantaneous data are processed to
calculate averages and 24-hour cumulative values. A test period covers 30 hours: the start of the first defrost of the
first case to the start of the second defrost of the last case. The instantaneous temperature of the "average" package
in a given case is the average of the instantaneous temperatures of all test packages in the case (12 for open cases
and 36 for reach-in cases). The instantaneous average values of the test packages are combined over 24 hours to
produce a single value "integrated-average" temperature (IAT) for each case. System and compressor energy data
are calculated across the defrost and running cycles of the first case. IATs and energy data for the individual cases
are calculated across the defrost and running cycles of each individual case. Temperature and pressure data for the
individual cases are averaged over the last three quarters of the running cycle for each individual case.
3. Results and Discussion
Warm-liquid defrost was tested at an evaporating temperature of -34.4°C and at several condensing
temperatures. This paper presents the results at 32.2°C condensing. Comparisons are made to tests performed with
ED at the same operating conditions. For Figures 3-6 and Figure 11, the legend nomenclature corresponds to the
nomenclature in Figure 2.
Figures 3 and 4 compare the coil temperatures of the same coil in an open display case for both WLD and
ED. For WLD, defrost was time-terminated at 1 hour. For ED, defrost was temperature-terminated, resulting in a
defrost cycle of 53 minutes. Coil temperatures for both methods of defrost exceed 0°C at the end of defrost, and the
temperatures plateau during the ice melting process as expected. Cycling of the warm liquid flow (MF_r) occurred
due to the response of the TXV to temperature swings measured by its bulb in the common suction line of the cases.
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.,3/1 I
>|-X
,.8/1 1
A-s cW"
1^1 llXlh^3 :Z/l l Q-

?

r
1-^,.8/r
..8/1 I
,.8/L
.,8/L
Figure 1: Schematic of Direct-Expansion Test Rig.
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Table 1: Nomenclature for Figure 1: Schematic of the Direct-Expansion Test Rig
c
Condenser water supply
CND
Condenser
CR
Condenser water return
C1,C2,C3
Compressors
D
Drop (pitch of pipe)
FD
Filter-drier
FMi
Flow meter at case i
Mdf.i
Defrost water from case i
mR
Refrigerant mass flowrate
Pd
Discharge manifold pressure
Pin,i
Refrigerant inlet pressure to case i
P out,i
Refrigerant outlet pressure of case i
Ps
Suction manifold pressure
OS
Oil separator
R
Receiver
RD
Refrigerant discharge
RFM
Refrigerant flow meter
RL
Refrigerant liquid line
RS
Refrigerant suction line
RV
Refrigerant valve after the receiver
sc
Subcooler water supply
SCR
Subcooler water return
SF
Suction filter
SGI
Sight-glass indicator
SV
Solenoid valves at the chiller
SI, S2, S3, S4
Solenoid valves at the case inlets
Tdj
Discharge temperature of compressor i
Tin,i
Inlet temperature of case i
T0ut,i
Outlet temperature of case i
Ts,i
Suction temperature to compressor i
T
1 subc,tn
Temperature at subcooler inlet
f Snbc,Ollt
Temperature at subcooler outlet
Twi
Water temperature into condenser
Tw2
Water temperature out of condenser
WM
Water flow meter
WRV
Water regulating valve
WS
Water strainer
wv
Water shut-off valve
Tx_OUT C
LIQUID BYPASS
— T34
' "T,2
DEFROST
HEATER
rTX^if t Iflow) t
1 ev_in |	rznzzmzinz
SUCTION-LINE
HEAT EXCHANGER
LEGEND
PL PRESSURE OF LIQUID ENTERING CASE
Ps PRESSURE OF VAPOR LEAVING CASE
SVB SOLENOID VALVE FOR LIQUID BYPASS
SVL SOLENOID VALVE FOR LIQUID LINE
SVs SOLENOID VALVE FOR SUCTION LINE
TXV, THERMOSTATIC EXPANSION VALVE FOR COIL
TXV2 THERMOSTATIC EXPANSION VALVE FOR
WARM-LIQUID DEFROST
Tev_in TEMPERATURE AT THE TXV, INLET
T„ ,N TEMPERATURE AT THE COIL INLET
T12 TEMPERATURE AT X OF THE COIL
Tm TEMPERATURE AT % OF THE COIL
Tx_out TEMPERATURE AT COIL OUTLET
T,n TEMPERATURE AT CASE INLET (RTD)
Tout TEMPERATURE AT CASE OUTLET (RTD)
svB
T
SVL
SVs



txv2
- Ps
" Pl
SUCTION LIQUID
LINE LINE
Figure 2. Schematic of a Display Case with Warm-Liquid Defrost.
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Tx in
Tx out
11 16 21 26 31 36 41 46 51 56 61 66 71 76 81
Time (minutes)
86 91
_ Figure 3. Open-Case Coil Temperatures During Warm-Liquid Defrost at 32.2°C Condensing
Temperature
140
25
130
20
120
110
100
80
70
§• -10
Tevjn
Tx in
»- -15
-20
40
-25
-30
20
	Tout
	MF r
-35
4 10
-40
6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86
1
Time (minutes)
i
Figure 4. Open-Case Coil Temperatures During Electric Defrost
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Figures 5 and 6 compare coil temperatures for the reach-in display case for both warm-liquid and electric
defrosts. As with the open case, WLD was time-terminated at 1 hour for the reach-in case. For ED, defrost was
temperature-terminated, resulting in a defrost cycle of 35 minutes for the reach-in case.
Lower warm-liquid flowrates were required for the reach-in cases compared to the open cases in order to
achieve defrost in 1 hour. For the cases shown in Figures 3 and 5, the reach-in case required 49 kg of warm liquid to
defrost the coils, and the open case required 71.4 kg. This correlates with the lower frost quantities that form in
cases with doors.
Figure 7 shows the impact of the defrost technologies on air temperatures in and out of the coil on the open
case. During ED, fans continue to operate to maintain the air curtain over the case. Air temperature out of the coil
peaks almost 11 K warmer for ED than with WLD. The corresponding impact on the product temperature is shown
in Figure 8. The data here have been overlapped so that minute 1 is the start of defrost, and the graph shows the
temperature deviation of the average package from its 24-hour IAT. During ED, the temperature of the "average"
package rises by about 7 K compared to a 5.6-K rise for the WLD. Also of interest is the temperature rise of the
warmest package in the case: for ED, the severest temperature swing is 11.7 K for a package near the air discharge
grill. This compares to a 7.8-K swing for the warmest package in WLD.
Figure 8 also shows that the product temperature begins to rise faster for ED than with WLD. This is
expected since ED operates by warming the air first; whereas, for WLD, the coil is warmed first. During pull-down,
the average package under WLD reaches its IAT (i.e., deviation = 0) about 45 minutes sooner than under ED. Thus
for open cases under ED, the packages are warmer for a longer period of time and peak at a higher temperature even
though the defrost time was about 10% shorter (7 minutes) than for WLD.
Figure 9 shows the air temperature in and out of the coil for a reach-in case during electric and warm-liquid
defrosts. For this case, the fans are shut off during defrost.' Air temperatures in ED are very high due to the
proximity of the weighted thermocouples to the electric heaters and due to lack of forced air circulation. Radiation
may also play a role. (Also note that the high air-out temperatures in this test lead to the discovery that the defrost
thermostat had been placed in the wrong location. After the sensor was placed correctly, peak air-out temperatures
dropped from about 77 to 63°C.) Air temperatures during WLD are significantly lower than during ED; however,
the temperatures remain high for a longer period of time due to the longer defrost time (1 hour as opposed to 35
minutes).
The impact on the temperature of the average product is shown in Figure 10. Minute 1 is the start of
defrost. Rate-of-temperature rise is comparable for both electric and warm-liquid defrosts. However, due to the
longer time in defrost for WLD, the package temperature rises higher than under ED. Under ED, the fans were off
for 35 minutes, and under WLD the fans were off for twice that period (1 hour during defrost, and 10 additional
minutes to allow the liquid remaining in the coil to drain through the TXV2). As a result, the package temperature
under WLD peaks almost twice as high as with ED (a 4.4-K rise compared to slightly over 2 K for ED). These
results emphasize the need to keep defrost time as short as possible.
One way to reduce defrost times in warm-liquid defrost is to use higher liquid flow rates through the coils.
Figure 11 shows the impact of a higher flow rate in the open case when TXV2 was set in the full-open position. The
average flow rate during defrost is 84 kg/hr compared to 68 kg/hr in Figure 3. All the coil temperatures exceed
10°F in about 40 minutes, although time-termination was still set for 1 hour.
One of the anticipated benefits is energy savings from elimination of the electric defrost heaters. The
preliminary results at 32.2°C did not show a reduction in energy usage due to an energy offset from the additional
runtime of the compressors during defrost. As Figures 3 and 5 show, mass flows during defrost are 2 to 3 times
higher than during normal operation.
An estimate was made of the percentage of the energy required to melt the ice relative to the total defrost
energy. This was done for both WLD and ED for the open case. The estimate takes into account only the energy
required to melt the ice: it does not include the energy needed to warm the ice to the melting point. Under ED, 1.6
kg of water was collected using 1.02 kWh of electric heat. This yields 14.5% for the utilized energy. For WLD, 1.4
kg of water was collected using the 71 kg of warm liquid. Using a liquid temperature drop of 23.9 K (23.9°C
entering the coil minus 0°C), this yields 0.68 kWh and correspondingly 18.3% for the utilized energy.
4. Conclusion
WLD was tested at a 32.2°C condensing temperature with defrost termination after 1 hour. During defrost,
coil temperatures were comparable to temperatures observed under ED for both open and reach-in cases. For the
open case, the average product temperature rose 5.6 K in WLD compared to a 7.2-K rise for ED, and the product
was also warm for a shorter period of time than with ED.
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*—Tev in
Tx in
Tx out
*—T12
MF r
g
~i	1	1	r-
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91
Time (minutes)
Figure 5. Reach-in Case Coil Temperatures During Warm-Liquid Defrost
at 32.2°C Condensing Temperature
o
¥
3
E
-------
-Air In ED
- Air Out ED
-Air In WLD
-Air Out WLD
o
o
-------
	Air In ED
— - Air Out ED
	Air In WLD
	Air Out WLD
o
o
0)
3
ra
0)
o.
E
0)
I-
-10
-20
-30
-40
91 101 111 121 131 141 151
11
21
31
41
51
61
71
81
1
Time (minutes)
Figure 9. Comparison of Air Temperatures In and Out of Coil for the Reach-in Case
(ED=Electric Defrost, WLD=Warm-Liquid Defrost)
5
Warm Liquid
Electric
4
3
2
1
0
1
¦2
1
31
61
91
121
151
181
211 241 271 301 331 361 391 421
Time (minutes)
Figure 10. Impact of Defrost on Average Package of a Reach-in Case
Devation of Temperature from 24-hour Integrated Average Temperature
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Tin
Tout
MF r 4
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96
Time (minutes)
Figure 11. Open-Case Coil in Warm-Liquid Defrost at 32 2°C Condensing
Higher Liquid Flow Rate at Defrost
For the reach-in case, the temperature rise of the average product under WLD was almost twice the value
observed for ED. This was probably a result of the longer defrost time in WLD compared to ED (1 hour as opposed
to 35 minutes). Higher liquid flow rates would reduce the defrost times.
In WLD, more of the energy is used to melt the ice, and less is wasted compared to ED.
5. Acknowledgements
The authors wish to acknowledge the significant equipment contributions that Hussrnann Corporation made
to establish the research facilities. We also acknowledge the support of Hill-Phoenix and CPC who were
instrumental in updating the control system. This work was jointly funded by the U.S. Department of Energy
(Office of Building Technology, State and Community Programs under contract DE-AC05-00OR22725 with UT-
Battelle, LLC) and the U.S. Environmental Protection Agency under contract 68-C-99-201.
6. References
Mei, Viung C., Fang C. Chen, and Ronald E. Domitrovic, "Supermarket Display Case Defrosting
Technologies," U.S. Department of Energy Display Case Workshop, Atlanta, GA, January 31, 2001a.
Mei, Viung C., Fang C. Chen, and Ronald E. Domitrovic, "Apparatus and Method for Evaporator Defrost,"
U.S. Patent #6,250,090, June 26, 2001b.
Standard 72-1998, "Method of Testing Open Refrigerators," American Society of Heating, Refrigerating
and Air-Conditioning Engineers, Atlanta, GA, 1998.
Standard 117-1992, "Method of Testing Closed Refrigerators," American Society of Heating,
Refrigerating and Air-Conditioning Engineers, Atlanta, GA, 1992.
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t,™ - roA TECHNICAL REPORT DATA
INKnKL-KIr-r-bol (Please read Instructions on thereverse before completing.
—
1. REPORT NO. 2.
EPA/600/A-02/086
3. RE
4. TITLE AND SUBTITLE
Warm-liquid Defrost for Commercial Food Display Cases:
Experimental Investigation at 32.2°C Condensing
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Cynthia L. Gage (EPA); Georgi S. Kazachki (ARCADIS)
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
ARCADIS Geraghty & Miller, Inc.
P.O. Box 13109
Research Triangle Park, North Carolina 27709
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-C-99-201 (ARCADIS)
DoE IAG DE-AC05-000R2 27 25
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Published paper;
14. SPONSORING AGENCY CODE
EPA/600/13
15. supplementary notes appq) Project officer is Cynthia L. Gage, E305-02, phone 929/541-
0590. For presentation at Illinois Conference - Commercial Refrigeration, Urbana, IL,
7/22-23/02.
16.abstract paper gives results of an experimental investigation at 32.2 C conden-
sing of warm-liquid defrost for commercial food display cases. A refrigeration test rig
with two open cases and two reach-in cases was tested using warm-liquid defrost (WLD)
at -34.4 C evaporating, 32.2 C condensing, and 4.4 K subcooling below the condensing
temperature. Results were compared to electric defrost (ED) at the same conditions.
For all cases, WLD at 32.2 C condensing performed as well as ED (e.g., coils reached
comparable temperatures at the end.of defrost). For the open, cases where defrost
times were comparable, WLD causes a 1 K smaller rise in product temperature than is
observed with ED. For the reach-in cases, defrost time for the WLD was 1 hour, com-
pared to 35 minutes for ED. As a result of the longer defrost time, the product tem-
perature was higher by 1.7 K with WLD than with ED. Increasing the flow rate of the
liquid would shorten the defrost time and improve the product conditions.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. cosati Field/Group
Air Pollution
Refrigerators
Defrosting
Food Storage
Energy
Pollution Control
Stationary Sources
13B
13A
13H
06H
14G
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
10
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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