Barbara Minor is an Engineering Fellow at DuPont Fluoroproducts, Wilmington, DE; Ward Wells is a Senior Engineer at DuPont

Fluoroproducts, Wilmington, DE. 

# Low GWP R-404A Alternatives for Commercial Refrigeration

Barbara Minor Ward Wells

Member ASHRAE Member ASHRAE

ABSTRACT
R-404A has been the global industry standard HFC re frigerant for commercial refrigeration in both smal l and large systems. Due to the high global warming potential of R-404A however (AR4 100 year GWP = 3922), intere st is growing to identify low GWP alternatives which match R-404A pe rformance. The new low GWP refrigerant HFO-1234yf (100 year GWP = 4) has been found to be a suitable component in mixtures which can be formulated to match R-404A performance while reducing GWP. Two approaches have been inv estigated. The first is to develop the lowest GWP R-404A alternative possible (GWP ~1400) while maintaining nonflammabil ity. This is desirable for large supermarket system s and for retrofit of existing R-404A systems in the field which were des igned for nonflammable refrigerants. The second ap proach is to drive to the lowest GWP (e.g. ~250), but with mild flammabil ity which meets ASHRAE A2L classification. This c ould be suitable for systems where a small degree of flammability may be allowed with smaller refrigerant charge sizes base d on industry codes and standards. These two approaches will be evalua ted by thermodynamic cycle analysis and through exp erimental testing in an R-404A self contained freezer. Several parame ters will be measured compared to an R-404A baselin e, including energy consumption, pressures and temperatures and mass fl ow rate.

INTRODUCTION

When replacements for chlorofluorocarbon (CFC) and hydrochlorofluorcarbon (HCFC) refrigerants were nee ded in the early 1990s due to the ozone issue, hydrofluorocarb on (HFC) refrigerants rapidly emerged as the leadin g alternatives. They had no ozone depletion potential because chlorine w as eliminated from their chemical structure. They had low toxicity, were non-flammable and thermally stable. And they offer ed a significantly lower global warming potential ( GWP) reduction versus CFC refrigerants. However, they still exhi bited relatively high GWPs compared to non-fluorina ted options. When climate change emerged as a leading environmental i ssue, HFCs came under challenge, particularly their use in more emissive applications such as automotive air condit ioning and large supermarket systems where leaks ar e more difficult to control. Commercial refrigeration is also of conc ern because the leading HFC refrigerant in use is R -404A with a 100 year GWP of 3922 (IPCC 2007), among the highest of the H FC refrigerants. The automotive industry selecte d a new low GWP hydrofluoroolefin HFO-1234yf (CF 3 CF 2 =CH 2 ) (SAE 2009) with a 100 year GWP of 4 (Minor and Sp atz 2008) as the preferred alternative for automotive air conditioni ng. They determined that even though HFO-1234yf was mildly flammable, because the charge size was small, it could be safe ly used in vehicles. There was therefore an opport unity to employ HFC1234yf as a component in developing low GWP mixture s to match R-404A performance. Two candidate refrigerants have been developed with performance similar to R-404A. The first, DR-7 ha s a GWP of
about 250 which is 94% lower than R-404A. It is mi ldly flammable (ASTM E681-04 2004) and therefore m ay have some limitations for use in larger charge systems. DR-3 3 is formulated to be nonflammable (ASTM E681-04 2 004 and ASHRAEStd 34-2010 2010), with a GWP of about 1400 which is about 65% lower than R-404A. Its GWP is also lo wer than other

alternatives for R-404A such as R-407A (36% lower) and R-407F (23% lower). Performance of both refrige rants has been evaluated in a reach-in freezer originally designed for R-404A.

THERMODYNAMIC PROPERTIES AND PERFORMANCE

A comparison of thermophysical properties of DR-7 a nd DR-33 compared to R-404A are shown in Table 1 below. The boiling points are very similar and the critica l points of DR7 and DR-33 are higher than R-404A. Vapor pressures and liquid densities are also similar. The vapor densi ties of DR-7 and R-33 are slightly lower than R-404 A which will reduce
mass flow rate. Table 1: Thermophysical Properties

R-404A DR-7 DR-33 Boiling Point °C (°F) -47 (-53) -48 (-54) -46 (-50) Critical Point °C (°F) 72 (162) 81 (178) 82 (180) Vapor Pressure at 25°C (77°F) in

kPa (Psia) 1254 (181) 1372 (199) 1271 (184)

Liquid Density at 25°C (77°F) in kg/m3 (lb/ft3) 1044 (65.2) 1133 (70.7) 1139 (71.1)

Vapor Density at 25°C (77°F) in kg/m3 (lb/ft3) 65.3 (4.07) 46.8 (2.92) 48.9 (3.05)

To evaluate the thermodynamic cooling performance, cycle modeling was performed for DR-7 and DR-33 ver sus R404A. The conditions were chosen to be fairly compa rable to the system conditions in the reach-in free zer tested at 32°C (90°F) ambient temperature: Evaporator temperatur e = -26°C (-15°F), Condenser temperature = 43°C (11 0°F), Subcool amount = 14K (25°F), Suction temperature = -4°C (25 °F) and compressor isentropic efficiency = 75%. Re sults are shown in
Table 2

.

Table 2: Thermodynamic Cycle Performance

Suction
Pressure
kPa (Psia) Discharge

Pressure kPa (Psia) Comp

Ratio Compressor Disch Temp C (F) Average

of Evap
and Cond Temp
Glide C (F) Capacity

kJ/m3 (Btu/ft3) Capacity

Rel to
R-404A COP COP

Rel to
R-404A

R-404A 242 (35) 1969 (286) 8.1 86 (187) 0.4 (0.7) 1 442 (38.8) 100% 2.03 100%

DR-7 230 (33) 1987 (288) 8.6 107 (225) 5.7 (10.3) 1 488 (40.0) 103% 2.05 101%

DR-33 207 (30) 1869 (271) 9.0 103 (217) 4.3 (7.7) 1 372 (36.9) 95% 2.07 102%

DR-7 and DR-33 both exhibit pressures and compressi on ratios comparable to R-404A. They both have hig her temperature glides than R-404A, but temperature gli des are comparable to some other commercial refrige rants in use today. For example, R-407C, which has been used successful ly for many years, under these conditions has a tem perature glide of 4.8°C (8.6°F). Compressor discharge temperatures ar e also somewhat higher than R-404A; however, the co oling capacity and energy efficiency is a very close match to R-404A.SYSTEM TESTING

In order to measure system performance of DR-7 and DR-33 in a system, a reach-in freezer was selected, fully instrumented and placed in a constant temperature r oom. The freezer is a 52 ft 3 (1.5 m 3 ) double-door unit designed for R404A with a refrigerant charge size of 1.15 kg (2.5 4 lb). It has a reciprocating compressor which use s a polyol ester (POE) 32 centistokes lubricant. The test setup was selec ted to conform as closely as possible to AHAM Stand ard HRF-1-2008 Energy and Internal Volume of Refrigerating Applian ces (AHAM 2008). It was not possible to use ANSI/A SHRAE Standard 72 because the system was not set up to op en and close freezer doors during the test. The re ach-in freezer is controlled with two thermostatic expansion valves ( TXVs). A picture of the unit is shown in Figure 1.

Figure 1 Picture of R-404A Reach-in freezer used in exper imental testing

The empty freezer (per AHAM HRF-1-2008) was placed in a constant temperature room and fully instrument ed with temperatures (T), pressures (P), mass flow rate (M) and energy measurement (W) as shown in Figure 2. Though humidity was not required to be controlled in the chamber (p er the standard Section 5.3.2) , tests at each cond ition for each refrigerant were repeated three times to insure results were re produced and any humidity variation was not impacti ng results.Figure 2 Schematic showing instrumentation of reach-in free zer

Tests were conducted at three ambient conditions 32 °C (90°F) required by the standard and also 24°C (7 5°F ) and 21°C F (70°). An R-404A baseline was tested at the th ree ambient conditions first. At each condition, t he evaporator exit superheat was controlled by adjusting the TXV to 3- 4K (6-7F). The evaporator superheat was calculated from the suction pressure and the temperature reading at the exit of the evaporator. The freezer internal average comp artment temperature (average of four thermocouple readings (placed thro ughout the freezer compartment) was controlled to - 17°C (1.4F). The compressor was allowed to cycle to maintain the cab inet set point temperature. Temperatures and press ures are reported as averages only while the compressor was running. Af ter testing, R404A was replaced with DR-7, then DR- 33. In each test, the system was flushed to remove lubricant and a ne w charge of the same type of POE lubricant was adde d to the system. No other changes were made to the system except to adj ust the evaporator superheat with the TXV to match R-404A superheat conditions. For both candidate refrigerants, the T XVs were closed one turn. Steady state results are shown in Table 3 for SI
units and in Table 4 for I-P units.Table 3. Reach-in Freezer Steady State Test Result s – SI Units

Energy Consumption (kWhr/day) Energy

Cons Rel to R404A Evap

Exit Sup Heat (K) Mass Flow Rate (kg/hr) Suction

T(°C) Suction

Pressure (kPa) Discharge

Pressure (kPa) Compr

Ratio Comp

Disch Temp (°C)

Ambient T = 32°C

R404A 33.46 100% 4 44 -5.2 211 2027 10 90 DR-7 30.81 92% 3 34 0.7 191 2133 11 105 DR-33 32.16 96% 4 30 4.6 172 1931 11 104

Ambient T = 24°C

R404A 25.11 100% 3 46 -9.7 198 1647 8 75 DR-7 23.91 95% 4 35 1.9 184 1728 9 91 DR-33 25.00 100% 3 31 0.6 172 1551 9 88

Ambient T = 21°C

R404A 23.39 100% 3 48 -13.1 199 1558 8 70 DR-7 22.62 97% 3 36 -0.7 179 1600 9 85 DR-33 23.41 100% 3.00 33 0.7 172 1448 8 83

Table 4. Reach-in Freezer Steady State Test Result s – I-P Units

Energy Consumption (kWhr/day) Energy

Cons Rel to R404A Evap

Exit Sup Heat (F) Mass

Flow Rate (lb/hr) Suction

T(°F) Suction

Pressure (Psia) Discharge

Pressure (Psia) Compr

Ratio Comp

Disch Temp (°F)

Ambient T = 90°F R404A 33.46 100% 7 96 22.6 31 294 10 194 DR-7 30.81 92% 6 75 33.3 28 309 11 221 DR-33 32.16 96% 7 65 40.3 25 280 11 219

Ambient T = 75°F R404A 25.11 100% 6 102 14.5 29 239 8 167 DR-7 23.91 95% 7 76 35.4 27 249 9 195 DR-33 25.00 100% 6 68 33.0 25 225 9 190

Ambient T = 70°F R404A 23.39 100% 6 105 8.4 29 226 8 158 DR-7 22.62 97% 6 78 30.8 26 232 9 184 DR-33 23.41 100% 6 73 33.3 25 210 8 182

Experimental results in the reach-in freezer show D R-7 and DR-33 exhibit similar performance compared to R404A, with both demonstrating improved energy effic iency at higher ambient conditions. DR-7 consume d 8% less energy at 32°C (90°F) ambient temperature and DR-33 4% les s energy. At lower ambient conditions, DR-7 still demonstrated 3-5% improved efficiency, whereas DR-33 was equivalent t o R-404A. For both refrigerants, the suction tempe rature was slightly higher than R-404A and for all three refrigerants, somewhat higher than expected. This may be due to the fact that the suction temperature thermocouple was located as clo se to the compressor as possible and conduction of heat from the compressor may have elevated the suction temperatur e readings. The mass flow rate was slightly lower and the dischargetemperature was slightly higher. However, the disc harge temperatures were still low enough to not req uire liquid injection

which is usually needed at temperatures great than 135°C (275°F). Evaporator superheats and compressi on ratios for both refrigerants were also similar to R-404A.

CONCLUSION

Two new low GWP refrigerants have been developed wh ich can potentially replace R-404A in commercial refrigeration. The first, DR-7 has a GWP of about 250 which is 94% lower than R-404A. It is mildly f lammable (2L) and therefore may have some limitations for use in larg er refrigerant charge systems such as supermarket r acks. However, it could be suitable for single condensing unit system s and stand-alone coolers and freezers. DR-33 is f ormulated to be nonflammable, with a GWP of about 1400 which is abo ut 65% lower than R-404A. It is suitable for large rack systems and other refrigeration units where use of flammable re frigerants is not preferred or is restricted. Perf ormance of both refrigerants has been evaluated in a reach-in freezer originally designed for R-404A. The only changes made to the system were a TXV adjustment to compare at equivalent evaporator supe rheat. Overall, energy consumption for both refrig erants is at least equivalent to R-404A while other performance parame ters are similar. At higher ambient conditions, DR -33 and DR-7 exhibit 4 and 8% lower energy consumption respectiv ely. Use of these low GWP refrigerants can effecti vely reduce the environmental impact of refrigeration systems by al lowing transition away from R-404A with minimal sys tem changes.

REFERENCES

AHAM HRF-1-2008. 2008. Energy and Internal Volume of Refrigerating Appliances: Association of Home Ap pliance
Manufacturers.
ASHRAE Standard 34-2010. 2010. Designation and Sa fety Classification of Refrigerants: American Socie ty of Heating, Refrigerating and Air-Conditioning Engineers (ASHRA E), Atlanta, GA, 2010.

ASTM E681-04. 2004. Standard Test Method for Conce ntration Limits of Flammability of Chemicals (Vapor s and Gases): American Society for Testing and Materials (ASTM), West Conshohocken, PA, 2004.

IPCC. 2007. Intergovernmental Panel on Climate Chan ge Fourth Assessment Report – Climate Change 2007: Synthesis Report. [Jallow, B.P., L. Kajfez-Bogataj, R. Bojaru , D. Hawkins, S. Diaz, H. Lee, A. Allali, I. Elgizo uli, D. Wratt, O. Hohmeyer, D. Griggs, and N. Leary (eds.)]. <http:// www.ipcc.ch/ipccreports/ar4-syr.htm>.
Nielson, O.J., Javadi, M.S., Sulbaek Andersen, M.P. , Hurley, M.D., Wallington, T. J., and R. Singh. 20 07. Atmospheric Chemistry of CF 3 CF=CH 2 : Kinetics and mechanisms of gas-phase reactions w ith Cl atoms, OH radicals and O 3 . Chemical Physics Letters, 439 (2007):18-22.

SAE. 2009. SAE International Releases Test Results of the Low GWP Refrigerant for Automotive Air Condi tioning. http://www.sae.org/servlets/pressRoom?OBJECT_TYPE=P ressReleases&PAGE=showRelease&RELEASE_ID=1109

, November 10, 2009.

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