This article was published in ASHRAE Journal, February 2010. Copyright 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org. In Retail Refrigeration By Richard Royal, P.E., Associate Member ASHRAE Most retail grocery stores refrigeration systems run 24 hours a day, seven days a week, consume one-third to one-half of the total electrical energy for the building and offer a constant supply of low quality heat. The term quality of heat is a reference to the comparative temperature available from the source system to the heat recovery application. Low quality heat, for this discussion, represents a comparative temperature difference of less than 100 F (55 C). case in a store. In a properly operating commercial refrigeration system, the low-temperature system will have an evaporator coil temperature of approximately 20 F ( 29 C), an expansion valve coil superheat range setting of 4 F to 6 F (2.2 C to 3.3 C) and a return gas temperature to the compressor suction header around 30 F ( 1 C). The mediumtemperature system will have an evaporator coil temperature near 20 F ( 7 C), an expansion valve coil superheat range setting of 5 F to 8 F (2.8 C to 4.4 C) Most commercial refrigeration systems use a direct expansion (DX) process with an expansion valve in the liquid line, to meter the amount of refrigerant into the evaporator coil, and a pressure regulating valve in the suction line, adjusted to control the temperature of the air off the evaporator coil from each refrigerated About the Author Richard Royal, P.E., is mechanical design manager, prototypical design/construction standards, at Wal- Mart in Bentonville, Ark. R-407A Properties Low Temperature Desuperheating 1 Inlet Day (Desuperheating) 65 140.9 190.0 203.2 174.3 95.8 4,602 320,875 85% 132,800 75 165.0 204.0 205.8 175.1 99.3 5,016 332,200 88% 153,905 85 192.1 219.0 208.6 175.8 103.0 5,494 343,525 91% 180,368 95 222.3 235.0 211.8 176.4 106.7 6,032 354,850 94% 213,326 105 256.1 250.0 214.7 176.9 110.6 6,666 366,175 97% 252,093 115 293.5 267.0 218.1 177.1 114.6 7,412 377,500 100% 303,711 Based on a low temperature system with a 378 MBh refrigeration load at 20 F saturated suction temperature and a medium temperature system with a 889 MBh refrigeration load at +20 F saturated suction temperature. One MBh is equal to 1,000 Btu/h. No natural subcooling or useful superheat included. Table 1: Desuperheating heat recovery for a R-407A system. 1 4 A S H R A E J o u r n a l a s h r a e. o r g F e b r u a r y 2 0 1 0
and a return gas temperature to the compressor suction header around 50 F (10 C). The heat available for recovery from a refrigeration system comes from the high-pressure, high-temperature refrigerant gas discharge of the compressors. The return gas temperature at the compressor suction header impacts the temperature of the discharge gas from the compressors, which impacts the heat available for recovery. Therefore, the documentation of the basis of design and the proper physical adjustment of the refrigeration systems are vital for effective and consistent heat recovery performance. The information in Tables 1 and 2 illustrate the relative design temperatures, refrigerant properties and heat energy available for refrigerants R-407A and R-404A. The quality of heat available from desuperheating is dependent on the temperature of the compressor discharge at a given condensing temperature. This desuperheated energy can be applied to a variety of heat recovery schemes, as it has higher heat quality. The amount of superheat available at a given condensing temperature is equal to the mass flow rate times the difference of the vapor enthalpies. The mass flow rate for a given refrigeration load varies by refrigerant and is based on the difference in the latent heat of vaporization (difference in enthalpies from saturated liquid to saturated vapor) for the refrigerant and the inlet quality to the evaporator coil. The condensing temperature is the measure of the liquid refrigerant. The compressor discharge temperature is the temperature of the superheated refrigerant at the exit of the compressor. As the condensing temperature of the system moves from design maximum to design minimum, the mass flow of the system adjusts to a relatively stable minimum based on the refrigerated case load. The electrical energy consumption of a refrigeration system is proportional to the mass flow rate of the refrigerant. The refrigerant mass flow required to meet the system load is the sum of the evaporator load plus the system condition loads, such as the condensing temperature, subcooler load, and parasitic piping losses. As the refrigerant temperature approaches R-404A Properties Low Temperature Desuperheating 1 Inlet Day (Desuperheating) 65 151.1 157.0 184.3 161.0 97.2 5,791 320,875 85% 135,182 75 175.7 167.0 185.9 161.9 100.8 6,412 332,200 88% 154,400 85 203.2 179.0 188.1 162.6 104.5 7,140 343,525 91% 181,509 95 233.8 192.0 190.4 163.3 108.3 8,008 354,850 94% 217,454 105 267.8 205.0 192.8 163.7 112.2 9,061 366,175 97% 263,331 115 305.4 218.0 195.1 163.9 116.3 10,397 377,500 100% 324,283 Based on a low temperature system with a 378 MBh refrigeration load at 20 F saturated suction temperature and a medium temperature system with a 889 MBh refrigeration load at +20 F saturated suction temperature. One MBh is equal to 1,000 Btu/h. No natural subcooling or useful superheat included. Table 2: Desuperheating heat recovery for a R-404A system. Fe b r u a r y 2 0 1 0 A S H R A E J o u r n a l 1 5
R-407A Properties Low Temperature Full 1 Inlet Day () 65 140.9 190.0 203.2 174.3 95.8 4,602 320,875 85% 494,032 75 165.0 204.0 205.8 175.1 99.3 5,016 332,200 88% 534,032 85 192.1 219.0 208.6 175.8 103.0 5,494 343,525 91% 580,323 95 222.3 235.0 211.8 176.4 106.7 6,032 354,850 94% 633,730 105 256.1 250.0 214.7 176.9 110.6 6,666 366,175 97% 693,743 115 293.5 267.0 218.1 177.1 114.6 7,412 377,500 100% 767,212 R-407A Properties Medium Temperature Full 1 Inlet Day () 65 140.9 128.0 188.5 174.3 95.8 8,305 621,950 70% 770,192 75 165.0 143.0 191.1 175.1 99.3 9,459 675,260 76% 868,218 85 192.1 159.0 193.9 175.8 103.0 10,763 728,570 82% 978,387 95 222.3 174.0 196.5 176.4 106.7 12,219 781,880 88% 1,097,125 105 256.1 188.0 198.8 176.9 110.6 13,899 835,190 94% 1,226,168 115 293.5 203.0 201.4 177.1 114.6 15,841 888,500 100% 1,375,440 The load of the refrigeration system adjusts primarily with the defrost schedule and in-store humidity conditions. The refrigeration effect per pound of refrigerant increases as the condensing pressure approaches minimum, effecting lower mass flow rates. Table 3: Full condensing heat recovery for a R-407A system. the saturated suction temperature (SST) of the evaporator, the refrigerant mass flow rate approaches a minimum. The data in Tables 1, 2, 3 and 4 illustrate the mass flow rate of the system is lower as the condensing temperature (the temperature of the liquid refrigerant out of the condenser) of the system gets colder. Therefore, the lowest energy position for a refrigeration system is to maintain the suction pressure as high as possible to meet the temperature of the coldest evaporator coil and to allow the head pressure (compressor discharge pressure) to run as low as possible, reducing the compression ratio of the compressors, which reduces electrical power requirements. There are other heat recovery schemes that use both superheat (sensible) energy, and the condensing (latent) energy of the refrigerant gas. Full condensing heat recovery, as shown in Tables 3 and 4, yields higher total heat recovery available, but with lower heat quality, since the quality of heat is based on the condensing temperature. Most full condensing heat recovery applications are designed for water-cooled condensing refrigeration systems. R-407A is a recent entry into commercial DX refrigeration design due to its significantly lower GWP (global warming potential) compared to R-404A. 2 Refrigerants with lower GWP are being considered because of the GHG (greenhouse gas) environmental impact of leaking refrigerant, which contributes to the carbon footprint of the system. Proper designs are measured by at least three basic metrics: cost, energy and carbon. A much longer discussion is warranted on the direct and indirect emission impacts of our HVAC and refrigeration systems, focusing on significant refrigerant charge reductions, and higher efficiency standards. Domestic Hot Water (DHW) The application of refrigeration heat recovery for heating water is governed by the temperature requirements of the hot water system. For a big box retailer, a typical hot water system may be a recirculated loop with storage tanks with a capacity of 3.33 gpm at 100 F (0.21 L/s at 55 C) temperature rise. The water temperature in the loop may be maintained by a mixing valve that mixes cold water (approximately 60 F [16 C]) with hot water from the heat recovery heat exchangers or supplemental heaters in the storage tanks. The temperature control of the mixing valve may be set to keep the loop temperature as high as 140 F (60 C). Based on the information in Tables 1 and 2, the amount of heat recovery available at minimum condensing temperature falls short (both in temperature and Btu/h) of the peak capacity of the hot water system. The water storage tanks are usually ordered with supplemental electrical heaters to handle the heating capacity during high demand periods. However, the hot water requirements are not at a constant peak demand and the recovered heat from the refrigeration system can meet the 1 6 A S H R A E J o u r n a l a s h r a e. o r g F e b r u a r y 2 0 1 0
water heating load most of the time without imposing electrical energy penalties. A holdback valve is a standard valve for most air-cooled refrigeration systems and is typically adjusted to maintain the minimum condensing pressure. If the holdback valve is intentionally adjusted to maintain a higher condensing temperature, for the purposes of higher quality heat, the electrical energy consumed by the refrigeration system may exceed the heat energy gained for the application. The application of domestic hot water heating by the refrigeration system is typically accomplished through a heat exchanger connected in the discharge line of the low-temperature compressors. The water is heated through the desuperheating of the primary refrigerant in R-404A Properties Low Temperature Full 1 Inlet Day () 65 151.1 157.0 184.3 161.0 97.2 5,791 320,875 85% 504,503 75 175.7 167.0 185.9 161.9 100.8 6,412 332,200 88% 545,929 85 203.2 179.0 188.1 162.6 104.5 7,140 343,525 91% 596,603 95 233.8 192.0 190.4 163.3 108.3 8,008 354,850 94% 657,603 105 267.8 205.0 192.8 163.7 112.2 9,061 366,175 97% 730,107 115 305.4 218.0 195.1 163.9 116.3 10,397 377,500 100% 819,514 R-404A Properties Medium Temperature Full 1 Day () 65 151.1 115.0 184.3 161.0 97.2 10,204 621,950 70% 888,992 75 175.7 128.0 185.9 161.9 100.8 11,774 675,260 76% 1,002,507 85 203.2 141.0 188.1 162.6 104.5 13,580 728,570 82% 1,134,656 95 233.8 153.0 190.4 163.3 108.3 15,685 781,880 88% 1,287,939 105 267.8 166.0 192.8 163.7 112.2 18,176 835,190 94% 1,464,490 115 305.4 178.0 195.1 163.9 116.3 21,231 888,500 100% 1,673,508 The load of the refrigeration system adjusts with the defrost schedule and in-store humidity conditions. The refrigeration effect per pound of refrigerant increases as the condensing pressure approaches minimum, effecting lower mass flow rates. Table 4: Full condensing heat recovery for a R-404A system. Hold Back Valve Receiver Medium Temperature System Air or Water Heat Reclaim s Air-Cooled the common discharge of the parallel piped compressors. The heat recovery for domestic hot water is typically controlled by an aqua stat located at the storage tank. The pressure drop in the heat recovery heat exchanger is sized for 5 psid (34 kpa). Figure 1 represents a typical air-cooled condensing, parallel rack refrigeration system with a heat reclaim heat Heat Reclaim Valve Medium Temperature s Oil Separator Subcooler Hold Back Valve Low Temperature System Air or Water Heat Reclaim s Low Temperature s Figure 1: Typical air-cooled condensing refrigeration system with heat reclaim. exchanger piped in series with the air-cooled condenser. For the application of heating water, as described earlier, the heat recovery is only accomplished by the low-temperature system. Heat recovery by the medium-temperature system will be discussed later in context with full condensing heat recovery. 1 8 A S H R A E J o u r n a l a s h r a e. o r g F e b r u a r y 2 0 1 0 Receiver Air-Cooled Heat Reclaim Valve Oil Separator
Preheating for OA Requirements The conditioning of the outdoor air to space temperature and humidity requirements represents a significant amount of the HVAC system load and energy consumption. The geographic location of the building impacts the design and selection of the building s HVAC and refrigeration systems. The building s HVAC heat load requirements and the available heat recovery from the refrigeration systems adjust to the outdoor climate. The climatic bin data in Table 5 lists the number of hours of space heating required at a given outdoor air dry-bulb temperature for the 13 climate zones shown in Figure 2. The bin data in Table 6 lists the amount of heat energy available for 13 climate zones. Climate Zones/Pilot Cities 1: Miami 8: Denver 2: Houston 9: Casper, Wyo. 3: Dallas 10: Las Vegas 4: Louisville, Ky. 11: Los Angeles 5: Chicago 12: Sacramento, Calif. 6: Minneapolis 13: Portland, Ore. 7: Phoenix This energy is the product of the THR (total heat of rejection) from the refrigeration system and the number of hours listed in Table 5. The heat energy calculation is based on the minimum condensing temperatures listed in Tables 3 and 4. The temperature data listed is the midpoint for five degree temperature ranges. The hourly data indicates that above Figure 2: Climate zone map with pilot cities. 3 47.5 F (8.6 C), there is no space heating requirements for all climate zones. This design data is based on a prototypical model for a big box retail store. Preheating outdoor air through a preheating coil in an air-handling unit or makeup air unit can be met with large amounts of relatively low quality heat. The typical applica- Advertisement formerly in this space. Advertisement formerly in this space. Fe b r u a r y 2 0 1 0 A S H R A E J o u r n a l 1 9
OA Bin R-407A Medium Temperature Full Climate Zone 1 2 3 4 5 6 7 8 9 10 11 12 13 Bin Hours 47.5 65 73 436 637 582 595 544 467 748 754 860 245 1,048 1,436 42.5 65 19 335 535 632 590 418 456 775 777 404 38 745 1,264 37.5 65 9 217 383 494 734 560 181 834 864 340 470 647 32.5 65 126 199 705 714 603 58 611 691 159 68 253 27.5 65 59 138 455 772 552 8 648 814 24 61 22.5 65 19 27 268 417 380 320 498 74 17.5 65 6 16 224 305 468 201 329 12.5 65 93 124 307 110 183 7.5 65 63 105 204 97 175 2.5 65 14 56 169 86 107 2.5 65 40 93 37 63 7.5 65 28 110 15 38 12.5 65 71 45 17.5 65 26 20 22.5 65 12 Total Hours 101 1,198 1,935 3,530 4,480 4,505 1,170 4,482 5,370 1,787 283 2,331 3,735 The data in Table 5 references the total number of hours for each climate zone, for a given ambient air temperature, as well as the number of hours at each temperature bracket. Table 5: Weather data of number of hours at OA temperatures, heating mode. No space heating is required at OA temperatures above 47 F. 4 tion requires approximately 35 gpm (2 L/s), with an entering water temperature (EWT) of 60 F (16 C) and a leaving water temperature (LWT) of 45 F (7 C). Based on the data in Tables 3, 4 and 5, the refrigeration systems should have the capacity to meet the need when the call for heating occurs at an outdoor temperature corresponding to the minimum condensing temperature for the refrigeration system. One refrigeration system configuration that best handles this application is a water-cooled condensing system, as shown in Figure 3, with a pump to circulate the warm glycol from the refrigeration system s condenser to the preheating coil and back. Receiver Balance Valve Oil Separator Subcooler Medium Temperature System Air-Cooled or Evaporative-Cooled Fluid Cooler and Heat Reclaim s Water-Cooled s Receiver Low Temperature System Fluid Pump Oil Separator s Medium Temperature s Heat Pump Application Requirements The application of heat from the refrigeration system into water source heat pumps may offer substantial gas energy savings. And, as with the preheating of outdoor air in an air-handling unit, the heat requirements can be met by using the large amount of low quality heat recovered from the refrigeration system. Unfortunately, the climate requiring the most heat in the building is Low Temperature s Figure 3: Typical water cooled condensing refrigeration system with heat reclaim. also the climate that offers the lowest condensing temperatures for the refrigeration system, which calculates to the lowest available amount of heat from the refrigeration system. As the refrigeration system s condensing temperature approaches a set minimum, the refrigeration system s compressors ap- 2 0 A S H R A E J o u r n a l a s h r a e. o r g F e b r u a r y 2 0 1 0
R-407A Medium Temperature Full 1 OA Bin THR Climate Zone 1 2 3 4 5 6 7 8 9 10 11 12 13 Bin (MBh) 47.5 770,192 56,224 335,804 490,612 448,251 458,264 418,984 359,679 576,103 580,724 662,365 188,697 807,161 1,105,995 42.5 770,192 14,634 258,014 412,052 486,761 454,413 321,940 351,207 596,898 598,439 311,157 29,267 573,793 973,522 37.5 770,192 6,932 167,132 294,983 380,475 565,321 431,307 139,405 642,340 665,445 261,865 0 361,990 498,314 32.5 770,192 0 97,044 153,268 542,985 549,917 464,425 44,671 470,587 532,202 122,460 0 52,373 194,858 27.5 770,192 0 45,441 106,286 350,437 594,588 425,146 6,162 499,084 626,936 18,485 0 0 46,982 22.5 770,192 0 14,634 20,795 206,411 321,170 292,673 0 246,461 383,555 0 0 0 56,994 17.5 770,192 0 4,621 12,323 172,523 234,908 360,450 0 154,808 253,393 0 0 0 0 12.5 770,192 0 0 0 71,628 95,504 236,449 0 84,721 140,945 0 0 0 0 7.5 770,192 0 0 0 48,522 80,870 157,119 0 74,709 134,784 0 0 0 0 2.5 770,192 0 0 0 10,783 43,131 130,162 0 66,236 82,410 0 0 0 0 2.5 770,192 0 0 0 0 30,808 71,628 0 28,497 48,522 0 0 0 0 7.5 770,192 0 0 0 0 21,565 84,721 0 11,553 29,267 0 0 0 0 12.5 770,192 0 0 0 0 0 54,684 0 0 34,659 0 0 0 0 17.5 770,192 0 0 0 0 0 20,025 0 0 15,404 0 0 0 0 22.5 770,192 0 0 0 0 0 0 0 0 9,242 0 0 0 0 Total Btu 77.8 Million 922.7 Million 1.5 2.7 3.5 3.5 90.1 Million 3.5 4.1 1.4 217 Million The data in Table 6 references the total amount of heat available from the refrigeration system at the range of ambient temperatures, as well as the amount of heat at each temperature bracket. Table 6: Refrigeration heat energy available by climate zone and outside air temperature. No space heating is required at outside air temperatures above 47 F (8 C) based on a building model with an average heating load of 2,100 MBh (609 kw). 1.8 2.9 proach maximum capacity due to lower compression ratios. Therefore, for a constant load on the refrigeration system, at minimum condensing temperatures there will be fewer compressors running, resulting in lower refrigerant mass flow (and, therefore, less heat available), to meet the heat load demand. This is a full condensing heat exchange for the refrigeration system, which is physically governed by the mass flow of the refrigeration system s refrigerant times its enthalpy difference from superheated vapor to saturated liquid. Considerations With air-cooled condensing refrigeration systems, the heat recovery design considerations deal with the temperature requirements for each application, the available energy for recovery, the refrigeration system s refrigerant charge and the control setpoints and sequences of operation. With water-cooled condensing refrigeration systems, the issues to address with the refrigeration system are the controls governing the condensing temperature, while most of the heat recovery considerations deal with the mechanical systems to support the water-cooled condensing: fluid coolers, water towers, water treatment, etc. Financial Considerations The rewards of good design are the financial merit of operational cost savings and low maintenance equipment that performs well. The financial balance of electrical and natural gas costs, along with the cost of equipment, installation and controls are collectively important for the engineer to understand. The information in Tables 7 and 8 illustrate the difference in financial merit for applying the same technology to the same store size in a different climate zone. In the first ROI calculation in Table 7, the heat reclaim was applied to a northern climate zone where the total bin hours for heat reclaim are 4,505 from Table 5. From the data in Table 5, we can see that there are 14 incremental temperature steps with a number of hours in each step. Table 6 presents a similar table of data relative to the same ambient outdoor air temperature except that the data are the product of the heat available from the refrigeration system times the number of bin hours at that ambient temperature. The total amount of heat available from the refrigeration system is summed at the end column. The sample ROI calculations use the average number of Btu/h available from the refrigeration system as calculated in Table 6. The ROI calculator also uses the heat losses in the Fe b r u a r y 2 0 1 0 A S H R A E J o u r n a l 2 1
AHU Energy Analysis Refrigeration Total Heat of Rejection Proto Size: 180,000 ft 2 Climate Zone 6: Minneapolis Pipe Material: Steel Heat Rejection : 445,397 Btu/h AHU Calculations Glycol Percentage: 50% Glycol Factor: 0.85 Specific Gravity: 1.041 Heat at Coil: 302,870 Btu/h Pump Energy Use Number of AHUs: 2 Coil DT: 10 F Piping Drop: 50 ft head Pump Efficiency: 0.60 Energy Costs: $0.085/kWh Pump Power: 1.98 kw Pump Energy Cost: $758 Natural Gas Savings Bin Hours for : 4,505 Hours Gas Cost: $1/therm Annual Savings: $17,055.35 Annual Energy Savings = $16,297.53 Simple ROI = 4.2 Years Table 7: Sample ROI calculator for northern climate heat recovery using the data in Tables 5 and 6. system to calculate the heat available at the heat reclaim coil. The amount of pump energy required to circulate the glycol around is also accounted for and added to the energy requirements. And with a cost per therm for natural gas, the measure of heat energy from the refrigeration system is calculated. As illustrated in Table 8, the savings from heat reclaim is not as good an investment based on the location of the store. Therefore, the application of heat recovery from refrigeration should be evaluated on a project-by-project basis. Conclusions Much work needs to be done to integrate the knowledge bases for refrigeration and HVAC so that a better understanding of the equipment and controls can develop into the mass production energy-frugal systems that reduce our building s energy requirements. These types of integrated technical developments when shared between the original equipment manufacturers (OEMs) and engineers of record (EORs), and if truly collaborated, will drive the HVAC and refrigeration industries to higher standards for energy conservation. This collaboration will challenge traditional barriers, and business models for both groups, as well as both industries. HVAC and refrigeration are often considered technically separate and independent. Also, the pursuit of control integration between both HVAC and refrigeration, in line with proposed energy codes and standards, could further yield important market driving changes to the industries. AHU Energy Analysis Refrigeration Total Heat of Rejection Proto Size: 180,000 ft 2 Climate Zone 2: Houston Pipe Material: Steel Heat Rejection : 471,314 Btu/h AHU Calculations Glycol Percentage: 50% Glycol Factor: 0.85 Specific Gravity: 1.041 Heat at Coil: 320,493 Btu/h Pump Energy Use Number of AHUs: 2 Coil DT: 10 F Piping Drop: 50 ft head Pump Efficiency: 0.60 Energy Costs: $0.085/kWh Pump Power: 2.09 kw Pump Energy Cost: $213 Natural Gas Savings Bin Hours for : 1,198 Hours Gas Cost: $1/therm Annual Savings: $4,799.39 Annual Energy Savings = $4,586.13 Simple ROI = 15.1 Years Table 8: Sample ROI calculator for southern climate heat recovery using the data in Tables 5 and 6. Caution should be used when considering integration of heat from the refrigeration system into HVAC. As we work to incorporate energy savings and green house gas reducing ideas into our designs, there is a need to carefully design systems in a way that can be technically and financially evaluated by system and subsystem. Otherwise, we unintentionally bundle price and performance as we bundle design and technology. Separate the HVAC and refrigeration systems into basic black boxes with subsystems. The variable frequency drive on the fan motor or compressor is a subsystem to the black box. Any addition to the basic systems should be considered a subsystem and evaluated as such. The technical and financial merit for any addition to the basic system must be clearly understood and evaluated in the design phase. This bundling effect can take what looks and sounds like a great energy saving addition to the design and wind up costing more for equipment and installation than it would have saved in the life cycle of the equipment. One of the fundamental measures of every good design is that it has to make good business sense, good environmental sense, be sustainable and is easy to maintain. References 1. NIST. 2007. Reference Fluid Thermodynamic and Transport Properties Database (REFPROP): Version 8.0. 2. Intergovernmental Panel on Climate Change. 2007. Fourth Assessment Report. 3. Wal-Mart Climate Zone Map. 4. InterEnergy Software. BinMaker PRO 2.0.4. 2 2 A S H R A E J o u r n a l a s h r a e. o r g F e b r u a r y 2 0 1 0