PART 3: CO 2 as a Refrigerant System Design, Commissioning, Operation Service This series continues with the introduction of transcritical, cascade and secondary systems; how each system works; and compares their advantages and disadvantages. BY ANDRE PATENAUDE Unless noted otherwise, images courtesy of Emerson Climate Technologies, Canada. Part 2 of this series provided an overview on the potential hazards of R-744, comparing CO 2 to other refrigerants, both traditional and new, and weighed its advantages and disadvantages. This article will cover system design, commissioning, operation and service as it relates to transcritical, cascade and secondary systems. The article will also review how each system works and compare their advantages and disadvantages. The properties of R-744 affect how the refrigerant is applied (see previous articles in the January and March issues for more details): gthe high density of R-744 compared to hydrofluorocarbons (HFCs) results in the requirement for less compressor displacement, typically one-fifth of that needed for R-404A. However, the motor size is similar since the work done is approximately the same. Smaller pipe diameters are another result, especially on the suction side of the system. gbecause of the higher pressures of R-744, all components require a higher maximum pressure rating. gthe high discharge temperatures of R-744 (because of the high index of compression) result in the need for two-stage compression for LT systems that reject heat to ambient air. gthe low critical temperature of R-744 results in differences in system design and control. In the retail sector this results in R-744 being used mainly in the following types of systems: 9 Transcritical systems: Systems are called transcritical when heat rejection takes place above the critical point of the refrigerant (for ambient temperatures above 25 C/77 F, assuming 6 C/10 F TD condenser/ gas cooler design) [see Figure 1]). 9 Booster systems: Systems with two temperature levels (e.g., low temp -30 C/-22 F and medium temp -7 C/+20 F evaporating temperature) and with low-stage and medium-stage compressors (see Figure 4). 9 Cascade systems: R-744 is the low-stage refrigerant in a cascade system in which the R-744 is always subcritical. Heat rejected by condensing R-744 is 30 RSES Journal MAY 2015 absorbed by the evaporating high-stage refrigerant. The high-stage system is usually a conventional system using HFC, hydrocarbon (HC) or ammonia refrigerants, known as hybrid cascade. In some systems R-744 is used in both the high and low stages. The R-744 in the low stage is always subcritical, but will be transcritical at high ambient conditions in the high stage. 9 Secondary systems: R-744 is used as a secondary fluid pumped through the heat exchangers (i.e., evaporators). The CO 2 partially evaporates and then is retuned to a heat exchanger to be fully condensed Figure 1 Simple transcritical system for medium temperature.
Figure 2 Pressure enthalpy chart showing transcritical operation. and chilled before being returned to the evaporators. Retail transcritical systems The diagram in Figure 1 is a simple single-stage transcritical system. The refrigerant discharged from the compressor flows into the gas cooler where heat is removed. The refrigerant does not condense in this part of the system if it is above the critical point. The refrigerant passes through a high-pressure reducing valve and condenses in the receiver/flash tank when its pressure drops below the critical point. Liquid is drawn off the bottom of the receiver to feed all of the evaporators. Electronic expansion valves provide evaporator superheat controls. The superheated gas is drawn back into thecompressor. In this simple system: gthe temperature of the refrigerant at the gas cooler exit depends on the size of the cooler; and gthe pressure of the refrigerant in the gas cooler depends on the quantity of refrigerant in the system and the ambient temperature. The capacity and efficiency of this type of system varies significantly with ambient temperature and the quantity of refrigerant in the system. Three example systems are shown on the pressure enthalpy chart in Figure 2. Each has identical evaporating conditions. In a subcritical system the refrigerant would de-superheat and then condense, rejecting heat at a constant temperature. In a transcritical system the R-744 does not condense; it rejects heat as a transcritical fluid, cooling during this process. Even with a wide temperature-glide HFC such as Circle Reader Service No. 42 MAY 2015 RSES Journal 31
Figure 3 Simple pressure control. Figure 4 Simple booster system without oil management. R-407A, the temperature change through the condenser is small compared to that of a gas cooler in a transcritical system. In each example, the R-744 exits the gas cooler at a temperature of 40 C/104 F. This exit temperature is a function of the size of the gas cooler and the ambient temperature, in the same way that condensing temperature is a function of the size of the condenser and the ambient temperature. 32 RSES Journal MAY 2015 System Advantages Disadvantages Transcritical Booster Cascade Secondary 9 One refrigerant 9 One system, lowest system costs 9 Better efficiency than HFC systems in mild climates 9 Two simple systems 9 LT with low R-744, the MT with a low- GWP HFC refrigerant 9 Standard HFC components for mediumand low-temperature cycles 9 Better efficiency in warm climates 9 Using R-744 as a secondary fluid using the latent heat, very low pump power required 9 Simple chiller system for the high stage with readily available components (separate chiller for LT and MT) 9 System works at constant pressure without any pressure pulsation 9 Option to combine LT and MT, pump circulation system for the MT using R-744 combined with an LT booster system 9 Chiller could use low-gwp HFCs or HCs 9 LT applications require two-stage compression 9 System faults in coupled systems affect MT and LT 9 High operation pressures 9 Lower efficiency as HFC systems in warm climates 9 Two refrigerants although R-744 can be used in the high stage 9 Temperature difference in the cascade heat exchanger reduces the efficiency slightly for the LT cycle 9 System faults in coupled systems affect MT and LT 9 Additional heat exchange and temperature difference slightly reduce the efficiency 9 R-744 pumps required 9 Pumps in this size are not readily available and are unfamiliar to many refrigeration engineers Table 1 Advantages and disadvantages of cascade and transcritical retail systems. The cooling capacity of each system varies significantly. For pressures shown in Figure 2, the cooling capacity reduces as the pressure reduces. This is the opposite of what happens in a subcritical system, where cooling capacity is greater at lower discharge pressures. The compressor power input of each system also varies. The lower the pressure, the lower the power input, as in subcritical systems. Variation in power input is not proportional to the variation in cooling capacity. For example, increasing the head pressure from condition 1 to condition 2 provides a significant increase in cooling capacity with a very low increase in compressor power input. Increasing the pressure from condition 2 to condition 3 increases cooling capacity less than the increase in compressor power input. Unlike subcritical systems, the maximum coefficient of performance (COP) does not occur at minimum condensing pressure. Optimum COP depends on evaporating conditions and gas cooler exit temperature, but is typically 90 100 bar (1,310 1,450 psi). In general, the pressure for optimum capacity is greater than that for optimum COP. In a retail transcritical system, gas cooler pressure is controlled to provide optimum capacity or optimum efficiency while maintaining the pressure below the maximum allowed at all times. Figure 3 shows how this pressure is controlled in a typical retail system with single-stage compression. Two additional valves in this system control the gas cooler and intermediate pressures: gthe gas cooler pressure valve 1, also called the high-pressure regulating valve, controls the pressure in the gas cooler. It is a pressure-reducing valve controlled from the R-744 pressure in the gas cooler and its exit temperature.
gthe receiver pressure valve 2, also called the mediumpressure regulating valve or the flash gas valve, controls the pressure of the refrigerant in the receiver and associated liquid distribution pipe work. It is controlled from the pressure in the receiver. This receiver is also called a flash tank. Note: Gas cooler pressure is usually selected for optimum COP unless greater capacity is needed, in which case a higher pressure would be selected. Subcritical operation The ambient temperature profile determines the proportion of time a system runs in transcritical mode. For many regions a proportion of the operation will be subcritical, typically when the ambient temperature is below 25 C/77 F, assuming 6 C/10 F TD condenser/gas cooler design). In this case, the gas cooler controller and valve usually control a user-defined subcooling setpoint. Retail booster systems Two-stage compression is used for transcritical low-temperature applications because the discharge temperature of R-744 is high and will potentially result in lubricant breakdown. Figure 4 shows a simple twostage booster or externally compounded system. The refrigerant from the low-temperature loads is drawn into the low-stage compressors. The discharge from these compressors goes into the suction of the high-stage compressors. The refrigerant from the medium-temperature (MT) loads is drawn into the suction of the high-stage compressors. The refrigerant from the receiver pressure-regulating valve is also drawn into the suction of the high-stage compressors. The Figure 5 Simple cascade system. I expect my ice machine to work as hard as I do. Whether purchasing a new ice machine, replacing an old one, or deciding to add another ice machine to your operation, Koolaire by Manitowoc might be the perfect solution for your ice machine needs. Koolaire ice machines provide the basic features you need with the reliability you expect at a price that fits your budget. Affordable reliability. Kool! Koolaire modular kube ice machines come in three convenient sizes 22, 30 and 48 with ice production from 250 lbs. up to 1350 lbs. per day. Plus, it s made by America s #1 selling brand of ice machine, Manitowoc Ice. What could be kooler than that? Let s talk reliable ice machines. 1-920-682-0161 KOOL-AIRE.COM Circle Reader Service No. 43 MAY 2015 RSES Journal 33
pumped around the load. It is volatile, so unlike a conventional secondary fluid such as glycol it does not remain as a liquid. Instead it partially evaporates, providing a significantly greater cooling capacity. This reduces the pump power required and the temperature difference needed at the heat exchanger. R-744 would typically be cooled to -3 C (26.6 F) for the MT load, and to -25 C (-13 F) for the LT load. The high-stage system is a simple chiller-type system, typically running on an HFC or HC or ammonia refrigerant. Selecting the best system The energy consumption is lower for both cascade and secondary systems, especially in Southern Europe where the ambient temperature is higher than Northern Europe. Berg (2009) reported the results of a study comparing HFC and CO 2 systems in Norway. The investment cost was 20% higher, but CO 2 systems showed better performance than HFC systems. Because of the impact of leakage of the HFC high stage in both cases, the overall impact on climate change is lower for the transcritical booster system. If a hydrocarbon refrigerant was used in the cascade and secondary high stage instead of an HFC, the overall environmental impact of these systems would be less than for a transcritical system. Figure 6 Simple pump system secondary. flash gas from the receiver pressure-regulating valve and the suction gas from the medium-temperature loads provide some interstage cooling. This is usually enough to maintain the discharge temperature of the high-stage compressors below the level at which the lubricant will deteriorate. Additional interstage cooling can also be provided if required. Retail cascade systems The cascade system comprises: gthe low stage, which provides cooling for the load. It uses R-744 and is always subcritical. gthe high stage, which absorbs heat from the condensing R-744 at the cascade heat exchanger Within the cascade heat exchanger the evaporating highstage refrigerant absorbs heat rejected by the condensing R-744. The condensing temperature is maintained below the critical point. The high stage is usually a simple, close-coupled system controlled by the pressure in the low-stage receiver. In this case the high stage provides cooling for the MT load as well as removing heat from the condensing R-744 in the low stage at the cascade heat exchanger. The high-stage refrigerant is usually an HFC, HC and sometimes ammonia, in which case the cascade is a hybrid system. In some systems R-744 is used in the high stage. It will be transcritical at ambient temperatures above 25 C/77 F. Secondary systems Figure 6 shows a simple secondary system. The high-stage system cools the liquid R-744 in the secondary circuit. The R-744 is 34 RSES Journal MAY 2015 Conclusion Transcritical operation is generally less efficient than subcritical operation, so system type selection is usually driven by the ambient temperature profile: gtranscritical systems are normally used in areas where the ambient temperature is generally low (i.e., predominantly below 25 C/77 F), such as northern Europe and Canada. gcascade and secondary systems (subcritical R-744) are usually used in high ambient areas such as Southern Europe, the U.S., much of Central and South America, Southeast Asia, Africa and Australia. The use of transcritical systems in high ambients generally results in low efficiency, hence cascade or secondary systems are preferred in these areas. That being said, component and equipment manufacturers continue to develop new products that are allowing improved efficiencies of transcritical systems in warmer climates.] The next two articles in this series will provide more detailed information about the design and components of R-744 cascade, transcritical booster and secondary systems. The content presented in this article first appeared in Commercial CO 2 Refrigeration Systems: Guide for Subcritical and Transcritical CO 2 Applications. Go to www.emerson climate.com to download the complete CO 2 Guide. Andre Patenaude is the Director of Marketing with Emerson Climate Technologies, Canada. He leads market strategy, planning and implementation of programs for Emerson Canada s refrigeration and A/C business. Patenaude is a certified Mechanical Engineering Technologist and has worked for Emerson since 1984 in a variety of technical and marketing positions, allowing him to gain a deep understanding of customers needs. For more information, email andre.patenaude@emerson.com or visit www.emersonclimate.com.