A Treatise on Liquid Subcooling

Transcription

1 A Treatise on Liquid Subcooling While the subject of this article is Liquid Refrigerant Subcooling, its affect on the operation of the thermostatic expansion valve (TEV), and ultimately on system performance and efficiency, is what makes it a relevant topic of discussion. As a refrigerant flow control, the TEV s sole purpose is to provide the evaporator with the precise mass flow of heat transfer medium (refrigerant) necessary to transfer the heat load from the refrigerated space. This vital function elevates the TEV to the status of heart of the refrigeration system. The advantage of the TEV, in comparison to fixed orifice expansion devices, is its ability to respond to changes in the heat load and system conditions while maintaining the proper refrigerant mass flow entering the evaporator. Most refrigeration applications are subject to wide load variations walk in box door openings, loading warm product in the refrigerated space, recovery after defrost, the indecisive shopper who requires the frozen food display case glass door to be open while browsing there is no such thing as a constant heat load. As the heat load increases/decreases, the TEV will open/close in an effort to maintain its superheat set point, thereby maintaining the correct refrigerant mass flow to the evaporator inlet. Variations in the heat load may be small or large. However; a heat load that is constantly varying results in a TEV that is constantly in a state of readjusting itself to meet the new heat load demand. When rapid and significant changes to the heat load occur, the previously adequate TEV opening (stroke) becomes completely inadequate for maintaining the correct refrigerant mass flow with respect to the change in load. While the TEV regulates refrigerant mass flow at the evaporator inlet, it is the superheat condition at the evaporator outlet which determines the valve opening. To say it more plainly: The TEV is a superheat control only, and it only responds to the superheat condition at the evaporator outlet. In a sense, the TEV is using old information as the basis for its control. There is a lag in time between the liquid refrigerant entering the evaporator, its change of state into a vapor as the heat load is transferred to it, and its eventual exit to the suction line. For example: Picture a walk in box, whose door is opened to load newly received product into the refrigerated space. The ambient air entering the space, along with the warm product, would cause a rapid increase to the heat load. Ideally, the increased heat load would trigger an increase in TEV opening (stroke), resulting in the necessary increase in refrigerant mass flow to meet the demand of the new heat load. In reality the TEV can only accomplish this after the superheat condition at the

2 evaporator outlet increases. The process which makes this happen is as follows: Additional heat enters the refrigerated space, and is transferred to the saturated refrigerant in the evaporator. The refrigerant then boils faster, completely vaporizing closer to the evaporator inlet, resulting in a higher superheat condition at the evaporator outlet. The increased superheat then causes the TEV to open, supplying more refrigerant to the evaporator in an effort to reduce the superheat back to the TEV set point. The built in time lag between an increase in the heat load and the resulting change in TEV stroke makes it difficult for the TEV to maintain a constant superheat condition at the evaporator outlet. Rapid load changes are one of several conditions that can lead to TEV hunting. TEV Capacity TEV capacity can be best described as the refrigeration effect (heat transfer capacity of the refrigerant at the condition the system is operating at, in Btu/lb) multiplied by the liquid refrigerant mass flow (in lb/min) delivered by the TEV to the evaporator inlet. The heat load being transferred to the saturated liquid refrigerant in the evaporator constitutes the greatest portion of the refrigeration effect. This causes the refrigerant to undergo a change of state; a latent heat transfer process. Refrigerant vapor experiencing a sensible heat gain (temperature increase) provides a very insignificant contribution to the refrigeration effect. The TEV, as a superheat control which regulates refrigerant mass flow at the evaporator inlet, also performs the function of converting the high pressure liquid refrigerant at its inlet to a low pressure mixture of liquid and vapor at its outlet. In a typical system where the four major components (compressor, condenser, TEV, and evaporator) are properly sized, the resulting low pressure mixture of liquid and vapor entering the evaporator should be at a saturation temperature that will be close to the design evaporator condition. When a liquid refrigerant experiences a reduction in pressure, it must also undergo a reduction in temperature and assume the new saturation temperature for the new pressure. This is accomplished as a percentage of the liquid refrigerant flowing through the TEV port flashes into a vapor, removing heat from the remaining liquid refrigerant in the process, achieving the temperature reduction. The difference between the temperature of the liquid refrigerant entering the TEV and refrigerant saturation temperature in the evaporator will determine what percentage of flashing occurs. It should be very clear that TEV capacity is not determined by the liquid refrigerant mass flow entering the TEV, but rather the usable refrigerant mass flow

3 exiting the TEV (and entering the evaporator). Given that refrigerant vapor contributes very little towards the refrigeration effect, the usable refrigerant mass flow should be defined as the mass flow of saturated liquid refrigerant. The mass flow of saturated liquid refrigerant available at the evaporator inlet (the TEV capacity) is determined by 3 things: (1) The thermodynamic properties of the refrigerant in use. (2) The physical dimensions of the TEV: pin angle, port diameter, and stroke. (3) The application conditions of the refrigerant. Any system condition which influences the amount of liquid flashing during the expansion process is relevant; the amount of flashing will determine the quality of refrigerant present at the evaporator inlet, which then determines the actual TEV capacity. The specific refrigerant conditions which determine the TEV capacity for a given refrigerant are: (1) Pressure Drop (ΔP) through the valve port. Mass flow through an orifice is proportional to the pressure drop experienced across the port. (2) Evaporator Temperature (saturation temperature of the refrigerant in the evaporator). This will partially determine the amount of flashing which occurs as the liquid refrigerant expands through the TEV port. As the difference between liquid temperature entering the TEV and the refrigerant saturation temperature in the evaporator increases, the percentage of liquid flashing will increase. (3) Liquid Refrigerant Temperature. As stated above, this also partially determines the amount of flashing that will occur across the TEV port. In addition, liquid refrigerant temperature at the TEV inlet will also determine the density of the liquid refrigerant. Lowering the liquid temperature will increase the density (lb/cu. ft.), which will increase the refrigerant mass flow as the refrigerant volume flow remains consistent at a given ΔP. The ARI rating condition for determining TEV capacity (for high pressure refrigerants such as R 22, R 404A, and R 507) is a 100 psi ΔP, 40ºF evaporator

4 temperature, and 100ºF liquid temperature. The nominal TEV capacity stamped on each valve and listed on the box is simply the valve s capacity at the ARI rating condition. If the conditions change, the valve capacity will also change. Referring to the Pressure Enthalpy diagram in Figure 1, refrigerant quality is amongst the many refrigerant properties which are displayed. It appears within the saturated region of the diagram (between the saturated liquid line on the left, and saturated vapor line on the right), and represents the percentage of vapor present for any given point plotted therein. The refrigerant quality will change as the system operating conditions change, and will be shown to have significance in determining TEV capacity. Figure 1 Refrigerant Properties The pressure enthalpy diagram in Figure 2 shows two systems plotted; a +40ºF airconditioning application and a 40ºF refrigeration application, both condensing at 100ºF. As stated above, the greater the percentage the quality is, the greater the percentage of vapor present in the saturated liquid/vapor mixture. The refrigerant quality at the evaporator inlet for the +40ºF and 40ºF applications are 33.5% and 60% respectively. Another way to look at this is in terms of how much liquid refrigerant remains after the expansion process. If the quality is 33.5% at +40ºF, then 66.5% of the

5 saturated mixture will be liquid. If the quality is 60% at 40ºF, then 40% of the saturated mixture will be liquid. To illustrate this point more plainly: For every 10 lbs/min of refrigerant in circulation, the evaporator inlet of the +40ºF and 40ºF applications will receive 6.65 lbs/min and 4 lbs/min respectively. This is the mass flow of saturated liquid refrigerant at the evaporator inlet, and is the only useful heat transfer medium in reducing the temperature of the refrigerated space. Figure 2 Comparative Refrigerant Qualities The amount of saturated liquid refrigerant remaining after the expansion process determines what the TEV capacity is. Referring to the TEV capacity chart in Figure 3, the nominal capacity of the SSE 4 is 4.08 tons at +40ºF, and 2.45 tons at 40ºF. The ratio of refrigerant quality between the 40ºF and +40ºF applications is.601 (40/66.5). This means that the available liquid mass flow at the inlet of the 40ºF evaporator is 60.1% of the liquid mass flow available at the inlet of the +40ºF evaporator. This should translate into a TEV capacity at 40ºF that is 60.1% of the capacity at +40ºF. Quickly

6 doing the math verifies this to be true; Figure 3 shows the capacity of the SSE 4 at +40ºF to be 4.08 tons. Multiplying 4.08 x 60.1% = 2.45 tons; precisely what is shown at the SSE 4 capacity at 40ºF. Valve Type Nominal Capacity 404A SC, SCP115 SZ. SZP 40º 20º 0º " 10º " 20º " 40º C S O Figure 3 TEV Capacity So, if you require a 4 ton TEV at the 40ºF evaporator condition, the nominal 4 ton TEV is too small. This is due to the large amount of liquid refrigerant flashing required to reduce the 100ºF saturated liquid at the TEV inlet to a 40ºF saturation temperature. After the flashing process the available liquid mass flow at the evaporator inlet is insufficient to provide 4 tons of heat transfer capacity, even though the refrigerant mass flow through the TEV port is greater than what is required for 4 tons of heat transfer. To realize 4 tons of refrigerant mass flow at the evaporator inlet for this application, more refrigerant must flow through the TEV port, and a larger port size is required for this to occur. Figure 2 shows the OSE 9 as having a capacity of 4.43 tons at a 40º evaporator temperature. As stated above, the amount of liquid refrigerant which flashes into a vapor during the TEV expansion process increases as the difference between the TEV entering liquid temperature and the evaporator refrigerant saturation temperature increases. Therefore, if the difference between these two temperatures can be reduced, the amount of usable liquid refrigerant available at the evaporator inlet will increase. When this is accomplished, it will allow the design heat transfer load to be achieved with a reduction in refrigerant mass flow, as the reduced liquid flashing provides for a more efficient use of the refrigerant entering the evaporator. A reduction in mass flow translates into a reduction in the required compressor capacity to achieve the

7 design condition. This reduction in compressor capacity can be achieved by: (1) operating a compressor sized for the design condition in an unloaded mode, (2) operating a smaller compressor instead of a larger compressor on a multi compressor rack, or (3) using a variable frequency drive on a compressor sized for the design load and reducing its capacity by reducing its motor speed. All of these methods result in a reduction in an increase in efficiency, coupled with a reduction in electrical consumption With most applications there isn t much of an opportunity to raise the saturated suction temperature; this is defined by the temperature requirement of the refrigerated space and the TD that the evaporator was selected to operate at (with TD being defined as the temperature difference between the evaporator entering air temperature, and the refrigerant saturation temperature in the evaporator). This fact leaves the lowering of the liquid refrigerant temperature entering the TEV as the only realistic means of reducing the amount of liquid flashing during the TEV expansion process which brings us to the subject of liquid subcooling. Liquid Subcooling By definition, a liquid exists in a subcooled state when its temperature is lower than the saturation temperature for its pressure. For example, the saturation temperature for R 404A at psig (at sea level) is 100ºF. It would be correct to state that liquid R 404A, at psig and 80ºF, is subcooled 20º; meaning that it is 20º below the saturation temperature for that pressure. There are various methods available to provide subcooled liquid for a given piece of refrigeration equipment; liquid to suction heat exchangers, dedicated refrigeration circuits/heat exchangers, or subcooling the liquid refrigerant in the condenser during periods of reduced ambient temperatures. While subcooling the liquid refrigerant before it enters the TEV will reduce the amount of liquid flashing required during the expansion process, if the benefit of the increased amount of liquid at the evaporator inlet does not offset the energy requirement to provide the subcooling, there is no net gain in system efficiency. For example, the liquid to suction heat exchanger shown in Figure 4 simply transfers some of the heat content of the liquid refrigerant to the suction vapor, resulting in a lower liquid temperature, but also a higher vapor temperature. This is a classic rob Peter to pay Paul scenario; the subcooled liquid does result in less flashing during the expansion process, which reduces the refrigerant mass flow requirement of the system..but

8 It is important to remember that a compressor is a volume pump, and will pump a constant volume of refrigerant consistent with the displacement of the compressor. Figure 4 Liquid to Suction Heat Exchanger The mass flow pumping capacity of the compressor in lbs/min is based on compressor displacement, compressor speed, and the refrigerant density. The resulting temperature increase of the suction vapor will result in an increase to the refrigerant s specific volume (or a reduction in density). To illustrate the importance of this, let s assume a compressor with a 300 cu. in. displacement operating at 3600 RPM. It will have a volume pumping capacity of 1,080,000 cu. in./min (300 X 3600). Dividing 1.080,000 by 1728 (1 cu. ft. = 1728 cu. in.) will convert the volume pumping capacity to 625 cfm. R 404A at approximately 35 psig has a saturation temperature of 40ºF, and a specific volume of cu. ft./lb. At 35 psig and the 65ºF suction vapor temperature that compressors are rated at, the specific volume is approximately 3 cu. ft./lb. Dividing the volume pumping capacity by the specific volume will yield the mass flow pumping capacity in lbs/min; 272 lb/min at 40ºF, and 208 lb/min at 40ºF. And therein lies the dilemma with the liquid to suction heat exchanger. While it does provide a lower liquid temperature, it comes at the expense of reduced compressor capacity no net efficiency gain. Its only benefit is the insurance that the liquid refrigerant entering the TEV is vapor free. A separate circuit on the compressor rack can be used to provide the refrigeration capacity needed to deliver subcooled liquid to the expansion valves. Again, this will provide the benefits of subcooled liquid as described above; reduced mass flow due

9 to less refrigerant flashing in the expansion process, and vapor free liquid at the TEV inlets. And once again, it will come with no net efficiency gain, as the equipment experiencing the benefit of the subcooled liquid is also providing the capacity to accomplish the subcooling once again it s a wash. Now, a typical supermarket will employ a rack of low temperature compressors which will operate at approximately 4 5 horsepower per ton, and a rack of medium temperature compressors, which will operate at approximately 2 3 horsepower per ton. It is a fact that lowering the liquid temperature will reduce the required refrigerant mass flow necessary to provide the design Btu capacity for all of the evaporators on a given system. When utilizing a subcooler for the low temperature rack, there is a great potential for efficiency gains when the medium temperature rack is used to provide the refrigeration capacity for the subcooler on the low temperature rack (See Figure 4). For example, if the subcooler load is 120,000 Btu for the low temperature rack, it would require approximately 45 horsepower from this same rack to provide the refrigeration capacity to achieve it ( hp/ton). The same subcooler load could be handled by 25 horsepower on the medium temperature rack. And there is the gain in efficiency; the medium temperature rack is providing 10 tons of refrigeration capacity for the low temperature rack, but accomplishing it with 20 horsepower less. Subcooler control configurations: Given the wide range of conditions..changes in loading, periods when refrigerated circuits have cycled off on temperature and/or defrost, and varying ambient temperatures that the refrigeration equipment will operate under throughout the year, the actual subcooler load will frequently be much less than the design subcooler load. Because of this, some flexibility in TEV application is required so that during those periods when the subcooler load is reduced, the heat exchanger will not operate with erratic superheat or floodback at its outlet. The typical piping/valve arrangement for a subcooler is shown in Figure 4, and consists of a single heat exchanger (typically a plate heat exchanger), with two TEVs piped in parallel supply refrigerant to the inlet of the heat exchanger. Each TEV has a solenoid valve at its inlet, which are controlled by the energy management system (in response to the heat exchanger outlet liquid temperature). Common practice is for one TEV to be sized to supply 1/3 of the heat exchanger s Btu capacity, with the other TEV supplying 2/3 of the heat exchanger s Btu capacity. An EPR valve is used to keep the saturation temperature in the heat exchanger constant. At its best, this arrangement is a compromise, and the poor liquid temperature control is illustrated in Figure 5.

10 Figure 4 Subcooler for L/T Rack, Refrigeration Provided by M/T Rack, Utilizing EEV Control Figure 5 Liquid Temperature Control from Two TEV Arrangement

11 A more reliable method for maintaining a constant liquid temperature at the heat exchanger outlet can be found in using a state of the art electronic controller, along with an electric expansion valve (see Figure 6). This method is somewhat unique; the controller utilizes a temperature probe and pressure transducer at the heat exchanger outlet to control the superheat, and a second temperature probe at the liquid outlet of the heat exchanger to control the leaving liquid temperature. Providing the actual superheat at the heat exchanger outlet is above the superheat set point, the controller will then drive the EEV open or closed in an attempt to maintain the liquid temperature set point. While not shown, an EPR can be used to maintain a constant saturation temperature in the heat exchanger, further enhancing the electronic controller s ability to maintain a constant liquid outlet temperature. Field testing has shown liquid temperatures maintained within +/ 3ºF (see Figure 7). Figure 6 Electronic Controller and EEV for Subcooler Maximizing System Evaporator Operation: As stated earlier, the heat load on the evaporator will undergo wide fluctuations throughout the day, and/or year this is not to be avoided. Every time the actual load varies from the design load condition, the TEV loading (ratio of the actual load divided by the TEV capacity) will change too. This affects the TEV superheat set point, and the ability of the TEV to maintain the desired superheat set point at all times, resulting in an evaporator operating less efficiently than desired.

12 In addition, whenever the TEV s entering liquid refrigeration temperature and/or the pressure drop across the TEV port changes, this too will change the TEV capacity, and in turn change the TEV loading, once again affecting the TEV superheat setpoint. Unlike the changes in the load condition, these system conditions can be controlled. And doing so, will allow for system TEVs and distributor tubes/nozzles that can be selected very precisely, and a very stable superheat condition a the evaporator outlet, resulting in exemplary evaporator performance and maximum efficiency. Figure 7 Liquid Temperature Control From Electronic Controller/EEV Arrangement This can be accomplished by using the above mentioned electronic superheat controller package, along with an outlet pressure regulator in the liquid line (at the receiver outlet). The combination of a fixed liquid pressure and constant liquid temperature of the refrigerant entering the TEV will yield the result described above.