This application guide provides a theoretical explanation of how to select the optimum high pressure valves and gas bypass valves for the application at hand, taking its starting point in a trans-critical booster system with gas bypass. The application guide uses a supermarket application as the case; the selection guideline, however, can also be used in other applications with minor modifications.
How to select high pressure valves
The function of the high pressure valve is to control the pressure according to the reference from the controller. The reference can be set to obtain the optimum COP, optimum capacity or something else. Normally the COP optimum is the goal, and the valve needs to be able to control the pressure in all situations.
In a high pressure control valve the pressure difference varies with the temperature out of the gas cooler and whether the system has heat recovery or not. Large variation in pressure difference is a challenge, and at the same time the mass flow varies, too. The variations in mass flow are due to changes in capacity, but also due to variations of the cycle. The difference in mass flow from winter to summer can be up to a factor of 2.
Figure 1: Summer condition
Figure 2: Winter condition
Because of the large variations in mass flow and differential pressure across the valve, the design point needs to be found by looking at the different factors and evaluating which of the factors has the most impact.
Figure 3 (Below): Pressures, valve size and mass flow as a function of temperature out of the gas cooler.
Figure 3 shows the differential pressure across the high pressure valve on the left side. At low ambient temperatures, the differential pressure is the lowest, and at the same time the mass flow is also reduced. The diagram shows the needed kv value of the valve under the circumstances. The maximum kv value is needed at the lowest temperature out of the gas
cooler, and that is normally the design point for the valve selection.
Some situations require special attention, even though they do not normally change the selection of valve size:
- In winter, the general mass flow of the plant is only approx. 50% of the max mass flow. This means that if all compressors are turned on for some reason and the valve is designed for the minimum gas bypass mass flow during winter condition, the valve is too small. This scenario is normally disregarded, because other components like the gas cooler and piping is not designed for the situation either. If the situation occurs, the valve will not be able to keep the desired pressure and the pressure in the gas cooler will increase. This situation will not cause any danger to the system or people, because the pressure will still stay within the designed pressure ratings. But worst case the HP-pressure switch will cut off the plant.
- Heat recovery is often used on transcritical systems and result in very low opening degrees. Therefore oversizing of the valve can potentially cause problems.
- Another relevant question is: how close do we want to go to the limit of the valve? If the valve is too small, this will mainly occur under winter conditions, where the load is typically far from the designed 100% load. Therefore, a valve that is too small is no likely to cause problems, and if it does, the system will run with slightly higher pressure for a period. Our recommendation is therefore to select a valve that is as small as possible to get good control capabilities in low load conditions.
How to Select Gas Bypass Valves
In summer, approximately 50% of the mass flow from the high pressure valve is gas. This gas needs to pass through the gas bypass valve. When the gas expands, approx. 2% liquid is formed in the flow. If this is a problem for the performance of the system, the liquid can be removed by installing suction line heat exchangers in the system. Normally, when using booster systems with LT load, problems with cold suction gas will not occur, but still needs to be considered in the design.
Things that need to be considered in the design:
- The design of the receiver needs careful attention. Often in Food Retail applications, the receiver only stores the liquid; in this type of system, however, the receiver also has to separate the gas and liquid flow coming from the gas cooler. If this is not done properly, the liquid carry-over will end in the MT compressors causing failure on the compressors.
- In some cases it might be beneficial to specify the valve for higher flow rates, but this is only in cases where the receiver pressure set-point is close to the design point of the receiver. If the receiver is designed for 40-45 bar, it would be reasonable to select the valve to accommodate the entire flow from the compressors. In case of failure on the high pressure side and if the high side pressure is too low, the system will go into a gas loop where all gas from the compressors will have to go through the gas ypass valve. If the receiver pressure valve is not large enough, the receiver pressure will go up. This will result in a safety valve opening. The trend is to go for higher design ressures, which will solve the problem. In most cases, even with 45 bar receivers, the gas bypass valve is selected for only the bypass gas, and not the compressor mass flow.
- Receiver size is also very important. The fluctuations in refrigerant charge are very big on trans-critical systems, especially when heat recovery is installed. If the receiver size is too small, there is a risk of low charge in the heat recovery mode or under summer conditions. If the system is charged for this situation and then returns to normal refrigeration mode, the receiver might overflow, and the liquid will go through the gas bypass valve into the compressors. It is the recommendation of Danfoss to use a high level sensor in the receiver. This sensor can then be used for alarm, input for load sheading or to stop the compressors.
- On systems with parallel compression, the gas bypass valve is only used under winter conditions, where the amount of gas is very limited. In summer, the parallel compressor will take over and the valve will be closed. In this type of applications it might make sense to select the valve only to be able to handle the winter situation. If you choose to do this, you need to consider the situation when the parallel compressor is not working. In this case, a backup solution is needed. In case of more parallel compressors (e.g. ejector systems), a smaller valve might be attractive.
Figure 4: Gas loop. Pressure 3 bar lower than optimum COP set point
Software can help you select the valves
Software for valve selection is available for this purpose. The selection software also calculates the refrigeration cycle. The selection tool works in the following way:
Figure 5: Screen dump from selection software
The high pressure section of the software provides different possibilities: the algorithm of the Danfoss controller should be used or you may use input from another controller. The temperature out of the gas cooler will be used as input for the algorithm, and the tool will return a pressure. If the check mark is unchecked an input box will appear where you can enter the required combination for temperature and pressure.
If there is additional sub-cooling after the gas cooler this can be put in °K. This sub-cooling can come from a suction line heat exchanger or from some kind of mechanical sub cooling from a chiller or another source. Minimum pressure in the gas cooler is the next value that needs to be entered into the selection tool. This is needed for the selection of the high pressure valve.
The valve type also needs to be selected. The possibilities are ICMTS, CCMT and 2 x CCMT in parallel. Te or suction pressure is needed to calculate the pressure difference for the receiver pressure valve, and it is also used to calculate the refrigeration cycle.
Superheat is also used to calculate the cycle, but the influence is very small. You have to enter the superheat of the compressor. Cooling capacity is required in order to calculate the mass flow of the compressor and the high pressure valve.
In the receiver and valve part of the selection tool you need to enter either pressure or temperature of the receiver.
There is also an option to select "Flash gas mass flow" and "Compressor mass flow". In most cases "Flash gas mass flow" is used. Valve selection can be ICM, CCM, CCMT or 2 x CCMT in parallel.