READER FAVOURITE FROM HPAC’S ARCHIVE: Pressure-regulating Valves: The Basics
June 2, 2014 | By DAVE DEMMA
Look at pressure-regulating valves as the consummate control freaks. Their job, as evidenced by their name, is to regulate (or control) pressure. In terms of desire, they are fairly self-absorbed and one-dimensional. They really do not care about anything else that is going on in the system other than the single function they are attempting to control. And there are times when this is not only beneficial, but actually crucial.
There are rules that these valves must abide by:
• As their name implies, pressure-regulating valves only control pressure.
• Pressure-regulating valves will either control an upstream pressure or a downstream pressure. But not both.
• Pressure-regulating valves will either prevent a pressure from falling below a predetermined level or from rising above a predetermined level. Again, but not both.
• Pressure-regulating valves control pressure without regard to what else is going on in the system.
There are four common types of pressure-regulating valves that one might come across in an HVAC/R system. Each type has its own specific function.
Crankcase Pressure Regulator
This valve is typically only applied in refrigeration systems, particularly low temperature applications. Because of the evaporator temperatures required for refrigeration applications, particularly low temperature applications, it is a foregone conclusion that during the normal system operation frost will accumulate on the surface of the evaporator tubes and fins. Frost accumulation acts as an insulator between the heat transfer surface (the evaporator) and the heat transfer medium (the refrigerant), reducing evaporator capacity and potentially leading to refrigerant flooding issues.
Frost accumulation necessitates periodic defrost cycles, which for low temperature applications will require the use of electric heaters, or flowing hot gas through the evaporator to melt the frost.
After the defrost cycle terminates the system is under a high load, meaning that the compressor will be under a high load. All compressor drive motors will have specified on their nameplate a rated load amperage (RLA) rating. This is an amperage that the motor should never exceed. The two system conditions that have direct affect on the compressor motor amperage are discharge pressure and suction pressure. It is the higher suction pressure during high load conditions after the termination of a defrost cycle which causes the compressor drive motor amperage to increase. At some point, if the suction pressure continues to rise, the amperage draw will exceed the RLA.
Here is where the crankcase pressure regulator (CPR) steps in to provide a solution. The CPR is a downstream pressure regulator and will prevent the downstream pressure (valve outlet pressure) from rising above a predetermined level. In this application what that predetermined level is is not the point. The proper method in setting this valve requires an electrical meter with amperage measuring capabilities. While the system is operating under a high load condition, measure the compressor amperage draw. (Note: it is important to make sure that the amperage being measured is for the compressor only, not the supply wire from the compressor contactor that might be feeding the compressor and the condenser fan motor). The valve should be adjusted to a point where the compressor drive motor amperage draw is right at or slightly below the RLA. Whatever the pressure that this occurs at is really immaterial.
Now, anytime the system is running under a high load condition, the CPR will prevent the compressor suction pressure from rising to a level such that it would cause the amperage draw for the drive motor to exceed the RLA.
Evaporator Pressure Regulator
Again, this valve is typically applied in refrigeration systems. There might be applications with a single compressor and single evaporator where maintaining a constant refrigerant saturated suction temperature (SST) in the evaporator is beneficial. These would be applications where the product has a high water content, such as fresh meat, produce or floral. If the system is allowed to operate at a high TD (difference between air entering the evaporator and SST) it will pull more of the water content out of the product. Not only will this result in a product that is less appealing to the consumer, but the business owner will literally see his profits going down the drain in the form of moisture taken out of his profit.
The evaporator pressure regulator (EPR) is an upstream pressure regulator that prevents the upstream pressure (valve inlet pressure) from falling below a predetermined level. In this application the valve’s set-point would be the pressure that corresponds to the SST requirement in the evaporator. For example, for an R-404A system where the SST requirement is 28F, the valve would be set for 66 psi. This will maintain a 28F coil temperature at all times.
In some larger refrigeration applications, the design might incorporate a multiplex compressor rack, where all of the compressors are piped together with common suction/discharge lines. It is typical in these applications to have several different evaporator systems connected to the common suction manifold of the compressor rack. One method of maintaining the design temperature for each individual evaporator system is to employ an EPR in each circuit. Each valve can be set to maintain the desired SST required for the circuit it is controlling, thereby maintaining consistent temperature in the refrigerated space.
Head Pressure Control Regulator(s)
For systems that are required to run 12 months out of the year, the varying ambient temperatures seen between the middle of summer and the dead of winter can cause wide fluctuations in discharge pressure. Given that the condenser is selected based upon the load requirements at the design ambient condition in the summer time, whenever the ambient temperature falls below the design ambient, the condenser becomes oversized.
As such, without any means of control, the head pressure will become lower during periods when the ambient temperature has fallen below the design ambient temperature. While lower head pressures allow the compressor(s) to operate more efficiently (with resulting higher capacity), if the head pressure falls to extremely low levels, you reach a condition where there isn’t adequate liquid refrigerant pressure to maintain the minimum pressure drop across the thermostatic expansion valve (TEV) port for the valve to operate at their required capacity. There is no magic level that the head pressure has to be maintained at, as it really depends on how the TEV was originally selected. The typical industry standards are 100 psi for low pressure refrigerants (R-134A, etc) and 180 psi for high-pressure refrigerants (R-404A, R-407A, R-507, etc).
The typical head pressure control application will utilize two valves. One is located at the outlet of the condenser and is normally referred to as the
“hold back” valve. It is an upstream pressure regulator, and is designed to keep the condenser pressure from falling below a predetermined level. If the pressure at the inlet of the valve is below its set-point, the valve remains closed. The compressor continues to pump refrigerant into the condenser; the condenser continues to remove heat from the discharge vapour and condense it into a liquid; and the liquid backs up in the condenser, reducing the effective size of the condenser. As the refrigerant backs up in the condenser or “floods” the condenser, the head pressure will rise. Once it rises to the set-point of the valve, the valve will start opening and allow refrigerant flow to the receiver.
This one valve by itself does not really accomplish what is desired, which is to maintain a minimum liquid refrigerant pressure to supply the TEVs. A second valve is required to maintain a constant pressure at the receiver, and is normally referred to as the “receiver pressurization” valve. This valve is an outlet pressure regulator, and is designed to keep the receiver pressure from falling below a predetermined level. It is piped between the discharge line (before it enters the condenser inlet) and the drop leg from the condenser.
If a minimum liquid pressure of 160 psi is required, then it would be typical to set the hold back valve at 180 psi, with the receiver pressurization valve set at 160 psi. It is important to note, that since this method of head pressure control relies on flooding the condenser with liquid refrigerant during the lower ambient periods of operation, it is necessary to have an adequate amount of refrigerant in the system to accommodate this. Charging a system to a full sight glass in the warmer months will not leave the system with adequate charge to properly flood the condenser and maintain a liquid seal at the receiver outlet during the lower ambient months.
Discharge Bypass Regulator
For systems that have wide fluctuations in load, this can cause fluctuations in the operating suction pressure of the system. There are two potential problems with this. First, when compressors operate at abnormally low suction pressure, the resulting higher compression ratio and high discharge temperature can be detrimental to long compressor life. Second, comfort cooling applications operating at SSTs much below 32F will result in frost buildup on the evaporator tubes and fins. Chillers are subject to the same fate but the minimum temperature will vary depending on the percentage concentration of glycol mixed with water. This result is particularly devastating with chillers. A freeze up can cause the chiller tubes to break and allow water to enter the refrigerant side of the system. Chiller barrel replacements are quite expensive.
The discharge bypass regulator, or hot gas bypass regulator (HGB), is another downstream regulator whose design is to keep the suction pressure from falling below a predetermined level. It accomplishes this by bypassing high-pressure discharge vapour to the low side of the system and artificially loading the compressor to prevent the suction pressure from falling below the predetermined minimum pressure.
Why not use an EPR for this application? Well, an EPR is essentially an adjustable restriction in the suction line. It will maintain a constant upstream pressure (the evaporator pressure), which will satisfy the second condition above. However, by the very nature of the valve being a restriction in the suction line, it will violate the first condition mentioned above. This illustrates how the pressure-regulating valves do not care about anything else other than what they are designed to do. The EPR is designed to maintain consistent pressure at its inlet or at the evaporator. It does so at the expense of the downstream pressure. To maintain a constant pressure in the evaporator while allowing the compressor suction pressure to fall outside of the safe operating range of the compressor would not be prudent course. The HGB satisfies both conditions listed above, and is the proper valve for this application.
While we have not explored the inner workings of the valves described above, this is a good overview of when and where to use the four common pressure-regulating valves available in the HVAC/R industry.
And remember: not all control freaks are bad guys. <>