Ice Breaker: Suction Line Pressure and System Capacity
Ways to Avoid Reduced Operating Capacity
An excessive pressure drop across the suction line will reduce the operating capacity of a refrigeration system.
Most systems are designed with a pressure drop not to exceed the equivalent of a 2°F change in saturation temperature. For example, on a medium-temperature hydrofluorocarbon (HFC)-134a system operating with a 20° coil temperature or saturation pressure of 18.4 psig, the pressure entering the compressor should not drop below a pressure equivalent to an 18° saturation temperature or saturation pressure of 17.0 psig. This allows only a 1.4 psig (18.4 psig – 17.0 psig) drop across the suction line.
If the system was a low-temperature HFC-404A system with a coil temperature of minus 10° or a saturation pressure of 23.6 psig, the pressure entering the compressor should not drop below a pressure equivalent of a minus 12° saturation temperature or saturation pressure of 22.0 psig. This allows only a 1.6 psig (23.6 psig – 22.0 psig) drop across the suction line.
So, how does an excessive suction line pressure drop decrease the capacity of the system?
As the refrigerant pressure drops, its specific volume increases and its density decreases. Compressors are constant displacement vapor pumps, meaning they pump a constant volume of refrigerant on each compression stroke. If the density of the refrigerant entering the compressor decreases, the compressor will pump less refrigerant (by weight) on each compression stroke. Less refrigerant will circulate through the system and the evaporator. Less refrigerant circulating through the evaporator will cause less heat energy to be absorbed by the refrigerant, and the overall system capacity will be reduced.
For example, suppose a system with a compressor designed for R-134a is installed on a medium-temperature system with an evaporating coil temperature of 20° (18.4 psig saturation pressure), a condensing temperature of 100°, and a capacity of 2,140 Btuh with a mass flow of 37 pounds per hour. If there is a 6.5 psig pressure drop across the suction line, the refrigerant pressure entering the compressor will be 11.9 psig (18.4 psig – 6.5 psig), or a saturation temperature of 10°.
If the condensing temperature remains constant, the capacity of the compressor will drop to 1,880 Btuh with a mass flow of 29 pounds per hour. That is approximately a 22 percent drop in the system’s capacity.
Avoiding the Drop
Excessive suction line pressure drops can be avoided by using the correctly sized suction line. If the suction line diameter is too small, it will create an excessive pressure drop. Always refer to the manufacturer’s piping charts when selecting the pipe sizes for an installation. Do not select the pipe diameter based on the stub connection on the compressor. The correct pipe diameter may be different based on the length of pipe run or the application.
Also remember: Bigger is not better. A larger diameter pipe may eliminate an excessive pressure drop but can cause issues with the refrigerant velocity and the system’s ability to return the oil to the compressor’s crankcase.
Also if there are any components installed in the suction line, make sure they are sized properly and are not creating an excessive pressure drop. Always refer to the system or component manufacturer’s instructions when selecting a component for a system.
Avoiding excessive suction line pressure will allow a system to operate at its rated capacity, consume less energy, and save money for your customer.
Publication date: 6/30/2014