GP1 Compressors Low vs High Vi

Issue:

GP1 compressors, low vs high Vi

Resolution:

The term to describe this compressor trait is the volume ratio. Volume ratio is the mathematical ratio of the volume of the pocket that is trapped by the suction port (the helix) divided by the volume of the pocket just as it is allowed to port to discharge. The volume ratio is abbreviated as Vi.

For GP1 compressors, we make low-volume ratio and high-volume ratio compressors. There is no difference in the displacement between a low vs a high volume ratio compressor. They are NOT low volume and high volume compressors. In GP1 compressors, the size of the pocket at suction is the same for low and high volume ratio machines; the difference is at the discharge port. The reason that we have a low and a high Vi compressor is to better match the compressors to applications. Units that impose a high-pressure ratio (like air cooled) on the compressor will have better compressor efficiency if a high Vi compressor is used. Units that impose lower pressure ratios will get better compressor efficiency if a low Vi compressor is used. The volume flow rate (which is proportional to capacity) will be the same for a low vs a high Vi compressor. When a high Vi compressor runs at low-pressure ratios, the efficiency will drop off, but it does not suffer the same drop off in minimum load capacity, so its discharge temperature will stay lower. For this reason, it is OK to replace a low Vi compressor with a high Vi compressor from a reliability standpoint; it will not be as efficient at low-pressure ratios, but it will not be at risk for minimum load discharge temperature "runaways".

A low volume ratio compressor has a deeper port that allows the compressor to travel from port to discharge sooner (while the pocket is larger in volume). A high volume ratio compressor has a smaller port which delays porting to discharge until the pocket is smaller. One thing to remember about low Vi compressors, is that they get less efficient at high-pressure ratio conditions, and their minimum load capacity drops off quickly as pressure ratio goes up; this can result in very high discharge temperatures.

All air cooled machines get high Vi compressors. Standard RTWA's get low Vi compressors, but high-pressure condenser RTWA's must have high Vi compressors to be reliable. Because we don't make very many low Vi compressors, there aren't very many available for re-build. We took a position early on, that when a service compressor was ordered for an RTWA, we would, in most cases, provide a high Vi compressor to assure reliability. In some cases where the customer objected to a high Vi compressor, we would produce a replacement low Vi compressor in Pueblo. A high Vi compressor should never be replaced by a low Vi compressor.

Notes:

Changes:

Other Question/Problem/Cause Statements:

• GP1 compressors, low vs high Vi

Facts:

• RTAA 70-125 Ton (Pueblo)
• RTWA