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A compressor’s operating envelope defines the range of saturated discharge temperatures (SDT) and saturated suction temperatures (SST) within which a compressor can safely operate. Operating a compressor outside this envelope for an extended period can lead to its premature failure. A compressor’s operating envelope is typically shown as a graphical representation of these conditions, as shown in Figure 1.
OPERATING ENVELOPE: The polygon on the graph represents the safe boundaries in which the compressor can operate. (Courtesy of Joe Marchese)
The X-axis of the graph represents the SST of refrigerant entering the compressor. The SST is obtained by converting the actual refrigerant pressure in psig to its equivalent saturation temperature using a standard P/T chart or app. For example, a compressor designed to use R-404A is operating at a 20 psig suction pressure, and its equivalent SST would be -14.6°F.
The Y axis of the graph represents the SDT of refrigerant leaving the compressor. If the compressor is operating at a 245 psig discharge pressure, its equivalent SDT would be 102.4°F. The polygon on the graph represents the safe boundaries in which the compressor can operate. Each compressor has a unique operating envelope that can be easily obtained from the manufacturer via an online search or through the manufacturer’s compressor app.
Compressor Failure
Identifying that a compressor has failed is usually straightforward, but determining the cause of the failure can be much more challenging. However, determining why a compressor has failed will help prevent the replacement compressor from also failing prematurely. Replacement compressors fail at a much higher rate than compressors used on new systems, which indicates that there is most likely a system issue that causes a replacement compressor to fail. Referencing the operating envelope of the compressor can be useful to determine if the compressor is operating outside its operating envelope, potentially leading to its failure.
Consider this example: you are working on a system with a plugged condenser and a failed compressor. After replacing the compressor and cleaning the condenser, you check the set point of the high-pressure switch. It is set to open at 400 psig, which equates to a SDT of slightly under 140°F. You check the operating envelope of the compressor and discover that it is operating at 140°F SDT, which is high but still within its operating envelope. You decide to check the operation of the high-pressure switch and discover it does not open until it gets to 500 psig, which equates to a SDT well over 150°F. You decide to change out the high-pressure switch to prevent the new compressor from operating above a SDT of 140°F for an extended period in the future.
Referencing a compressor’s operating envelope can also be helpful when selecting a compressor for a new application or a replacement for a discontinued compressor that does not have a cross-reference replacement listed. Referencing a compressor’s operating envelope can help to ensure the replacement compressor selected will operate satisfactorily for the application.
Selecting the right compressor for a job and preventing repeat compressor failures is a must for any successful service contractor. Referencing the compressor operating envelope is just another tool a contactor can use to be sure the replacement compressor will not fail. Replacing multiple compressors on a system not only takes away from a contractor’s profit margin, but it also tarnishes their professional image. Using the right compressor and repairing the system properly not only keeps the customer happy, but it also keeps a contractor profitable.