If a centrifugal chiller has a suction pressure of 18 in HG vacuum and a discharge pressure of 6 PSIG, what is the compression ratio?

To determine the compression ratio of a centrifugal chiller, it's essential to understand how this ratio is calculated. The compression ratio is the ratio of the discharge pressure to the suction pressure. It helps in understanding the performance of the chiller in terms of how much the refrigerant is compressed during the refrigeration cycle.

In this scenario, we have a suction pressure of 18 inches of mercury (HG) vacuum and a discharge pressure of 6 pounds per square inch gauge (PSIG). First, we need to convert the suction pressure from vacuum (in HG) to absolute pressure, and discharge from gauge to absolute pressure as well.

1. Convert Suction Pressure:

The suction pressure in vacuum (18 in HG) means that the pressure is below the atmospheric pressure. Atmospheric pressure is typically around 14.7 pounds per square inch (PSI) at sea level.

• The absolute suction pressure can be calculated as:

• Absolute suction pressure = Atmospheric pressure - Vacuum pressure

• Vacuum pressure in PSI = 18 in HG * (1 PSI / 2.036 in HG) ≈ 8.84 PSI

• Absolute suction pressure = 14.7 PSI - 8.84 PSI