Net Oil Pressure - Part III

Calculating Net Oil Pressure

Last month's Tech Tip discussed the variables which can affect net oil pressure. This month we will discuss how to calculate net oil pressure. First, to recap:

Net Oil Pressure = Oil Pump Discharge Pressure - Crankcase Pressure

With some refrigeration compressors it is possible for the crankcase pressure to be below atmospheric pressure. In these compressors, for example the Dunham-Bush^® 20- and 30-HP "Big 4" compressors, the suction line feeds into the bell housing of the crankcase. There are pros and cons to this arrangement, but regardless the effect is the crankcase pressure in these compressors is near the suction pressure of the system.

Examples Of Net Oil Pressure Computation
1. Assume a case where the compressor crankcase (suction) pressure gage reads 7 PSIG and the oil pressure gage reads 24 PSIG.
   
   The net oil pressure is 24 PSIG - 7 PSIG, or 17 PSI.
2. Assume a case where the compressor crankcase (suction) pressure gage reads 18 INCHES HG vacuum (-8.84 PSIG) and the oil pressure gage reads 13 PSIG.
   
   The net oil pressure is 13 PSIG - (-8.84 PSIG), or 21.84 PSI
   
   In this instance the 18 INCHES HG (suction) is below 0 PSIG (atmospheric pressure) and is converted to a negative Gage pressure. Remember that when subtracting a negative number, you add the value.
3. To illustrate there can still be a decent net oil pressure to the compressor bearings at extreme conditions: Assume a crankcase (suction) pressure of a 29 INCHES HG vacuum (-14.24 PSIG) and an oil pressure gage reading of .5 PSIG.
   
   The net oil pressure is .5 PSIG - (-14.24 PSIG), or 14.74 PSI.
   
   Note that with an oil pressure switch set to cut-out at 12 PSI, as Dunham-Bush^® recommends on the "Big 4" series machines, even the above conditions would not cause the compressor to shut-down on low oil pressure.

PSIG, Pounds per Square Inch Gage; a pressure which references atmospheric pressure as Zero and positive values as greater than atmospheric pressure.

PSIA, Pounds per Square Inch Absolute; a pressure which references absolute vacuum as Zero. On this scale atmospheric pressure at sea level is generally about 14.7 PSIA.

INCHES HG (suction), inches of mercury; a measure of suction which references atmospheric pressure as Zero and positive values as less than atmospheric pressure. A suction given in inches of mercury can be converted to a negative gage pressure or a positive absolute pressure.

PSI, Pounds per Square Inch; the differential between two pressure readings given using the same scale, both PSIG or both PSIA.