Net Oil Pressure Part I – Description - 2011-12

Net Oil Pressure - Part I: Description

What is Net Oil Pressure?

Net oil pressure (NOP), sometimes referred to as useful oil pressure, is the difference between the oil pump discharge pressure and the crankcase pressure. Only this differential pressure is capable of useful work.

Many larger refrigeration compressors have forced oiling systems. These compressors are usually over 5 hp. They contain an oil pump located at the end of the compressor opposite the motor.

When dealing with compressors that employ an oil pump there can be confusion between net oil pressure and oil pump discharge pressure. The oil pump picks up oil at crankcase pressure from the compressor's sump through a screen. The oil pump's rotating gear or eccentric then adds pressure to the oil pumped through the crankshaft. The added pressure is considered "net" oil pressure. Net oil pressure is not the pressure measured at the discharge of the oil pump. The oil pump discharge port's pressure includes both crankcase pressure and oil pressure.

A gauge at the oil pump's discharge port would register a combination of crankcase pressure and oil pump pressure. This is why oil safety controllers are called differential-type controllers. They sense the difference between oil pump discharge pressure and crankcase pressure. These controllers have a capillary tube connected to the discharge of the oil pump and a capillary in the crankcase of the compressor; they sense the difference between these pressures, the "net oil pressure".


NOP and the Differential Oil Pressure Switch

The oil pump discharge pressure acts to open the differential pressure switch. Conversely, the crankcase pressure acts to close the switch. The differential between these two pressures is the net oil pressure. If the differential is too low, the compressor will shut down to protect itself from lack of lubrication.

If there is a fall in net oil pressure below 10 pounds per square inch differential (psid), the pressure differential switch will close and a heater in series with the switch will be energized. There is usually a two minute delay while the heater warps a bimetallic strip. This warping action will open the timing switch contacts, which are in series with the motor starter or contact coil. This in turn takes the motor out of service and shuts down the compressor.

The reason for the two minute time delay before the switch opens is to prevent nuisance trips of the oil safety controller. There are often times when the crankcase may have liquid refrigerant in it from an momentary fluctuation in the system. The two minute delay gives the crankcase time to clear unwanted refrigerant when refrigerant migration or flooding has occurred. The two minute delay also avoids shutdowns during short fluctuations in net oil pressure which can occur on start-up.

The timing switch must be manually reset on most controls. Manually resetting the timing switch will close the switch contacts after the bimetal strip cools down. There is a short delay before the switch can be reset to prevent "short cycling".

Next month we will discuss the variables that affect net oil pressure.


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These technical tips are for use by qualified maintenance personnel who are familiar with their specific freeze drying equipment. These tips are intended only as general guidelines. Lyophilization equipment is frequently custom configured and some tips may not be appropriate for all freeze dryers. Always read and follow the directions of your equipment's maintenance manual. If you would like to discuss one of our tech tips, please contact Dave Clayton at 215-672-7800 ext-1376.


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