Determining Useful Oil Pressure and Discharge Pressure
When dealing with refrigeration or air conditioning compressors that employ an oil pump, many service technicians confuse net oil pressure with oil pump discharge pressure. However, it is of utmost importance to understand the difference between these two pressures when servicing compressors with oil pumps.
Many larger compressors in the refrigeration and air conditioning field have forced oiling systems. These compressors are usually over 5 hp, and they contain an oil pump located at the end of the compressor’s crankshaft. The crankshaft is actually connected to the oil pump and supplies power, which turns the oil pump.
Oil pumps force oil through drilled holes — called oil galleys — in the crankshaft and deliver it to bearings and connecting rods. The oil then drops to the crankcase to be filtered and picked up again by the oil pump. Smaller compressors usually have some sort of splash-type oiling system. These systems may have an oil scoop that scoops and flings the oil throughout the crankcase, causing an oil fog as the crankshaft rotates.
USEFUL OR NET OIL PRESSURE
Oil pumps can be of the gear or eccentric type. The oil pump’s rotating gear or eccentric adds a certain pressure to the oil pumped through the crankshaft. This pressure is considered net oil pressure. Net oil pressure is not the pressure that can be measured with a gauge at the discharge of the oil pump.
The oil pump picks up oil (at crankcase pressure) from the compressor’s crankcase through a screen or filter. The oil pump’s discharge port pressure includes both crankcase pressure and oil pump gear pressure added to the oil. That is why net oil pressure cannot be measured directly with a gauge. A gauge at the oil pump’s discharge port would register a combination of crankcase pressure and oil pump gear pressure. Technicians must understand this and subtract the crankcase pressure from the oil pump discharge port’s pressure to get the net oil pressure. Net oil pressure, or useful oil pressure, is the difference between the oil pump discharge pressure and the crankcase pressure. Refer to the equation below for the subtraction or differential calculation:
oil pump discharge pressure
- crankcase pressure
= net oil pressure
Problem: The oil pump discharge pressure is 70 psig. The crankcase pressure is 15 psig. What would be the net oil pressure?
Solution: Simply subtract the crankcase pressure from the oil pump discharge pressure to get net oil pressure: 70 psi – 15 psi = 55 psi net oil pressure
This means the oil pump’s gear or eccentric is actually putting 55 psi of pressure into the oil when delivering it into the crankshaft’s drilled passages.
In rare cases, the compressor’s crankcase may be operating in a vacuum. In this case, the crankcase pressure is negative. Remember, every 2 inches of mercury vacuum is equivalent to -1 psi.
Problem: What is the net oil pressure if the oil pump discharge pressure is 35 psig and the crankcase pressure is 6 inches of vacuum (-3 psi)?
Solution: Again, using the equation above, subtract the crankcase pressure from the oil pump discharge pressure to get the net oil pressure: 35 psi – (-3 psi) = 38 psi of net oil pressure.
This means that the oil pump’s gear or eccentric is delivering 38 psi of net oil pressure through the crankshaft and bearings.
HOW MUCH NET OIL PRESSURE?
Net oil pressures usually range from 20 to 40 psi and vary from compressor to compressor. Most oil pressure safety controllers will shut down the compressor if the net oil pressure falls below 10 psi. Variables that affect the net oil pressure are the compressor’s size, temperature of the oil, bearing clearance, and viscosity of the oil.
Larger compressors need more net oil pressure because they have more surface areas to lubricate. The oil pumps must also pump and carry the oil greater distances within the larger compressor. In addition, as the oil gets hotter and its viscosity drops, the net oil pressure will also usually drop. As a compressor wears, its tolerances will become greater and easier for the oil to escape through its clearances. However, if there is a fall in net oil pressure below 9 pounds per square inch differential (psid), the pressure differential switch will close, and a heater in series with the pressure differential switch will be energized.
There is usually a two-minute delay before the heater will warp a bimetallic strip. This warping action will open the timing switch contacts, which are in series with the motor starter or contactor coil. This action takes the motor out of service and must be manually reset on most controls. The reason for the two-minute time delay 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 imperfect system. The two-minute delay gives the crankcase time to clear any unwanted refrigerant during periods when refrigerant migration or flooding has occurred. It also avoids shutdowns during short fluctuations in net oil pressure on startups.
Here are some causes of why an oil safety controller may shut down a compressor:
- Clogged oil screen or strainer;
- Oil pressure safety controller malfunction;
- Worn bearings or connecting rods;
- Excessively hot oil (low viscosity);
- Wrong oil in the compressor;
- Oil pump malfunction;
- Low oil level in crankcase;
- Old compressor (three-phase reversal); and
- Broken oil line or blown gasket.
OIL SAFETY CONTROLLER TYPES
Oil safety controllers can be either mechanical (bellows) or electronic (transducer). Bellows-type controllers sense both crankcase pressure and oil pump discharge pressure, usually through some type of tubing, and then transmit the pressure to flexible bellows. The tubing connected to the bellows is usually copper capillary tubes, high-pressure rubber hose, or a type of high-pressure plastic with a braided aluminum shrouding.
Transducer-type controllers use a pressure transducer, which senses a combination of oil pump discharge pressure and crankcase pressure. The pressure transducer has two separate ports to sense both crankcase pressure and oil pump discharge pressure. The subtraction, or difference, between these two pressures (net oil pressure) is accomplished by the transducer mechanically.
The pressure transducer is connected to an electronic controller by wires and transforms a pressure signal to an electrical signal for the electronic controller to process. Both types of oil safety controllers are referred to as differential-type controllers. The nomenclature comes from the fact that they sense and act on two different pressures. Those pressures are crankcase pressure and oil pump discharge pressure.
Publication date: 4/30/2018