Oil Pressure Problems

1. Oil Pressure Problems What to look for

2. Compressor Oil Level Check the compressor sight glass for sufficient oil. • The sight glass should be at least one-third full (for Copeland Compressors) • Too much oil can cause crankcase turbulence which can cause low oil pressure. Remove excess oil. • On start up, low oil pressure can occur in large systems with long piping runs or in an evaporator with a lot of tubing • If this is the case wait for the system to settle down and the oil level should recover • If the system is new you must wait 24 hours after start up to add any oil

3. Refrigerant Piping • Is there a suction riser? Does it have a trap at the bottom? If not, corrections must be made • Tube diameter must be consistent with the length of run and BTU capacity • If the tube is too large refrigerant velocity in the pipe becomes insufficient to carry the oil with it • Tubing should be slopped one inch per ten feet of run towards the compressor to aid in oil return and be properly insulated

4. Compressor Suction Superheat • This is the measure of how much above it’s saturated temperature the refrigerant is being heated • A high superheat causes low gas density which causes the system to run at low suction pressure • This causes the compressor to pump at an inefficient rate causing gas velocity to drop. • The gas is not dense enough to carry the oil back • To correct, open the expansion valve to allow more gas flow • Check the suction superheat and adjust to 20* at the compressor

5. Refrigerant Charge • If the charge is low there is not enough refrigerantto return the oil to the compressor • Determine why the system is low on charge. • When refrigerant leaks, oil will leak also which will reduce the oil level • Improperly set fan cycle control reduces system pressures affecting oil return

6. Low TXV Superheat • This condition causes floodback which washes oil out of the compressor • Oil washout causes bearing seizure resulting in compressor motor burnout which is often misdiagnosed as an electrical failure • Indicated by excessive foaming of the oil in the compressor sight glass • Causes can include improperly set TXV, iced evaporator, restricted air flow or an evaporator fan motor that is not running • Copeland recommends superheat of 10 - 12* for high temp, 5 – 8* for medium temp and 2 – 5* for low temp

7. Burned out Crankcase Heater • Boils off liquid refrigerant in the crankcase to prevent gathering • Without it, on start up, the compressor will attempt to pump a liquid which results in failure • Large amounts of liquid in the crankcase causes oil to be pumped out and violent oil foaming • This reduces lower crankcase pressure and the liquid is trying to boil off

8. System Defrost • This is critical! • Iced evaporators will cause poor oil return • A pump down on defrost system helps avoid this problem by introducing the liquid refrigerant to the warm evaporator after defrost which increases it’s volume and helps carry oil back to the compressor

9. TXV Distributer • The distributer contains a nozzle that is sized for the application upon installation • Improper nozzle sizing causes refrigerant velocity loss with resulting in poor oil return • As a result it is uncommon to have the incorrect nozzle but it should not be overlooked

10. Oil Pump • A worn pump will cause low oil pressure • The possibility of a worn pump is remote unless it is very old • It is possible that the pump is not getting sufficient oil due to a clogged oil pump inlet screen • To correct remove the screen from the pump, clean and reinstall • Reverse the compressor rotation to allow oil flow through the unused, clean oil port

11. Low Pressure Control • An improperly set LPC can cause the oil control to trip unnecessarily. • If the differential is set too close the compressor will short cycle causing the OFC to trip • Check the manufacturer’s recommended settings