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Compressors can have a plethora of problems most stemming from what the system is doing to it. In a rare instance, the compressor is just worn out.
Top of my list of compressor killers is liquid flood back. Liquid flood back, or slugging, is when liquid refrigerant is returned to the compressor from the evaporator. The two significant problems it causes are; it displaces the oil causing loss of lubrication and it won’t compress so, it dead heads the compressor piston.
Those problems lead to the following symptoms:
1. Noisy operation, when the piston hits short of stroke, the head and cylinder walls take the brunt of the force rattling the compressor to the bone.
2. Due to symptom (1), you can probably guess there will be vibration. If the compressor makes it long enough, I’ve seen it snap copper lines.
3. The compressor is going to overheat. The friction goes up because there is little or no oil. The metal to metal contact makes heat.
4. The system suffers capacity problems. Getting liquid into the compressor reduces the head and alters the saturated condensing temperature reducing the refrigeration effect.
5. High saturated suction temperature. All the liquid isn’t evaporated so the temperature of the gas isn’t lowered as much.
6. Superheat will be close to nothing, but when it’s even about 3 – 5 you should worry.
What to look for when you have these conditions might seem obvious, overcharge! And it would be my first guess too.
But there are some other culprits:
1. Evaporator air-flow is too low. Which can be any number of problems itself.
a. Loose or broken drive belt.
b. Plugged air-filter.
c. All the supply registers closed.
d. Duct is blocked with insulation.
e. Duct is crushed.
f. Incorrect speed selection or drive sheave.
2. The expansion valve could be stuck open. Could possibly only freeze at night.
3. On large flooded evaporators, the level could be set too high.
Those are the more common ones. Less common are the design issues. Unit is oversized, or equipment miss-matched.
Next on the list, flooded startup. Causes the same problems as liquid flood back, but it clears up after the unit runs for a bit. Can be tough to spot unless you are there when it starts up and know to look for it.
A clue can be that the system displays oil faults and low oil levels. If you had to add oil more than once, suspect a flooded start.
On hermetic compressors, it harder to catch without a sight glass for the oil, but they will probably be rattling from the damage, just like one that has been overcharged, just no other current symptoms.
There are a few suspects:
1. Bad crankcase heater. The heater comes on when the compressor goes off. It is intended to boil out the liquid that gathers in the sump before the compressor starts.
2. Pump down controls have failed. The pump down solenoid shuts off the liquid to the evaporator just before the unit shuts off. This operation is intended to remove excess refrigerant from the evaporator and suction line to prevent a flooded start up. Often found on outdoor package units.
3. The pump down controls may be working fine, but the solenoid isn’t closing. Refrigerant is leaking through during the off cycle. If this is the case, you’ll be able to watch the pressure rise on the suction line after the unit shuts off. Should rise some, but not equal to the condenser pressure.
Oil loss, oil loss is definitely a compressor killer, and can sometimes be hard to remedy.
Liquid flood back, and flooded starts are the most common reasons for oil loss, but here are a few more:
1. Oil isn’t returning to the compressor due to improper line sizing. The suction line velocity should stay above 1500 foot per minute to carry oil back.
2. Wrong oil with the refrigerant being used. Some refrigerants don’t like some oils and it just won’t carry it back. Sometimes adding small amounts of an oil the refrigerant likes will fix it, but it could also lead to sludge. Check the manufacturer for a solution.
3. Failed separator filter or return trap float. Some oil separators use a coalescing filter to strip the oil out of the gas stream. These materials, over time, get contaminated and quit working. Some also have a float that returns the oil to the compressor, the float can stick or the seat can get blocked off.
4. Bad oil return solenoid.
5. Bad heating element in the oil still. Large system with oil recovery stills use heat to drive out refrigerant from the oil. If the heater goes bad, you get back refrigerant, not oil.
6. Oil skimmers not on or evaporator level to high/low. Also on larger systems, the evaporator can have a series of pipes that should align with the refrigerant level in the evaporator. If they don’t the level control might not be working or needed adjusted.
Running the compressor to hot will lead to a slow death of the compressor. Compressors are tough, but they can only take so much.
Compressors need to operate at the manufacturers stated operating temperature. Here is a list of what to run down if it’s running too hot.
1. Dirty condenser, any surprise?
2. Condenser water or air-flow is low.
3. Excessive superheat. Can be set that way, or be a problem with the metering device. Can also be caused by overloading the evaporator with too much heat.
4. Over-charge and under-charge can cause it.
5. Low oil, or wrong oil.
6. Line voltage or current problems.
7. Mechanical wear. Usually from oil loss, but possibly from wear, I’ve seen compressors still functioning after 30 years in a quality design and installation.
Electrical failures happen but not as often as you would expect.
Symptom: compressor has power but will not run
1. Open winding.
2. Open overload.
3. Open thermal overload.
4. Burnt off terminal.
5. Running a scroll compressor in a vacuum lower than 15 inches of water cause a winding failure.
6. Lightning strikes, lol, so I’ve heard.
If the compressor trips the breaker, check for a shorted winding or a grounded circuit, do not try to operate.
Last but not least, contamination. Technicians are usually really good at keeping stuff out of the refrigerant system. Sometimes it happens through. Problem is, it can be hard to spot since it causes symptoms of other more common problems. Regular oil sampling is the best way to catch it. But here is a list of the symptoms.
1. Compressor won’t reach condensing pressure doesn’t move enough refrigerant.
2. Valves stick open or closed.
3. Compressor overheats.
4. Foul odors from the oil.
5. Unusually high condenser pressure but system is still adequate.
6. High temperature difference on the liquid line dryer.
7. Discolored sight glass.
8. Milky oil.
9. Elevated evaporator temperature.
10. No oil return.
Hope this helps prevent a premature compressor change out.
I worked for over thirty years in the HVACR industry. I have designed, installed, serviced, and trouble shot units of various types throughout the years. The posts here are information based on that experience, I hope you find them useful. If you have a different experience, please comment.