HERMETIC COMPRESSORS FAULTS

By AC Compressors

Factory faults in refrigeration & air-conditioning compressors are extremely rare & infrequent. Depending on the compressors make, they usually have a legitimate failure rate between 0.001% to 0.1%. It is therefore prudent, to have a good look at the whole system when faced with a faulty compressor, rather than to blame the compressor by default.

Often you will spot something obvious, like an evaporator fan not turning, or a broken gas pipe – for example, and you will therefore know why your compressor ceased to function. More often, however, there will be no obvious clues. That is when you should have your compressor cut-open and inspected to find the cause of its failure.

There are no nationwide & global statistics or information on the most common system faults leading to compressors
failure available, (or at least I have never come across one). Therefore, we have kept our own records for the last 20 years or so.
According to our statistics, electrical related failures are easily the most common on the list of system faults leading to a compressors failure. This is mainly attributed to the loadshedding we currently face in South Africa. In the past, it used to be high discharge temperature & liquid floodback/slugging. It is important to remember though, that HDT may develop from liquid floodback. (The floodback section further down, gives a comprehensive & detailed explanation regarding this).

On this page, you will find the list of compressor failures along with the explanations & the possible causes.

1. HIGH DISCHARGE TEMPERATURE
   In this cause of failure, the temperature in a compressors head and cylinders gets so high that it thins or carbonizes the
   lubricant.
   Possible symptoms of high discharge temperature, usually include the following:

• · Motor windings being down to earth
• · High amperage draw
• · Blown out terminals

• Typical Damage(s) seen on HDT:
• · Carbonized oil
• · Burnt out / carbonized valve reeds
• · Worn cylinders, pistons and rings
• · Broken con-rods
• · Worn, scored, seized bushes and crankshaft
• · Burnt electric windings
• · Dragging rotor
• · Blown out terminals
• · Etcetera

The correct temperature of the motor and cylinder heads are directly related to the suction gas
temperature. The higher the latter, the lower the cooling of the compressors gear and electric
motor.

The following two system problems will lead to high suction gas temperature:

• · High Suction Superheat
• · High Discharge Pressure

Lets have a look at possible system faults leading to these problems:

• High Suction Gas Superheat
  System Short of gas

Restricted refrigeration flow results in:

• TXV problem (stuck in closed position)
• Excessively high load
• Inefficient compression

• High Condensing Pressure

• System overcharged with refrigerant (TXV)

Warning: When a compressor fails under these conditions, it will almost certainly lead to
contamination of the entire system. Detergent cleaning and burnout kit are essential before
starting up replacement compressor.

Typical symptoms:

• Compressor will not pump
• Compressor will not start
• Compressor noisy
• Electric motor down to earth, burned

Typical Damage:

• Worn, scored bearings
• Scored, broken con-rods (especially big end)
• Scored crankshaft
• Dragged rotor
• Broken, carbonized reeds, broken discharge pipe
• Carbonized oil
• Burned electric windings
• Etcetera

Liquid refrigerant returns to the compressor during the ‘On’ cycle. At the beginning of the
process some of the droplets evaporate and the other mix with oil, lowering its lubricating
properties. This mixture wears bearings, scores rods and shafts, leads to excessive working
clearances – causing compression inefficiency and/or rotor drag. The droplets that evaporate,
lower the compressor’s temperature. As the temperature goes down, more droplets escape
evaporation and travel straight into the cylinders.

This is when liquid floodback might cause damage similar to slugging. Not all of the droplets in
this torrent will evaporate, and those that do not, can not be pumped. Thus they are forced via
the discharge reeds to the discharge heads and farther to the discharge pipes. Discharge reeds are
designed to take some hydraulic hammering, but discharge pipes and suction reeds are not,
therefore one of them brakes. It is usually the discharge pipe – it breaks of at a joint due to
strong vibrations. Now, our compressor is not only poorly lubricated, it also partially discharges
back into suction side. Now, the picture changes dramatically. The compressor – from a
refrigeration point of view – becomes entirely inefficient. There is no – or very little – liquid
refrigerant passing through an expansion device, therefore no or very little cooling action takes
place. There are no more liquid refrigerant droplets coming in; the temperature in crankcase rises
and suction line super-heat goes sky high. A perfect HDT condition.
It is important to mention here, that actually, more often than not, a Floodback turns into a High
Discharge Temperature without breaking any parts – just wearing them down, and by doing so
constantly decreasing the compressors pumping efficiency. The final effect is the same.

The following can cause liquid refrigerant to enter a compressor:

• System overcharged with refrigerant (capillary tube)
• Dirty or blocked evaporator air filter
• Evaporator fan out of order
• TXV wrongly adjusted or loose bulb
• TXV oversized
• Faulty or too low set thermostat
• System running outside manufacturer recommendations*
• Etcetera

*System running on too low/high condensing or evaporating pressure/temperature.
What to look for to detect this problem during service:
An easy one to spot, even without any measuring instruments: The compressor’s body feels cold
to the touch of hand. You should be especially alarmed if it feels cold at the crankcase.
Correction should be easy if you check the system for the “a” to “g” faults.

3. LIQUID SLUGGING
Slugging with liquid refrigerant or oil – or most often a combination of both, in the compressor’s
cylinder(s). Like in the previous chapter, the refrigerant migration is solely responsible for this
type of damage; the difference being that a larger quantity of refrigerant migrates into the
crankcase bringing the oil/refrigerant close to the cylinders intake or even above. During the
start-up this mixture is driven directly into cylinders with disastrous results.

Typical symptoms:

• Compressor will not pump
• Noisy run on compressor

Typical damage:

• · Broken valve reeds
• · Damaged gaskets
• · Loose discharge valve bolts
• · Broken connecting rods
• · Broken crankshaft
• · Etc.

The following can lead to refrigerant migration:

• Lack of /or faulty crankcase heater
• Leaking liquid line solenoid valve
• TXV problem or oversized
• Etc.

What to look for to detect this problem during service:
An elementary condition to notice: Terrible, rattling noise during start-up – which tells you that
you got there too late and you are looking at an already destroyed compressor.
Because slugging destroys a compressor so quickly and efficiently – its inside usually remains
pretty clean, which is not the case with washout or floodback.
For the same reason as in liquid washout, damage to the electric motor is possible.

4.LIQUID WASHOUT
A result of refrigerant migrating during the Off cycle, when liquid refrigerant drips into
compressor’s crankcase. During the start-up, pressure in crankcase lowers sharply which causes
refrigerant/oil mixture to foam. This foam gets sucked into the cylinders and washes oil off most
of the lubricated surfaces, but mostly the cylinder assemblies.

Typical Damage:

• Compressor will not pump
• Worn pistons and rings
• Worn cylinders
• Scored piston pins
• Scored, broken rods
• Scored shaft and bearings
• The following can lead to refrigerant migration:
• Lack of /or faulty crankcase heater
• Leaking liquid line solenoid valve
• TXV problem or oversized
• Etc.

What to look for to detect this problem during service:
A noise during the compressor start-up is a give-away. Another clue is the compressor’s
temperature after a few minutes of running time. It should feel warmish to touch (+/- human
body temperature). If it runs cold, the presence of liquid refrigerant should be suspected, either
due to migration or floodback. It is important to realize that liquid washout may lead to winding burnout when the bearing seizes
on shaft, thus locking the rotor, or when neatly broken off con-rods let the motor run without
cooling.

5.LUBRICATION PROBLEMS
I cover under this heading problems caused by lack of lubricant rather than the lack of lubrication
in spite of lubricant presence. Lack of, or not enough oil leads to excessive wear and tear of running gear.

• Typical Symptoms:
• · Compressor noisy

• Windings DTE

• Typical Damage:
• · Scored bearings
• · Broken rods
• · Scored crankshaft
• · Burnt electric motor

There is a certain amount of oil pumped out of the compressor with each stroke of the

piston
(usually 3% of discharged volume). The same amount of oil must return to oil-sump if the
compressor is to be sufficiently lubricated. In a properly designed air-conditioning or
refrigeration system the oil travels along with refrigerant, completing the full circle.

The following can lead to lack of lubricant in the crankcase:

• Gas velocity too small to pick-up the oil:
• Due to lack of refrigerant in the system or
• Due to oversized suction line
• Compressor short cycling
• Blockage in the system
• Lack of oil traps in long vertical pipe run
• What to look for to detect this problem during a service visit:
• The noise is good enough to give you an indication, but unfortunately, as in the case of any other
• mechanical damage, usually it is too late to correct the problem.

6.MOISTURE
The presence of moisture or water in the compressor or system.

Typical Symptoms:

• Trips earth leakage
• Typical damage:
• · Copper plating or/and rust on steel surfaces
• · Scored bearings
• · Scored crankshaft
• · Burnt electric motor

Presence of moisture in the system plus high temperature triggers a chemical brake-down of oil
and refrigerant, which in turn causes corrosion of copper and brass parts. This corrosion initiates
a chemical process that finally washes out copper from all affected parts. The washed out
particles travel with the oil to plate out on compressor metal parts. Plating on non-working parts
does not matter, but it often rubs itself into bearings; scoring them and reducing working
clearances. Bearings’ seizure follows.

The following can cause moisture/water presence in the system:
· Careless, sloppy installation / compressor replacement
· Neglecting to vacuum the system
· Failure to install dehydrating filters

8. ELECTRIC MOTOR PROBLEMS
   Single phase motors:
   8.1 START WINDING BURN
   Sadly, this is the second biggest compressor killer and it usually happens during the
   replacement’s start-up. Unfortunately, this is a grave testimony to “technicians” unprofessionalism.
   Typical symptoms:
   Only the starting winding has burnt due to excessive current flow through it.
   This is by far the most common single-phase motor failure and can initiate in the following ways:
   · Terminals mis-wired

· Faulty or too small capacitor
· Loose terminals
· Motor overloaded due to pressures not equalised
· Low voltage
· Compressor short-cycling
· Compressor not turning
· Etc.

What to look for to detect this problem during service:
Burnt winding is a good indication. Of course, there is nothing one can do, except discarding
this compressor and buying a new one.

There is still a lot one can do to avoid the same
happening again: Read Compressor’s Terminals Markings! It is always there. Wire the
windings accordingly! Then go through the above list and correct whatever needs to be
corrected.

8.2 RUN WINDING BURN
Although only run winding is burnt, it often appears as a general burnout. This is due to the fact
that the temperature of the burning run winding is so high that it partially burns the start-winding
insulation too.
The following can lead to this type of burn:
· Lack of cooling
· High load
· Mechanical problems
· Faulty run capacitor
· Loose terminals
· Faulty contactor
· Under or Over Voltage
Usually, the absence of mechanical damage suggests electric network problem.
8.3 SPOT BURN
This is a short, either between windings, between wires, within a winding or to ground. The
damage is confined to a small area. Usually, the reason is difficult to trace, but look for the
following:
· Contaminants
· Metal shavings or shrapnel bombarding windings
· Vibration
· Voltage surge (lightning, starting capacitor discharge)
Three Phase Motors:
8.4 GENERAL BURN
All three windings uniformly overheated or burned. This can be either a system problem or an
electrical network problem.
· Lack of cooling
· High load
· Mechanical problems
· Compressor short cycling
· Under or over voltage
· Loose terminals
· Faulty contactor
8.5 SINGLE PHASE BURN
Two phases of a three-phase motor are burned or overheated. This is a result of a lack of current
flow through the unburned phase that caused an overload of the remaining phases.
One of the following can lead to this damage:
· Faulty contactor
· Loose compressor terminal (inside/outside)
· Blown fuse
· Unbalanced voltage
8.6 SPOT BURN
See Single Phase Motors.

SUMMARY
More than 99.9% of compressor problems, mechanical as well as electric motor related, are
initiated by refrigeration / air-conditioning system problems.
Careful and patient examination of a system during the service visit should reveal most of the
looming problems and lead to their correction. If, however, the compressor is lost, the first
priority is to determine the cause of failure and its correction – after all it is senseless to have two
compressors fail in a row – for the same reason! (If you are not equipped: to do this kind of
analyses, AC Compressors will assist).