An Ounce Of Prevention For Years Of Service

Even though reciprocating compressors are very dependable and usually have a long life, premature failure can occur. Here's how to prevent that from happening

BY NORM CHRISTOPHERSON

The compressor is the heart of the air conditioning and refrigeration mechanical cycle and the most costly component. More time and money is spent replacing compressors than any other system component.

When a compressor is replaced the chances that the replacement will fail is several times greater than the first unit failing. Even though these compressors are very dependable and rarely fail, it happens. Fortunately, most failures are preventable.

When a compressor fails it's usually because of an easily preventable problem that if taken care of will eliminate the need for replacement. Replacing the compressor does not solve the cause of the problem, which is why so many replacement compressors also fail. Reciprocating compressor failures usually can be traced to refrigerant floodback, lack of lubrication, system contamination, electrical problems and overheating.

A brief look at each of these causes will help when performing preventive maintenance as well as when determining the cause of a compressor failure at the time of a compressor replacement.

A compressor runs more than 50 percent of the time and at full speed. There are 8,760 hours in a year so the compressor runs more than half of those hours. To make a conservative comparison, assume that a compressor runs just 4,000 hours a year. Many reciprocating compressors run for 15 or more years without a failure. If our compressor runs for 4,000 hours per year for 15 years it would have operated for 60,000 hours.

The reciprocating compressor does this without an oil change, new parts or a tuneup; no other electromechanical device can achieve such a record. Two basic preventive maintenance tasks will greatly improve the performance and longevity of any compressor: keeping the condenser clean and clear of dirt and regularly replacing evaporator air filters. Additional maintenance will improve the performance and longevity even more.

Refrigerant floodback occurs when liquid refrigerant doesn't completely vaporize in the evaporator but instead returns to the compressor down the suction line. This condition can cause liquid refrigerant to enter the compressor cylinder.

Reciprocating compressors can't compress a liquid and this can cause pressures in excess of 1,000 psig to occur in the cylinder. The result may be a broken valve, head or piston; a bent or broken crank; or a host of other mechanical failures.

Refrigerant floodback is preventable. The most common causes include:

• Oversized metering device.
  
• Thermostatic expansion valve (TXV) superheat set too low.
  
• TXV thermal bulb loose or not thermally tight.
  
• Head pressure too high and/or low pressure too low.
  
• Low evaporator air flow.
  
• Refrigerant overcharge.
  
  

Oversized metering device:
An oversized metering device can allow more refrigerant to enter the evaporator than the evaporator can vaporize. Therefore, the extra liquid refrigerant will pass down the liquid line and enter the compressor.

TXV superheat set too low:
The superheat must be allowed to change in order for the sensing bulb to make corresponding adjustments to the valve opening while it maintains the set superheat. A low superheat setting can allow the superheat to fall to zero. This condition will cause the valve to close and the superheat will soon rise well above its setting.

This in turn can cause the valve to reopen wider than necessary and overshoot the superheat setting. This condition is called hunting. On one of the valve's swings to the wide open position, liquid refrigerant can flood the suction line and allow liquid to enter the compressor, a potentially damaging condition.

TXV thermal bulb loose or not thermally tight:
A loose thermostatic sensing bulb will sense warmer air temperature rather than the cooler refrigerant temperature leaving the evaporator. The warmer bulb will move the valve to a position opening larger than normal and allow liquid to flood back to the compressor.

Not only should the thermal sensing bulb be mechanically and thermally tight on the line, it's always a good idea to insulate the bulb and the line where the bulb is located. Remember, both the sensing bulb and the refrigerant line are round; very little surface area of each actually contact each other. It's amazing that so little bulb-to-line contact actually works without insulation covering them.

Head pressure too high and/or low pressure too low: The pressures across the metering device affect the amount of refrigerant that will be forced through the metering device. As the high pressure increases or the low pressure decreases more refrigerant will tend to enter the evaporator. A higher-than-normal high-side pressure and/or a lower-than-normal low-side pressure oversizes the metering device. It's similar to having the wrong size metering device installed in the first place.

It's possible for a liquid floodback condition to occur if the high-side pressure is too high, the low-side pressure is too low or if there's a combination of the two. Airflow restrictions of all kinds on both the evaporator and condenser coils are common causes of higher-than-normal head pressures and lower-than-normal suction pressures. Each of these could cause liquid floodback and the loss of the compressor.

Low evaporator airflow:
Having discussed how low evaporator airflow can effectively increase the operating capacity of the metering device and cause liquid floodback, it's still important to look at how the evaporator air flow can cause liquid floodback for yet another reason.

The air moving across the evaporator contains the heat that vaporizes the liquid entering the evaporator. In order for all the liquid refrigerant in the evaporator to vaporize completely, there must be enough air bringing enough heat to the evaporator to do so. The lack of air across the evaporator also means less heat is available to vaporize all of the liquid. The remainder of the liquid refrigerant can cause floodback and potentially cause mechanical damage.

Refrigerant overcharge:
An overcharge of refrigerant simply means that more refrigerant exists in the system than the evaporator and condenser were designed to operate with. More refrigerant is in the evaporator than the evaporator has surface area to vaporize. The result is the extra refrigerant flooding back to the compressor.

Lubrication Problems

Lack of lubrication, another reason for compressor failure, is caused by oil loss, oil traps, refrigerant migration diluting the oil, oil pump failure and overcharge.

Oil loss:
If oil is lost, there's a leak in the system. Never add oil to a system to make up for a low oil level unless the leak is located.

Oil traps:
| An oil trap is any place where oil can collect. If the suction line is too large in diameter, the refrigerant velocity will be too low to carry along the oil particles contained in the refrigerant moving back to the compressor. The heavier oil particles will drop out and deposit in the bottom of the suction line. Eventually this oil will return to the compressor as an oil slug.A suction line with low spots, such as a suction line drooping between ceiling joists as it runs across the ceiling, can trap oil and eventually oil slug the compressor. In addition, the colder the evaporator, the thicker the oil will be.

All compressors pump oil, which is easily carried out through hot discharge line and into the high side of the system. The oil mixes with the warm liquid refrigerant in the condenser and is carried through the liquid line to the evaporator where the cold evaporator allows the oil to become thicker.

The thicker oil is unable to flow back to the compressor and can build up in the low side of the system, often accumulating in a low spot (an oil trap). Eventually, the oil will accumulate to the point that it will be blown back down the suction line as an oil slug and possibly damage the compressor.

Refrigerant migration:
Just as pressure moves from high to low and heat moves from higher to lower, refrigerant migrates to the coldest part of the system. When the compressor shuts off, refrigerant moves to the lower pressure, lower temperature evaporator. This means when the compressor restarts the possibility of liquid floodback to the compressor is greater. If the system has been overcharged, the possibility is greater still.

When the compressor shuts down, the compressor crankcase, if located outside, may be the coldest part of the system. In this case it's possible for the refrigerant to migrate to the compressor crankcase and condense into the oil. Many refrigerants and oils are miscible, which means the two mix well.

When the compressor restarts, the refrigerant boils out of the oil as the crankcase pressure drops. The boiling refrigerant carries oil with it and much oil is removed from the crankcase leaving less oil for compressor lubrication.

In addition, the mixture of refrigerant and oil results in a diluted lubricant, and compressor wear likely will be immediate due to the poor lubrication capability of the diluted mixture. The common solution to this problem is the use of a crankcase heater, which keeps the crankcase warm enough to prevent refrigerant migration to the crankcase in the first place.

When a compressor has been shut down for an extended length of time the crankcase heater should be turned on for 12 to 24 hours prior to allowing the compressor to start. The use of an automatic pump-down system can help prevent refrigerant migration from occurring and help keep the oil returning from the evaporator to the compressor on a regular basis and in small, harmless quantities.

All compressors using oil pumps should have an oil failure control installed and in proper working order

Oil pump failure:
The failure of an oil pump to provide sufficient oil pressure to properly lubricate the compressor can also lead to a lubrication failure. All compressors using oil pumps should have an oil failure control installed and in proper working order.

The oil pump pressure may be checked by reading the crankcase or low-side pressure and the oil pump discharge pressure. The actual oil pressure is found by subtracting the low-side pressure from the pump discharge pressure. If the oil pressure isn't sufficient for that particular compressor, the oil pump may be the
problem.

Other problems can also cause the oil pump to fail to create sufficient oil pressure. If the oil pump strainer in the compressor crankcase is plugged or if the oil pump pressure regulator is broken or stuck open, a perfectly good oil pump won't create sufficient oil pressure. It's also possible that the compressor is so worn that oil leaks out of tolerance bearing surfaces, making it impossible for even a good oil pump to keep the oil pressure at a normal level.

An easy test to determine if the problem is with the oil pump or a worn compressor is to reverse the compressor's rotation. Reversing the rotation will cause the oil pump to operate in reverse and to use the unused sides of the gears that pump the oil. If after reversing the rotation the oil pressure improves, the oil pump was the problem. If the oil pressure fails to improve, look for one of the other causes as the problem.

Overcharge:
An overcharge of refrigerant will increase refrigerant migration and increase the possibility of refrigerant migrating to the crankcase.

Worn compressor:
As stated earlier, a worn compressor will allow oil to blow past the larger openings between the bearing surfaces and make it difficult for the oil pump to keep the oil pressure up to normal. The only solution for this is to replace or rebuild the compressor.

Causes of System Contamination

The causes of system contamination leading to compressor failure are air, moisture, dirt/flux/powder and acid.

Air:
Air in a system goes to the condenser and takes up space used by the refrigerant to give up heat. In addition, the air has its own pressure, which is added to the refrigerant pressure. This raises the high-side pressure and can cause the metering device to overfeed. Floodback to the compressor may result.

Moisture:
The air also contains moisture which may chemically combine with the other materials in the system and break down to form acids. The acids in turn will begin eating away at the metal surfaces in the compressor as well as destroying the insulation on the motor windings, possibly leading to an electrical failure.

Both air and moisture can be kept out of the system by having a leak-free system and using good evacuation techniques. Acids are more easily formed with the application of heat and pressure, both of which are plentiful in the mechanical refrigeration system

Dirt/ flux/ powder:
It's equally important to practice good service procedures and keep dirt from entering the system when it's open. Overuse of flux can allow flux to enter the system as well. The powder inside certain insulation can get inside lines if they're not sealed when sliding the insulation on them. Dirt, flux and powder each can combine with the air, moisture, oil and refrigerant to form acids.

Causes Of Electrical Failure

There are six primary causes of electrical failure, including improper voltage, under-voltage condition, over-voltage condition, single phasing, voltage imbalance and current imbalance.

Improper voltage:
Obviously, no motor will operate long if it has the improper voltage. Not so obvious is a situation where the 208-volt motor that has 230 volts applied to it. Such a possible condition should be checked because it can lead to replacement of an existing compressor which has had an electrical failure.

Under-voltage/over-voltage conditions:
Motors are designed to operate within 10 percent of their rated voltage. A quick check of the actual voltage at the compressor terminal will indicate if the actual voltage is within the 10 percent limit.

Single phasing:
Single phasing occurs when a three-phase motor loses one of the three power lines to the compressor. The required voltage should be available across all three legs of each of the three-phase power lines.

Voltage imbalance:
A voltage imbalance only occurs on three-phase systems. If a voltage check between each of the three power lines shows that the voltages are not all equal, the voltage is not in balance. A small voltage imbalance (3 percent or less) may not be harmful to the compressor motor. An imbalance greater than this can cause the motor to overheat and fail.

Current imbalance:
A current imbalance only occurs on three-phase systems. If a current check on each of the three-phase power lines shows that there are different amperages in any of the three legs, a current imbalance exists. Small current imbalances may not cause a problem. But if the current becomes too imbalanced, the motor may overheat or a winding may open.

Causes Of Overheating

The primary causes of overheating are lack of suction cooling, lack of condenser cooling, air in the system, system restriction and electrical problems.

Lack of suction cooling:
Hermetic and semi-hermetic compressors depend upon the cool suction gas entering the compressor crankcase for cooling. Generally, if the suction gas temperature at the suction service valve is greater than 65° F, the compressor motor will overheat and a failure may result.

Many times simply insulating the suction line will lower the high suction gas temperature sufficiently. Other causes of high suction gas temperatures may be a system undercharge or the use of a liquid to suction line heat exchanger where it's not warranted.

Lack of condenser cooling:
Insufficient air or water flow over a condenser will raise the head-side pressure and temperature, and may cause the compressor to run in an overheated condition. It's important to determine that sufficient air or water is available for condenser cooling. In addition to causing potential compressor damage, high head pressures lower the compressor capacity, increase running time and increase power consumption.

It bears repeating that if the cause of a compressor failure is not determined and rectified, the replacement also will fail

Air in the system:
Just as air in the system can cause the metering device to push too much refrigerant into the evaporator and cause a liquid floodback condition, air also raises the high-side pressure and temperature, and may cause the compressor to overheat. It's one more reason to practice good evacuation techniques.

Restriction:
A restriction usually occurs in the filter-drier or at the metering device. A restriction can raise the high-side pressure and temperature and may cause the compressor to overheat.

In conclusion, it¹s not uncommon for more than one of these conditions to exist at the same time. Together, each may contribute to the failure of the compressor and its eventual replacement. It bears repeating here that if the cause of a compressor failure is not determined and rectified, the replacement compressor will also fail. It's not difficult to check for these conditions and take corrective action to prevent a failure. Remember, compressors are very reliable and few actually fail. Service technicians only see those units that do fail and can be led to believe that they're not reliable.

A final note. Not everything in this article, which is intended to serve as a relatively comprehensive overview of reciprocating compressor failure prevention, applies to every compressor product. Contact the manufacturer for information on a specific product.