OTR - Prevost Nashville

[johnklopp]

I’m not sure exactly what overhauled means. At the minimum it may have been a major service with belt and seal replacement. Whatever the case may be its important to run an open drive compressor at least monthly for 15 min or so. The reason is periodic lubrication of the shaft seal located behind the clutch on the compressor. If the compressor is properly cared for with all of the high and low pressure safeties operational your open drive compressor air conditioning systems should run several years without problems.
Compressors should be just as dependable as they are in your home air conditioning system or your late model car. A compressor is similar to an engine with cylinders and an oil sump. Rooftop electric compressors will typically be rotary rather than reciprocating. The bottom line is they are dependable and 99% of the time fail for external causes that could have or should have been preventable.

Typical preventable compressor failures include the following:

Los of oil and refrigerant. If you have a refrigerant leak you also have an oil leak since the oil is entrained with the refrigerant and circulates thru the entire system. Many leaks are found by locating oil at connection points or shaft seals. The oil may not be apparent but the collection of dust on the oil leak is a dead giveaway.

Oil needs to be added carefully. The open drive compressor on OTR air will have a sight glass were you can see the oil level. Just because the oil is at the bottom of the sight glass doesn’t mean its low. The oil needs to be checked after the system has operated half an hour or more at high load. The reason is that the compressor has unloaders that reduce the refrigerant mass flow at light loads. Oil will tend to collect in some areas of the system and lower the oil level in the compressor. When the load returns to high, the additional mass flow of the refrigerant will return the missing oil to the compressor. If someone has added oil to a system that is not low, the cooling capacity will be reduced. The reason is the additional percentage of oil in the entrained refrigerant will insulate the refrigerant from the coils and reduce heat transfer. I’m probably getting close to information overload so I will leave it at that.

Florescent dye is commonly used by technicians in the search for leaks. It is easily added to an operational system and allows a technician with an ultraviolet flashlight to see tiny leaks that now glow in the flashlight beam. You can purchase a portable florescent leak detection kit made by “yellow jacket” at most refrigerant supply stores.

High discharge pressure caused by a dirty condenser coil will increase the compressor discharge pressure. In the short term the affect will be reduction in cooling capacity and a substantial increase in power to drive or run the compressor.

If the compressor is for OTR air, the unloaders seals life will be reduced. If a technician ever indicates your compressor is worn out, ask him to do maintenance on it. The unloaders seals, shaft seals, cylinder liners etc. are all replicable. If he doesn’t have the knowledge, purchase a rebuilt unit from one of many companies that remanufacture the Carrier open drive bus compressors. You should be able to get one complete with a new clutch for less than $2000.

If the compressor is an electrically driven roof mount or bay mount, your compressor can not be rebuilt. The damage that will occur is a breakdown of the motor windings. Eventually the motor will fail.

Dirty evaporator filters will cause the refrigerant to have an incomplete phase change from liquid to gas. If it’s bad enough some liquid refrigerants will get back to the compressor. Since liquid doesn’t compress, the compressors valves will break.

Eclectic compressor short cycling will cause motor winding failure due to lack of cooling. You electric compressor should have a short cycle time delay. If it’s been removed or the time interval has been changed overheating and premature failure will result. The reason is your electric compressor is cooled by return refrigerant gas. Each time a compressor is started a large amount of power is used to overcome static friction and then increase the motor to operational speed. This puts residual heat into the motor that takes around 5 minutes of normal operation to remove. No or very low cooling is taking place while the compressor is off. Repetitive short cycling of the compressor will progressively heat the motor winding until a failure occurs.

This represents a partial summary of the preventable failures that your air conditioners are experiencing.

Its time for an adult beverage