When servicing refrigeration or air conditioning systems that incorporate reciprocating or piston-type compressors, worn compressor rings and/or leaky compressor suction or discharge valves can cause a lack of cooling in both types of applications.
Service technicians are often called when the refrigeration or air conditioning equipment is operating, but the product or room temperature is suffering from being too warm. These calls can be very challenging, because the system is still producing cooling, but not quite enough cooling to achieve the desired space temperature. In refrigeration, this means that fruits and vegetables will spoil more quickly or low-temperature products will not be frozen solid. In air conditioning applications, people will be uncomfortable — and unhappy — from warm temperatures and higher humidity.
In these scenarios, service technicians will usually find the system to have a higher-than-normal suction pressure along with a lower-than-normal head pressure. They will also notice lower amperage draws from the compressor, normal-to-a-bit-high evaporator superheats and condenser subcooling, and often a higher-than-normal compressor discharge temperature. Otherwise, the system will be running but cooling inefficiently.
Depending on the severity of the compressor’s valve or ring damage, service technicians must perform a complete service checklist to properly troubleshoot these challenging situations.
System Check for Bad Valves
In the system check below, the R-134a system has compressor valves that are not seating properly. It is important to note that a system with worn rings will show similar symptoms as leaky valves. When the compressor rings are worn, high-side discharge gases will leak through them during the compression stroke, giving the system a lower head pressure. Because discharge gases have leaked through the rings and into the crankcase, the suction pressure will also be higher than normal. The resulting symptom will be a lower head pressure with a higher suction pressure.
|Compressor discharge temperature||300°F|
|Condenser outlet temperature||74°F|
|Evaporator outlet temperature||25°F|
|Compressor inlet temperature||55°F|
|Refrigerated space temperature||27°F|
|Low-side pressure||12.5 psig/11°F|
|High-side pressure||95.0 psig/85°F|
|Superheat at compressor (total)||45°F|
System Check Analysis
Low condensing (head) pressures: Once the compressor’s valves start to leak and some of the discharge gases are being short-cycled in and out of the compressor's cylinder, there will be a low refrigerant flow rate through the condenser. This will make for a reduced heat-rejection load on the condenser, thus reduced condensing (head) pressures and temperatures. This will cause the condenser split to also be lowered.
High evaporator (suction) pressure: Refrigerant vapor will be drawn from the suction line into the compressor's cylinder during the down stroke of the compressor. However, during the upstroke, this same refrigerant may sneak back into the suction line because of the suction valve not seating properly from oil sludge or other oil breakdown byproducts adhering to its surface. The results are high suction pressures. Either suction or discharge valves may also be warped from a compressor overheating problem.
Low amp draw: Low amp draw is caused from the reduced refrigerant flow rate through the compressor. During the compression stroke, some of the refrigerant will leak through the suction valve and back into the suction line, reducing the refrigerant flow. During the suction stroke, some of the refrigerant will sneak through the discharge valve (because of it not seating properly) and get back into the compressor's cylinder. In both situations, there is a reduced refrigerant flow rate, causing the amp draw to be lowered. The low head pressure that the compressor has to pump against will also reduce the amp draw.
Normal-to-high condenser subcooling: There will be reduced refrigerant flow through the condenser — and thus through the entire system — because of system components being in series. Most of the refrigerant will be in the condenser and receiver, and this may give the condenser a bit higher subcooling.
Normal-to-high superheats: Because of the reduced refrigerant flow through the system, the thermostatic expansion valve (TXV) may not be getting the refrigerant flow rate it needs. High superheats may be the result; however, the superheats may be normal if the valve problem is not severe.
Higher-than-normal discharge temperatures: A discharge valve that isn't seating properly because it has been damaged or sludged will cause the head pressure to be low. The reason is that refrigerant vapor will be forced out of the cylinder and into the discharge line during the upstroke of the compressor. On the down stroke, this same refrigerant that is now in the discharge line and compressed will be drawn back into the cylinder because of the discharge valve not seating properly. This short-cycling of refrigerant will cause heating of the discharge gases over and over again, causing higher-than-normal discharge temperatures. However, if the valve problem has progressed to where there is hardly any refrigerant flow rate through the system, there will be a lower discharge temperature from the extremely low flow rate.
The temperature limit for any discharge line temperature measured two inches from the compressor is 250°F. This means that the actual discharge valve temperature would be approximately 375°F (300° + 75°F). The equation shows that by simply adding 75°F to the discharge line, temperature reading will give the technician an approximate discharge valve temperature.
Mineral oil lubricants will start to decompose at 350°F and polyol ester (POE) lubricants at 400°F. Any increase in temperature above these points causes polymerization of the oil, which occurs when the lubricant’s molecules start to combine into larger and larger molecules. The end product is thick dark oil, then sludge, and finally a solid powder. This is referred to as oil breakdown. Compressor bearing failure and loss of lubrication to moving parts will occur from plugged oil inlet screens and oil galleys (lubrication passages) that are manufactured within the compressor. Oil sludge and other byproducts of oil breakdown can also attach themselves to internal surfaces, including the suction and discharge valves and valve plates.
In conclusion, a refrigeration or air conditioning system that is experiencing leaky valves may be the most challenging troubleshooting scenario to service because of the varying severity of the valve problems that can occur. The low head pressure, while simultaneously having high suction pressures, is a dead giveaway that something is wrong with the compressor’s valve or ring structure.