Refrigeration / Compressors

The Professor: Diagnosing Bad HVAC Compressor Valves

When Low Head and High Suction Pressures Collide

John TomczykMany service technicians experience service calls where the compressor has both a low head pressure and a high suction pressure. Often, the refrigeration equipment is still running, but the product temperature is suffering about 7-10°F. The compressor is still cooling, but not to its rated capacity, which can cause refrigerated products to spoil faster and frozen products to begin to thaw.

There are three main reasons why a compressor will simultaneously have a low head pressure and a high suction pressure:

1. Bad (leaky) compressor valves;

2. Worn compressor rings; and

3. Leaky oil separator.

Leaky Compressor Valves

A compressor’s valves may become inefficient because of valve warping from overheating or lack of lubrication, or from having carbon and/or sludge deposits on them preventing them from sealing properly. Here are a few additional reasons why such incidents occur:

• Slugging of refrigerant and/or oil;

• Moisture and heat, causing sludging problems;

• Refrigerant migration problems;

• Refrigerant flooding problems;

• Overheating the compressor, which may warp the valves;

• Acids and/or sludge in the system deteriorating parts;

• TXV set incorrectly — too little superheat, causing flooding or slugging;

• TXV set incorrectly — too much superheat, causing compressor overheating;

• Undercharge causing high superheat and compressor overheating; and

• Low load on the evaporator from a frozen coil or fan not working, causing slugging or flooding of the compressor.

Here is a service checklist for a compressor with valves that are not sealing:

Compressor with Leaky Valves

(Measured values)

• Compressor discharge temperature: 225˚F

• Condenser outlet temperature: 75˚

• Evaporator outlet temperature: 25˚

• Compressor inlet temperature: 55˚

• Ambient temperature: 75˚

• Box temperature: 25˚

• Compressor volts: 230

• Compressor amps: Low

• Lowside (evaporating) pressure (psig): 11.6 psig (10˚)

• Highside (condensing) pressure (psig): 95 psig (85˚)

(Calculated values ˚F)

• Condenser split: 10˚

• Condenser subcooling: 10˚

• Evaporator superheat: 15˚

• Compressor superheat: 45˚


• Higher-than-normal discharge temperatures;

• Low condensing (head) pressures and temperatures;

• Normal-to-high condenser subcooling;

• Normal-to-high superheats;

• High evaporator (suction) pressures; and

• Low amp draw.

Higher-than-normal discharge temperatures — A discharge valve that isn’t seating properly because it has been damaged will cause the head pressure to be low. 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 low flow rate.

Low condensing (head) pressures — Because 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 to the condenser. This will make for a reduced heat load on the condenser, thus reducing condensing (head) pressures and temperatures.

Normal-to-high condenser subcooling — There will be reduced refrigerant flow through the condenser, thus through the entire system, because of components being in series. Most of the refrigerant will be in the condenser and receiver. This may give the condenser a bit higher subcooling.

Normal-to-high superheats — Because of the reduced refrigerant flow through the system, the TXV may not be getting the refrigerant flow rate it needs. High superheat may be the result. However, the superheat may be normal if the valve problem is not severe.

High evaporator (suction) pressure — Refrigerant vapor will be drawn from the suction line into the compressor’s cylinder during the downstroke 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. The results are high suction pressures.

Low amp draw — Low amp draw is caused by 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.

Worn Compressor Rings

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, suction pressure will also be higher than normal. The resulting symptom will be a lower head pressure with a higher suction pressure. The symptoms for worn rings on a compressor are very similar to leaky valves.

Leaky Oil Separator

When the oil level in the oil separator becomes high enough to raise a float, an oil return needle is opened, and the oil is returned to the compressor crankcase through a small return line. The pressure difference between the high and low sides of the refrigeration system is the driving force for the oil to travel from the oil separator to the compressor’s crankcase. The oil separator is in the high side of the system and the compressor crankcase in the low side. The float-operated oil return needle valve is located high enough in the oil sump to allow clean oil to automatically return to the compressor’s crankcase. Only a small amount of oil is needed to actuate the float mechanism, which ensures that only a small amount of oil is ever absent from the compressor crankcase at any given time. When the oil level in the sump of the oil separator drops to a certain level, the float forces the needle valve closed. When the ball and float mechanism on an oil separator goes bad, it may bypass hot discharge gas directly into the compressor’s crankcase. The needle valve may also get stuck partially open from grit in the oil. This will cause high pressure to go directly into the compressor’s crankcase, causing high low-side pressures and low high-side pressures.

Publication date: 10/7/2013

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