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Service Tip: Evaporator vs. System Superheat

May 1, 2009
Q: What is the difference between evaporator superheat and system superheat? A: Superheat varies within the system, depending on where it is measured.

Q: What is the difference between evaporator superheat and system superheat?

A: Superheat varies within the system, depending on where it is measured. The superheat that the thermal expansion valve (TXV) controls is the evaporator superheat. This is measured at the outlet of the evaporator. The refrigerant gains superheat as it travels through the evaporator, basically starting at 0. The refrigerant enters the evaporator, travels through the evaporator absorbing heat and reaches a maximum at the outlet. System superheat refers to the superheat entering the suction of the compressor. Some people confuse system superheat with ‘return gas temperature.’

Superheat varies as the saturated suction pressure of the refrigerant varies. Return gas temperature is a temperature value measured by a thermometer or other temperature-sensing device. It does not vary due to pressure changes.

Q: How much system superheat should I see at the compressor inlet?

A: Compressor manufacturers like to see a minimum of about 20F of superheat at the compressor inlet. This is to assure them that no liquid refrigerant is entering the compressor.

Q: What is refrigerant floodback?

A: Refrigerant floodback is a result of liquid refrigerant returning to the compressor during the running cycle. The oil is diluted with refrigerant to the point that it cannot properly lubricate the load-bearing surfaces.

Q: What are the signs of refrigerant floodback in an air-cooled compressor?

A: Signs of refrigerant floodback in an air-cooled compressor include worn pistons and cylinders; and no evidence of overheating

Q: How does refrigerant floodback happen in an air-cooled compressor?

A: Refrigerant floodback in an air-cooled compressor happens when the liquid washes the oil off of the pistons and cylinders during the suction stroke, causing them to wear during the compression stroke.

Q: What are the signs of refrigerant floodback in a refrigerant-cooled compressor?

A: Signs of refrigerant floodback in a refrigerant-cooled compressor include the center and rear bearings being worn or seized, a dragging rotor and shorted stator, a progressively scored crankshaft, and worn or broken rods.

© Emerson Climate Technologies. Used by permission.