HVAC Residential Market / HVAC Light Commercial Market / HVAC Commercial Market / Refrigeration

The Professor: Superheat

[Editor’s Note: This column is the first of two on evaporator superheat.]

Superheat is probably the most talked about yet misunderstood technical term used by service technicians in the field. Superheat is a measured value. It is the difference between two temperatures.

Superheat is measured as the difference between the actual temperature of refrigerant vapor at a certain point and the saturation temperature of the refrigerant. Superheat on the system’s low side can be divided into two types: evaporator superheat, and total or (compressor) superheat.

Evaporator Superheat

Evaporator superheat starts at the 100 percent saturated vapor point in the evaporator and ends at the outlet of the evaporator. The 100 percent saturated vapor point is the point where all the liquid has just turned to vapor. The temperature at this point can be obtained from a pressure/temperature chart. The evaporator outlet is where the remote bulb of the TXV is located.

Technicians usually put a thermistor or thermocouple at the evaporator outlet to get the evaporator outlet temperature. A pressure gauge at that same point as the temperature reading will give the technician the saturated vapor temperature. Most manufacturers of larger evaporators supply a Schrader fitting at the evaporator outlet pretty close to the remote bulb of the TXV for measuring pressure.

Here is an example with regards to an R-134a refrigeration system:

The low side gauge reading at evaporator outlet equals 20 psig or 23˚F. (See excerpt from temperature/pressure chart.) The evaporator outlet temperature (thermistor reading) equals 30˚.

So the evaporator superheat calculation is:

30˚ evaporator outlet temperature

– 23˚ saturation temperature at evaporator outlet

= 7˚ evaporator superheat

If a technician was to measure the pressure at the compressor instead of the evaporator outlet, a higher, fictitious superheat value will be read. As the refrigerant travels the length of the suction line, there will be associated pressure drop from friction and/or restrictions caused from long runs of suction piping, oil accumulation, filters, or valves. This will cause the pressure at the compressor to be lower than the pressure at the evaporator outlet. This higher, fictitious superheat reading may lead the technician to adjust the TXV stem clockwise (open) more to compensate for the fictitious high superheat reading. This could cause compressor damage from liquid flooding or slugging from too low of a superheat setting.

For an example of this, assume a 5 psi pressure drop from the evaporator outlet to the compressor. Again, this applies to an R-134a refrigeration system. The low side gauge reading at compressor inlet equals 15 psig or 15˚. (See excerpted chart.) The evaporator outlet temperature (thermistor reading) equals 30˚.

So, the evaporator superheat calculation is:

30˚ evaporator outlet temperature

– 15˚ degree saturation temperature at compressor inlet

= 15˚ superheat

The superheat changed from 7˚ to 15˚ simply by reading the pressure at the compressor inlet instead of the evaporator outlet. The correct evaporator superheat would be 7˚. It is best to measure the pressure at the same location as you measured the temperature to exclude any system pressure drops.

Publication date: 11/12/2012
 

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