What is return gas temperature?

[david2008]

When I read the product specification from a compressor manufacture, they give the test condition of return gas temperature at 40F. What does return gas refer to? Thanks.

[US Iceman]

Return gas temperature (RGT) is the temperature of the refrigerant vapor returning to the compressor; i.e., the suction vapor.

You may see this specified as 40°F gas temperature or a maximum of 65°F of superheat (or something like this. It's been a long time since I have worked on small equipment). Some of the high and medium temperature compressors will probably have the RGT = 40°F, while the low temperature compressors will probably be listed as 65° of superheat.

This is where it helps to understand the difference between evaporator superheat and suction superheat. At the compressor the gas temperature is almost always warmer than the superheat temperature from the evaporator. The additional heat comes from heat picked up by the suction line before the gas returns to the compressor.

Neither of these two descriptions indicate they are preferred or normal, although many systems run like this on supermarket racks. It is a simple way of describing how the compressor performance is determined for the published capacities.

[GXMPLX]

It is the temperature of the gas entering the compressor.

When you test compressors to measure capacity, the easiest way is to use an elecric heat exchanger and heat the refrigerant to a given temperature.

If you reach a point of equilibrium in temperatures and pressure, the power you input to the heater equals the capacity of the compressor.

It does not mater if the resulting point is dangerous to the compressor. In the lab you cool the headers or even don't care if the compressor will last only few hours working in this condition.

So be very careful, test conditions may not be working conditions you want.

[david2008]

Thank you all. Explanation are perfect!

[david2008]

Another question is that testing data from compressor manufacturer seems like

Cond temp 100
Evap temp 10F
Btu/h 1234btu/h
Watt 321w

For different evap temp and cond temp, btu/h and watt will have different number. We assume this compressor is a 0.3HP. I wonder that what does 1234btu/h and 0.3HP mean? 321W should be the electrical energy compressor consumed. Thanks.

[US Iceman]

Now you are getting into the area of smoke and mirrors.

Compressor capacities change when the suction and/or discharge pressure vary, when the liquid refrigerant temperature to the TXV increases or decreases. Any of this can occur or be modified by the actual design of the system or selection of the evaporator or condenser.

HP or watts as electrical input power for a compressor do not provide any information about the capability of a compressor. This data only says : the compressor has "x" HP or "x" watts available to keep the compressor running.

[GXMPLX]

Usually the Watt indication is for power input to the compressor to produce the BTU/h that is "like"the power output of the compressor.

If the Watt indication is nominal power it's less usefull.

But the meaning must be clearly said in the catalog or like a map without a scale the catalog is almost useless.

[david2008]

Hi Iceman and GXMPLX, I understand your posts. But why compressor manufacture will provide testing data at different evap and cond temp? How people can use them when select a compressor? Thanks.

[US Iceman]

I believe this is based on ARI (Air Conditioning & Refrigeration Institute) rating procedures the manufacturers have agreed to base their compressor performance on.

Therefore, when you compare one compressor to another (at the same rated conditions) you can see the difference in performance (assuming all else; speed, displacement, etc) are the same.

These ratings do not indicate how the compressors perform in the real world; only on the catalog sheets! The ranges (on the catalog sheets) simply provide a range of performance data.