Does Frost Mean Floodback?

November 1, 2000
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Figure 1. Frost coming back to a suction gas-cooled compressor.


Figure 2. Frost coming back to an air-cooled compressor.
Many service technicians believe that frost on a suction line or on the compressor itself indicates that liquid refrigerant is coming back to the compressor. This simply is not true.

All that the presence of frost means is that the suction line or compressor is below freezing, and the moisture in the air has reached its dewpoint temperature and condensed. This condensed moisture has then frozen to ice because the temperature is below 32°F.

Figures 1 and 2 show frost coming back to a suction gas-cooled compressor and an air-cooled compressor, respectively. Figure 3 shows a frosted suction line filter and accumulator. In all three figures, the evaporators had over 9° of superheat and each compressor had over 25° of superheat.

Compressor superheat is the assurance that there is no liquid refrigerant present at the compressor, and that the saturated vapor in the evaporator has gained 25° of sensible heat before reaching the compressor. All three condensing units were low-temperature applications running -10° box temperatures. With -10° box temperatures, the evaporator temperature averaged about -24°.

With -24° evaporating temperatures and the system having 25° of compressor superheat, the compressor return gas temperatures were about 1° (Equation 1). Dew water) will freeze at this temperature (1°) and become frost on the lines and compressor.

Publication date: 11/06/2000

Frosted suction line filter and accumulator.

Gas-Cooled or Air? Know the Difference

It is important for service technicians to understand the difference between suction gas-cooled and air-cooled compressors.

In an air-cooled compressor (Figure 2), the suction return gas does not pass over the windings of the compressor. The return gas simply enters the compressor through the suction service valve on the side of the compressor.

This gas enters the suction valve and cylinders right away, without seeing any other heat source. If there is any liquid (refrigerant or oil) entrained in this suction gas, the valves and/or piston rods themselves can be seriously damaged.

This is not the case for refrigerant gas-cooled compressors (Fig-ure 1). Liquid refrigerant coming back to the compressor must first pass around or through the motor windings. There is a good chance that the windings will produce enough heat to vaporize any liquid refrigerant before it is sucked up through the suction cavities to the valve structures.

Figure 4 shows a cutaway view of a suction gas-cooled compressor. Notice that any refrigerant must travel in close proximity to the motor windings before it flows uphill and enters the valve structures and cylinders.

The only way a service technician can tell if liquid refrigerant is coming back to the compressor (floodback) is to measure the compressor superheat at the set and convert it to a temperature with a pressure-temperature chart.

Next, with a thermometer or thermistor, measure the compressor temperature on the suction line about 6 in. from the compressor (Figure 1). The compressor in temperature should always be warmer than the evaporating temperature.

If it is at the same temperature or colder, liquid refrigerant is probably present in the compressor. To figure compressor superheat, subtract the evaporator temperature from the compressor in temperature (Equation 2).

Tomczyk is a professor of hvac at Ferris State University, Big Rapids, MI, and author of the book Troubleshooting and Servicing Modern Air Conditioning and Refrigeration Systems, published by Business News Publishing Co. To order, call 800-837-1037.

Publication date: 11/06/2000

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