The Professor: Refrigerant Undercharge

February 4, 2008
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My Jan. 14 column in The NEWS focused on system overcharges. This column will look at an undercharge of refrigerant.

In the example to be used, we will look at a medium-temperature refrigeration system running on HFC-134a and having a thermal expansion valve (TXV) and liquid receiver.

Here are the checklist’s measured values:

• Compressor discharge temperature: 195°F

• Condenser outlet temperature: 78°

• Evaporator outlet temperature: 10°

• Compressor inlet temperature: 50°

• Ambient temperature: 70°

• Box temperature: 20°

• Compressor volts: 230

• Compressor amps: Low

• Low side (evaporating) pressure (psig): 3.94 inches Hg at -20°

• High side (condensing) pressure (psig): 86.4 at 80°

The calculated valves are:

• Condenser split: 10°F

• Condenser subcooling: 2°

• Evaporator superheat: 30°

• Compressor superheat: 70°

Symptoms include:

• Medium to high discharge temperatures

• High evaporator superheat

• High compressor superheat

• Low condenser subcooling

• Low compressor amps

• Low evaporating temperatures and pressures

• Low condensing temperatures and pressures

IN DETAIL

Now let’s look at each symptom in detail.

Compressor discharge: This temperature is very high compared to normal system operations. The 195° discharge temperature is caused by the evaporator and compressor running high superheat along with high compression ratios. When undercharged, do not expect the TXV to control superheat. The TXV may be seeing a combination of vapor and liquid at its entrance. The evaporator will be starved of refrigerant and running high superheat. The compressor now sees high superheat and, with the compression stroke, will superheat the refrigerant even more.

Compression ratios will also be elevated, giving the system a higher-than-normal heat of compression. Compression ratios will be high from low evaporator pressures. This will give the system very low volumetric efficiencies and cause unwanted inefficiencies with low refrigerant flow rates. The compressor will now have to compress much lower pressure vapors coming from the suction line to the condensing pressure. This requires a greater compression range and a higher compression ratio.

This greater compression range from the lower evaporator pressure to the condensing pressure is what causes compression work and generates some heat of compression. This increased heat may be seen by the high compressor discharge temperature.

However, because of the lower flow rates from the lower volumetric efficiencies, compressors see a somewhat low load. This low load is what keeps the discharge temperature from getting too hot.

Higher compression ratios and higher superheats are what cause the discharge temperature to be somewhat high. Remember, the discharge line sees all of the superheat coming to the compressor, the motor heat generated, and the heat of compression.

The absolute limit for any discharge temperature measured about 3 inches from the compressor on the discharge line is 225°. The back of the discharge valve is usually about 50° to 75° hotter than the discharge line. This would make the back of the discharge valve about 275° to 300°. This could vaporize oil around the cylinders and cause excessive wear. At 350°, oil will breakdown. Overheating of the compressor will soon occur. Compressor overheating is one of today’s most serious field problems. Try to keep discharge temperature below 225° for longer compressor life.

High evaporator superheats: Since the evaporator is starved of refrigerant from the undercharge, there will be high evaporator superheats. This in turn will lead to high compressor (total) superheats. The receiver is not getting enough liquid refrigerant from the condenser because of the shortage of refrigerant in the system. This will starve the liquid line and may even bubble a sight glass if the condition is severe enough. This TXV is not seeing normal pressures and may even be trying to pass liquid and vapor from the starved liquid line. The TXV will also be starved and cannot be expected to control superheat.

High compressor superheats: Since the liquid line, TXV, and evaporator are being starved of refrigerants from the undercharge, so will the compressor. This can be seen in the high compressor superheat reading.

Low condenser subcooling: Because the compressor is seeing very hot vapors from the high superheat readings, the gases entering the compressor will be very expanded and have a low density. This compression ratio will be high from the low suction pressure causing low volumetric efficiencies. The compressor is simply not pumping much refrigerant. All components in the system will be starved of refrigerant.

The 100 percent saturated liquid point in the condenser will be very low. This will cause a low condenser subcooling. The condenser is simply not receiving enough refrigerant vapor to condense it to a liquid and feed the receiver. Condenser subcooling is a good indicator of how much refrigerant charge is in the system. A low condenser subcooling can mean a low charge. A high condenser subcooling can mean an overcharge, but not always.

A capillary tube system having no receiver can run high subcooling simply from a restriction in the cap tube or liquid line. The excess refrigerant will accumulate in the condenser causing high subcooling and high head pressures. If a TXV receiver system is restricted in the liquid line, most of the refrigerant will accumulate in the receiver, with a bit in the condenser. This will cause low subcooling and low head pressure.

Low compressor amps: High superheat will cause compressor inlet vapors from the suction line to be very expanded. This will decrease their density. Low-density vapors entering the compressor will mean low refrigerant flow through the compressor. This will cause a low amp draw because the compressor doesn’t have to work as hard compressing the low-density vapors. Low refrigerant flow will also cause refrigerant-cooled compressors to overheat.

Low evaporator pressure: Low evaporator pressure is caused from the compressor being starved of refrigerant. The compressor is trying to draw refrigerant into its cylinders, but there is not enough refrigerant to satisfy it. The entire low side of the system will experience low pressure.

Low condensing pressure: Because the evaporator and compressor are being starved of refrigerant, the condenser will also be starved. Starving the condenser will reduce the heat load on the condenser because it isn’t seeing as much refrigerant to reject any heat. With not as much heat to accept and thus reject from the compressor, the condenser will be at a lower temperature. This lower temperature will cause a lower pressure in the condenser because of the pressure-temperature relationship at saturation.

CONDENSER SPLIT

The temperature difference between the condensing temperature and the ambient is called the condenser’s Delta tee or split. As shown here:

Condensing Temperature
-  Ambient Temperature
Condenser Delta Tee (Split)

As the condenser sees less and less heat from the compressor because of it being starved from the undercharge of refrigerant, the condenser split will decrease. No matter what the ambient is, the condenser split (or the difference between the condensing temperature and ambient) will remain the same if the load remains the same on the evaporator. Condenser split will change if the load on the evaporator changes.

Publication Date: 02/04/2008

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