Troubleshooting An Undercharge of Refrigerant

May 5, 2002
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Troubleshooting modern refrigeration and/or air conditioning systems requires a thorough knowledge of the refrigeration system and its functions. However, to be a good troubleshooter, a technician must use the proper instrumentation and, more importantly, have certain organizational skills.

Organizational skills will help the technician more quickly solve even the toughest problems encountered in the field.

One of the best-known ways for a service technician to organize his or her thoughts is through the use of a service checklist like the one shown here.

SERVICE CHECKLIST
Measured values:
Compressor discharge temperature*
Condenser outlet temperature*
Evaporator outlet temperature*
Compressor in temperature*
Ambient temperature*
Box temperature*
Compressor volts
Compressor amps
Low-side (evaporating) pressure (psig)
High-side (condensing) pressure (psig)

Calculated values (degrees F):
Condenser split
Condenser subcooling
Evaporator superheat
Compressor superheat

By simply measuring and recording the above six temperatures (noted above with *) with thermistors or thermocouples, getting the low- and high-side pressure readings from a gauge manifold set, and determining the compressor’s voltage and amperage, a technician will be able to thoroughly troubleshoot any refrigeration system quickly. These six temperatures and two pressures give the technician evaporator superheat, compressor superheat, condenser subcooling, and condenser split for the system.

TROUBLESHOOTING

Referring to the checklist, a technician can analyze a system for faster systematic troubleshooting. Let’s take the categories of the service checklist one by one for an undercharged system, as detailed in the next checklist. Temperatures are in degrees Fahrenheit (degrees F). The system is a low-temperature refrigeration system with R-134a as the refrigerant. The system incorporates a liquid receiver and a TXV for a metering device. In all cases, let’s assume that this system has components that were originally sized properly and are still on the system.

REFRIGERANT UNDERCHARGE
Measured values:
Compressor discharge temperature…...220
Condenser outlet temperature………….78
Evaporator outlet temperature…………10
Compressor in temperature……………50
Ambient temperature………………….70
Box temperature…………………….…20
Compressor volts …………………….230
Compressor amps ……………………Low
Low-side (evaporating)
pressure (psig)…3.94 in. Hg (-20 degrees)
High-side (condensing)
pressure (psig)…....86.4 psig (80 degrees)

Calculated values (degrees F):
Condenser split….……………………10
Condenser subcooling…………………2
Evaporator superheat………………....30
Compressor superheat………………..70

SYMPTOMS

  • Medium to high compressor discharge temperature;
  • High evaporator superheat;
  • High compressor superheat;
  • Low condenser subcooling;
  • Low compressor amps;
  • Low evaporator pressure; and
  • Low condensing temperature.

    COMPRESSOR DISCHARGE

    This temperature is very high compared to normal system operations. The 220 degree discharge temperature is caused from 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 vapor and liquid at its entrance. The evaporator will be starved of refrigerant and running high superheat. The compressor now sees high superheat coming to it, and with the compression stroke will superheat the vapors even more.

    COMPRESSION RATIOS

    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 more compression work and generates more heat of compression. This increased heat may be seen by the high compressor discharge temperature.

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

    The absolute limit that any discharge temperature should be when measured at a distance of about 3 inches from the compressor on the discharge line is 225 degrees. The back of the discharge valve is usually about 50 to 75 degrees hotter than this point on the discharge line. This would make the back of the discharge valve about 275 to 300 degrees. This could vaporize oil around the cylinders and cause excessive wear. At 350 degrees, oil will break down. Overheating of the compressor will soon occur. Compressor overheating is one of today’s most serious field problems. Try to keep your discharge temperatures below 225 degrees for longer compressor life. (Note: For more information on compressor discharge temperatures, refer to “A Look At Compressor Discharge Temperatures” in the March 4, 2002 issue of The News.)

    HIGH EVAPORATOR SUPERHEATS

    Since the TXV and evaporator are 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 sightglass if the condition is severe enough. The TXV is not seeing normal pressures and may even be trying to pass a liquid and vapor mixture from the starved liquid line. The TXV cannot be expected to control evaporator superheat under these conditions.

    HIGH COMPRESSOR SUPERHEATS

    Again, since the liquid line, TXV, and evaporator are being starved of refrigerant from the undercharge, so too will be the compressor. This can be seen in the high compressor superheat reading.

    LOW CONDENSER SUBCOOLING

    TXV systems: Because the compressor is seeing much warmer vapors from the high superheat readings, the gases entering the compressor will be very expanded and have a low density. The 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% 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.

    Capillary tube systems: This is not true for capillary tube systems because the majority of them have no receiver. A capillary tube system can run high subcooling simply from a restriction in the capillary 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, causing low to normal subcooling and low head pressure.

    LOW COMPRESSOR AMPS

    High superheats will cause compressor inlet vapors from the suction line to be very expanded. This will decrease their density. Low-density vapors will enter the compressor and 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, 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.

    The temperature difference between the condensing temperature and the ambient is called the condenser’s delta T or split. (See Equation 1.) The service industry often refers to this as the condenser split. The term condenser split will be used from here on in this article in place of delta T.

    Equation 1:
    Condensing temperature - Ambient temperature = Condenser split

    As the condenser sees less and less heat from the evaporator and compressor because both of them are starved of refrigerant from the undercharge, the condenser split will decrease. No matter what the ambient temperature is, the condenser split (or, in other words, the difference between the condensing temperature and the 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. Some common condenser splits for refrigeration applications are listed below in Table 1. Box temperatures will tell the technician what evaporator load the system is under. A low box temperature means a low load and a high box temperature means a high load.

    CONCLUSION

    As one can see, systematic troubleshooting entitles a thorough knowledge of the refrigeration system and its components. How and why temperatures and pressures respond the way they do when something goes wrong with the system can only be understood through a thorough analysis of a system checklist. System checklists let service technicians organize their thoughts and help them find the answers to tough problems faster.

    Tomczyk is a professor of hvac at Ferris State University, Big Rapids, MI, and author of the book Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached at tomczykj@tucker-usa.com.

    Publication date: 05/06/2002

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