Restricted Liquid Lines

October 5, 2003
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Service technicians must be able to distinguish between a restricted liquid line and a system overcharge. Adding refrigerant to a system that has a restriction before the receiver can lead to a dangerous high-pressure situation, especially if the high-pressure control is not set or is not operating properly.

When a restriction occurs before the receiver in a low-temp commercial refrigeration system, the evaporator and compressor will be starved. Any refrigerant that does enter the condenser will be trapped there, causing high condenser subcooling.

This high subcooling will take up valuable condenser surface volume, causing the condensing (head) pressure to elevate.

System And Symptoms

The system we’re going to examine in this article has a thermostatic expansion valve metering device with a receiver. The refrigerant is R-134a.

Table 1 shows a system checklist for a restricted liquid line before the receiver. Table 2 shows the calculated superheat, subcooling, and condenser split values.

Table 1. Checklist for a restricted liquid line before the receiver.

Table 2. Values for superheat, subcooling, and the condenser split.
Symptoms include:

  • High discharge temperatures.

  • High condensing (head) pressures.

  • High condenser subcooling.

  • High condenser splits.

  • Low evaporator pressures.

  • Low amp draws.

  • Bubbly sight glass.

  • Short cycling of the low-pressure control (LPC).

    High Discharge Temperature
    High discharge pressures result from high compression ratios. High compressor ratios result from the elevated condensing temperatures. High condensing temperatures result from the liquid backed up in the condenser from the restriction between the condenser and the receiver.

    High Condensing (Head) Pressure
    High head pressures result from the liquid being backed up in the condenser, causing a reduced condensing surface area. The restriction before the receiver causes most of the liquid to remain in the condenser, and slowly get into the receiver.

    This is far different than if the restriction was after the receiver. A restriction after the receiver would cause most of the refrigerant to be in the receiver. This would result in low head pressures because the receiver is designed to carry all of the refrigerant, plus a 25 percent refrigerant vapor head for safety purposes.

    High Condenser Subcooling
    With the restriction in the liquid line between the receiver and condenser, liquid will primarily remain in the condenser and cause high condenser subcooling.

    Do not confuse this with an overcharge of refrigerant. An overcharged system will not have low evaporator pressures and high superheats. (For more information on system overcharge, please refer to “The Symptoms Of Refrigerant Overcharge,” Sept. 1, 2003.)

    High Condenser Splits
    Elevated condensing pressures and temperatures will cause the temperature difference between the surrounding ambient and the condensing temperature to increase. This temperature difference is called the condenser split.

    Low Evaporator Pressures
    Because of the restriction in the liquid line, the receiver, TXV, and evaporator will be starved. The compressor will keep pumping, trying to get vapors from the evaporator. This will draw the evaporator into a low pressure. How low the suction pressure gets will depend on the severity of the restriction.

    Low Amp Draw
    Amp draws will usually be low in this situation because the compressor is starved of refrigerant. Also, the low density of the small amount of refrigerant vapors coming into the compressor will cause low amp draws.

    Bubbly Sight Glass
    The sight glass will have bubbles because of the starved receiver, as well as the liquid line after the receiver. Depending on the severity of the restriction, the slight glass may have very little refrigerant running through it and look empty.

    Short Cycling On LPC
    Because the compressor is starved of refrigerant, the low-pressure control (LPC) will cycle the compressor off and on from the low (evaporator) suction pressures.

    John Tomczyk is a professor of HVAC at Ferris State University, Big Rapids, Mich., and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. Tomczyk can be reached at

    Publication date: 10/06/2003

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