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May 24, 2023

Determining the Cause of a Restricted Liquid Line

The liquid line in a refrigeration system begins at the outlet of the receiver and includes the filter drier, sight glass, metering device, and any other components located between the receiver and

The liquid line in a refrigeration system begins at the outlet of the receiver and includes the filter drier, sight glass, metering device, and any other components located between the receiver and TXV. The filter drier is a very likely component to become restricted from moisture and/or debris accumulation because it is designed to absorb and store debris and moisture. A restricted filter drier will give the same symptoms as a restricted TXV since it, too, is in the liquid line.

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There are multiple ways a TXV can become restricted. Remember that a TXV becoming partially restricted will have the same symptoms as a restriction anywhere in the liquid line. The following are the many ways the metering device (TXV) can become restricted:

Liquid line restrictions cause the evaporator, compressor, and condenser to be starved of refrigerant. The mass flow rate of refrigerant through the system will decrease, and liquid refrigerant will accumulate in the bottom of the condenser and receiver, causing condenser subcooling readings to be normal-to-high. The system will have symptoms of low suction pressures, high evaporator and compressor superheats, normal-to-high condenser subcooling, low compressor amp draws, and low head pressures.

Here is a service checklist for a restricted liquid line after the receiver and before the filter drier:

Many technicians believe that when any part of the systems high side is restricted or plugged, head pressures will elevate. This is simply not the case, especially on a TXV/receiver system. A restricted liquid line will starve the evaporator of refrigerant, causing low evaporator pressures. The compressor and condenser will also be starved, since they are in series with one another, which will cause both condenser and evaporator pressures to be lowered.

A liquid line restriction may exhibit the following symptoms:

Higher-than-normal discharge temperature. High discharge temperatures are caused from high compressor superheats, as a result of the starved evaporator. High compression ratios from the low evaporator pressure will cause high heat of compressions, thus high discharge temperatures.

High superheats. Both evaporator and compressor superheats will be high as a result of the TXV, evaporator, and compressor being starved of refrigerant from the liquid line restriction. Most of the refrigerant will be in the receiver, with some in the condenser.

Low evaporator pressure. The low evaporator pressure is caused from the compressor being starved of refrigerant. The compressor will try to draw refrigerant from the evaporator through the suction line, but the liquid line restriction will prevent refrigerant from entering the evaporator. This will cause the compressor to put the evaporator in a low-pressure situation.

Low condensing pressure. Since both the evaporator and compressor are starved of refrigerant, the condenser will be, too. Reduced refrigerant to the evaporator will cause a reduced heat load to be delivered to the condenser, which in turn will not have to elevate its temperature and pressure to reject heat.

Normal condenser subcooling. Since the condenser is being starved, it is not condensing much vapor to liquid. All of the liquid in the condenser will probably sit there for a while and subcool because of the low refrigerant flow caused from the restriction. The receiver will have a reduced flow in and out of it, and most of the refrigerant will be in the receiver with some in the condenser. If the receiver is in a hot ambient, subcooling may be lost as refrigerant sits in the receiver. This is why some commercial systems have receiver bypasses for certain situations. Receiver bypasses are a liquid line solenoid valve controlled by a thermostat, which will bypass liquid around the receiver to the liquid line.

Low condenser splits. Because the condenser is being somewhat starved, there is not much heat to reject, which will cause low condenser splits.

Local cold spot or frost where restriction occurs. Liquid refrigerant flashing to vapor might occur at the restriction if the restriction is severe enough. Simply running your hand along the liquid line and on the filter drier may find a local cold spot. A thermistor on the liquid line about 12 inches before the entrance of the TXV should not be colder than the ambient that surrounds it. If it is, there is a sure restriction somewhere upstream.

There are many times when a filter drier or line may be partially plugged and the technician cannot feel a temperature difference. Because of this, many filter driers restrictions go unchecked, but the use of a sight glass after the filter drier will assist the technician with this issue, as well as with system charging. A moisture-indicating sight glass will alert the technician if the system is contaminated with moisture by changing colors.

Bubbles in sight glass. With a restriction in the liquid line before the sight glass, bubbles are sure to occur in the sight glass. Many technicians believe that a bubbling sight glass means nothing but an undercharge of refrigerant, but this is not true. On startup of some refrigeration systems when there is a large load on the system, bubbling and flashing could occur in the sight glass downstream of the receiver. This bubbling is caused from a pressure drop at the entrance of the outlet tube of the receiver.

Bubbling could also occur during rapid increases in loads. The TXV could be opened wide during an increase in load and some flashing could occur even though the receiver has sufficient liquid. Also, sudden changes in head pressure control systems, which may dump hot gas into the receiver to build up head pressure, often will bubble a sight glass even though there is sufficient liquid in the receiver to form a seal on the receiver's dip tube outlet. A sight glass on the receiver prevents technicians from overcharging in this case.

A sight glass on the liquid line before the TXV would also help let the technician know if any liquid flashing is occurring before the TXV. This flashing could be from loss of subcooling or too much static and/or friction pressure drop in the liquid line before it reaches the TXV.

There is a big difference between a bubbling sight glass and a low flow rate sight glass. If bubbles are entrained in the liquid, this is sign of a pressure drop causing liquid flashing, or an undercharge, or refrigerant causing vapor and liquid to exit the receiver because of no subcooling. Remember, the condenser subcooling will be low if an undercharge is causing the bubbling of the sight glass. Otherwise, the bubbling sight glass could mean a restricted liquid line, restricted filter drier, loss of receiver or liquid line subcooling from hot ambient, or static and friction losses in the liquid line are too great.

On the other hand, a low flow rate sight glass is an indication that the system is about ready to cycle off, because the box temperature has pulled down to a low enough temperature. It is at these times that the system is at its lowest heat loads and the refrigerant flow rate through the system will be the lowest. The sight glass may be only one-quarter to one-half full with no entrained bubbles. This situation is especially true with horizontal liquid lines. Do not add refrigerant in this situation, because you will overcharge the system, which will be noticed at higher heat loads. The low heat loads cause the system to be at its lowest suction pressure, so the density of refrigerant vapors entering the compressor will be lowest. Because of the lowest evaporator pressures, the compression ratio will be high, causing low volumetric efficiencies and thus low refrigerant flow rates. There is usually plenty of subcooling in the condenser, but the sight glass will only be partially filled.

A sight glass after the filter drier is a good method to tell if the drier is starting to plug, because of the refrigerant flash from the added pressure drop in the restricted drier.

Low amp draw. Because the compressor is being starved of refrigerant from the restriction in the liquid line, it will not have to work as hard in compressing what vapors do pass through it. The low density of the vapors from the low evaporator pressure will require less work from the compressor, requiring a low amp draw.

Short cycle the low pressure control. The LPC will cycle the compressor off and on from the low evaporator (suction) pressures. Once off, refrigerant will slowly enter the evaporator and cycle the compressor back on. This on and off of the compressor will continue until the problem is fixed.

Given the many different reasons why a liquid line may become restricted, it is incumbent upon the technician to thoroughly troubleshoot the system to find the root cause and then fix it.

John Tomczyk is HVACR professor emeritus, Ferris State University, Big Rapids, Michigan, and coauthor of Refrigeration & Air Conditioning Technology, published by Cengage Learning. Contact him at [email protected].