Refrigeration systems can become restricted in both their high and low sides for a variety of reasons, including the following:

  • Restricted thermostatic expansion valve (TXV) screen or orifice;
  • Kinked liquid line;
  • Kinked U-bend in liquid line;
  • Foreign material in metering device orifice;
  • Oil-logged TXV from refrigerant flooding the compressor;
  • Too much oil in the system;
  • TXV adjusted too far closed;
  • Manufacturer’s defect in the metering device valve;
  • Plugged inlet screen on TXV;
  • Wax buildup in TXV valve from wrong oil in system;
  • Sludge buildup in TXV from the byproducts of a compressor burnout;
  • Partial TXV orifice freeze-up from excessive moisture in the system;
  • Restricted filter drier from moisture, dirt, oil, or sludge;
  • Restricted liquid line solder joint;
  • Receiver outlet valve partially closed off or sludged; and/or
  • Restricted solenoid valve.

Notice that the above causes for system restrictions deal with the liquid line, which is located in the high side of the refrigeration system. While most restrictions occur in the system’s liquid line, low-side restrictions do occur and will be covered later in this article.



The liquid line begins at the outlet of the receiver and includes the filter drier, sight glass, and any other components located between the receiver and TXV, including solenoid valves and hand valves. A system with a restricted metering device has the very same symptoms as a system with a liquid line restriction. This is because the TXV is actually part of the liquid line.

All liquid line restrictions cause the evaporator, compressor, and condenser to be starved of refrigerant. This will cause symptoms of low suction pressures, high evaporator and compressor superheats, normal to a bit high condenser subcooling, high compressor discharge temperatures, low compressor amp draws, low head pressures, and low condenser splits. Often, the system will cycle on the low-pressure control.

The filter drier is a very likely component to become restricted from moisture and/or debris accumulation. It will result in the same symptoms as a restricted TXV, since it, too, is in the liquid line. However, if the filter drier is restricted enough, it will sometimes feel cool or cold to the touch. This phenomenon happens because some of the liquid refrigerant experiences a slight pressure drop and expands into vapor as it travels through the filter drier. Vapor bubbles in the sight glass downstream of the filter drier can also be observed if this phenomenon occurs. This is why it is of utmost importance to have the liquid sight glass downstream, not upstream, of the filter drier.

Filter driers are designed to remove foreign material from a refrigeration or air conditioning system, including moisture, dirt, sand paper grit, soldering flux, small solder beads, and acid. However, it is excessive moisture that causes most filter driers to become restricted. Sources of moisture in refrigeration and air conditioning systems can include:

  • Improper handling of hygroscopic system lubricants;
  • Improper handling of system piping at installation;
  • Improper brazing or soldering techniques;
  • Leaky systems; and
  • Improper evacuation techniques.

As the fine particles accumulate over time, they can cause enough pressure drop so as to cause the liquid refrigerant passing through to flash to a vapor. In addition to a local cold spot on the filter drier, condensation (sweating) may occur on the outer surface of the filter drier’s body. The sight glass will also bubble due to the refrigerant flashing to a vapor. Do not mistake this with a bubbling sight glass associated with an undercharge of refrigerant.

There is a big difference between a bubbling sight glass and a low flow rate sight glass. If bubbles are entrained in the liquid within the sight glass, this is sign of a pressure drop causing liquid flashing or an undercharge of refrigerant causing refrigerant vapor and liquid to exit the receiver and enter the liquid line 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 a hot ambient or static and friction losses in the liquid line are too great.

On the other hand, a low refrigerant 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. A low flow rate sight glass may be only one-quarter to one-half full with no entrained bubbles, especially with horizontal liquid lines. Do not add refrigerant in this situation because you will overcharge the system, and this will be noticeable at the higher heat loads. The low heat loads cause the system to be at its lowest suction pressure; thus, the density of refrigerant vapors entering the compressor will be lowest. Because of the low evaporator pressures, the compression ratio will be high, causing low volumetric efficiencies and low refrigerant flow rates. There is usually plenty of subcooling in the condenser, but the sight glass will only be partially filled when this scenario occurs. So do not confuse a low refrigerant flow rate sight glass with a bubbly sight glass, which has bubbles entrained in the liquid.

As mentioned earlier, there are many times when a filter drier may be partially plugged, and the technician cannot feel a temperature difference across it with their hands. Because of this, many filter drier restrictions go unchecked, which is why the use of a sight glass after the filter drier is important.

Sight glasses can also assist in system charging. Moisture-indicating sight glasses will change color if the system is contaminated with moisture. On startup, with some refrigeration systems, if 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.

Sudden changes in head pressure control systems, which may dump hot gas into the receiver to build up head pressure, can also 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 can prevent technicians from overcharging in this case but would cost the manufacturer a bit more money initially. A sight glass on the liquid line before the TXV would also 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.



On new installations, a suction filter will likely be installed in the suction line, and this filter can become restricted just like the liquid line filter. Compressor manufacturers also often place a fine screen or mesh in the suction inlet of the compressor, and if this screen becomes plugged, low suction pressures will be noticed, and compressor capacities will suffer.

The low side of a refrigeration system can also become restricted due to an oil-logged evaporator. Ideally, oil in a refrigeration or air conditioning system should stay inside the compressor’s crankcase to lubricate the compressor’s moving parts. However, this doesn’t always happen, due to ever-changing heat loads on the system and varying system conditions caused from refrigerant undercharges or overcharges, inoperative valves, faulty or misadjusted metering devices, dirty condensers, dirty evaporators, fan motor failures, or plugged filter driers.

Inevitably, the compressor’s lubricating oil will escape the compressor’s crankcase and enter the system’s tubing. Hopefully, this escaped oil will eventually find its way through the condenser, receiver, liquid line, metering device, evaporator, and suction line and end up back in the compressor’s crankcase.

It is the velocity of the refrigerant traveling through the system and the piping arrangements that brings the oil back to the compressor’s crankcase. That is why it is so important to have the proper line and coil sizes to support the correct refrigerant velocity for proper oil return. Just as important are precisely sloped lines and accurate location of P-traps for proper oil return to the compressor.

Refrigerant migration and system flooding conditions are often two of the reasons why system lubricant or oil escapes the compressor’s crankcase and enters the system. Refrigerant migration is when the refrigerant migrates back to the compressor’s crankcase during the off cycle. This occurs due to a pressure difference between the oil in the compressor’s crankcase and the refrigerant.

Oil has a very low vapor pressure and will attract the refrigerant in both the vapor and liquid states. Refrigerant migration can cause the compressor’s crankcase to lose its oil, thus circulating the oil throughout the refrigeration system. This oil in circulation usually gets caught in the evaporator and can cause an oil-logged evaporator, which is actually a restriction in the low side.

Refrigerant flooding refers to liquid refrigerant entering the compressor’s crankcase during the “on,” or running, cycle. Flooding can cause flashing of the oil in the compressor’s crankcase due to liquid refrigerant boiling and also cause excessive pressures in the crankcase. This phenomenon can also cause the compressor to lose its oil and circulate it throughout the refrigeration system. Excessive oil in the system will again get caught in the evaporator and cause it to become oil logged.

Some of the ways in which an evaporator’s inside tubing can become oil logged include:

  • Wrong type or viscosity of oil;
  • A liquid refrigerant flooded compressor circulating oil at startup;
  • Liquid refrigerant migration during the “off” cycle, causing crankcase oil foaming on startups;
  • Too much oil in the system;
  • System not piped correctly (no oil traps or piping too large);
  • TXV out of adjustment (too little superheat, causing a refrigerant-flooded compressor flooding); and
  • Not enough defrost periods for low-temperature-application machines.

Oil usually logs in the evaporator because it is the coldest component with the largest tubes, so it has the slowest refrigerant velocity. Oil logged in the evaporator will coat the inner walls of the coil and reduce the heat transfer through the walls, which will cause a loss of capacity and poor performance. The compressor will be robbed of some of its crankcase oil and run with a lower-than-normal oil level, which may score or ruin mechanical parts in the compressor. Some compressors have an oil sight glass for visual inspection of the oil level in the crankcase.

If the viscosity (thickness) of the oil is too high, it can also be hard to return from an evaporator, which can cause oil logging. Usually, the heat from the defrost heaters will warm and thin the oil in the evaporator, so it can be returned to the compressor once it starts up. This will happen only if the right viscosity of oil is used. Also, if a suction line is oversized, the refrigerant velocity will be decreased, which will prevent the oil from moving through the suction line to the compressor’s crankcase.

Remember, it is the refrigerant velocity that moves the oil through the refrigeration system’s piping. Making sure it is designed and configured properly will ensure proper oil return to the compressor, reducing the possibility of restrictions.

Publication date: 10/1/2018

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