A restricted metering device — thermostatic expansion valve or TXV — can cause a number of symptoms in a system. Here are the ways a TXV can become restricted:
- Wax buildup in the valve because the wrong oil was used in the system;
- Sludge from the byproducts of a compressor burnout;
- Partial TXV orifice freeze-up from excessive moisture in the system;
- Foreign material in the orifice;
- Oil-logged TXV from refrigerant flooding the compressor;
- Too much oil in the system;
- TXV is adjusted too far closed;
- Manufacturer’s defect in the valve; or
- Plugged inlet screen on TXV (see Figure No. 1).
A system with a restricted metering device will display the same symptoms as a system with a liquid-line restriction. This is because the TXV is actually part of the liquid line. 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. Other components may include solenoid valves, hand valves, and the like.
The filter drier is a very likely component to become restricted from moisture and/or debris accumulation. It will give the same symptoms as a restricted TXV since it, too, is in the liquid line. However, if the filter drier is restricted enough, sometimes it will feel cool or cold to a technician’s touch. This phenomenon happens from some of the liquid refrigerant experiencing a slight pressure drop and expanding 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 happens. This is why it’s of the utmost importance to have the liquid sight glass downstream — not upstream — of the filter drier. Figure No. 2 shows a cut-away of a liquid-line filter drier.
These liquid-line restrictions all cause the evaporator, compressor, and condenser to be starved of refrigerant. This will cause the following symptoms:
- Low evaporator (suction) pressure;
- High evaporator and compressor superheats;
- Low compressor amp draw;
- Short-cycling on the low-pressure control;
- Somewhat high discharge temperature;
- Low condensing (head) pressure;
- Low condenser split; and
- Normal to a bit high condenser subcooling.
THE REASONS FOR THE SYMPTOMS
Let’s look at why a restricted metering device will cause the symptoms that it does:
Low Evaporator Pressures — Because the evaporator is starved of refrigerant, the compressor will be starving also and will pull itself into a low-pressure situation. It is the amount and rate of refrigerant vaporizing in the evaporator that keeps the pressure up. A small amount of refrigerant vaporizing will cause a lower pressure.
High Superheats — High superheats also are caused by the evaporator and compressor being starved of refrigerant. With the TXV restricted, the evaporator will become inactive and run high superheat. This will cause the compressor superheat to be high. The 100 percent saturated vapor point in the evaporator will climb up the evaporator coil causing high superheats.
Low Amp Draw — High compressor superheats and low suction pressures will cause low-density vapors to enter the compressor. Also, the compressor will be partly starved from the TXV being restricted. These factors will put a very light load on the compressor and cause the amp draw to be low.
Short Cycling on the Low-pressure Control — The compressor may short cycle on the low-pressure control, depending on how severe the restriction in the TXV is. The low suction pressures may cycle the compressor off prematurely. After a short period of time, the evaporator pressure will slowly rise from the small amounts of refrigerant in it and the heat load on it. This will cycle the compressor back on. This short cycling may keep occurring until the compressor overheats. Short cycling is hard on controls, capacitors, and motor windings.
High Discharge Temperature — Somewhat high discharge temperatures are caused by the higher superheats, which are caused by the evaporator being starved of refrigerant. The compressor is now seeing a lot of sensible heat coming from the evaporator and suction line, along with its heat of compression and motor heat. The compressor will probably overheat from the lack of refrigerant cooling if it is a refrigerant-cooled compressor.
Low Condensing (Head) Pressures — Since the evaporator and compressor are being starved of refrigerant, so will the condenser because of these components being in series with one another. There will be little heat to eject to the ambient surrounding the condenser. This allows the condenser to operate at a lower temperature and pressure.
Lower Condenser Splits — Because the condenser is being starved of refrigerant, it can operate at a lower temperature and pressure. This is because it does not need a large temperature difference between the ambient and the condensing temperature to reject the small amount of heat it is getting from the evaporator, suction line, and compressor. This temperature difference is referred to as the condenser split. If there were large amounts of heat to reject in the condenser, the condenser would accumulate heat until the condenser split was high enough to reject this large amount of heat. High heat loads on the condenser mean large condenser splits. Low heat loads on the condenser mean low condenser splits.
Normal to a Bit High Condenser Subcooling — Most of the refrigerant will be in the receiver, with some in the condenser. The condenser subcooling will be normal to a bit high because of this. The refrigerant flow rate will be low through the system from the restriction. This will cause what refrigerant that is in the condenser to remain there longer and gain more subcooling. Note that an undercharge of refrigerant will cause low subcooling.
IT’S NOT A REFRIGERANT UNDERCHARGE
The symptoms of a system with a restricted thermostatic expansion valve (TXV) are very similar to those of a system with a refrigerant undercharge. However, the undercharged system will have low condenser subcooling levels. Service technicians often confuse an undercharged system with a restricted metering device. Adding refrigerant to a system with a restricted metering device will only raise the condenser subcooling amounts to a level where the head pressure may elevate. This is caused from a lack of internal volume in the condenser to hold the added refrigerant. Even the receiver may overfill if too much refrigerant is added. If the system is then pumped down for any reason, dangerous head pressures will result.
Publication date: 1/9/2017