The Professor: Restricted Metering Device
March 10, 2008
This column explores how a restricted metering device will affect system performance and efficiency. The system is a commercial refrigeration system employing a receiver and a thermostatic expansion valve (TXV) as the metering device. The refrigerant is R-134a.
Here are ways a metering device (TXV) can become restricted:
• Plugged inlet screen.
• Foreign material in TXV’s orifice.
• Oil logged from refrigerant flooding the compressor.
• TXV adjusted too far closed.
• Wax buildup in valve from wrong oil in system.
• Sludge from the byproducts of a compressor burnout.
• Partial TXV orifice freeze-up from excessive moisture in the system.
• Manufacturer defect in the valve.
A system with a restricted metering device has the very same symptoms as a system with a liquid line restriction that occurred after the receiver. This is because the TXV is actually part of the liquid line. A TXV being restricted will cause the evaporator, compressor, and condenser to be starved of refrigerant. This will cause low suction pressures, high superheats, low amp draws, and low head pressures.
Also, the symptoms of a restricted TXV system are very similar to a system with a refrigerant undercharge. However, the undercharged system will have low subcooling levels. Service technicians often confuse an undercharged system with a restricted metering device.
Adding refrigerant to a system with a restricted meter device will only raise the subcooling amounts in the condenser to a level where the head pressure may elevate. This is caused from a lack of internal volume of the condenser to hold the added refrigerant. Even the receiver may overfill if too much refrigerant is added.
A CHECKLISTThe following is a checklist for a system with a restricted metering device:
Compressor discharge temperature: 200°F
Condenser outlet temperature: 70°
Evaporator outlet temperature: 30°
Compressor inlet temperature: 65°
Ambient temperature: 70°
Box temperature: 30°
Compressor volts: 230
Compressor amps: Low
Low side (evaporating) pressure: 1.8 psig (-10°)
High side (condensing) pressure: 104.2 psig (85°)
Condenser split: 15°
Condenser subcooling: 15°
Evaporator superheat: 40°
Compressor superheat: 75°
SYMPTOMSSymptoms can be:
• Somewhat high discharge temperature.
• Low condensing (head) pressure.
• Low condenser split.
• Normal to a bit high condenser subcooling.
• Low evaporator (suction) pressure.
• High superheat.
• Low amp draw.
• Short cycle on low-pressure control (LPC).
Here are those symptoms in more detail.
Somewhat high discharge temperature - Somewhat high discharge temperatures are caused from the higher superheats from 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) pressure - 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.
Low condenser split - Since 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 means 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 subcool more. Note that an undercharge of refrigerant will cause low subcooling.
Low evaporator (suction) pressure - Since 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 the lower pressure.
High superheat - High superheats are caused again from the evaporator and compressor being starved of refrigerant. With the TXV restricted, the evaporator will became 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 causing the amp draw to be low.
Short cycle on low-pressure control (LPC) - The compressor may short cycle on the LPC 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. The short cycling may keep occurring until the compressor overheats. Short cycling is hard on controls, capacitors, and motor windings.
Publication date: 03/10/2008