November 5, 2007
The problem we are going to deal with in this and next month’s columns is a restricted liquid line after the receiver. In this column, we will look at eight possible symptoms. In the Dec. 3 column, we will look at a couple of other issues. For both columns, we will use a service checklist that shows that the system has a TXV valve, filter drier, and sightglass. The refrigerant is R-134a.
Here are the measured values:
• Compressor discharge temperature is 125°F.
• Condenser outlet temperature is 70°.
• Evaporator outlet temperature is 30°.
• Compressor inlet temperature is 60°.
• Ambient temperature is 70°.
• Box temperature is 30°.
• Compressor voltage is 230.
• Compressor amperage is low.
• Lowside (evaporating) pressure is 1.8 pounds per square inch gauge at -10°.
• Highside (condensing) pressure is 95 psig at 85°.
Calculated values in °F are:
• Condenser split, 15°.
• Condenser subcooling, 15°.
• Evaporator superheat, 40°.
• Compressor superheat, 70°.
Causes for restricted liquid lines or restricted components in the liquid line could be:
• Restricted filter-drier from moisture, dirt, oil, or sludge.
• Restricted TXV screen or orifice.
• Kinked liquid line.
• Restricted liquid line solder joint.
• Receiver outlet valve (King Valve) partially closed off.
Many technicians believe that when any part of the system’s 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. With a starved evaporator, the compressor and condenser will also be starved. This will cause a low condensing pressure. Most of the refrigerant will now be stored in the condenser and receiver simulating a pumped-down system.
SYMPTOMSSome of the symptoms could be:
• Higher-than-normal discharge temperature.
• High superheats.
• Low evaporator pressures.
• Low condensing pressures.
• Normal to a bit high condenser subcooling.
• Low condenser splits.
• Low amp draw.
• Short cycle of the low-pressure control (LPC).
Higher than normal discharge temperature: High discharge temperatures are caused from high compressor superheats. A starved evaporator will cause the high superheats. High compression ratios from the low evaporator pressure will cause high heat of compressions, thus high discharge temperatures. This is assuming there is still some mass flow rate of refrigerant. The severity of the restriction will determine how high the discharge temperature will be. If the system becomes completely restricted, the compressor will pump down the system and stay off on the low-pressure control.
High superheats: Both evaporator and compressor superheats will be high. This is caused from 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 TXV and compressor being starved of refrigerant. The compressor is trying to draw refrigerant from the evaporator through the suction line, but the liquid line restriction is preventing 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 being starved of refrigerant, so will the condenser. Reduced refrigerant to the evaporator will cause a reduced heat load to be delivered to the condenser. The condenser in turn does not have to elevate its temperature and pressure to reject heat.
Normal to a bit high 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 sub-cool because of the low refrigerant flow caused from the restriction. The receiver will have a reduced flow in and out of it. Most of the refrigerant will be in the receiver with some in the condenser. If the receiver is in a hot ambient, sub-cooling may be lost as refrigerant sits in the receiver. This is why some commercial systems have receiver bypasses for certain situations. Receiver bypasses are nothing more than 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. This will cause low condenser splits. Remember, the split is the temperature difference between the condensing temperature and the ambient.
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 of 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.
Publication Date: 11/05/2007