The Professor: Systematic Troubleshooting
July 7, 2008
[Editor’s note: This is the third in a series of columns on systematic air conditioning troubleshooting and diagnostics that began in the May 5 issue of The NEWS and continued in the June 2 issue. This column deals with liquid subcooling, liquid and suction line restrictions, overcharges, and undercharges.]
Subcooling is defined as the difference between the measured liquid temperature and the saturation temperature at a given pressure. Condenser subcooling can be measured at the condenser outlet with a thermometer or thermocouple and a pressure gauge. Simply subtract the condenser outlet temperature from the saturation temperature at the condenser outlet to get the amount of liquid subcooling in the condenser.
The saturation pressure has to be measured at the condenser outlet and converted to a temperature. Always take the pressure at the same point the temperature is taken. This will alleviate any pressure drop error through the condenser.
A forced air condenser should have from 6° to 10°F of liquid subcooling if charged properly. However, the amount of condenser subcooling depends on the static and friction line pressure losses in the liquid line, and will vary from system to system. The 6° to 10° of liquid subcooling is assuming no liquid amplification pump is pressurizing the liquid out of the condenser. Condenser subcooling can be an indicator of the refrigerant charge in the system.
For receiverless systems, the less the refrigerant charge, the less the subcooling. The rated EER will have little or no effect on condenser subcooling.
Another factor that will affect condenser subcooling is the air entering the condenser. As the condenser air entering temperature (CAET) increases, the liquid subcooling will decrease. This is because higher condensing (head) pressures will force more of the subcooled liquid through the metering device to the evaporator. This will also affect evaporator superheat. The evaporator superheat will be less from the increased flow rate through its coil, assuming the system has a capillary tube or restrictive orifice as a metering device. TXV metering devices should hold a constant superheat under all conditions as long as the valve’s rated conditions are not exceeded.
LIQUID LINE RESTRICTIONLiquid line restrictions can be caused by a multitude of things, such as a restricted filter drier; a restricted TXV screen; a kinked liquid line; a kinked or bent U-bend on lower condenser coil; a restricting solder joint in the liquid line; or an oil-logged capillary tube.
A restricted liquid line will starve the evaporator of refrigerant, thus causing low pressures in the evaporator. If the evaporator is starved of refrigerant, the compressor and condenser will also be starved. The evaporator will not be absorbing very much heat for the condenser to reject.
However, most of the refrigerant will be in the condenser and not necessarily causing high head pressures because of the reduced heat load on the evaporator. Because most of the refrigerant charge is in the condenser, liquid subcooling in the condenser will increase. This is a big difference from an undercharge of refrigerant. An undercharge of refrigerant will have low condenser subcooling. If the system has a receiver, most of the refrigerant will be in the receiver causing lower than normal head.
Other symptoms of a liquid line restriction are:
• Local cool spot after a severe restriction from expansion of the refrigerant at the local pressure drop.
• Low ampere draw at the compressor from the reduced refrigerant flow through it.
• Bubbles in the sight glass if the restriction is before the sight glass. (Sight glasses are optional on some systems).
• High superheats from a starved evaporator.
• Low to normal head pressures from a low mass flow rate of refrigerant.
RESTRICTED SUCTION LINEThe suction line is a much more sensitive refrigerant line than the liquid line. This is because much less dense refrigerant - vapor instead of liquid - flows through it than the liquid line. A restricted suction line will cause low suction pressures along with a starved compressor and condenser. A starved compressor will lead to low compressor amp draw because of its lightened load.
The condensing pressure will also be low from the condenser’s light load. Since suction line restrictions starve compressors of refrigerant, the entire mass flow rate of refrigerant will decrease through the system causing high superheats from inactive evaporators. Restricted and/or dirty suction filters are the major cause of suction line restrictions.
Liquid subcooling in the condenser will be normal to a bit high since a lot of refrigerant will be in the condenser coil but not be circulated very fast. The condenser subcooling may be normal to a bit high if there is a receiver in the system. This subcooling in the condenser tells us that there is refrigerant in the system and that an undercharge of refrigerant can be ruled out.
UNDERCHARGEUndercharged systems mean less mass flow rate throughout the entire system. Low suction and discharge pressures with high superheat in the evaporator are all indications of an undercharge. Severely undercharged systems will run very low condenser subcooling because of no refrigerant to subcool.
If the subcooling drops to zero, the hot gas in the condenser will start to leave the condenser with some liquid, thus bubbles will form in the sight glass if the system is fortunate enough to have one. Compressor amp draw will be low because of the decreased refrigerant flow.
Service technicians will usually confuse themselves over an undercharge of refrigerant and a liquid line restriction.
Remember, a liquid line restriction will give the system a lot of subcooling in the condenser, where an undercharge will not. Otherwise, symptoms are very similar.
OVERCHARGEA system with an overcharge of refrigerant will have higher-than-normal condensing temperatures because of liquid backing up in the condenser and robbing the condenser of useful condensing area. In reciprocating compressors, the elevated head pressure causes the volumetric efficiency of the compressor to decrease because of higher pressures of the re-expanding clearance volume vapors in the clearance pocket of the compressor. The amp draw of the compressor would increase from the higher head pressure, creating higher compression ratios. The entire system would have reduced capacities.
If the system has a TXV metering device, the TXV will still try to maintain its superheat and the evaporator pressure will be normal to slightly high, depending on the amount of overcharge. The higher evaporator pressure would be caused from the decreased mass flow rate from the higher compression ratio, and the evaporator would have a hard time keeping up with the higher heat load of the warmer entering air temperature. The TXV will have a tendency to overfeed on its opening strokes due to the high head pressures.
If we are dealing with a capillary tube-metering device, the same symptoms occur with exceptions to evaporator superheat. Remember, one reason a capillary tube system is critically charged is to prevent flooding of the compressor on low evaporator loads. The higher head pressures of an overcharged capillary tube system will have a tendency to overfeed the evaporator, thus decreasing the superheat. If the system is more than 10 percent overcharged, liquid can enter the suction line and get to the suction valves or crankcase. This will result in compressor damage and soon failure.
Publication date: 07/07/2008