The Professor: Air Conditioning System Troubleshooting
August 4, 2008
This is the final article in a series covering inefficient compressors, noncondensibles in the system, low and high condenser entering air temperatures, and different metering devices as they apply to troubleshooting air conditioning systems.
Inefficient compressors certainly decrease the heat transfer ability of the air conditioning system since they are responsible for circulating the refrigerant through the system while it absorbs heat and then rejects it. In reciprocating compressors, leaky valves or worn piston rings are two of the major problems that lead to inefficiencies associated with compressors.
Symptoms for an inefficient compressor are high suction pressures and low discharge (head) pressures. If the compressor is inefficient, the evaporator cannot handle the high heat load due to a decreased refrigerant flow rate and the conditioned space temperature will start to rise. This rise in return air temperature will overload the evaporator with heat causing high suction pressures and higher-than-normal superheats.
Piston ring blow-by and reed valve leakage can also cause high suction pressures from re-circulation of refrigerant. This is also a cause of low refrigerant flow rate. The condenser will also see a reduced heat load to reject from the decreased mass flow rate of refrigerant being circulated through it. The reduced condenser load will cause a low condensing temperature and pressure.
The compressor amp draw will be lowered from less work having to be expended with the low mass flow rate of refrigerant from re-circulated refrigerant. Subcooling in the condenser should be a bit low from the reduced heat load on the condenser.
Symptoms for an inefficient reciprocating compressor with bad valves and leaky rings could include:
• High suction pressures;
• Low head pressures;
• Low compressor amp draw;
• High superheat (capillary tube and orifice);
• High return air temperature;
• Condenser subcooling.
NONCONDENSIBLESAir and water vapor are probably the best known noncondensible in a refrigeration or air conditioning system. Noncondensibles usually enter a system through poor service practices and/or leaks.
A technician forgetting to purge hoses can let air and water vapors into a system. The air and water vapor will pass through the evaporator and compressor because the compressor is a vapor pump. Once the air gets to the condenser, it will remain at its top and not condense. The subcooled liquid seal at the condenser’s bottom will prevent the air from passing out of the condenser. This air and water vapor will take up valuable condenser surface area and cause high-head pressures. Subcooling will be high because of the high-head pressures causing a greater temperature difference between the liquid temperature in the condenser and the ambient.
Noncondensibles in a system and an overcharge of refrigerant have very similar symptoms when a TXV metering device is used.
Symptoms of noncondensibles in a system could be:
• High head (condensing) pressures;
• High subcooling;
• High compression ratios;
• High discharge temperatures.
LOW CONDENSER ENTERING AIR TEMPERATURELow condenser entering air temperature will cause a low-head pressure from the excessive heat transfer between this cool ambient and the condenser coil. Low-head pressure may reduce flow through metering devices that have capacity ratings dependant on the pressure differences across them. This reduced refrigerant flow causes a starved evaporator that will cause low suction pressures and high superheats. However this could be offset by increased subcooling at these lower ambients.
This entire drop in capacity may decrease the air conditioner’s heat removal abilities if it is not designed for it. If not designed properly, liquid will start to back up in the condenser. But, because of a low heat transfer rate caused from the lower condenser temperature, the liquid temperature in the bottom of the condenser will be low, causing liquid subcooling in the condenser to be increased.
Also, less refrigerant circulated means less work for the entire system to perform, so the ampere draw of the compressor will be lowered.
However, if the system is set up for this reduced condenser air entering temperature, the head pressure can be designed to “float” or change with the changing ambient temperature. This will give lower head pressures and increased efficiencies. A properly matched TXV to handle reduced pressure drops across its orifice may have to be incorporated into the design.
Symptoms of low condenser entering temperature could be:
• Low entering air temperature;
• Low suction pressure if not designed for low-head pressure at the TXV;
• Low discharge (condensing) pressure;
• High superheat if not designed for low-head pressure at TXV;
• Low amp draw;
• High subcooling.
HIGH CONDENSER ENTERING AIR TEMPERATUREHigh ambients will have much different effects on an air conditioning system. The higher outdoor ambients will cause head pressure to elevate in order to complete the heat rejection task. The temperature difference (TD) between the condensing temperature and the ambient will go down and the refrigerant gas will not condense until the head pressure rises. The condenser cannot reject as much heat at this lower TD, and thus will accumulate the heat.
The accumulated heat forces the condensing temperature to elevate to a TD where the heat can be rejected. Remember, the temperature difference is the driving potential for heat transfer. However, this heat rejection happens at a higher condensing temperature forcing the system to have higher compression ratios and lower efficiencies.
High-head pressures cause the compression ratio to increase causing low volumetric efficiencies from higher-pressure vapors re-expanding in the clearance volume of the piston cylinder on each down stroke.
As volumetric efficiencies decrease, mass flow rates decrease and the compressor is less efficient. High-head pressures also elevate liquid temperatures entering the metering device, which will increase evaporator flash gas and thus decrease the net refrigeration effect. Because of these inefficiencies, the suction pressure may be a bit higher. The system will have a hard time maintaining temperature and humidity of the conditioned space.
METERING DEVICESEvaporator superheats will vary depending on the type of metering device.
Flow rates through a capillary tube-metering device or any fixed orifice-metering device depend on the pressure difference across the metering device. Higher head pressures will increase the flow rate through this metering device pushing the subcooled liquid at the condenser’s bottom through the metering device at a faster rate. Because of this, condensing subcooling will decrease. Evaporator superheat will also decrease because of a flooded evaporator coil with a lot of flash gas at its entrance.
TXV systems will try to maintain evaporator superheat even though the pressure drop across the valve may be out of its control range at the highest-ambient temperatures. Here, the condenser subcooling may be normal.
FINAL THOUGHTAs one can see, air conditioning system diagnosis isn’t easy. A service technician must be a trained professional to diagnose a system efficiently and correctly. No longer can we rely on rules of thumb for coil temperatures or pressures.
Publication date: 08/04/2008