EDITOR’S NOTE: This is part two of a two-part article on air conditioning system troubleshooting and commissioning. Part one, which ran on Page 25 of the March 7 issue of The NEWS, covered air- and refrigerant-flow problems, cooling coil size, condensers, subcooling, and low-condenser-entering air temperatures. This month’s article will cover liquid and suction line restrictions, undercharge and overcharge, inefficient compressors, and non-condensables in the system.
Diagnosing an air conditioning system isn’t easy. A service technician must be a trained professional to diagnose a system efficiently and correctly — no longer can a tech rely on rules of thumb for coil temperatures or pressures. So, if a tech encounters balky systems as he or she performs spring a/c system tuneups, here are some troubleshooting tips to help the professional diagnose and repair systems so customers remain cool all summer.
LIQUID LINE RESTRICTIONS
Liquid line restrictions can be caused by a multitude of things, including:
• A restricted filter drier;
• A restricted thermostatic expansion valve (TXV) screen;
• A kinked liquid line;
• A kinked or bent U-bend on the 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, 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 present as low condenser subcooling. If the system has a receiver, most of the refrigerant will be in the receiver, which will cause decreased head pressures.
Other symptoms of a liquid line restriction include:
• A local “cool” spot just after a severe restriction. This is caused by expansion of the refrigerant from the restriction’s local pressure drop;
• Low ampere draw at the compressor from the reduced refrigerant flow through it;
• A hot compressor, especially if it is refrigerant-cooled;
• 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; and
• Low to normal head pressures from liquid backed up in the condenser if the system has a receiver.
RESTRICTED SUCTION LINE
The 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 and also may starve the compressor and condenser. A starved compressor will lead to low compressor amp draws because of its lightened load. The condensing pressure will also be low. Because suction line restrictions starve compressors of refrigerant, the entire mass flow rate of the 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. If there is a suction line restriction, liquid subcooling in the condenser will be normal to a bit high since a lot of refrigerant will be in the condenser coil and it won’t be circulated through the system very quickly. The condenser subcooling may be normal to a bit high if there is a receiver in the system. This subcooling in the condenser lets a technician know there is refrigerant in the system and that an undercharge of refrigerant can be ruled out.
Undercharged systems cause less mass flow rate of refrigerant throughout the entire cooling system. Low suction and discharge pressures with high superheat in the evaporator are all indications of an undercharge. Low suction pressures can cause evaporator temperatures to be below the freezing point. This phenomenon will cause evaporator coils to ice up (see Figure 1 below). The system now experiences even lower temperatures and pressures on its low side, which may cause more unwanted inefficiencies.
Severely undercharged systems will run very low condenser subcooling because there is no refrigerant to subcool. If the subcooling drops to zero, the hot superheated gas entering the top of the condenser will start to exit the condenser and enter the liquid line. This will cause bubbles to 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 rate. It can often be difficult to differentiate an undercharge of refrigerant from 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, the symptoms are very similar.
A system with an overcharge of refrigerant will have higher than normal condensing temperatures because the liquid is backing up in the condenser and robbing the condenser of useful condensing area. 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 will increase from the higher head pressure. Higher head pressures also create higher compression ratios, and the entire system will 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 will be caused by the decreased mass flow rate from the higher compression ratio, and the evaporator will have a hard time keeping up with the higher heat load of the warm entering-air temperature. Some TXVs will have a tendency to overfeed on their opening strokes because of high head pressures.
If we are dealing with a capillary-tube metering device, the same symptoms occur — with the exception being the evaporator superheat, which won’t be held constant. 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, ultimately, failure.
Inefficient compressors certainly decrease the heat transfer ability of the air conditioning system since they are responsible for circulating the refrigerant through the system.
Leaky valves or worn piston rings are two of the major problems that lead to inefficiencies associated with compressors. A telling sign of an inefficient compressor is high suction pressures combined with low discharge pressures. Again, the evaporator cannot handle the load because of a decreased refrigerant flow, 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. The condenser also will see a reduced load from the decreased mass flow rate of refrigerant being circulated through it. The reduced condenser load will cause a low condensing pressure. The compressor amp draw will be lowered from less work having to be expended with the low mass flow rate of refrigerant. Subcooling in the condenser could be a bit low from the reduced heat rejection load on the condenser.
Symptoms of compressor inefficiencies include:
• High suction pressures;
• Low head pressures;
• Low compressor amp draw;
• High superheat;
• High return air temperature; and
• Condenser subcooling (low to normal).
NON-CONDENSABLES IN THE SYSTEM
Air and water vapor are probably the best-known non-condensables in a refrigeration or air conditioning system. Non-condensables usually enter a system through poor service practices and/or leaks. A technician forgetting to purge his or her hoses can let air and water vapor 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 high head pressures, which causes a greater temperature difference between the liquid temperature in the condenser and the ambient. Non-condensables in a system and an overcharge of refrigerant have very similar symptoms when a TXV metering device is used.
Symptoms of non-condensables in a system are:
• High head (condensing) pressures;
• High subcooling;
• High compression ratios; and
• High discharge temperatures.
HIGH-CONDENSER-ENTERING AIR TEMPERATURE
High ambient temperatures will have much different effects on an air conditioning system. Higher outdoor ambient temperatures will cause head pressures to elevate in order to complete the heat-rejection task. The temperature difference (TD) between condensing temperature and the hotter ambient will decrease. The refrigerant gas will not condense until the condensing pressure and condensing temperature rise. 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, which forces the system to have higher head pressures, higher compression ratios, and lower efficiencies.
High head pressures cause the compression ratio to increase, which causes 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 the 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 the temperature and humidity of the conditioned space. Evaporator superheats will vary depending on the type of metering device.
ORIFICES AND CAPILLARY TUBES
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, condenser 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 higher ambient temperatures. Here, the condenser subcooling may be normal.
As we said at the start, air conditioning system diagnosis is difficult and requires a knowledgeable, trained professional. It is also a uniquely rewarding experience to correctly identify and repair a perplexing system problem.
Publication date: 4/11/2016