Troubleshooting an air conditioning system often concerns refrigerant, airflow, and mechanical problems, either individually or in combination. The next series of articles will deal with many of these problems, but this one will cover air temperatures entering the condenser, inefficient compressors, and noncondensables in the refrigerant system.
AIR TEMPERATURE AND CONDENSERS
Low entering air temperature in the condenser will cause a low head pressure from the excessive heat transfer between this cool ambient air and the refrigerant in the condenser coil. Low head pressures can reduce refrigerant mass flow rates through metering devices, which have capacity ratings dependent on the pressure differences across them. The lower this pressure difference is, the less flow through the metering device. This reduced refrigerant flow rate can cause a starved evaporator, which will, in turn, cause low suction pressures and high superheats. However, these system inefficiencies may be offset by increased subcooling at these lower ambient air temperatures entering the condenser coil.
This entire drop in system 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, causing liquid subcooling in the condenser to be increased. Also, less refrigerant circulated means less work for the compressor, so the ampere draw of the compressor will be lowered. 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 with increased efficiencies.
A properly matched thermostatic expansion valve (TXV) to handle these reduced pressure drops across its orifice may have to be incorporated into the design to keep refrigerant flow rates acceptable. A TXV with a balanced-port design is often used in these scenarios.
The symptoms of low condenser entering air temperature include:
- Low suction pressure (if not designed for low head pressure at TXV);
- Low head (condensing) pressure;
- High superheat (if not designed for low head pressure at TXV);
- Low amp draw; and
- Higher condenser subcooling.
High entering air temperature in the condenser will have much different effects on an air conditioning system. Higher outdoor ambient air will cause head pressures to elevate in order to complete the heat rejection task. The temperature difference (TD) between the condensing temperature and the ambient air 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 will therefore accumulate the heat. The accumulated heat forces the condensing temperature to elevate to a TD where the heat can be rejected at the proper rate. Remember, the temperature difference is the driving potential for heat transfer. However, at higher entering air temperatures, 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, resulting in low volumetric efficiencies. 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 (NRE). Because of these inefficiencies, the suction pressure may be a bit higher, and the system will have a hard time maintaining designed temperature and humidity of the conditioned space. Evaporator superheats will vary depending on the type of metering device.
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; however, 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, as will evaporator superheat, because the flooded evaporator coil will have a lot of flash gas at its entrance.
Because they are responsible for circulating refrigerant through the system, inefficient compressors can decrease the heat transfer ability of an air conditioning system. In reciprocating compressors, leaky valves or worn piston rings are two of the major problems that can lead to inefficiencies.
One of the symptoms of an inefficient compressor is high suction pressures along with 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 recirculation of refrigerant. This is also a cause of low refrigerant flow rate. The condenser will see a reduced heat load to reject from the decreased mass flow rate of refrigerant being circulated through it, which will cause a low condensing temperature and pressure. The compressor ampere draw will be lowered from less work having to be expended with the low mass flow rate of refrigerant from recirculated refrigerant. Subcooling in the condenser should also be a bit low from the reduced heat load on the condenser.
Symptoms for an inefficient reciprocating compressor with bad valves or leaky rings include:
- High suction pressures;
- Low head pressures;
- Low compressor amp draw;
- High return air temperature
- High superheat (capillary tube and orifice); and
- Condenser subcooling (low to normal).
Air and water vapor are probably the best known noncondensables in air conditioning systems. Noncondensables usually enter a system through leaks and/or poor service practices; for example, a technician may forget to purge the hoses, which lets air and water vapor into a system.
Air and water vapor will pass through the evaporator and compressor, because the compressor is a vapor pump; however, 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 surrounding ambient air.
Symptoms of noncondensables in a system include:
- High head (condensing) pressures;
- High compression ratios;
- High subcooling; and
- High discharge temperatures.
Notice that noncondensables in a system and an overcharge of refrigerant have very similar symptoms when a TXV metering device is used. If noncondensables are suspected in the system, recover the refrigerant, evacuate to the appropriate vacuum levels (500 microns), and recharge with refrigerant.
However, if charging with the same refrigerant just recovered, let the recovered cylinder stabilize its pressure to the surrounding ambient temperature. Once stabilized, slowly recover vapor from the top of the recovery cylinder using the recovery device. This will rid the cylinder of any noncondensables that have accumulated on top of the liquid in the recovery cylinder. It is then possible to charge the system with the recovered refrigerant, which is now free of noncondensables. The spare cylinder with the noncondensables can then be sent to a reclaimer.
Publication date: 7/1/2019