Let's take a look at a system checklist for a low-temperature refrigeration system using R-134a. (See Table 1.) The system has a dirty condenser or restricted airflow over the condenser. The metering device is a thermostatic expansion valve (TXV), and there is a receiver.
Symptoms in this system include high discharge temperatures, high condensing pressures, high condenser splits, normal-to-high condenser subcooling, normal-to-high evaporator pressures, normal superheats, high compression ratios, and high amp draw.
The condensing pressure rises. Because of the refrigerant's pressure-temperature relationship, the condensing temperature will also rise. The high heat of compression from the high compression ratio also causes the discharge temperature to be high.
High condensing pressures: Because heat from the evaporator, suction line, compressor motor, and heat of compression is rejected into the condenser, the condenser coil must be kept clean with the proper amount of airflow through it. A dirty condenser or restricted airflow across the coil cannot reject this heat fast enough. The condensing temperature and pressure will elevate.
Once the temperature is elevated, the condenser split will become greater and heat can be rejected at the required rate. (The condenser split is the difference between the condensing temperature and the ambient or surrounding temperature.) However, the system is operating at elevated condensing temperatures and pressures, which results in high compression ratios and operating inefficiencies.
High condenser splits: As we already mentioned, as the condensing temperature rises farther above the ambient, the temperature difference between the ambient and condensing temperatures becomes greater. It's a higher condenser split. Heat can be rejected at higher splits because a greater temperature difference will enhance heat transfer.
However, the system will suffer at these higher condensing pressures and temperatures because of the unwanted low volumetric inefficiencies from the higher compression ratios.
Normal-to-moderately high condenser subcooling: High condensing pressures cause high compression ratios, which in turn cause low volumetric efficiencies. Low volumetric efficiencies cause low refrigerant flow rates. Low flow rates will not create much subcooling. However, whatever subcooling is formed in the condenser will be at an elevated temperature and will reject heat to the ambient faster because of the higher condenser split.
Because of this faster heat rejection, liquid in the condenser will cool faster and have a greater temperature difference when compared to the condensing temperature. This is one of the big differences between an overcharge of refrigerant and a blocked condenser: An overcharge of refrigerant can cause very high condenser subcooling, while a blocked condenser will not.
Normal-to-high evaporator pressure: Again, the TXV will try to maintain a constant amount of evaporator superheat. Because of the low refrigerant flows caused by low volumetric efficiencies, the evaporator may not be able to keep up with the heat load. This could cause high box temperatures and thus higher evaporator pressures.
The TXV could be letting out a bit too much refrigerant during the beginning of its opening stroke because the higher head pressures are causing higher evaporator pressures. Otherwise, because the TXV is maintaining superheat and doing its job, there could be normal evaporator pressures. This depends on the severity of the condenser's condition.
Normal superheat: The TXV will maintain the set evaporator superheat unless the condensing pressure exceeds the range of the valve. Each TXV has a pressure range that it can operate within. Read the nomenclature on the top of the TXV or consult with the manufacturer for more precise information on TXV temperature and pressure ranges.
High compression ratios: The higher condensing pressures cause higher compression ratios, which lead to lower volumetric efficiencies.
High amp draws: Higher compression ratios result in a greater pressure range in which suction vapors are compressed to the condensing pressure. This requires more work from the compressor, which in turn increases amp draw.
Clean condensers and proper airflow over the condenser coil will result in a system with lower condensing pressures, lower discharge temperatures, lower compression ratios, lower amp draw, higher volumetric efficiencies, and high refrigerant mass flow rates.
John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich., and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached by e-mail at firstname.lastname@example.org.
Publication date: 03/14/2005