Condensing is system dependent and usually takes place in the lower two-thirds of the condenser. Once the saturation or condensing temperature is reached in the condenser and the refrigerant gas has reached 100% saturated vapor, condensation can take place if any more heat is removed. As more heat is taken away from the 100% saturated vapor, it will force the vapor to become a liquid, or to condense.

When condensing, the vapor will gradually change phases to liquid until 100% liquid is all that remains. This phase change, or change of state, is an example of a latent heat rejection process, as the heat removed is latent heat not sensible heat. This phase change will happen at one temperature even though heat is being removed. This one temperature is the saturation temperature corresponding to the saturation pressure in the condenser. (An exception to this are ASHRAE 400-series blends of refrigerants.) This pressure can be measured anywhere on the high side of the refrigeration system, as long as line and valve pressure drops and losses are negligible.

The second function of the condenser is desuperheating. The first passes of the condenser desuperheat the discharge line gases sent from the compressor, which prepares these high-pressure superheated vapors for condensation, or phase change, from a vapor to a liquid. Remember, these superheated gases must lose all of their superheat before reaching the condensing temperature for a certain condensing pressure.

Once the initial passes of the condenser have rejected enough superheat and the condensing temperature has been reached, the gases are referred to as saturated vapors. The refrigerant is then said to have reached the 100% saturated vapor point.

The third function of the condenser is to subcool the liquid refrigerant, which means any sensible heat taken away from 100% saturated liquid. Technically, subcooling is defined as the difference between the measured liquid temperature and the liquid saturation temperature at a given pressure. Once the saturated vapor in the condenser has changed phases to saturated liquid, the 100% saturated liquid point has been reached. If any more heat is removed, the liquid will go through a sensible heat rejection process and lose temperature as it loses heat. The liquid that is cooler than the saturated liquid in the condenser is subcooled liquid.

Subcooling is an important process because it starts to lower the liquid temperature closer to the evaporator temperature. This reduction in temperature will reduce liquid flash loss in the evaporator so that more of the vaporization of the liquid in the evaporator can be used for useful cooling of the product load.

Dirty Condensers

Dirty or fouled condenser coils are one of the most frequent service problems in the commercial refrigeration and air conditioning fields today. If a condenser coil is dirty or fouled, its ability to reject heat is severely affected.

Remember, one of the main functions of the condenser is to condense the refrigerant vapor to liquid. If a condenser becomes damaged, dirty, or fouled, less heat transfer can take place from the refrigerant to the surrounding ambient. If less heat can be rejected to the surrounding air, the heat will start to accumulate in the condenser, which will make the condensing temperature rise.

Once the condensing temperature starts to rise, there will come a point where the temperature difference between the condensing temperature and the surrounding ambient (delta T, or ΔT) is great enough to reject heat from the dirty or fouled condenser. The temperature difference is the driving potential for heat transfer to take place between anything; the greater the temperature difference, the greater the heat transfer.

The dirty condenser will reject enough heat at the elevated ΔT to keep the system running; however, the system will run very inefficiently due to the higher condensing temperature and pressure, causing high compression ratios. Even the subcooled liquid temperature coming out of the condenser will be at a higher temperature when the condenser is damaged, fouled, or dirty. This means that the liquid temperature out of the condenser will be further from the evaporating temperature, which will cause more flash gas at the metering device and a lower net refrigeration effect.

The compressor’s discharge temperature will also run hotter because of the higher condensing temperature and pressure, which cause a higher compression ratio. The compressor will then have to put more energy in compressing the suction pressure vapors to the higher condensing or discharge pressure. This added energy will be reflected in higher discharge temperatures and higher amperage draws.

System Diagnosis

Shown below is a system diagnosis checklist for a system with a dirty condenser. The system is a low-temperature refrigeration application with R-134a refrigerant and a TXV with a receiver.

Symptoms

• High discharge temperatures;
• High condensing pressures;
• High condenser splits;
• High-to-normal condenser subcooling;
• Normal-to-high evaporator pressures;
• Normal superheats;
• High compression ratios; and
• High compressor amp draw

System Analysis

High discharge temperatures: With the condenser coil plugged or an inoperative condenser fan motor or damaged fan blade, the discharge temperature will be high. This is caused by the refrigeration system not being able to reject heat in the condenser. The condensing pressure will rise, and because of the pressure/temperature relationship, the condensing temperature will then rise. The high heat of compression from the high compression ratio also causes the discharge temperature to be high. This discharge temperature should never exceed 225°F, as carbonization and oil breakdown can occur if compressor discharge temperatures exceed this temperature.

High condensing pressures: Since heat from the evaporator, suction line, compressor motor, and heat of compression is rejected in 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 condenser cannot reject this heat fast enough, which will cause the condensing temperature and pressure to elevate. Once this temperature is elevated, the condenser split will become greater and heat can now be rejected at a needed rate. However, the system is operating at elevated condensing temperatures and pressures, causing unwanted inefficiencies from high compression ratios

High condenser splits: As the condensing temperature rises farther above the ambient, the temperature difference between the ambient and the condensing temperature will become greater. This is defined as a higher condenser split or ΔT. At the higher splits, heat can now be rejected because a greater temperature difference will enhance the 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.

High-to-normal condenser subcooling: High condensing pressures cause high compression ratios, which in turn, cause low volumetric efficiencies. Low volumetric efficiencies then cause low refrigerant flow rates through the system, which will not form much liquid subcooling in the high side. However, what 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, the 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, where a blocked condenser’s subcooling will not be as high.

Normal-to-high evaporator pressures: The TXV will try to maintain a constant amount of evaporator superheat as it is designed to do. Because of the low refrigerant flows caused by low volumetric efficiencies, the evaporator may not keep up with the heat load. This could cause high refrigerated space temperatures and thus, a bit higher evaporator pressures. Again, the TXV could be letting a bit too much refrigerant out during the beginning of its opening stroke from the higher head pressures causing a bit higher evaporator pressures. Otherwise, because the TXV is maintaining superheat and doing its job, there could be normal evaporator pressures. This all depends on the severity of the blocked condenser's condition.

Normal superheats: The TXV will be maintaining 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 top of the TXV or consult with the manufacturer for more precise information on TXV temperature and pressure ranges.

Over time, condenser coils can become dirty or fouled, which will lead to poor airflow. Keeping the condenser clean will help keep a system operating properly.