How a Dirty or Blocked Condenser Effects System Efficiency

As its name states, one of the main functions of the condenser is to condense the refrigerant sent to it from the compressor. However, the condenser also has other functions. Desuperheating and subcooling are other important functions of the condenser.

In summary, the three main functions of the condenser are:

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 phase change to liquid until 100% liquid is all that remains. (See Figure 1.)

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. 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. Table 1 is a pressure/temperature chart of HFC-134a.

(Note: Exceptions to this are near-azeotropic blends [ASHRAE 400 series blends] of refrigerants. With these blends, there can be a noticeable temperature glide or range of temperatures when the blend is phase changing.)

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. (See Figure 1.)

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

Remember, a 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 condenser is now rejecting enough heat at the elevated Delta T to keep the system running with a dirty condenser. However, the system is now running very inefficiently because of the higher condensing temperature and pressure causing high compression ratios.

Sometimes you come across an air conditioning condensing unit that has started to become overrun by ferns. You may also see clothes drier lint stuck to the coil. The placement of the clothes drier vent can cause any small or large pieces of lint escaping the vent to get sucked into the condenser coil. A combination of fern leaves and lint can partially block the condenser’s airflow, which causes high condenser pressures and inefficiencies.

For example: Let’s say an R-134a air-cooled condenser is running at a condensing pressure of 147 psig (110?F) at an ambient of 90?. (See Table 1.) This is a Delta T of 20?. If this condenser becomes dirty or is blocked, the condensing pressure might rise to 215 psig (135?) at the same 90? ambient. The Delta T or temperature difference between the condensing temperature and the ambient is now 45?. The condenser can reject heat at this Delta T, but it makes the entire system very inefficient. Often, if a high-pressure control is not protecting the system, a compressor burnout can occur with time.

A condensing unit may recycle some hot discharge air because of the house’s overhang. The hot air discharged out the top of the condenser may hit the overhang and be recycled back into the side of the condenser. Try to position these types of condensing units away from protruding overhangs if possible.

Condensing units located on the east side of a building will usually experience shade during the hottest times of the day. This helps keep condensing pressures down. Also, on condensing units that discharge air from their sides, never face a condensing unit’s fan directly into the prevailing wind direction. This will hinder the airflow out of the condensing unit on a windy day. It also may rotate the fan on the off-cycle and cause fan motor starting problems.

Tomczyk is a professor of hvacr at Ferris State University, Big Rapids, MI, and author of the book Troubleshooting and Servicing Modern Air Conditioning and Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696.He can be reached by e-mail at [email protected].

Publication date: 09/03/2001