Desuperheating;

Condensing; and

Subcooling.

Desuperheating

The first passes of the condenser desuperheat the discharge line gases. This prepares the high-pressure superheated vapors coming from the compressor’s discharge line for condensation, or the phase change from vapor to 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, these gases are referred to as saturated vapor. The refrigerant is then said to have reached the 100% saturated vapor point. (See Figure 1.)

Condensation

As mentioned earlier, one of the main functions of the condenser is to condense the refrigerant vapor to liquid. 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 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 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.)

Subcooling

The last function of the condenser is to subcool the liquid refrigerant. Subcooling is defined as 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 phase changed 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. (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.

A Dirty or Blocked Condenser

If a condenser becomes dirty or fouled, less heat transfer can take place from the refrigerant to the surrounding ambient. Dirty or blocked condensers are one of the most frequent service problems in commercial refrigeration and summer air conditioning fields today. If less heat can be rejected to the surrounding air with an air-cooled condenser, the heat will start to accumulate in the condenser. This accumulation of heat in the condenser will make the condensing temperature rise. Now that the condensing temperature is rising, there will come a point where the temperature difference between the condensing temperature and the surrounding ambient (Delta T) is great enough to reject heat from the condenser.

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.

Clearance Space

Piston-type compressors all have some clearance space between the valve plate and the piston’s face to avoid a collision of the two. Modern compressor technology has lessened this clearance space, but some always exists. At the piston’s top dead center position, discharge gas gets trapped in this clearance space. When the piston starts its downstroke, this trapped discharge gas must re-expand to the suction line pressure before the suction valve will open. If a condenser is running with a high head pressure, a higher-pressure gas will be trapped in the clearance space. This requires more re-expansion of discharge gas that must take place before its pressure reaches the suction line pressure, which allows for the suction valve to open. The bottom line is a wasted portion of the suction stroke to re-expansion of discharge gas. This causes low volumetric efficiencies. The system will then run longer, be less efficient, and draw higher amps.

Higher Subcooled Liquid Temperatures

With a dirty or blocked condenser, even the subcooled liquid temperature coming out of the condenser will be at a higher temperature. This means that the liquid temperature out of the condenser will be further from the evaporating temperature. This will cause more flash gas at the metering device and a lower net refrigeration effect in the evaporator.

Higher Discharge Temperatures

The compressor’s discharge temperature will also run hotter because of the higher condensing temperature and pressure causing a higher compression ratio. The compressor will now have to put more energy in compressing the suction pressure vapors to the higher condensing or discharge pressure. This added energy is reflected in higher discharge temperatures and higher amperage draws. The compressor discharge temperature should never exceed 225? about 6 in. from the compressor. Discharge hotter than 225? will cause compressor failures in the near future.

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 tomczykj@tucker-usa.com.

Publication date: 09/03/2001