ACHRNEWS

The Professor: Condensing Temperatures and Condensing Splits

July 2, 2012
Condensing temperatures often give service technicians valuable hints as to what the problem may be within a refrigeration system. In fact, most service technicians would rather troubleshoot the refrigeration system’s high side than the system’s low side. This is because almost all of the heat absorbed in the system is rejected in the condenser. All the heat absorbed in the evaporator and the suction line is rejected in the condenser. Also, the compressor’s motor heat and the heat generated in the compression stroke, often referred to as heat of compression, also has to be rejected in the condenser. The three functions of the condenser are desuperheating, condensation, and subcooling.

Desuperheating

The compressor delivers high-pressure, superheated vapor to the condenser through the discharge line. In a standard condenser, the first passes desuperheat the discharge line gases. This prepares the high-pressure superheated vapors for condensation or phase change from vapor to liquid because it takes the sensible (measurable) heat away from them and shrinks their volume.

Remember that 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 or saturation temperature has been reached, these gases are referred to as 100 percent saturated vapor. The refrigerant is said to have reached the 100 percent saturated vapor point in the condenser. This point is the end of the desuperheating process.

Condensation

Now the vapor is ready to condense if any more heat is lost. In fact, condensation, or changing vapor to liquid, is the main function of the condenser. 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 percent saturated vapor, condensation can take place if more heat is removed. As more heat is taken away from the 100 percent saturated vapor, it will force the vapor to become a liquid (condense). When condensing, the vapor will gradually phase change to liquid until all that remains is 100 percent liquid. This phase change, or change of state, is an example of a latent heat rejection process. This is because the heat that is being removed while phase changing is latent heat, not sensible heat. This phase change from vapor to liquid will happen at one temperature. In other words, the temperature will remain constant while phase changing even though heat is being removed.

Note: An exception to this is the 400 series refrigerant blends where there is a temperature change (temperature glide) when phase changing. This one temperature is the saturation temperature corresponding to the saturation pressure in the condenser. Remember, it is only at saturation in a phase changing region that there is a pressure/temperature relationship and the service technician can use a pressure/temperature chart. 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.

Subcooling

The last function of the condenser is to subcool the liquid refrigerant. Subcooling is defined as any sensible heat taken away from the 100 percent 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 percent 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 evaporating temperature before it reaches the metering device. 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. In other words, the net refrigeration effect is increased.

Condenser Splits

When talking about air-cooled condensers, the temperature difference between the ambient and the condensing temperature is often referred to as the condenser split. For example, if the condensing temperature is 110˚F and the ambient is 80˚, the condenser split would be 30˚ (110–80). The condensing temperature in any system is obtained from the condensing pressure using a pressure/temperature chart. Condenser splits can range from 10˚ to 30˚ depending on whether the condensing unit is a standard-, mid-, or high-efficiency condensing unit. The higher the efficiency, the more coil surface area there will be, thus the lower the condenser split will be. For the balance of this article, I will deal with a standard efficiency condenser that normally runs a 20-30˚ split.

Condenser splits are not affected by ambient changes. If there is an increase in the ambient temperature, there will also be an increase in the condensing temperature, but the condenser split (difference between the two temperatures) will remain the same.

On the other hand, condensing temperatures for a single condenser can vary depending on two factors: ambient swings and evaporator heat loading.

Ambient Swings

As the ambient temperature increases, less heat can be rejected from the air-cooled condenser to the hotter ambient. This means that more of the heat absorbed by the evaporator and suction line, and the heat of compression generated by the compressor, will remain in the condenser. This will increase the condenser’s internal temperature and pressure. The condenser is now operating at an elevated condensing temperature for the elevated ambient, but the difference between the condensing temperature and the ambient (condenser split) has remained the same.

Evaporator Heat Loading

As the evaporator sees more heat load, more heat has to be rejected to the condenser. As more heat is rejected to the condenser, its condensing temperature increases. With an increased condensing temperature, the condenser split is now increased because ambient temperature remained the same.

Low Condenser Splits

If a service technician experiences a low condenser split, say 7˚F, the technician immediately knows that not much heat is being absorbed in the evaporator. The low split tells the technician that the refrigerator or freezer is not working very hard. This holds true no matter what the ambient or condensing temperature is. It could mean a frosted evaporator coil, evaporator fan out, high superheat condition in the evaporator from undercharge or starving metering device, inefficient compressor, a plugged filter drier, or similar. The low condenser split is a reflection of these problems.

High Condenser Splits

On the other hand, if the service technician experiences a high condenser split, the technician immediately knows that the refrigerator or freezer is rejecting a lot of heat out of the condenser. This means that there must be a lot of heat being absorbed in the evaporator. It could be from a door opening, warm product in the box, just after a defrost period, or similar.

Because what happens in the condenser is a direct reflection on what is happening in the rest of the refrigeration system, the service technician should never ignore troubleshooting the high side of the refrigeration system. The high side of the refrigeration system will often give valuable information to the wise service technician.

Publication date: 7/2/2012