Condensing Temperature Clues

March 30, 2004
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Condensing temperatures often give technicians valuable hints as to what the problem may be within a refrigeration system. The high side of the refrigeration system offers valuable information to the wise technician.

Most technicians would rather troubleshoot the refrigeration system's high side than the low side. This is because almost all 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 heat generated in the compression stroke, often referred to as the heat of compression, has to be rejected in the condenser.

What The Condenser Does

The three functions of the condenser are desuperheating, condensing, and subcooling the refrigerant.

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

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 or saturation temperature have been reached, these gases are referred to as 100-percent saturated vapor.

When the refrigerant has reached the 100-percent saturated vapor point in the condenser (Point 2 in Figure 1), this is the end of the desuperheating process.

Condensation: Now the vapor is ready to condense if any more heat is lost. Indeed, condensation (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 a 100-percent saturated vapor state, condensation can take place. As more heat is taken away from the 100-percent saturated vapor, it forces the vapor to a liquid state; it condenses.

The condensation process happens between Points 2 and 3 in Figure 1. When condensing, the vapor gradually changes its state 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. The heat being removed during this phase change is latent heat, not sensible heat. This change from vapor to liquid happens at one temperature; the temperature remains constant while phase changing, even though heat is being removed. (Note: An exception to this occurs in the 400 series refrigerant blend, which has a temperature change [glide] when phase changing.)

This one temperature is the saturation temperature. It corresponds to the saturation pressure in the condenser. Remember, only at saturation in a phase-changing region is there a pressure-temperature relationship and the 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 vapor pressure drops and losses are negligible.

Subcooling: The last function of the condenser is to subcool the liquid refrigerant. Subcooling can be defined as any sensible heat taken away from the 100-percent saturated liquid. Technically, subcooling is 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 its phase to saturated liquid, and 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. Its temperature will drop as it loses heat. The liquid that is cooler than the saturated liquid in the condenser is called subcooled liquid. The condenser subcooling process starts at Point 3 and continues to the end of the condenser.

Subcooling is an important process, because it starts to lower the liquid temperature closer to the evaporating temperature before the refrigerant reaches the metering device. This reduces flash loss in the evaporator, so more of the vaporization of the liquid in the evaporator can be used for cooling the product load. In other words, the net refrigeration effect is increased.

Figure 1. A diagram of a basic refrigeration system showing various functions of the condenser.


Splits, Swings, And Loads

In air-cooled condensers, the temperature difference between the ambient and the condensing temperature is referred to as the condenser split. For example, if the condensing temperature is 110 degrees F and the ambient is 80 degrees, the condenser split would be 30 degrees.

(The condensing temperature in any system is figured off of the condensing pressure using a pressure-temperature chart.)

Condenser splits can range from 15 degrees to 30 degrees, depending on whether the condenser is a standard-, mid-, or high-efficiency unit. The higher the efficiency, the more coil surface area there will be, thus the lower the condenser split will be.

In this article we will discuss a standard-efficiency condenser that normally runs a 25 degree to 30 degree split. Note that condenser splits are not affected by ambient temperature 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: the ambient swing and the evaporator heating load.

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

On the other hand, if the evaporator sees more of a heat load, more heat has to be rejected to the condenser; its condensing temperature increases. With an increased condensing temperature, the condenser split is increased because the ambient temperature remained the same.

High And Low Splits

If you find a low condenser split, say 7 degrees, you will immediately know that not much heat is being absorbed in the evaporator. The low split tells you that the refrigerator or freezer is not working very hard. This is true no matter what the ambient or condensing temperatures are.

If you measure a low condenser split, potential system problems could include:

  • Frosted evaporator coil.

  • Malfunctioning evaporator fan.

  • High superheat condition in the evaporator (from undercharge or starving metering device).

  • Inefficient compressor.

  • Plugged filter-drier.

    On the other hand, if you find a high condenser split, you will immediately know that the refrigerator or freezer is rejecting a lot of heat out of the condenser. There must be a lot of heat being absorbed in the evaporator. Causes could include:

  • A door opening.

  • Warm products in the box.

  • A recent defrost period.

    What happens in the condenser is a direct reflection of what is happening in the rest of the refrigeration system. Never ignore troubleshooting the high side of the refrigeration system.

    Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich. He can be reached by e-mail at tomczykj@tucker-usa.com.

    Publication date: 04/05/2004

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