Also rejected to the condenser is the compressor’s motor heat, as well as the heat generated in the compression stroke, often referred to as heat of compression.

By looking at condensing temperature over ambient (CTOA) and the SEER of a cooling unit, it is possible to determine what the proper condensing temperature (pressure) should be.

GETTING BACK TO THE BASICS

Every experienced service technician knows that the condenser’s three functions are to desuperheat, condense, and subcool the refrigerant.

The compressor delivers high-pressure, superheated vapor to the condenser through the discharge line, and in a standard condenser, the first passes desuperheat this gas. 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. Then, the vapor is ready to condense if any more heat is lost.

Condensation, or changing vapor to liquid, is the main function of the condenser.

Condensing of refrigerant 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. That is because the heat 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 latent heat is being removed. (Note: An exception to this is the 400-series refrigerant blends, where there is a temperature glide when phase-changing.) This one temperature is the saturation temperature corresponding to the saturation pressure in the condenser. Remember, only at saturation in a phase-changing region is there 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.

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 (net refrigeration effect) of the product load. In other words, the net refrigeration effect is increased.

CTOA AND SEER

Service technicians often wonder what the proper condensing temperature (pressure) should be for a certain cooling application. Keep in mind that the condensing temperature must be warmer than the air passing through it in order for it to be able to reject heat to this air. In other words, there must be a temperature difference for heat transfer to take place between the refrigerant and the air. But just how much warmer must the condensing temperature be relative to the air passing through it for the system to be functional and efficient? The answer lies in the efficiency — or SEER — of the system.

A SEER rating is the total cooling output of the system divided by the actual power input. So, the higher the SEER, the higher the Btu output for the same energy input. CTOA is how much hotter the condensing temperature is over the air temperature passing through the condenser.

With air-cooled condensers, it used to be that the temperature difference between the ambient air and the condensing temperature was referred to as the condenser split or temperature difference.

In today’s world of varying system efficiencies and equipment age, it is more proper to use CTOA to express how much hotter the condensing temperature is than the air passing through it. For example, if the condensing temperature is 100°F and the ambient is 80°, the CTOA would be 20° (100°– 80°).

The condensing temperature in any system is derived from the condensing pressure using a pressure/temperature chart. CTOAs can range from 12° to 30°, depending on the condensing unit’s SEER rating or whether it is a standard-, mid-, or high-efficiency unit. As shown in Figure 1, the higher the SEER, the lower the CTOA, due to the condenser’s ability to reject heat more efficiently.

CTOAs 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 CTOA (difference between the two temperatures) will remain the same. On the other hand, condensing temperatures for a single condenser can vary depending on ambient swings and evaporator heat loading.

As the ambient temperature increases, less heat can be rejected from the air-cooled condenser to the hotter ambient air passing through it. 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 then operates at an elevated condensing temperature for the elevated ambient, but the difference between the condensing temperature and the ambient air (CTOA) remains the same.

As the evaporator sees more heat load, more heat has to be rejected to the condenser, which causes the condensing temperature to increase. With an increased condensing temperature, the CTOA is now increased because ambient temperature remained the same.

Consider a scenario in which the system has a low CTOA of 7°, which immediately tells the technician that not much heat is being absorbed in the evaporator. The low split signals the technician that the refrigerator or freezer is not working very hard, and this holds true regardless of the ambient or condensing temperature.

The problem could be a frosted evaporator coil, an evaporator fan is out, there is a high superheat condition in the evaporator due to undercharge or starving metering device, the compressor is inefficient, there is a plugged filter-drier, etc.

On the other hand, if a system has a high CTOA of 40°, the technician immediately knows that the refrigerator or freezer is trying to reject a lot of heat out of the condenser.

This could mean that there is a lot of heat being absorbed in the evaporator, or it could be due to a door opening, a warm product in the box, a defrost period just occurred, or that it is simply a very inefficient system.

A dirty condenser impeding heat transfer from the condenser could also be the culprit behind a high CTOA.

As you can see, measuring the CTOA is important, as it can help technicians pinpoint problems in a refrigeration system.

Publication date: 4/1/2019

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