Condensing temperatures often give service technicians valuable hints as to what the problem may be within a refrigeration system. What happens in the condenser is a direct reflection of what is happening in the rest of the refrigeration system.
It is important to remember that almost all of the heat absorbed in the system is rejected in the condenser; in fact, all the heat absorbed in the evaporator and the suction line is rejected in the condenser. In addition, the compressor’s motor heat and the heat generated in the compression stroke, often referred to as heat of compression, are rejected in the condenser. The service technician should never ignore troubleshooting the high side of the refrigeration system, because the high side will often provide valuable information.
Every experienced service technician should know that the condenser’s three functions are to desuperheat, condense, and subcool the refrigerant.
Once the compressor’s superheated discharge gases are delivered to the top of the condenser, the first passes of the condenser start to desuperheat the discharge line gases. This prepares the high-pressure superheated vapors for condensation or phase change from vapor to liquid. This occurs by taking away the sensible (measurable) heat from the superheated gases, which drops their temperature and shrinks their volume.
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% saturated vapor, condensation can take place if any more heat is removed. As more heat is taken away from the 100% 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% 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 latent heat is being removed. 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, 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.
The last function of the condenser is to subcool the liquid refrigerant. Subcooling is defined as any sensible heat taken away from the 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.
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.
It's a Mystery
Service technicians often wonder what the proper condensing temperature (pressure) should be for a certain cooling application. One sure fact is 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 here is where the mystery lies: 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 seasonal energy efficiency ratio (SEER) of the system.
Over the years, manufacturers have made condensers and condensing units more and more efficient. The sizes of the condensers have increased, and the materials they are made of have been tweaked. Advanced technology in extended surfaces, which include fins and fin-like extrusions, have also increased the heat transfer abilities of condensers. In addition, different routings of the refrigerant passes through the condenser coil have increased efficiencies in condensers, while compressor technology has advanced in leaps and bounds.
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. The condensing temperature over the ambient (CTOA) tells us how much hotter the condensing temperature is over the air temperature passing through the condenser. In the past, when talking about air-cooled condensers, the temperature difference between the ambient air and the condensing temperature was often referred to as the condenser split or temperature difference. In today’s world of varying system efficiencies and ages of equipment, it is more proper to use CTOA when expressing 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°F, the CTOA would be 20°F (100°F – 80°F). The condensing temperature in any system is derived from the condensing pressure using a pressure/temperature chart. CTOAs can range from 12°F to 30°F depending on the condensing unit’s SEER rating or whether the condensing unit is a standard, mid, or high efficiency condensing unit. The higher the SEER, the lower the CTOA because of the condenser’s ability to reject heat more efficiently. Refer to the chart below for estimated SEER to CTOA relationships.
|6 to 9 SEER||30°F|
|10 to 12 SEER||25°F|
|13 to 15 SEER||20°F|
More About CTOAs
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 ambient swings and evaporator heat loading.
If 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 is now operating at an elevated condensing temperature for the elevated ambient, but the difference between the CTOA has remained the same.
If the evaporator sees more heat load, more heat has to be rejected to the condenser. As more heat is rejected to the condenser, the condensing temperature increases. With an increased condensing temperature, the CTOA is now increased because ambient temperature remained the same.
If a service technician experiences a low CTOA of, say 7°F, the technician immediately knows that not much heat is being absorbed in the evaporator. The low CTOA 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 the like. The low CTOAs are a reflection of these problems.
On the other hand, if the service technician experiences high CTOAs of, say, 40°F, 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, which could be from a door opening, warm product in the box, just after a defrost period, a very inefficient system, etc.
A dirty condenser impeding heat transfer from the condenser could also be the culprit of a high CTOA. Dirty or fouled condenser coils are one of the most frequent service problems in the commercial refrigeration and summer air conditioning fields today. If a condenser coil is dirty or fouled, its ability to reject heat is severely affected.
If a condenser becomes damaged, dirty, or fouled, less heat transfer can take place from the refrigerant to the surrounding ambient. If less heat is rejected to the surrounding air, the heat will start to accumulate in the condenser, which will make the condensing temperature rise. Once the condensing temperature starts rising, there will come a point where the temperature difference between the condensing temperature and the surrounding ambient (CTOA) is great enough to reject heat from the dirty or fouled 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 CTOA” 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.
Even the subcooled liquid temperature coming out of the condenser will be at a higher temperature when the condenser is damaged, fouled, or dirty. This means that the liquid temperature out of the condenser will be further from the evaporating temperature, which will cause more flash gas at the metering device and a lower net refrigeration effect.
The compressor’s discharge temperature will also run hotter because of the higher condensing temperature and pressure, which will cause 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.
When the condensing temperature is high because of a fouled condenser, the compressor must compress the refrigerant from the low-side (evaporating) pressure to an elevated high-side (condensing) pressure. This will make the heat of compression be higher, thus the compressor’s discharge temperature will be higher. However, there are many causes for high condensing temperatures that also cause high discharge temperatures. Some of the causes for high condensing pressures (temperatures) include:
- Recirculated air over the condenser;
- Wrong refrigerant;
- Damaged condenser fan blade;
- Wrong condenser fan blade;
- Burned out condenser fan motor;
- Restricted air flow over condenser;
- Refrigerant overcharge;
- High heat load on evaporator;
- Dirty condenser;
- High ambient temperature;
- Non-condensable (air) in the system;
- Broken fan belts; and
- Undersized condenser coils.
The bottom line is that service technicians should always thoroughly troubleshoot the high side of the refrigeration system, because condensing temperatures offer helpful information in determining what the problem may be within a refrigeration system.