The condenser is the most underrated component in an air conditioning or refrigeration system. Besides being a simple heat exchanger, the condenser has to desuperheat, condense, and subcool refrigerant from the system’s compressor. In fact, all the heat absorbed in the evaporator and the suction line is rejected in the condenser.
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. That is why service technicians should never ignore troubleshooting this component in the high side of the refrigeration system. The high side of the refrigeration system will often give valuable information to even the most experienced service technician. Because of this, the condenser must be serviced, cleaned, and maintained to provide good HVACR system health.
As noted above, the three main functions of the condenser are condensing, desuperheating, and subcooling.
Condensing is system dependent and usually takes place in the lower two-thirds of the condenser. One of the main functions of the condenser is to condense the refrigerant vapor to liquid. 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. 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. Except for 400-series refrigerant blends, 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 pressure losses and valve pressure drops are negligible.
The second function of the condenser is desuperheating. The first passes of the condenser desuperheat the discharge line gases, which prepares the high-pressure superheated vapors coming from the 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.
The last function of the condenser is subcooling 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. 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.
Dirty or Undersized
If a condenser becomes dirty or fouled, or if it is undersized, less heat transfer can take place from the refrigerant to the surrounding ambient. Dirty 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, which will make the condensing temperature rise. Once the condensing temperature starts to rise, there will come a point where the temperature difference between the condensing temperature and the surrounding ambient is great enough to reject heat from the condenser. This is called condensing temperature over ambient (CTOA).
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 will be rejecting enough heat at the elevated CTOA to keep the system running with a dirty condenser; however, the system will run very inefficiently because of the higher condensing temperature and pressure causing high compression ratios.
For example: if an R-134a air-cooled condenser is running at a condensing pressure of 147 psig (110°F) at an ambient of 90°F, this is a CTOA of 20 degrees. If this condenser becomes dirty, the condensing pressure might rise to 215 psig (135°F) at the same 90°F ambient. The CTOA or temperature difference is now 45°F. The condenser can reject heat at this Delta T, but it makes the entire system very inefficient. In fact, if a high-pressure control is not protecting the system, a compressor burnout can occur with time.
With an elevated condensing temperature and larger CTOA, 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 (NRE), which means lower system capacity and efficiency.
The compressor’s discharge temperature will also run hotter because of the higher condensing temperature and pressure. Higher condensing (discharge) pressures cause higher compression ratio. The compressor will have to put more energy in compressing the suction pressure vapors to the higher condensing or discharge pressure. This added energy will be reflected in higher discharge temperatures and higher amperage draws.
The compressor’s superheated discharge temperature is often an overlooked temperature when troubleshooting a refrigeration or air conditioning system. However, the discharge temperature is very important because it is an indication of the amount of heat absorbed in the evaporator and suction line, and any heat of compression generated by the compression process. Since the compressor’s discharge temperature is superheated, a pressure/temperature relationship does not exist and it must be read directly on the discharge line by some sort of temperature measuring device.
The compressor’s discharge temperature should be taken about 1 to 2 inches away from the compressor on the discharge line. This discharge temperature should never exceed 225° F, as carbonization and oil breakdown can occur if compressor discharge temperatures exceed 225°F.
Temperatures and Pressures
The main cause for high compression ratios and high compressor discharge temperatures in today’s service fields are high condensing pressures caused from dirty condensers. The three causes for high discharge temperatures are:
- High condensing temperature;
- Low evaporator temperatures; and
- High compression ratios.
As mentioned earlier, when the condensing temperature and pressure are high, the compressor must compress the refrigerant from the low side (evaporating) pressure to an elevated high side (condensing) pressure. This added work of compression done by the compressor will make the heat of compression be higher. Thus, the compressor’s superheated discharge temperature will be higher.
Remember, the condensing temperature is the temperature the refrigerant is changing phase from a vapor to a liquid in the condenser. There is a pressure/temperature relationship with the condensing temperature because of the phase change. A gauge reading on the high side of the system is all that is needed to find the condensing temperature. Convert this pressure to a temperature using a pressure/temperature chart, and this will be the condensing temperature.
There are many causes for high condensing pressures and temperatures, which also cause high discharge temperatures. Listed below are some causes:
- Dirty condenser;
- High ambient temperature;
- Misplaced condensing unit;
- Recirculated condenser air;
- Non-condensable (air) in the system;
- Condenser fan out;
- Restricted airflow over condenser;
- Refrigerant overcharge;
- Wrong refrigerant; and
- High heat load on evaporator.
Higher condensing pressures cause high compression ratios or pressure ratios. Pressure ratio or compression ratio is defined as the absolute discharge pressure divided by the absolute suction pressure. A gauge is calibrated to read zero at atmosphere pressure, but in reality, there is 14.7 psi (standard atmospheric pressure) of pressure being applied to it. This is called a gauge pressure and it is not a true pressure. To attain the true (absolute) pressure, one must add 14.7 to the gauge reading. A true or “absolute” pressure must be used when using any equation like the one below.
Compression ratio = (Absolute discharge pressure) / (Absolute suction pressure)
For example, determine the compression ratio for a system with the following pressures:
Discharge pressure = 145 psig
Suction pressure = 5 psig
Compression Ratio = (145 psig + 14.7 psi = 159.7 psia) / (5 psig + 14.7 psi = 19.7 psia) = 8.1:1
A compression ratio of 8 to 1 is expressed as 8:1 and simply means that the discharge or condensing pressure is 8 times the magnitude of the suction pressure. Again, a compression ratio of 12.3:1 simply indicates to the technician that the "absolute" or true discharge pressure is 12.3 times as great as the absolute suction pressure.
High compression ratios are a result of high condensing pressures or low evaporator pressures or both. Any time there are high condensing pressures or low evaporator pressures, or both, there will be high compression ratios, thus more work will be added to the compression stroke of the compressor. More work on the compression stroke will cause the heat of compression to increase, causing the compressor to have a higher discharge temperature. The main cause for high compression ratios in today’s service fields are high condensing pressures caused from dirty condensers.
For piston-type compressors, volumetric efficiency is defined as the ratio of the actual volume of the refrigerant gas pumped by the compressor to the volume displaced by the compressor pistons. The volumetric efficiency is expressed as a percentage from 0% to 100%. A high volumetric efficiency means that more of the piston's cylinder volume is being filled with new refrigerant from the suction line and not re-expanded clearance volume gases.
The higher the volumetric efficiency, the greater the amount of new refrigerant that will be introduced into the cylinder each down-stroke of the piston; thus, more refrigerant will be circulated each revolution of the crankshaft. The system will now have better capacity and a higher efficiency. So, the lower the discharge pressure, the less re-expansion of discharge gases to suction pressure. Also, the higher the suction pressure, the less re-expansion of discharge gases. This happens because the discharge gases will experience less re-expansion to the higher suction pressure, causing the suction valve to open sooner.
The compressor’s volumetric efficiency depends mainly on system pressures. In fact, the farther apart the discharge pressure's magnitude is from the suction pressure's magnitude, the lower the volumetric efficiency will be. As stated earlier, compression ratio is the ratio that measures how many times greater the discharge pressure is than the suction pressure, in other words, their relative magnitudes. A clean coil will keep condensing pressures lower, which will in turn, cause lower compression ratios. Lower compression ratios will cause higher volumetric efficiencies and lower discharge temperatures.
So, keep those compression ratios as low as possible by keeping condensing pressures low and evaporator pressures high, or both.