Pressures, States, And Conditions
Figure 1 illustrates the basic refrigeration system. The circled numbers in the diagram refer to 10 locations within a system that are the focus of this article. An explanation of each of those point's pressure, condition, and state should clarify any system weaknesses. Here are some assumptions: The refrigerant is R-134a; the discharge (condensing) pressure is 124 psig (100 degrees F); the suction (evaporating) pressure is 6 psig (0 degrees); the discharge temperature is 180 degrees; the condenser outlet temperature is 90 degrees; the thermostatic expansion valve (TXV) inlet temperature is 80 degrees; the evaporator outlet temperature is 10 degrees; and the compressor inlet temperature is 40 degrees.
Point 1Compressor Discharge
The refrigeration compressor is a vapor pump and not a liquid pump. The vapors leaving the compressor will be high-pressure, superheated vapor. Since the compressor is one of the two components in the system that separate the high-pressure side from the low-pressure side of the system, the compressor's discharge will be high pressure. This leaves the compressor's suction as low pressure. The compressor's discharge vapor receives its superheat from sensible heat coming from the evaporator, suction line, motor windings, and friction, as well as internal heat of compression from the compression stroke. Since the vapor is superheated, no pressure-temperature relationship exists. Its temperature is well above the saturation temperature of 100 degrees for the saturation (condensing) pressure of 124 psig.
Point 2Condenser Inlet
As high-pressure and superheated refrigerant leaves the compressor, it instantly begins to lose superheat and cool in temperature. Its heat is usually given up to the surroundings. This process is called desuperheating. Even though the refrigerant vapor is going through a desuperheating process, it is still superheated vapor.
Since pressure acts equally in all directions, the vapor will also be high pressure, or the same pressure as the compressor's discharge. This is assuming that any line and valve pressure drops are ignored. Remember, the refrigerant is superheated and not saturated, so there is no pressure-temperature relationship to work with. This high-pressure, superheated vapor is also above its saturation temperature of 100 degrees for the given discharge of condensing pressure of 124 psig. This point can be referred to as high-pressure, superheated vapor. This process of desuperheating will continue until the 100 percent saturated vapor point in the condenser is reached.
Point 3100% Saturated Vapor Point
Once all of the superheat is rejected from the refrigerant gas, the saturation temperature of 100 degrees is finally reached for the condensing pressure of 124 psig. (See Table 1.) The vapor has now reached the 100 percent saturated vapor point. The refrigerant vapor has reached the lowest temperature it can have and still remain as a vapor. This temperature is referred to as the saturation temperature, and a pressure-temperature relationship exists. This temperature is also the condensing temperature. Any heat lost past the 100 percent saturated vapor point will gradually phase change the vapor to liquid or condense it.
The heat removed from the vapor turning to liquid is referred to as latent heat, and happens at a constant temperature of 100 degrees. As the vapor condenses to liquid, refrigerant molecules are actually becoming denser and getting closer together. This molecular joining is what gives up most of the latent heat energy. This point is on the high side of the refrigeration system and is referred to as high-pressure, saturated vapor.
Point 4100% Saturated Liquid Point
Soon all of the vapor will give up its latent heat and turn to saturated liquid at a constant condensing temperature of 100 degrees. The point is referred to as the 100 percent saturated liquid point. Any more heat given up by the refrigerant after this point will be sensible heat since the phase change from vapor to liquid is complete. This point is still on the high side of the refrigeration system and can be referred to as high pressure saturated liquid. The entire condensing process takes place between the 100 percent saturated vapor point and the 100 percent saturated liquid point. Any heat lost past the 100 percent saturated liquid point is referred to as subcooling.
Point 5Condenser Outlet
Once the 100 percent saturated liquid point is reached in the condenser, subcooling of the liquid occurs. Remember, any heat lost in the liquid past its 100 percent saturated liquid point is subcooling. Liquid subcooling can continue all the way to the metering device's entrance if conditions are right. Since this point is on the high side of the system and is all subcooled liquid, it will be referred to as high-pressure, subcooled liquid. There is no pressure-temperature relationship at the subcooled condition, only at saturation.
The temperature of the 100 percent saturated liquid point in the condenser corresponding to its pressure of 124 psig was 100 degrees. The difference between 100 degrees and the condenser outlet temperature of 90 degrees figures out to be 10 degrees of condenser subcooling (100 degrees minus 90 degrees equals 10 degrees of condenser subcooling).
Point 6TXV Inlet
The inlet to the TXV is on the high side of the system and consists of subcooled liquid. Hopefully, subcooling will continue from the 100 percent saturated liquid point in the condenser. The tubing from the condenser outlet to the TXV inlet is often referred to as the liquid line. The liquid line may be exposed to very high or low roof temperatures, depending on the seasons of the year. This will seriously affect whether or not subcooling line takes place and to what magnitude. If the liquid line is exposed to high temperatures, liquid line flashing may occur. Since this point is on the high side of the system and is subcooled liquid, it will be referred to as high-pressure, subcooled liquid.
Point 7Middle Of Evaporator
Once the subcooled liquid enters the TXV, flashing of the liquid will occur. Once in the evaporator, the liquid refrigerant will experience a severe drop in pressure to the new saturation (evaporator) pressure of 6 psig. This pressure decrease will cause some of the liquid to flash to vapor in order to reach the new saturation temperature in the evaporator of 0 degrees. Once this new evaporator temperature is reached, the liquid/vapor mixture will start absorbing that from the product load and continue to change from liquid to vapor. This process happens at the new saturation (evaporator) pressure of 6 psig.
This is a classic example of heat absorbed by the refrigerant without increasing in temperature. It is a latent heat process of vaporization. The heat energy absorbed in the refrigerant went into breaking up the liquid molecules to vapor molecules instead of increasing its temperature. Since the refrigerant is both saturated liquid and vapor, and is on the low side of the refrigeration system, it will be referred to as a low-pressure, saturated liquid and vapor.
Point 8100% Saturated Vapor Point
Once all of the liquid has changed to vapor in the evaporator, the 100 percent saturated vapor point has been reached. This point is still at the evaporator's saturation temperature of 0 degrees. Any more heat absorbed by the refrigerant vapor will now result in temperature rises of the refrigerant. This heat energy now goes into increasing the velocity of the vapor's molecules and expanding them. This is because there isn't any more liquid to be vaporized. This increase in molecular velocity can be measured in degrees. Any heat added past this 100 percent saturated vapor point is superheat. Since this 100 percent saturated vapor point is in the low side of the system and is saturated vapor, we will refer to it as low-pressure, saturated vapor.
Point 9Evaporator Outlet
The evaporator outlet temperature is used for evaporator superheat calculations. This point is located at the evaporator outlet next to the TXV remote bulb. Because it is located downstream of the 100 percent saturated vapor point, it will be superheated. This point is in the low side of the refrigeration system and will be referred to as low-pressure, superheated vapor. The difference between the 100 percent saturated vapor temperature of 0 degrees and the evaporator outlet temperature of 10 degrees is called evaporator superheat. In this example, there is 10 degrees of evaporator superheat (10 degrees minus 0 degrees equals 10 degrees of evaporator superheat).
Point 10Compressor Inlet
The compressor inlet consists of low-pressure, superheated vapor. These vapors feed the compressor. As the refrigerant travels from the evaporator outlet down the suction line to the compressor, more superheat is gained. As mentioned before, superheat ensures that no liquid refrigerant will enter the compressor at low evaporator loadings when TXV valves are known to lose control of superheat settings. Because this point is superheated, no refrigerant pressure-temperature relationship exists.
John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich., and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached by e-mail at email@example.com.
Publication date: 08/01/2005