PRESSURE AND TEMPERATURE RELATIONSHIP
Upon entering the evaporator, the liquid refrigerant's temperature is between pressure from the compressor causes the temperature of the refrigerant to rise. Traditional PT charts list the saturated refrigerant pressure, in psig, with a or the pressure-temperature relationship for saturated liquid and saturated vapor. ponent refrigerant will either evapo- use of pressure-temperature (PT) charts and liquid. Subcooled liquid. Superheat vapor. Glide. Boiling. Boiling point.
If we also measure 16 psig 1.
How to read a pressure temperature chart for super-heat and sub-cooling calculations
This is the temperature that we would be able to measure if we placed a thermocouple in the refrigerant at the point where it is changing from a vapor to a liquid. At this point, there is no difference between the measured temperature and the saturation temperature. Therefore, the refrigerant is saturated, or in other words, at the boiling point.
In our example we also measure psig When a system employs the use of a liquid receiver, there can be no subcooling at the surface of the liquid in the receiver. The reason is that when liquid refrigerant and vapor exist together, they must obey the P-T relationship or the refrigerant must be saturated.
In our example the measured pressure in the receiver is psig Once a solid column of liquid is formed, subcooling of the refrigerant can take place by lowering its temperature with the use of liquidsuction heat exchangers, subcoolers, or from lower ambient temperatures surrounding the line.
Subcooling is a lowering of a temperature below the saturation point or boiling point. Of course, it is important to maintain some liquid subcooling in the liquid line to prevent flash gas from forming in the liquid line and entering the thermostatic expansion valve. Liquid and vapor are present together when the measured temperature corresponds to the P-T relationship.Basic Principles of pressure and temperature
The amount of superheat is indicated by the difference. Subcooled liquid is present when the measured temperature is below the saturation temperature corresponding to the P-T relationship. The amount of subcooling is represented by the difference. Practical limitation to gauge locations In our illustration we have located gauges at points in the system where it is not always feasible to do so on an actual installation. Because of this, we must oftentimes make deductions and assumptions when dealing with an actual system.
As an example, we would normally assume that the psig That is, we assume that there is no pressure loss of any consequence between the compressor discharge and the condenser. If an undersized discharge line or other restrictions are suspected, we cannot make this assumption and other pressure taps may be necessary to locate the troublesome area. It is also common practice to assume that the pressure measured at the suction service valve of the compressor is the same pressure that exists at the outlet of the evaporator at the expansion valve bulb location.
This is particularly true on closecoupled systems where it has been determined that the suction line is of the proper size. By making this assumption, we can determine the expansion valve superheat without installing an additional pressure tap at the bulb location.
However, to eliminate any doubt as to the amount of suction line pressure drop and to be absolutely precise in measuring superheat, a gauge must be installed in the suction line at the bulb location. Care must be taken to make a reasonable allowance for pressure drops within the system.
Excessive pressure drops can be detected by applying the principles of the P-T relationship. That would mean that there is a pressure drop of 29 psi 2. While this would be considered excessive on a single-circuit evaporator, it should be remembered that on multicircuit evaporators there will be a pressure drop through the refrigerant distributor assembly. A pressure drop through the distributor assembly on Ra may be in the vicinity of 25 psi 1.
Checking on noncondensables The proper use of the P-T relationship can be helpful in discovering the presence of air or other noncondensable gases in the system. These undesirable gases will accumulate in the condenser and add their pressure to that produced by the refrigerant, resulting in a higher total pressure. Traditional PT charts list the saturated refrigerant pressure, in psig, with a column for temperature down the left side. Single-component refrigerants and azeotropes boil and condense at one temperature for a given pressure.
Therefore, only one column is needed to show the pressure-temperature relationship for any phase-change process in a system see Figure 1.
The properties of the new zeotropic blends are somewhat different than the traditional refrigerants.
Zeotropic blends shift in composition during the boiling or condensing process see Figure 2. As the blend changes phase, more of one component will transfer to the other phase faster than the rest. This property is called fractionation.
The changing composition of the liquid causes the boiling point temperature to shift as well.
Using P-T Analysis as a Service Tool | Refrigeration - Parker Sporlan
The overall shift of temperature from one side of the heat exchanger to the other is called the temperature glide. Zeotropic blends cannot be defined by a single pressure-temperature relationship. The temperature glide will cause different values for temperature at a given pressure, depending on how much refrigerant is liquid and how much is vapor.
The most important values for checking superheat and subcool are the end points of the glide or the pressure-temperature relationship for saturated liquid and saturated vapor. The saturated liquid condition is often referred to as the bubble point. Imagine a pot of liquid sitting on a stove; as it begins to boil it forms bubbles in the liquid.
The saturated vapor condition is referred to as the dew point. Imagine a room full of vapor and dew drops forming on the furniture.