The thermal state of a refrigerant can be illustrated by its thermodynamic properties table (see the attached table for the saturation thermal properties of commonly used refrigerants) or by a pressure-enthalpy diagram. The pressure-enthalpy diagram (lgP-h diagram) is a thermal diagram with the logarithmic value of absolute pressure lgP as the ordinate and the enthalpy value as the abscissa. The purpose of using the logarithmic value lgP (instead of P) as the ordinate is to reduce the size of the graph and improve the accuracy of the low-pressure region, but it is still sufficient to read the value of P directly from the graph when it is used.
1. Structure of pressure-enthalpy diagram (lgP—h diagram).
There are two coarse curves in the pressure-enthalpy diagram, one on the left is the saturated liquid line (dryness χ=0) and the other on the right is the dry saturated steam line (dryness χ=1), and the two lines intersect at one point K, and the graph is divided into three regions. where K is called the critical point, the left side of the saturated liquid line is the supercooled liquid area, the right side of the dry saturated steam line is the superheated steam area, and the wet steam area is between the two lines.
Pressure-enthalpy diagram
There are six isostate parameter lines in the pressure-enthalpy diagram, as shown in the figure above:
(1) Isobar P: horizontal thin straight line.
(2) Enthalpy line H: vertical and thin straight line.
(3) Isotherm T: dotted and dashed line, which is a vertical line in the supercooled liquid area, a horizontal line in the wet steam area, and a curve slightly bent to the lower right in the superheated steam area.
(4) Isentropic line S: is a solid line that curves slightly upwards from left to right.
(5) Equal specific volume line υ: In the wet steam area and the superheated steam area, it is a dashed line that bends slightly upward from left to right, but it is flatter than the isentropic line, and there is no equal specific volume line in the liquid area, because the liquid volume under different pressures does not change much.
(6) Isodryness line χ: It only exists in the wet steam area and the superheated steam area, and the direction is basically the same as that of the saturated liquid line or the dry saturated steam line.
Each point on the pressure-enthalpy diagram represents a certain state of the refrigerant, and among the six state parameters of temperature, pressure, specific volume, enthalpy, entropy, and dryness, as long as you know any two of the independent state parameters, you can determine the state point in the diagram, so as to find out several other state parameters.
In refrigeration engineering, the middle part of the high-pressure area and the wet steam area is rarely used, so these two parts are often deleted and not drawn in some pressure enthalpy diagrams. Different refrigerants have different shapes of the pressure-enthalpy diagram (lgP—h diagram).
In engineering calculations, the saturated thermal properties of the refrigerant can be checked according to the need, and according to one state parameter, other state parameters of the saturated liquid or dry saturated steam of the refrigerant can be checked.
Applications of pressure-enthalpy diagram (lgP—h diagram):
The pressure-enthalpy diagram (lgP—h diagram) is an important tool for the analysis and calculation of the refrigeration cycle, and the working parameters of the cycle must be determined before the thermodynamic analysis and calculation of the refrigeration cycle, so that the pressure-enthalpy diagram can be used to determine the parameter values of the relevant state points of the cycle, as shown in the figure below.
Point 1: The state in which the refrigerant vapor enters the compressor. If the cooling loss of the pipeline is not considered, the suction temperature t1 of the compressor is the temperature t0 when the refrigerant leaves the evaporator, that is, t1=t0, and in the ideal case, the refrigerant vapor entering the compressor is saturated. If the evaporation temperature t0 is known, the refrigerant evaporation pressure P0 can be known, so that point 1 can be obtained from the intersection of the isobaric line and the dry saturated steam line of P0=C.
Point 2: The state of the compressor for the refrigerant is also the state of the condenser. Process L-2 is the process of adiabatic compression of the refrigerant in the compressor. The entropy does not change in the adiabatic process, i.e., S1=S2, and the process proceeds along the isomoisture line of point 1, and the intersection point of its isobar with Pk=C is point 2.
Point 5: It is the state in which the refrigerant condenses into a saturated liquid in the condenser. It can be obtained by intersecting the isobaric line of Pk=C with the saturated liquid line.
Point 3: It is the state of the refrigerant liquid after supercooling. Because the refrigerant liquid in the process of supercooling is equal to the condensing pressure Pk, and its temperature is lower than the condensing temperature, the intersection point of the isobar of Pk=C and the isotherm of tg=C is point 3.
Point 4: The state of the throttle valve (expansion valve) for the refrigerant is also the initial state of the evaporator. Because the enthalpy value before and after throttling remains unchanged, and the pressure decreases to the evaporation pressure P0, and the temperature is the evaporation temperature t0, the point 4 is obtained by the intersection of the perpendicular line (i.e., the isenthalpy line) from point 3 and the isotherm of t0=C.
4-1: It is the heat absorption process of refrigerant vaporization in the evaporator. In this way, according to the state points obtained on the figure, the thermal parameter values of each state point can be found.
Example 2-1 What is the state of ammonia with an absolute pressure of 2 bar and a specific volume of 0.7m3/kg?
Solution: The state sought is the intersection point A of the horizontal line of P=2 bar and the equispecific line of υ=0.7m3/kg on the 1gP-h graph (see figure above). Because point A is in the superheated zone, the state of ammonia is superheated steam, the temperature at this state point is 20 °C, and the enthalpy value is about 1470 kJ/kg.
Example 2-2 What is the state of Freon-22 with an absolute pressure of 10 bar and a temperature of 20°C?
Solution: The desired state can be represented by the intersection point B of the isobar at 10 bar and the isotherm at 20 °C (see the left side of the figure below). Because point B is in the supercooled region, the state of Freon-22 is a supercooled liquid with an enthalpy of 224.08 kJ/kg. Example 2-3 Freon-22 What is the temperature of the final state of compression when the vapor sucked in by the compressor is -5°C dry saturated steam, if it is insulated and compressed to a PK of 12 bar?
Solution: The compressor suction state can be determined by the intersection point C of the -5°C isotherm and the dry saturated steam line (see figure below). The entropy of point C S=1.76 kJ/kg· K, because it is an adiabatic compression process, the entropy value of the compression process does not change. Therefore, the compression end point D is the isobar with pressure PK=12 bar and S=1.76kJ/kg· The intersection of the isentropic lines of K. The temperature Td=47°C found from the figure is the final state temperature of the compression.
To sum up, the pressure-enthalpy diagram can not only easily determine the state parameters of the refrigerant, but also represent the changes in the parameters and energy changes in the refrigeration cycle and process, and it can use the length of the line segment to represent the amount of energy. Since the endothermic and exothermic processes of the refrigerant in the evaporator and condenser are carried out at constant pressure, and the change of heat in the process of constant pressure and the work consumed by the compressor in the process of adiabatic compression can be calculated by the enthalpy difference, and the enthalpy value of the refrigerant before and after the throttle valve remains unchanged, it is most convenient to use the 1gP-h diagram to analyze the refrigeration cycle and perform thermal calculation.