The pressure-enthalpy diagram and the basic process

1. The pressure-enthalpy diagram (log ph diagram)

The thermodynamic properties of a refrigerant are often represented in a pressure-enthalpy diagram. Here, the logarithm of pressure is plotted as a function of enthalpy, with the various thermodynamic properties as parameters (see Figure 1). The main components are:

◆ The thick line (boiling line) represents the saturated liquid and the double line (dew line) the saturated vapor. Instead of the pressure, the saturation temperature can be specified. Both lines intersect at the critical point, which is marked by a circle. The enthalpy difference between the two lines is the latent heat. The area to the left of the black line represents the supercooled liquid and the area to the right of the black double line represents the superheated steam. In between is a mixture of saturated liquid and vapor.

L Soloconcentration lines show the states of equal vapor content of the liquid-vapor mixture.

◆ Isotherms are the lines of equal temperature in the supercooled liquid and superheated steam.

◆ lsentropen represent changes in state without heat transfer between the fluid and the environment, eg the compression of the refrigerant.

◆ Isochores (lines of constant volume) are occasionally shown.

2. The basic process

Figure 1 shows the basic chiller process both as a cycle in the pressure-enthalpy diagram and in its physical components. To discuss this process, we could start at any point. A good starting point in our example is the slightly supercooled liquid refrigerant of 35 ° C and a pressure of 15.33 bar, which corresponds to a saturation temperature of 40 ° C at R22. This is point A in Figure 1. It is a convenient starting point because, in spite of the modifications of the basic process described later, it generally varies only slightly.

A - B. The liquid expands in the expansion valve. There is no energy - thermal or mechanical - exchanged with the environment. The expansion is isenthalp. This is shown in Fig.1 with a straight, vertical state change.

When the pressure decreases, nothing happens at first. The temperature of the liquid remains (almost) constant until the saturation curve is reached. A further reduction in pressure means that the temperature must also be lower. Otherwise, the liquid would overheat resulting in a thermodynamically unstable state. The liquid is thus cooled and the energy released evaporates some of the liquid or, in other words, the evaporating liquid cools the remaining liquid. The lower the pressure, the more liquid evaporates.

B . The liquid has reached the final pressure. The proportion of vaporized refrigerant can be read off the lines for constant vapor content. In the example, the refrigerant has expanded to 1.63 bar / -30 ° C, with 33.9% being evaporated.

B - D. The partially vaporized refrigerant enters the evaporator. Here, the remaining liquid refrigerant evaporates, producing the desired cooling effect. The refrigerant first reaches point C, where there is 100% saturated vapor, and leaves the evaporator slightly overheated at point D.

D. The vapor leaves the evaporator at -25 ° C overheated at 1.63 bar / -30 ° c.

D - E . The steam is compressed in the compressor to the condensing pressure. The compression should be as ideal as possible, ie mechanical, but no thermal energy is supplied to the steam until the pressure has reached he required height (in the example 15.3 bar / 40 ° C).

When this condition is met, the densification process proceeds along the lsentropic D - E ·. Note the difference to expansion A - B. There is no energy exchange with the environment, so the process is isenthalp. Here, mechanical but no thermal energy is supplied, so the state change of the steam takes place along the lentropic. The compression increases the temperature, as the diagram shows. The temperature increase precedes the pressure increase, ie the refrigerant not only remains in vapor form, but is still overheated.

The compression is not ideal. There is internal friction between the moving parts of the steam, friction energy in the lubricating oil, pressure steam flows back to the suction side, etc. All this means that additional heat is supplied to the steam. The compression process thus does not run along the lsentropic D - E ', but along an undefined path to the higher end temperature at E. This additional energy depends on the compressor grade η.

So:

H E   - H 0  = (H E '  - H 0 ) / η (the real compressor power)

With knowledge of η (from the manufacturer), H E & H 0 (diagram), H E can be calculated and, taking into account the final pressure, the outlet temperature can be determined (diagram).

E - F. The superheated steam leaves the compressor at a relatively high temperature. The steam represents energy that is too valuable to be wasted. So the steam could be re-heated in a special heat exchanger and the heat used for hot water or space heating.

F - A. The steam enters the actual condenser, probably a little overheated (slightly to the right of point F), and condenses. Normally, the condensate does not exactly saturate the condenser, but leaves it slightly subcooled. We have now reached the starting point A again: 15.33 bar / 40 ° C, supercooled liquid at 35 ° C.

Fig. 01: The basic chiller process

The task of a refrigeration system is to extract heat from a process fluid or air at a low temperature and to return it to a receiver medium, water or air.

The picture shows schematically a refrigeration system consisting of evaporator, compressor, condenser, expansion valve and piping. These are the minimum required components of a compression refrigeration cycle. The pressure is represented as a function of the enthalpy of liquid and vapor. To the left of the boiling line is liquid and to the right is the dew line steam. Between both lines is the two-phase area. The lines intersect at the critical point. Other properties can still be drawn as parameters, eg. B. Isotherms - lines - constant temperature.

The picture shows an isotherm for -25 ° C. It runs approximately vertically in the liquid area, because the specific heat capacity of the liquid is hardly pressure-dependent. By contrast, the specific heat capacity in the vapor phase is very much dependent on pressure (and temperature), which is why the isotherm here has an arcuate and inclined course.

The picture also shows lsentrope - a state change in which there is no heat transfer between the fluid and the environment. An ideal compression would follow this line (D - E '). Due to the inevitably released frictional energy (D - E), the real course and a higher end temperature is reached.

What: Alfa Laval