Refrigeration & Air-Conditioning

Module 1. Fundamentals of refrigeration

Lesson 3
REPRESENTATION OF VAPOUR COMPRESSION REFRIGERATION CYCLE ON P-V, T-Ф AND P-H DIAGRAMS, USE OF REFRIGERANT PROPERTIES TABLES AND P-H (MOLLIOR) CHARTS

3.1 Introduction

The properties of refrigerant do not remain constant during the operation of the system. Refrigerant properties tables are used to know the thermodynamic properties of refrigerant at saturation points at various temperatures and pressures. The properties of refrigerant in any thermodynamic state can be represented on Pressure-Enthalpy (P-H) chart. These charts (Mollier Diagrams) are plotted for 1.0 kg mass of refrigerant. The values of enthalpy and entropy are based on an arbitrarily chosen datum. The refrigerant properties tables and charts are used to determine theoretical refrigerating effect, work of compression and co-efficient of performance (C.O.P.) of the refrigeration system operating under different conditions.

In order to understand the presentation of the working cycle on P-V, T- ф and P-H diagrams, the block diagram of the system which is shown in lesson 2 is again indicated in Fig. 3.1.

The various processes involved in the cycle are as under.

1-2, Compression

2-3, Condensation

3-4, Expansion

4-1, Evaporation

The above mentioned processes are indicated on P-V, T- ф and P-H in Fig. 3.2, Fig. 3.3 and Fig. 3.4 respectively.

In order to get the values of enthalpy, entropy, specific volume, temperature etc, it is necessary to locate the points on actual diagrams corresponding to the operating suction and discharge pressure of the system.

3.2 Refrigerant Properties Tables

Refrigerant properties table are used to obtain values of pressure, volume, enthalpy and entropy of refrigerant at different temperatures under saturated condition. Properties tables of R-22 and R-717 are given bellow. The thermodynamic properties of refrigerant under superheated, sub cooled and two phase conditions can not be obtained from these tables but can be obtained from P-H (Mollier chart) diagram of the respective refrigerants.

3.3 Refrigerant P-H Diagram

P-H diagram are used to obtain the thermodynamic properties of refrigerants at any point during operation of refrigeration system. When the operating suction and discharge pressure are known we can get values like enthalpy, specific volume and corresponding temperature etc. from the diagram. The theoretical COP can be calculated by reading the enthalpy values at different stages. The P-H diagram of R-717 and R-22 are given in Figure 3.5 and 3.6 respectively.

3.4 How to Use Refrigerant Property Tables and P-H Charts?

Suppose, an ammonia VCR plant operates at suction pressure of 2 bar (absolute) and condensing pressure of 12 bar (absolute) so the corresponding values of temperature, specific volume, specific enthalpy and specific entropy of both gas and liquid can be read in the front of the pressure value in table. (It is highlighted in the ammonia properties table.)

The operating cycle for above mentioned conditions for ammonia is marked in the chart.

From the chart

h₁ = 1420 kJ/ kg (enthalpy at the suction of compressor).

h₂ = 1650 kJ/ kg (enthalpy at the end of compression)

h₃ = 325 kJ/ kg (enthalpy at the end of condensation)

The values of h₁ and h ₃ can be obtained from chart as well as from table but the value of h₂ can only be obtained from P-H chart.

Table 3.1 R-22 Properties table

Table 3.2 R-22 Refrigeration Table

Table 3.3 Ammonia Properties table

Table 3.4 R-66 Refrigeration Table

Fig. 3.1 P-H diagram of ammonia refrigerant

Fig. 3.2 P-H diagram of R-22 refrigerant