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Working substance in refrigeration is

vapor.  Gas laws do not apply to vapors.

Vapors have complicated equations of state.

These are laborious for analysis. Hence

charts and tables are in use.

  1. Martin- How Equation Of State

                   P =RT/ (v-B) + (A2+B2+C2 e-kT/TC)/ (v-B)2

                      + (A3+B3+C3 e-kT/TC)/ (v-B)3 +——

                    WHERE B, A2, B2, C2, A3, B3, C3, A4 —are 11 constants

                     Tc is the critical temperature

                      R is the refrigerant constant

       2.Redlich-Kwong Equation Of State

                      P = RT/ (V-b) —a/ [T0.5 V (V+b)]

                      Where V is molar volume

                     Tc is the critical temperature

                     pc is the critical pressure ‘

                     a = 0.4278R2 Tc2.5/pc

                      b = 0.0867RTc/pc

These equations require lot of time for calculating the various thermodynamic properties. Therefore to save time in the analysis, standard tables and charts are available in literature. Use these in the analysis of the vapor compression refrigeration systems.

As entropy is a thermodynamic property. It is like other thermodynamic properties such as pressure/internal energy/enthalpy. Entropy is abscissa (x –co-ordinate). ABSOLUTE temperature is as the ordinate. This chart is very useful in comparing the performance of various heat engine and refrigeration cycles.

Although entropy is an extensive (depends on mass) thermodynamic property. Yet, its concept comes from the second law of thermodynamics (Clausius inequality). Conventionally it is in maximum use as an intensive property in kJ/kg K.


Fig. Temperature Entropy Chart

Constant Pressure Line is ABCD

Another Constant Pressure Line is EFGH

Constant temperature Line is Horizontal.

Constant Entropy Line is Vertical.

Area on this diagram heat quantity.



Fig: Pressure Enthalpy chart

p-h chart contains

(i) Iso-thermal lines

vertical in sub-cooled region

horizontal in two phase region

curve like in super-heated region

FGHI is Constant Temperature Line.

JKLM is another Constant Temperature Line.

(ii) Constant pressure lines—————Horizontal Lines

(iii) Constant  specific entropy lines——– more Inclined to horizontal and are

plotted in the super-heated region

NO and PQ are Constant Entropy Lines.

(iv) Constant specific volume lines————-less inclined to horizontal and are

in the super-heated region.

RS and TU are Constant Specific Volume Lines.

(v) Vertical Lines are Constant Enthalpy Lines.

(vi) Critical Point—–Point ‘C’ is the critical point.

(vii) Saturated Liquid curve- Curve a b c

(viii) Saturated vapor Line—–Curve CDE is the saturated vapor line.

(ix) SUB-COOLED REGION—Towards left of Saturated Liquid Line.

(x) TWO-PHASE REGION—In between Saturated Liquid Line and Saturated Vapor Line.

(xi) SUPER-HEATED REGION——–Towards Right of Saturated Vapor Line.

(xii) GASES REGION———Towards Right of Saturated Vapor Region where Degree of Super-heat is >50C


These contain the values of specific properties of saturated liquid and dry saturated vapors. Calculate the properties of subcooled liquid and super heated vapors using specific properties of saturated liquid and dry saturated vapors.


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