Other Types of Vapour Compression Cycles

In the previous chapter, we have seen the ideal/standard VCR cycle, where the:

• Refrigerant vapour is at saturated state (point 1 on the saturation curve)

• Refrigerant is getting compressed isentropically from saturated state to superheated state.

Let's have a look at a few other common cycles.

A. Theoretical Vapour Compression Cycle with Dry Saturated Vapour after Compression

A vapour compression cycle with dry saturated vapour after compression is shown on T-s and p-h diagrams below.

1. Compression process: The vapour refrigerant at low pressure p1 and temperature Tl is compressed isentropically to dry saturated vapour as shown by the vertical line 1-2 on T-s diagram and by the curve 1-2 on p-h diagram. The pressure and temperature rises from pl to p2 and T1 to T2 respectively.

The Work done during isentropic compression per kg of refrigerant is given by: w = h2-h1

Where h1 = Enthalpy of vapour refrigerant at temperature TI, i.e. at suction of the compressor, and h2 = Enthalpy of the vapour refrigerant at temperature T2, is i.e. at discharge of the compressor.

2. Condensing process: The high pressure and temperature vapour refrigerant from the compressor is passed through the condenser where it is completely condensed at constant pressure p2 and temperature T2, as shown by the horizontal line 2-3 on T-s and p-h diagrams. The vapour refrigerant is changed into liquid refrigerant. The refrigerant, while passing through the condenser, gives its latent heat to the surrounding condensing medium.

3. Expansion process: The liquid refrigerant at pressure p3 = p2 and temperature T3 = T2 is expanded by throttling process through the expansion valve to a low pressure p4 = p1 and temperature T4 = T1 , as shown by the curve 3-4 on T-s diagram and by the vertical line 3-4 on p-h diagram. We have already discussed that some of the liquid refrigerant evaporates as it passes through the expansion valve; but the greater portion is vaporised in the evaporator. We know that during the throttling process, no heat is absorbed or rejected by the liquid refrigerant.

Notes:

(a) In case an expansion cylinder is used in place of throttle or expansion valve to expand the liquid refrigerant, then the refrigerant will expand isentropically as shown by dotted vertical line on T-s diagram. The isentropic expansion reduces the external work being expanded in running the compressor and increases the refrigerating effect. Thus, the net result of using the expansion cylinder is to increase the coefficient of performance.

Since the expansion cylinder system of expanding the liquid refrigerant is quite complicated and involves greater initial cost, therefore its use is not justified for small gain in cooling capacity. Moreover, the flow rate of the refrigerant can be controlled with throttle valve which is not possible in case of expansion cylinder which has a fixed cylinder volume.

(b) In modern domestic refrigerators, a capillary (small bore tube) is used in place of an expansion valve.

4. Vaporising process: The liquid-vapour mixture of the refrigerant at pressure p4 = p1 and temperature T4=T1 is evaporated and changed into vapour refrigerant at constant pressure and temperature, as shown by the horizontal line 4-1 on T-s and p-h diagrams. During evaporation, the liquid-vapour refrigerant absorbs its latent heat of vaporisation from the medium (air, water or brine) which is to be cooled. This heat which is absorbed by the refrigerant is called refrigerating effect and it is briefly written as Re. The process of vaporisation continues up to point 1 which is the starting point and thus the cycle is completed.

The refrigerating effect or the heat absorbed or extracted by the liquid-vapour refrigerant during evaporation per kg of refrigerant is given by:

Where Hf3 is the Sensible heat at temperature T3, i.e. enthalpy of Liquid refrigerant leaving the condenser

It can be noticed from the cycle that the liquid-vapour refrigerant has extracted heat during evaporation and the work will be done by the compressor for isentropic compression of the high pressure and temperature vapour refrigerant.

COP = Refrigerating effect / Work Done = (h1 - h4) / (h2 - h1)

B. Theoretical Vapour Compression Cycle with Wet Vapour after Compression

A vapour compression cycle with wet vapour after compression is shown on T-s and p-h diagrams below. In this cycle, the enthalpy at point 2 is found out with the help of dryness fraction at this point. The dryness fraction at points 1 and 2 may be obtained by equating entropies at points 1 and 2.

The remaining cycle is the same as discussed above, and the COP can be determined in the same manner.

Comparison with Simple VCR Cycle

A little consideration will show that the superheating in simpler VCR cycle increases the refrigerating effect and the amount of work done in the compressor.

Since the increase in refrigerating effect is less as compared to the increase in work done, therefore, the net effect of superheating is a lower coefficient of performance.