The refrigeration cycle

Heat is put into the fluid at the lower temperature and pressure and provides the latent heat to make it boil and change to a vapour. This vapour is then mechanically compressed to a higher pressure and a corresponding saturation temperature at which its latent heat can be rejected so that it changes back to a liquid. 

Enthalpy of fluid entering evaporator = 91.4 kJ/kg Enthalpy of saturated gas leaving evaporator = 249.9 kJ/kg Cooling effect = 249.9 - 91.4 = 158.5 kJ/kg 

Since the vapour compression cycle uses energy to move energy, the ratio of these two quantities can be used directly as a measure of the performance of the system. This ratio, the coefficient of performance, was first expressed by Sadi Carnot in 1824 for an 

The distance of D between the two parts of the curve indicates the proportion of flash gas at that point. The condenser receives the high-pressure superheated gas, cools it down to saturation temperature, condenses it to liquid, and finally suhcools it slightly. The energy removed in the condenser is seen to he the refrigerating effect plus the heat of compression. 

Transfer of heat through the walls of the evaporator and condenser requires a temperature difference, and the larger these heat exchangers are, the lower will he the temperature differences and so the closer the fluid temperatures will he to those of the load and condensing medium. The closer this approach, the nearer the cycle will he to the ideal reversed Carnot cycle. (See Table 2.1.) 

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Tc = temperature at which heat is taken into the refrigeration cycle (K)... 

It must be emphasized that Eq. (6.7) is only an approximate method for calculating the performance of refrigeration cycles. If greater accuracy is required, the refrigeration cycle must be followed using thermodynamic properties of the refrigerant being used. °... 

A simplified flow diagram of a modern H2SO4 alkylation unit is shown in Eigure 1. Excess isobutane is suppHed as recycle to the reactor section to suppress polymerization and other undesirable side reactions. The isobutane is suppHed both by fractionation and by the return of flashed reactor effluent from the refrigeration cycle. 

The refrigeration cycle is shown hy the process lines ABCD (Figure 2.7). Compression is assumed to he adiahatic, hut this will alter... 

The higher the coefficient of performance, the more efficient is the refrigeration cycle. An ideal coefficient of performance can be defined by ... 

It is obvious from Equation 24.20 that the larger the temperature difference across the refrigeration cycle (Tcond Tevap), the lower will be the coefficient of performance and the higher will be the power requirements for a given cooling duty. 

For the performance of practical refrigeration cycles, rather than assuming 0.6 of the ideal performance, the refrigeration cycle can be followed using thermodynamic properties of the refrigerant to obtain a more accurate calculation of the coefficient of performance. This will be discussed later. 

The desirable energy output of the refrigeration cycle is the heat added to the evaporator (or heat removed from the inner space or the low-temperature reservoir of the refrigerator). The energy input to the cycle is the compressor work required. The energy produced is the turbine work. The net work (fTnet) required to operate the cycle is (lVi2 + Thus, the coefficient of performance (COP) of the cycle is... 

What is the area enclosed by the refrigeration cycle on a T-s diagram ... 

Draw a simple scheme for the refrigeration cycle discussed in Figure 3.3. The compressor power for the production of 494 kW ice is 1802 kW. 

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