Refrigeration power requirement

The family of short curves in Fig. 29-45 shows the power efficiency of conventional refrigeration systems. The curves for the latter are taken from the Engineering Data Book, Gas Processors Suppliers Association, Tulsa, Oklahoma. The data refer to the evaporator temperature as the point at which refrigeration is removed. If the refrigeration is used to cool a stream over a temperature interval, the efficiency is obviously somewhat less. The short curves in Fig. 29-45 are for several refrigeration-temperature intervals. A comparison of these curves with the expander curve shows that the refrigeration power requirement by expansion compares favorably with mechanical refrigeration below 360° R (—100° F). The expander efficiency is favored by lower temperature at which heat is to be removed. 

To evaluate the impact of each design option, a quick and reliable method for estimating refrigeration power requirements can be very useful8,9. The benefits from setting targets for refrigeration power are to... 

Pressure level. The choice of pressure level for the mixed refrigerant evaporation affects the temperature difference between the process cooling curve and refrigerant evaporation curve. Increasing the overall temperature difference will increase the refrigeration power requirements. 

The cost of power required to run a refrigeration system can be estimated approximately as a multiple of the power required for an ideal system. Thus, for an ideal system,... 

Looking at this low temperature refrigeration as to power requirement, one expander horsepower removes its heat equivalent to 2,545 Btu/lir, as eompared with 12,000 Btu/hr, about 4.7 times as mueh. This is refened to as a ton of refrigeration. Thus, the turhoexpander must develop 4.7 hp to generate a ton of refrigeration however, it delivers 4.7 hp haek as power. 

Example 28.3 Refrigerant For the same duty, liquid R.22 enters the expansion valve at 33°C, evaporates at 5°C, and leaves the cooler at 9°C. Fan power is 0.9 kW. What mass flow of refrigerant is required ... 

The performance of refrigeration cycles is measured as a coefficient of performance (COPref), as illustrated in Figure 24.25. The coefficient of performance is the ratio of cooling duty performed per unit power required. 

Example 24.1 Estimate the coefficient of performance and power requirement for a refrigeration cycle with Tevap = — 30°C, Tcond = 40°C and Qevap = 3 MW. 

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. 

Stream 2. At 200 K, either ethane or ethylene would be suitable refrigerants, with ethane being slightly better from the point of view of the power requirements. As for Stream 1, a cascade system would be required for heat rejection to ambient temperature. 

Stream 4. At 245 K, chlorine, ammonia, propylene and propane could all be chosen. In principle, ethane and ethylene could also have been included but at 245 K they are too close to their critical temperature and would require significantly higher refrigeration power than the other options. The safety problems associated with chlorine are likely to be greater than ammonia. Thus, ammonia might be a suitable choice of refrigerant. Choosing a component already in the process would be desirable. 

Calculation of the power requirements for the three refrigerants requires the flowrate to be calculated for a duty of 3 MW. This can be calculated from the enthalpy difference across the evaporator (H2 — H1). The enthalpy difference across the evaporator is assumed to be the difference between the saturated vapor enthalpy at the evaporator pressure and the saturated liquid enthalpy at the condenser pressure. This assumes no subcooling of the refrigerant. 

In principle, ammonia is the best refrigerant fluid in terms of power requirement. However, this conclusion disregards the potential practical problems associated with compression. There is little to choose between propylene and propane in terms of the power requirements. 

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