EML4304C HW4 solution

EML4304C HW4 solution - 11.17 A refrigerator in my 20oC...

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11.17 A refrigerator in my 20 o C kitchen uses R-12 and I want to make ice cubes at –5 o C. What is the minimum high P and the maximum low P it can use? Since the R-12 must give heat transfer out to the kitchen air at 20 o C, it must at least be that hot at state 3. From Table B.3.1: P 3 = P 2 = P sat = 567 kPa is minimum high P. Since the R-12 must absorb heat transfer at the freezers –5 o C, it must at least be that cold at state 4. From Table B.3.1: P 1 = P 4 = P sat = 261 kPa is maximum low P. 11.18 How many parameters are needed to completely determine a standard vapor compression refrigeration cycle? Two parameters : The high pressure and the low pressure. This assumes the exit of the condenser is saturated liquid and the exit of the evaporator is saturated vapor. 11.19 Why would one consider a combined cycle system for a power plant? For a heat pump or refrigerator? Dual cycle or combined cycle systems have the advantage of a smaller difference between the high and low ranges for P and T. The heat can be added at several different temperatures reducing the difference between the energy source T and the working substance T. The working substance vapor pressure at the

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