P5.1. An ordinary vapor compression cycle is to operate a refrigerator on R134a between –40°C and 40°C (condenser temperatures). Compute the coefficient of performance and the heat removed from the refrigerator per day if the power used by the refrigerator is 9000 J per day. (ANS. 1.76)
P5.2. An ordinary vapor compression cycle is to be operated on methane to cool a chamber to –260°F. Heat will be rejected to liquid ethylene at –165°F. The temperatures in the condenser and evaporator are –160°F and –280°F. Compute the coefficient of performance. (ANS. 0.86)
P5.3. A simple Rankine cycle is to operate on steam between 200°C and 99.6°C, with saturated steam exhausting from the turbine. What is the maximum possible value for its thermodynamic efficiency? (ANS. 7.8%)
P5.4. An ordinary vapor compression cycle is to be operated on methane to cool a chamber to 112 K. Heat is rejected to liquid ethylene at 160 K. The temperatures in the coils are 168 K and 100 K. (ANS. online)
a. Write the relevant energy and entropy balances for the compression step.
b. Estimate the minimal work requirement (J/g) for the compressor assuming the ideal gas law.
c. Estimate the coefficient of performance (COP) for this OVC cycle.
d. Estimate the COP by the Carnot guideline.
e. Estimate the minimal work requirement for the compressor using the chart in the text.
P5.5. A house has an effective heat loss of 100,000 Btu/hr. During the heating season of 160 days the average inside temperature should be 70°F while the outside is 45°F. Freon-134a is the working fluid and an ordinary vapor-compression cycle is used. A 10°F approach on each side may be assumed. Electricity costs $0.14/KW-hr.
a. What is the cost in $/hr if the compressor is 100% efficient?
b. What is the cost if the compressor is 80% efficient? (ANS. ~0.37, 0.45)
P5.6. An adiabatic turbine is supplied with steam at 300 psia and 550°F that exhausts at atmospheric pressure. The quality of the exhaust steam is 95%.
a. What is the efficiency of the turbine? (ANS. 76%)
b. What would be the thermodynamic efficiency of a Rankine cycle operated using this turbine at these conditions? (ANS. 17%)
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