Chapter 11: Problem 99
A thermoelectric cooler has a COP of 0.18 and the power input to the cooler is 1.8 hp. Determine the rate of heat removed from the refrigerated space, in \(\mathrm{Btu} / \mathrm{min}\).
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A thermoelectric refrigerator removes heat from a refrigerated space at \(-5^{\circ} \mathrm{C}\) at a rate of \(130 \mathrm{W}\) and rejects it to an environment at \(20^{\circ} \mathrm{C}\). Determine the maximum coefficient of performance this thermoelectric refrigerator can have and the minimum required power input.
A gas refrigeration system using air as the working fluid has a pressure ratio of \(5 .\) Air enters the compressor at \(0^{\circ} \mathrm{C} .\) The high- pressure air is cooled to \(35^{\circ} \mathrm{C}\) by rejecting heat to the surroundings. The refrigerant leaves the turbine at \(-80^{\circ} \mathrm{C}\) and enters the refrigerated space where it absorbs heat before entering the regenerator. The mass flow rate of air is \(0.4 \mathrm{kg} / \mathrm{s}\). Assuming isentropic efficiencies of 80 percent for the compressor and 85 percent for the turbine and using variable specific heats, determine ( \(a\) ) the effectiveness of the regenerator, \((b)\) the rate of heat removal from the refrigerated space, and \((c)\) the \(\mathrm{COP}\) of the cycle. Also, determine \((d)\) the refrigeration load and the COP if this system operated on the simple gas refrigeration cycle. Use the same compressor inlet temperature as given, the same turbine inlet temperature as calculated, and the same compressor and turbine efficiencies.
Describe the Seebeck and the Peltier effects.
Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at \(0.20 \mathrm{MPa}\) and \(-5^{\circ} \mathrm{C}\) at a rate of \(0.07 \mathrm{kg} / \mathrm{s},\) and it leaves at \(1.2 \mathrm{MPa}\) and \(70^{\circ} \mathrm{C}\). The refrigerant is cooled in the condenser to \(44^{\circ} \mathrm{C}\) and \(1.15 \mathrm{MPa}\), and it is throttled to 0.21 MPa. Disregarding any heat transfer and pressure drops in the connecting lines between the components, show the cycle on a \(T\) -s diagram with respect to saturation lines, and determine ( \(a\) ) the rate of heat removal from the refrigerated space and the power input to the compressor, \((b)\) the isentropic efficiency of the compressor, and \((c)\) the \(C O P\) of the refrigerator.
Consider a regenerative gas refrigeration cycle using helium as the working fluid. Helium enters the compressor at \(100 \mathrm{kPa}\) and \(-10^{\circ} \mathrm{C}\) and is compressed to \(300 \mathrm{kPa}\). Helium is then cooled to \(20^{\circ} \mathrm{C}\) by water. It then enters the regenerator where it is cooled further before it enters the turbine. Helium leaves the refrigerated space at \(-25^{\circ} \mathrm{C}\) and enters the regenerator. Assuming both the turbine and the compressor to be isentropic, determine ( \(a\) ) the temperature of the helium at the turbine inlet, ( \(b\) ) the coefficient of performance of the cycle, and ( \(c\) ) the net power input required for a mass flow rate of \(0.45 \mathrm{kg} / \mathrm{s}\).
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