Chapter 4: 4.10 (page 129)
Suppose that heat leaks into your kitchen refrigerator at an average rate of 300 watts. Assuming ideal operation, how much power must it draw from the wall?
Short Answer
The power drawn from wall is 57.69 W.
Chapter 4: 4.10 (page 129)
Suppose that heat leaks into your kitchen refrigerator at an average rate of 300 watts. Assuming ideal operation, how much power must it draw from the wall?
The power drawn from wall is 57.69 W.
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Get started for freeConsider an ideal Hampson-Linde cycle in which no heat is lost to the environment.
(a) Argue that the combination of the throttling valve and the heat exchanger is a constant-enthalpy device, so that the total enthalpy of the fluid coming out of this combination is the same as the enthalpy of the fluid going in.
(b) Let be the fraction of the fluid that liquefies on each pass through the cycle. Show that
where is the enthalpy of each mole of compressed gas that goes into the heat exchanger, is the enthalpy of each mole of low-pressure gas that comes out of the heat exchanger, and is the enthalpy of each mole of liquid produced.
(c) Use the data in Table to calculate the fraction of nitrogen liquefied on each pass through a Hampson-Linde cycle operating between 1 bar and 100 bars, with an input temperature of . Assume that the heat exchanger works perfectly, so the temperature of the low-pressure gas coming out of it is the same as the temperature of the high-pressure gas going in. Repeat the calculation for an input temperature of .
Derive a formula for the efficiency of the Diesel cycle, in terms of the compression ratio V1/ V2and the cutoff ratio V3/ V2. Show that for a given compression ratio, the Diesel cycle is less efficient than the Otto cycle. Evaluate the theoretical efficiency of a Diesel engine with a compression ratio of 18 and a cutoff ratio of 2.
Why must you put an air conditioner in the window of a building, rather than in the middle of a room?
A common (but imprecise) way of stating the third law of thermodynamics is "You can't reach absolute zero." Discuss how the third law, as stated in Section 3.2, puts limits on how low a temperature can be attained by various refrigeration techniques.
Suppose you are told to design a household air conditioner using
HFC-134a as its working substance. Over what range of pressures would you have it operate? Explain your reasoning. Calculate the COP for your design, and compare to the COP of an ideal Carnot refrigerator operating between the same extreme temperatures.
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