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Chapter 4: Q. 4.12 (page 129)

Explain why a rectangular PVcycle, as considered in Problems 1.34 and 4.1, cannot be used (in reverse) for refrigeration.

Short Answer

Expert verified

The rectangular PV cycle doesn't show the property of The range of temperature at which working substance absorbs heat must be less than the range off temperature at which working substance rejects heat. Therefore it cannot be used for refrigeration.

Step by step solution

01

Concept Introduction

For cyclic process, PV diagram is a closed loop. The area of the PV diagram represents the amount of work done during the process.

02

Explanation

The range of temperature at which working substance absorbs heat must be less than the range off temperature at which working substance rejects heat. Since the rectangular PV cycle doesn't show this property, therefore it cannot be used for refrigeration.

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Most popular questions from this chapter

Consider 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 xbe the fraction of the fluid that liquefies on each pass through the cycle. Show that

x=Hout-HinHout-Hliq,

where Hinis the enthalpy of each mole of compressed gas that goes into the heat exchanger, Houtis the enthalpy of each mole of low-pressure gas that comes out of the heat exchanger, and Hliqis the enthalpy of each mole of liquid produced.

(c) Use the data in Table 4.5to 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 300K. 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 200K.

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.

Estimate the maximum possible COP of a household air conditioner. Use any reasonable values for the reservoir temperatures.

Table 4.5 gives experimental values of the molar enthalpy of nitrogen at 1 bar and 100 bars. Use this data to answer the following questions about a nitrogen throttling process operating between these two pressures.

(a) If the initial temperature is 300K, what is the final temperature? (Hint: You'll have to do an interpolation between the tabulated values.)

(b) If the initial temperature is 200K, what is the final temperature?

(c) If the initial temperature is 100K, what is the final temperature? What fraction of the nitrogen ends up as a liquid in this case?

(d) What is the highest initial temperature at which some liquefaction takes place?

(e) What would happen if the initial temperature were 600K? Explain.
See all solutions

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