A freezer is maintained at 20^∘ F by removing heat from it at a rate of 75 Btu / min. The power

step 1 This question here we consider a freezer is maintained at the rate of 20 degree Fahrenheit by re

A freezer is maintained at 20^∘ F by removing heat from it...

A freezer is maintained at $20^{\circ} \mathrm{F}$ by removing heat from it at a rate of $75 \mathrm{Btu} / \mathrm{min}$. The power input to the freezer is $0.70 \mathrm{hp},$ and the surrounding air is at $75^{\circ} \mathrm{F}$. Determine (a) the reversible power, (b) the irreversibility, and (c) the second-law efficiency of this freezer. Answers: (o) 0.20 hp,

A freezer is maintained at $20^{\circ} \mathrm{F}$ by removing heat from it at a rate of $75 \mathrm{Btu} / \mathrm{min}$. The power input to the freezer is $0.70 \mathrm{hp},$ and the surrounding air is at $75^{\circ} \mathrm{F}$. Determine
(a) the reversible power, (b) the irreversibility, and (c) the second-law efficiency of this freezer. Answers: (o) 0.20 hp,

Key Concepts

First Law of Thermodynamics in Refrigeration

This concept involves the energy balance for a refrigerator where the heat removed from the cold reservoir and the net work input are accounted for. It establishes that the energy extracted as heat from the freezer plus the work input must match the energy delivered to the surroundings, providing a basic framework for analyzing the performance of refrigeration systems.

Second Law of Thermodynamics in Refrigeration

The second law introduces the concept of entropy, indicating that real processes come with inefficiencies due to irreversibility. In refrigeration systems, it allows us to evaluate the ideal, reversible scenario versus the actual performance, quantifying losses in performance and the exergy (or available work) destruction during the process.

Reversible Process Concept

A reversible process in thermodynamics is an idealized process that occurs infinitely slowly without any entropy generation. In refrigeration, the reversible process represents the theoretical optimum where the minimum work input is required to remove a given quantity of heat, serving as a benchmark for comparing actual system performance.

Irreversibility and Exergy Destruction

Irreversibility refers to the losses in a process due to factors such as friction, non-equilibrium heat transfer, and other real-world imperfections. In the context of refrigeration, irreversibility is a measure of energy that is degraded and cannot be converted into useful work, and it is evaluated as the difference between the actual work input and the work required in the ideal, reversible case.

Second-law Efficiency

This is a performance measure that compares the actual refrigeration effect achieved by a system to the maximum possible effect (obtained in a reversible process) as dictated by the second law. It is defined as the ratio of the ideal, reversible performance to that of the actual system, thereby quantifying how effectively the system approaches the theoretical limit.

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