RAC Lecture 9 - Lesson 9 Air cycle refrigeration systems 1...

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Lesson 9 Air cycle refrigeration systems Version 1 ME, IIT Kharagpur 1
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The specific objectives of the lesson: This lesson discusses various gas cycle refrigeration systems based on air, namely: 1. Reverse Carnot cycle & its limitations ( Section 9.4 ) 2. Reverse Brayton cycle – Ideal & Actual ( Section 9.5 ) 3. Aircraft refrigeration cycles, namely Simple system, Bootstrap system, Regenerative system, etc. ( Section 9.6 ) At the end of the lesson the student should be able to: 1. Describe various air cycle refrigeration systems ( Section 9.1-9.6 ) 2. State the assumptions made in the analyses of air cycle systems ( Section 9.2 ) 3. Show the cycles on T-s diagrams ( Section 9.4-9.6 ) 4. Perform various cycle calculations ( Section 9.3-9.6 ) 5. State the significance of Dry Air Rated Temperature ( Section 9.6 ) 9.1. Introduction Air cycle refrigeration systems belong to the general class of gas cycle refrigeration systems, in which a gas is used as the working fluid. The gas does not undergo any phase change during the cycle, consequently, all the internal heat transfer processes are sensible heat transfer processes. Gas cycle refrigeration systems find applications in air craft cabin cooling and also in the liquefaction of various gases. In the present chapter gas cycle refrigeration systems based on air are discussed. 9.2. Air Standard Cycle analysis Air cycle refrigeration system analysis is considerably simplified if one makes the following assumptions: i. The working fluid is a fixed mass of air that behaves as an ideal gas ii. The cycle is assumed to be a closed loop cycle with all inlet and exhaust processes of open loop cycles being replaced by heat transfer processes to or from the environment iii. All the processes within the cycle are reversible, i.e., the cycle is internally reversible iv. The specific heat of air remains constant throughout the cycle An analysis with the above assumptions is called as cold Air Standard Cycle (ASC) analysis. This analysis yields reasonably accurate results for most of the cycles and processes encountered in air cycle refrigeration systems. However, the analysis fails when one considers a cycle consisting of a throttling process, as the temperature drop during throttling is zero for an ideal gas, whereas the actual cycles depend exclusively on the real gas behavior to produce refrigeration during throttling. Version 1 ME, IIT Kharagpur 2
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9.3. Basic concepts The temperature of an ideal gas can be reduced either by making the gas to do work in an isentropic process or by sensible heat exchange with a cooler environment. When the gas does adiabatic work in a closed system by say, expanding against a piston, its internal energy drops. Since the internal energy of the ideal gas depends only on its temperature, the temperature of the gas also drops during the process, i.e., ) T T ( mc ) u u ( m W 2 1 v 2 1 = = (9.1) where m is the mass of the gas, u 1 and u 2 are the initial and final internal energies of the gas, T 1 and T 2 are the initial and final temperatures and c v is the specific heat at constant volume. If the expansion is reversible and adiabatic, by using the ideal gas
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This note was uploaded on 12/04/2010 for the course ME N/A taught by Professor N/a during the Spring '10 term at Indian Institute of Technology, Kharagpur.

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RAC Lecture 9 - Lesson 9 Air cycle refrigeration systems 1...

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