UNIT-I MODULE-I Air Refrigeration Cycles 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. 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. 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., (1) where m is the mass of the gas, u1 and u2 are the initial and final internal energies of the gas, T1 and T2 are the initial and final temperatures and cv is the specific heat at constant volume. If the expansion is reversible and adiabatic, by using the ideal gas equation and the equation for isentropic process the final temperature (2) is related to the initial temperature (T1) and initial and final pressures (P1 and P2) by the equation: (3) where γ is the coefficient of isentropic expansion given by: Isentropic expansion of the gas can also be carried out in a steady flow in a turbine which gives a net work output. Neglecting potential and kinetic energy changes, the work output of the turbine is given by: (4) The final temperature is related to the initial temperature and initial and final pressures by Eq. (2) Reversed Carnot cycle employing a gas Reversed Carnot cycle is an ideal refrigeration cycle for constant temperature external heat source and heat sinks. Figure 9.1(a) shows the schematic of a reversed Carnot refrigeration system using a gas as the working fluid along with the cycle diagram on T-s and P-Vcoordinates. As shown, the cycle consists of the following four processes: Process 1-2: Reversible, adiabatic compression in a compressor Process 2-3: Reversible, isothermal heat rejection in a compressor Process 3-4: Reversible, adiabatic expansion in a turbine Process 4-1: Reversible, isothermal heat absorption in a turbine
Fig.1(a). Schematic of a reverse Carnot refrigeration system Fig. 1(b). Reverse Carnot refrigeration system in P-v and T-s coordinates The heat transferred during isothermal processes 2-3 and 4-1 are given by:
Thus the COP of the Carnot system depends only on the refrigeration (Tl) and heat rejection (Th) temperatures only. Limitations of Carnot cycle: Carnot cycle is an idealization and it suffers from several practical limitations. One of the main difficulties with Carnot cycle employing a gas is the difficulty of achieving isothermal heat transfer during processes 2-3 and 4-1. For a gas to have heat transfer isothermally, it is essential to carry out work transfer from or to the system when heat is transferred to the system (process 4-1) or from the system (process 2-3). This is difficult to achieve in practice. In addition, the volumetric refrigeration capacity of the Carnot system is very small leading to large compressor displacement, which gives rise to large frictional effects. All actual processes are irreversible, hence completely reversible cycles are idealizations only.