# HW9S - HW 9 Solution 1 7-24 Air is compressed steadily by a...

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HW 9 Solution 1. 7-24 Air is compressed steadily by a compressor. The air temperature is maintained constant by heat rejection to the surroundings. The rate of entropy change of air is to be determined. Assumptions 1 This is a steady-flow process since there is no change with time. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas. 4 The process involves no internal irreversibilities such as friction, and thus it is an isothermal, internally reversible process. Properties Noting that h = h ( T ) for ideal gases, we have h 1 = h 2 since T 1 = T 2 = 25 ° C. Analysis We take the compressor as the system. Noting that the enthalpy of air remains constant, the energy balance for this steady-flow system can be expressed in the rate form as out in out in energies etc. potential, kinetic, internal, in change of Rate (steady) 0 system mass and work, heat, by nsfer energy tra net of Rate out in 0 Q W E E E E E ± ± ± ± ² ²³ ² ²´ µ ± µ ± ± = = = Δ = Ê Therefore, kW 30 in out = = W Q ± ± Noting that the process is assumed to be an isothermal and internally reversible process, the rate of entropy change of air is determined to be kW/K 0.0101 = = = Δ K 298 kW 30 sys air out, air T Q S ± ± AIR T = const. P 2 P 1 Q · 30 kW 2. 7-29 R-134a enters an evaporator as a saturated liquid-vapor at a specified pressure. Heat is transferred to the refrigerant from the cooled space, and the liquid is vaporized. The entropy change of the refrigerant, the entropy change of the cooled space, and the total entropy change for this process are to be determined. Assumptions 1 Both the refrigerant and the cooled space involve no internal irreversibilities such as friction. 2 Any temperature change occurs within the wall of the tube, and thus both the refrigerant and the cooled space remain isothermal during this process. Thus it is an isothermal, internally reversible process.

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## This note was uploaded on 01/03/2011 for the course MEEN 315 taught by Professor Ramussen during the Fall '07 term at Texas A&M.

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HW9S - HW 9 Solution 1 7-24 Air is compressed steadily by a...

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