Power and Refrigeration Cycles – Applications (YAC: Ch. 7) Most devices operate on cycles (open or closed) of two common types: Power Cycles: Produce net work or power, e.g. Power plants, I.C. engines. Refrigeration Cycles: Transfer heat from a cold space (reservoir ) to a hot one. E.g. Refrigerators, heat pumps, A/C’s Cycles commonly classified as: Vapor cycles – Working fluid in liquid and vapor phases in different portions of the cycle. Gas cycles – Working fluid in gas phase in all portions of the cycle. Also classified as: Closed cycles – The same working fluid is re-circulated through the entire cycle, I.e. the fluid undergoes a thermodynamic cycle. E.g. Steam Vapor Power Plants, the refrigerant in heat pumps and A/C’s Open cycles – The working fluid is replaced by new fluid at the end of each cycle, e.g. I.C. engines Modifed: 11/6/01

Power and Refrigeration Cycles – Idealizations Most real devices operate on cycles which are usually very complex. We analyze such real devices by making simplifying assumptions, which lead to idealized cycles Some common simplifying assumptions: 1.All compression and expansion processes are reversible (quasi-equilibrium) 2.Friction is negligible, i.e. no pressure losses in pipes, heat exchangers, etc. 3.Perfect insulation, i.e. no heat losses in pipes or other components. (Note: this does not mean that there is no heat transfer transfer between the working fluid and thermal reservoirs) 4.Changes in Kinetic energy and Potential energy usually neglected. (Q: Any exceptions ?) What is the use of studying such idealized cycles ? They allow us to study the influence of major parameters on the behavior of real cycles. E.g. Increasing T H or decreasing T L will increase the efficiency of idealized and real cycles. Modifed: 11/6/01

Simple Steam Vapor Power Plant Modifed: 11/6/01