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’sOpen 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 – IdealizationsMost real devices operate on cycles which are usually very complex.We analyze such real devices by making simplifying assumptions, which lead to idealized cyclesSome 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 TH or decreasing TL will increase the efficiency of idealized and real cycles.

Modifed: 11/6/01

Simple Steam Vapor Power Plant

Modifed: 11/6/01