Power and Refrigeration Cycles

1
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/Cs
• 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/Cs
• 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
2
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
• All compression and expansion processes are
  reversible (quasi-equilibrium)
• Friction is negligible, i.e. no pressure losses
  in pipes, heat exchangers, etc.
• 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)
• 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
3
Simple Steam Vapor Power Plant
Modifed 11/6/01