1. Objectives Empathize with refrigerant Describe refrigeration cycles Analyze cycles on T-s diagrams Compare real cycles to ideal cycles Basis for discussion of components

3. Efficiency First Law Coefficient of performance, COP COP = useful refrigerating effect/net energy supplied COP = q r /w net Second law Refrigerating efficiency, η R η R = COP/COP rev Comparison to ideal reversible cycle

4. Carnot Cycle No cycle can have a higher COP All reversible cycles operating at the same temperatures (T 0, T R ) will have the same COP For constant temp processes dq = Tds COP = T R /(T 0 – T R )

5. Get Real Assume no heat transfer or potential or kinetic energy transfer in expansion valve COP = (h 3 -h 2 )/(h 4 -h 3 )

6. Area Analysis of Work and Efficiency

7. Comparison Between Single-Stage and Carnot Cycles Figure 3.6

8. Example R-22 condensing temp of 30 °C and evaporating temp of 0 °C Determine a) q carnot w carnot b) Diminished q R and excess w for real cycle caused by throttling and superheat horn c) η R

10. Subcooling and Superheating Refrigerant may be subcooled in condenser or in liquid line Temperature goes below saturation temperature Refrigerant may be superheated in evaporator or in vapor (suction) line Temperature goes above saturation temperature Neither of this results in useful heat transfer Loss of efficiency

12. Multistage Compression Cycles Combine multiple cycles to improve efficiency Prevents excessive compressor discharge temperature Allows low evaporating temperatures (cryogenics)

13. Summary Theoretical and “real” vapor compression cycles Analysis on T-s diagram Chapter 3 (focus on beginning of chapter) Next: Refrigerants, compressors, expansion devices Chapter 4