2. Objectives Cooling Systems
Point out why are they very important for buildings
Describe vapor compression cycle basics
Draw cycle on T-s diagrams
Compare real cycles to ideal cycles

3. Vapor Compression Cycle
Expansion Valve

5. Efficiency First Law Second law Coefficient of performance, COP
COP = useful refrigerating effect/net energy supplied
COP = qr/wnet
Second law
Refrigerating efficiency, ηR
ηR = COP/COPrev
Comparison to ideal reversible cycle

6. Carnot Cycle No cycle can have a higher COP
All reversible cycles operating at the same temperatures (T0, TR) will have the same COP
For constant temp processes
dq = Tds
COP = TR/(T0 – TR)

7. Real Cycles
Assume no heat transfer or potential or kinetic energy transfer in expansion valve
COP = (h3-h2)/(h4-h3)
Compressor displacement = mv3

8. Example
R-22 condensing temp of 30 °C (86F) and evaporating temp of 0°C (32 F)
Determine
qcarnot wcarnot ηR

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

11. 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

12. Two stage systems

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

14. What are desirable properties of refrigerants?
Pressure and boiling point
Critical temperature
Latent heat of vaporization
Heat transfer properties
Viscosity
Stability