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

Vapor Compression Cycle Expansion Valve

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

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)

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

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

Comparison Between Single-Stage and Carnot Cycles Figure 3.6

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

Two stage systems

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

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