Lecture Objectives:

• Learn more about cooling cycles

Vapor Compression Cycle

Expansion Valve

Efficiency

• First 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)

Get Real

• 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)

• Determinea) qcarnot wcarnot

b) Diminished qR and excess w for real cycle caused by throttling and superheat horn

c) ηR

Comparison Between Single-Stage and Carnot Cycles

• Figure 3.6

carnot

carnotR

wAA

qA

21

2

1

1

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)