Objectives

• Cooling Cycles – Examples

• Cooling system components

• Refrigerants

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 CycleNo 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)

• 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)

Example 1

• R-22 condensing temp of 30 °C and evaporating temp of 0°C

• Determinea) qcarnot wcarnot

b) COPcarnot

c) ηR

Example 2

• R-22 condensing temp of 60 °C and evaporating temp of 0°C

• Determine the ηR

Example 3• Two stage compressor • R-22 condensing temp of 60 °C • Evaporating temp of compressor A

30°C and for compressor B 0°C.

• Determine the ηR

Assume that condenser temperature for compressor B is the same like evaporation temperature of compressor A

Systems: residential

Indoor Air

Outdoor Air

Large building cooling systems(chillers with cooling towers)

Cooling tower

25oC

P

Weather parameters

DBT=30oCWBT=20oC

35oC

11oC

Waterrefrigerant

6oC

System componentsLarge building system

Plate heat exchanger

Water formcooling tower

Water tocooling tower

6oC 11oC

Water to building

Water from building

25oC 35oC

Chiller

Compressors

Reciprocating compressor

Scroll compressor

http://www.youtube.com/watch?v=f_6xolDoqs0

Screw compressor

http://www.youtube.com/watch?v=xO7IhhzImMU&feature=related

Rotary Compressor

Refrigerants

What are desirable properties of refrigerants?

• Pressure and boiling point• Critical temperature• Latent heat of vaporization• Heat transfer properties• Viscosity• Stability

In Addition….

• Toxicity• Flammability• Ozone-depletion• Greenhouse potential• Cost• Leak detection• Oil solubility• Water solubility