Objectives Cooling Cycles –Examples Cooling system components Refrigerants

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

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 )

Real Cycles Assume no heat transfer or potential or kinetic energy transfer in expansion valve COP = (h 3 -h 2 )/(h 4 -h 3 ) Compressor displacement = mv 3

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

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)

Example 1 R-22 condensing temp of 30 °C and evaporating temp of 0°C Determine a) q carnot w carnot b) COP carnot c) η R

Example 2 R-22 condensing temp of 60 °C and evaporating temp of 0°C Determine the η R

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 η R 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 25 o C P Weather parameters DBT=30 o C WBT=20 o C 35 o C 11 o C Water refrigerant 6oC6oC

System components Large building system Plate heat exchanger Water form cooling tower Water to cooling tower 6oC6oC11 o C Water to building Water from building 25 o C35 o C

Chiller

Compressors

Reciprocating compressor

Scroll compressor

Screw compressor

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