Objectives Cooling Cycles Cooling system components Refrigerants Examples Cooling system components Refrigerants

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 qcarnot wcarnot COPcarnot 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 Outdoor Air Indoor Air

Large building cooling systems (chillers with cooling towers) Weather parameters DBT=30oC WBT=20oC 25oC 35oC Water P refrigerant 6oC 11oC

System components Large building system Water to cooling tower Water form cooling tower 25oC 35oC Plate heat exchanger 6oC 11oC Water to building Water from building

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

Refrigerants What does R-12 mean? ASHRAE classifications From right to left ← # fluorine atoms # hydrogen atoms +1 # C atoms – 1 (omit if zero) # C=C double bonds (omit if zero) B at end means bromine instead of chlorine a or b at end means different isomer