Objectives • Cooling Cycles – Examples • Cooling system components • Refrigerants
Feb 24, 2016
Objectives
• Cooling Cycles – 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
• 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
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