Objectives

1
Objectives

• Cooling Cycles
• Examples
• Cooling system components
• Refrigerants

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Efficiency

• First Law
• Coefficient of performance, COP
• COP useful refrigerating effect/net energy
  supplied
• COP qr/wnet
• Second law
• Refrigerating efficiency, ?R
• ?R COP/COPrev
• Comparison to ideal reversible cycle

3
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)

4
Real Cycles

• Assume no heat transfer or potential or kinetic
  energy transfer in expansion valve
• COP (h3-h2)/(h4-h3)
• Compressor displacement mv3

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Example

• R-22 condensing temp of 30 C (86F) and
  evaporating temp of 0C (32 F)
• Determine
• qcarnot wcarnot
• Diminished qR and excess w for real cycle caused
  by throttling and superheat horn
• ?R

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Comparison Between Single-Stage and Carnot Cycles

• Figure 3.6

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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

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Two stage systems
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Multistage Compression Cycles

• Combine multiple cycles to improve efficiency
• Prevents excessive compressor discharge
  temperature
• Allows low evaporating temperatures (cryogenics)

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Example 1

• R-22 condensing temp of 30 C and evaporating
  temp of 0C
• Determine
• qcarnot wcarnot
• COPcarnot
• ?R

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Example 2

• R-22 condensing temp of 60 C and evaporating
  temp of 0C
• Determine the ?R

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Example 3

• Two stage compressor
• R-22 condensing temp of 60 C
• Evaporating temp of compressor A 30C and for
  compressor B 0C.
• Determine the ?R
• Assume that condenser temperature for compressor
  B is the same like evaporation temperature of
  compressor A

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Systems residential
Outdoor Air
Indoor Air
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Large building cooling systems(chillers with
cooling towers)
Cooling tower
Weather parameters
DBT30oC WBT20oC
25oC
35oC
Water
P
refrigerant
6oC
11oC
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System componentsLarge building system
Water to cooling tower
Water form cooling tower
25oC
35oC
Plate heat exchanger
6oC
11oC
Water to building
Water from building
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Chiller
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Compressors
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Reciprocating compressor
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Scroll compressor
http//www.youtube.com/watch?vf_6xolDoqs0
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Screw compressor
http//www.youtube.com/watch?vxO7IhhzImMUfeature
related
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Rotary Compressor
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Refrigerants
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What are desirable properties of refrigerants?

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

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In Addition.

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