Objectives - PowerPoint PPT Presentation

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Objectives

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... System components Large building system Chiller Compressors Reciprocating compressor Scroll compressor Screw compressor Rotary Compressor Refrigerants What ... – PowerPoint PPT presentation

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Title: Objectives


1
Objectives
  • Cooling Cycles
  • Examples
  • Cooling system components
  • Refrigerants

2
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

5
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

6
(No Transcript)
7
Comparison Between Single-Stage and Carnot Cycles
  • Figure 3.6

8
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

9
Two stage systems
10
Multistage Compression Cycles
  • Combine multiple cycles to improve efficiency
  • Prevents excessive compressor discharge
    temperature
  • Allows low evaporating temperatures (cryogenics)

11
Example 1
  • R-22 condensing temp of 30 C and evaporating
    temp of 0C
  • Determine
  • qcarnot wcarnot
  • COPcarnot
  • ?R

12
Example 2
  • R-22 condensing temp of 60 C and evaporating
    temp of 0C
  • Determine the ?R

13
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

14
Systems residential
Outdoor Air
Indoor Air
15
Large building cooling systems(chillers with
cooling towers)
Cooling tower
Weather parameters
DBT30oC WBT20oC
25oC
35oC
Water
P
refrigerant
6oC
11oC
16
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
17
Chiller
18
Compressors
19
Reciprocating compressor
20
Scroll compressor
http//www.youtube.com/watch?vf_6xolDoqs0
21
Screw compressor
http//www.youtube.com/watch?vxO7IhhzImMUfeature
related
22
Rotary Compressor
23
Refrigerants
24
What are desirable properties of refrigerants?
  • Pressure and boiling point
  • Critical temperature
  • Latent heat of vaporization
  • Heat transfer properties
  • Viscosity
  • Stability

25
In Addition.
  • Toxicity
  • Flammability
  • Ozone-depletion
  • Greenhouse potential
  • Cost
  • Leak detection
  • Oil solubility
  • Water solubility
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