HW2

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HW2

• AHU problems
• Book 8.5, 8.25, 8.27, 8.28, 8.22
•   
• Cooling Cycles Problems
• - Book 3.1 (page 69),  - Book 3.5 ((page 70),
  - Out of book Same like 3.5 for R22 with no
  intercooler- Book 3.9 (pages 70-71)

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Objectives

• Learn about Cooling towers
• Cooling cycles

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Cooling Tower

• Similar to an evaporative cooler, but the purpose
  is often to cool water
• Widely used for heat rejection in HVAC systems
• Also used to reject industrial process heat

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Cooling Tower
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Solution

• Can get from Stevens diagram (page 272)
• Can also be used to determine
• Minimum water temperature
• Volume of tower required
• Can be evaluated as a heat exchanger by
  conducting NTU analysis

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Summary

• Heat rejection is often accomplished with devices
  that have direct contact between air and water
• Evaporative cooling
• Can construct analysis of these devices
• Requires parameters which need to be measured for
  a specific system

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Vapor Compression Cycle
Expansion Valve
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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

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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

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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)

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Carnot Vapor-Compression Cycle

• Figure 3.2

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Get Real

• 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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Area Analysis of Work and Efficiency
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Comparison Between Single-Stage and Carnot Cycles
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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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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