ChemE 260 Vapor-Compression Refrigeration Cycles

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ChemE 260 Vapor-Compression Refrigeration
Cycles Enhanced VCR Cycles

• Dr. William Baratuci
• Senior Lecturer
• Chemical Engineering Department
• University of Washington
• TCD 10 A BSO 8 8

May 27, 2005
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Applications

• V-C Refrigeration Cycle
• Food preservation
• Air conditioning
• Industrial cooling processes
• Gas Refrigeration Cycle
• Air conditioning on passenger aircraft
• Cryogenic refrigeration (very low temperatures, lt
  150 K)
• Thermoelectric Refrigeration Systems
• Peltier Effect
• Very small and lightweight
• Very low efficiency

Baratuci ChemE 260 May 27, 2005
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Ammonia Absorption Refrigeration

• Ammonia Absorption Refrigeration
• No moving parts
• No work input
• Well-suited to remote and portable applications
• Locations where electric power is not available
• RVs and campers
• Invented by a student in Sweden
• See external flow diagram or visithttp//www.nh3
  tech.org/absorption.htmlfor more details.

Baratuci ChemE 260 May 27, 2005
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NH3 Absorption Refrigeration Diagram
Baratuci ChemE 260 May 27, 2005
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Ideal V-C Refrigeration Cycle

• The Ideal V-C Refrigeration Cycle
• Step 1-2 Evaporator Heat absorbed at constant
  pressure
• Step 2-3 Compressor Isentropic compression
• Step 3-4 Condenser Heat rejected at constant
  pressure
• Step 4-1 Expansion Valve Isenthalpic expansion

Baratuci ChemE 260 May 27, 2005
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TS Diagram
Baratuci ChemE 260 May 27, 2005
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Irreversibilities in V-C Refrig. Cycle
Baratuci ChemE 260 May 27, 2005
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Refrigerant Selection

• Selection Criteria
• Cost
• P vs T Relationship
• DHvap
• Chemical Stability
• Corrosiveness
• Toxicity
• Flammability

Baratuci ChemE 260 May 27, 2005
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P T Relationship

• Condenser
• Tsat(Phigh) gt Thot res because the working fluid
  must reject heat to the hot reservoir in the
  condenser.
• Rule of thumb is at least Tsat(Phigh) - Thot res
  10oC
• Evaporator
• Tsat(Plow) lt Tcold res because the working fluid
  must absorb heat from the cold reservoir in the
  evaporator.
• Rule of thumb is at least Tcold res -
  Tsat(Plow) 10oC
• Vapor Pressure (Saturation Pressure)
• Not too high makes equipment expensive and
  dangerous.
• Not too low must be greater than atmospheric
  pressure
• Air leaks into the system drastically reduce COPR.

Baratuci ChemE 260 May 27, 2005
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Cascade VCR Cycles

• Cascade V-C Refrigeration Cycles
• Two separate refrigeration cycles
• Analogous to Binary Vapor Power Cycles
• One provides cooling to the other
• Usually two different refrigerants
• High COPR
• High cost
• Can reach very low temperatures

Baratuci ChemE 260 May 27, 2005
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Cascade VCR Flow Diagram
Baratuci ChemE 260 May 27, 2005
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Cascade VCR TS Diagram
Baratuci ChemE 260 May 27, 2005
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Multi-Stage VCR Cycles

• Similar to Cascade V-C Refrigeration
• Two cycles use the same refrigerant
• Instead of exchanging heat between two cycles,
  the refrigerant streams are mixed.
• This is more efficient than heat exchange.
• COPR increases (slightly better than Cascade VCR)
• Cost increases (but not as expensive as Cascade
  VCR)
• Can reach very low temperatures

• Cannot reach as temperatures as cold as Cascade
  VCR

Baratuci ChemE 260 May 27, 2005
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Multistage VCR Flow Diagram
Baratuci ChemE 260 May 27, 2005
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Multistage VCR TS Diagram
Baratuci ChemE 260 May 27, 2005
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Next Class

• Heat Pumps
• Almost identical to VCR Cycles
• The objective is QH not QC
• Home heat pumps function as both heating and
  air-conditioning systems
• The key is the Reversing Valve
• Gas Refrigeration Systems
• Less efficient, but very lightweight
• Passenger aircraft air-conditioning
• Brayton Cycle operating in reverse
• All of the improvements to the Brayton Cycle can
  be used.
• Capable of reaching very low, even cryogenic
  temperatures
• Can be used to liquefy many gases.

Baratuci ChemE 260 May 27, 2005