ChemE 260 Heat Pump Systems

Gas Refrigeration Systems

May 31, 2005

Dr. William BaratuciSenior Lecturer

Chemical Engineering Department

University of Washington

TCD 10: D & ECB 10: 6 & 8

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Heat Pumps - ReviewHeat Pumps - Review

• Goal: Transfer into a hot reservoir

• Requires work and must take in heat from cold a reservoir

H HHP

cycle H C C H

Q Q 1COP

W Q Q 1 Q / Q

• HP’s and Ref’s operate with thermal reservoirs at very similar temperatures– As a result, they use the same refrigerants. R-134a is popular.

• Heat pump operate on essentially the same cycle as a V-C refrigerator, only in reverse.

BaratuciChemE 260May 31, 2005

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Process Flow and TS DiagramsProcess Flow and TS Diagrams

BaratuciChemE 260May 31, 2005

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The Reversing ValveThe Reversing Valve

• Home heat pumps can function as both heating and air-conditioning systems

• Heat pump mode (HP)– Indoor air is the hot reservoir

• Air-conditioning mode (AC)– Indoor air is the cold reservoir

• Reversing Valve– Clever way to switch from HP to AC mode without

physically moving the HEX’s !

BaratuciChemE 260May 31, 2005

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Reversing Valve & TS DiagramReversing Valve & TS Diagram

• HP Mode:

• AC Mode:

BaratuciChemE 260May 31, 2005

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AS Gas Refrigeration CycleAS Gas Refrigeration Cycle• Less efficient than V-C Refrigeration Cycles• Lightweight• Capable of reaching temperatures below 100 K with a

regenerator.• Reverse Air-Standard Brayton Cycle• Air-Standard Assumptions

– Air is the working fluid and it behaves as an ideal gas.– The GRC is modeled as as a closed cycle.– All processes are internally reversible.

• Cold Air-Standard Assumption– The heat capacities of air are constant and always have the

values determined at 25oC.BaratuciChemE 260May 31, 2005

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Process Flow and TS DiagramsProcess Flow and TS Diagrams

• “Ideal” GRC– Internally reversible– Pump & Compressor are

adiabatic and internally reversible.

• Reverse Brayton Cycle• Heat exchange at constant

pressure

BaratuciChemE 260May 31, 2005

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IrreversibilitesIrreversibilites

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Regeneration Regeneration • The purpose of regeneration is to allow the GRC to reach the

lowest possible temperature.

– It does increase the COPR of the cycle, but that is not the main

objective.

• Key: “pre-cool” the turbine feed so that when it expands through the turbine, it will reach a lower temperature than without pre-cooling.

• Multiple regenerative GRC’s in series are used to reach temperatures below 100 K.

• Check out: http://www.stirling.nl

– They use the reverse Stirling Cycle, but it is similar to the reverse Brayton Cycle.

BaratuciChemE 260May 31, 2005

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Regeneration Flow DiagramRegeneration Flow Diagram

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Regeneration TS DiagramRegeneration TS Diagram

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Next Class …• Problem Session

• After that… PLANT TRIP !

• After that…– Course evaluations

– Prepare for Final Exam

• After that …– Final Exam, Tue 12/14 at 2:30 – 4:30 in Loew 201

• After that– Sleep, Freedom, Life, etc.

BaratuciChemE 260May 31, 2005