ChemE 260
Heat Pump Systems
Gas Refrigeration Systems
Dr. William Baratuci
Senior Lecturer
Chemical Engineering Department
University of Washington
TCD 10: D & E
CB 10: 6 & 8
May 31, 2005
Heat Pumps - Review
• Goal: Transfer into a hot reservoir
• Requires work and must take in heat from cold a reservoir
COPHP
•
QH
QH
1



Wcycle Q H  QC 1  Q C / Q H
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.
Baratuci
ChemE 260
May 31, 2005
2
Process Flow and TS Diagrams
Baratuci
ChemE 260
May 31, 2005
3
The Reversing Valve
• Home heat pumps can function as both heating and airconditioning 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 !
Baratuci
ChemE 260
May 31, 2005
4
Reversing Valve & TS Diagram
•
HP Mode:
•
AC Mode:
Baratuci
ChemE 260
May 31, 2005
5
AS 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.
Baratuci
ChemE 260
May 31, 2005
6
Process Flow and TS Diagrams
•
•
•
Baratuci
ChemE 260
May 31, 2005
Reverse Brayton Cycle
Heat exchange at constant
pressure
“Ideal” GRC
– Internally reversible
– Pump & Compressor are
adiabatic and internally
reversible.
7
Irreversibilites
Baratuci
ChemE 260
May 31, 2005
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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.
Baratuci
ChemE 260
May 31, 2005
9
Regeneration Flow Diagram
Baratuci
ChemE 260
May 31, 2005
10
Regeneration TS Diagram
Baratuci
ChemE 260
May 31, 2005
11
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.
Baratuci
ChemE 260
May 31, 2005
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