ChemE 260 Heat Pump Systems Gas Refrigeration Systems

1
ChemE 260 Heat Pump Systems Gas Refrigeration
Systems

• Dr. William Baratuci
• Senior Lecturer
• Chemical Engineering Department
• University of Washington
• TCD 10 D ECB 10 6 8

May 31, 2005
2
Heat Pumps - Review

• Goal Transfer into a hot reservoir
• Requires work and must take in heat from cold a
  reservoir

• HPs and Refs 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
3
Process Flow and TS Diagrams
Baratuci ChemE 260 May 31, 2005
4
The 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 HEXs !

Baratuci ChemE 260 May 31, 2005
5
Reversing Valve TS Diagram

• HP Mode

• AC Mode

Baratuci ChemE 260 May 31, 2005
6
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
7
Process Flow and TS Diagrams

• Reverse Brayton Cycle
• Heat exchange at constant pressure

• Ideal GRC
• Internally reversible
• Pump Compressor are adiabatic and internally
  reversible.

Baratuci ChemE 260 May 31, 2005
8
Irreversibilites
Baratuci ChemE 260 May 31, 2005
9
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 GRCs 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
10
Regeneration Flow Diagram
Baratuci ChemE 260 May 31, 2005
11
Regeneration TS Diagram
Baratuci ChemE 260 May 31, 2005
12
Next Class

• Problem Session
• After that PLANT TRIP !
• After that
• Course evaluations
• Prepare for Final Exam
• After that
• Final Exam, Tue 12/14 at 230 430 in Loew 201
• After that
• Sleep, Freedom, Life, etc.

Baratuci ChemE 260 May 31, 2005