ChemE 260 Heat Pump Systems Gas Refrigeration Systems May 31, 2005 Dr. William Baratuci Senior Lecturer Chemical Engineering Department University of Washington TCD 10: D & E CB 10: 6 & 8

2 Heat Pumps - Review Goal: Transfer into a hot reservoir Requires work and must take in heat from cold a reservoir 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

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 HEX’s ! 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 25 o C. Baratuci ChemE 260 May 31, 2005

7 Process Flow and TS Diagrams “Ideal” GRC –Internally reversible –Pump & Compressor are adiabatic and internally reversible. Reverse Brayton Cycle Heat exchange at constant pressure 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 COP R 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: –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 2:30 – 4:30 in Loew 201 After that –Sleep, Freedom, Life, etc. Baratuci ChemE 260 May 31, 2005