Chem E 260 Heat Pump Systems Gas Refrigeration

Chem. E 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 • HP’s and Ref’s operate with thermal reservoirs at very similar temperatures – As a result, they use the same refrigerants. R-134 a is popular. • Heat pump operate on essentially the same cycle as a V-C refrigerator, only in reverse. Baratuci Chem. E 260 May 31, 2005 2

Process Flow and TS Diagrams Baratuci Chem. E 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 Chem. E 260 May 31, 2005 4

Reversing Valve & TS Diagram • HP Mode: • AC Mode: Baratuci Chem. E 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 25 o. C. Baratuci Chem. E 260 May 31, 2005 6

Process Flow and TS Diagrams • Reverse Brayton Cycle • Heat exchange at constant pressure • Baratuci Chem. E 260 May 31, 2005 “Ideal” GRC – Internally reversible – Pump & Compressor are adiabatic and internally reversible. 7

Irreversibilites Baratuci Chem. E 260 May 31, 2005 8

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 Chem. E 260 May 31, 2005 9

Regeneration Flow Diagram Baratuci Chem. E 260 May 31, 2005 10

Regeneration TS Diagram Baratuci Chem. E 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 Chem. E 260 May 31, 2005 12