Chem E 260 VaporCompression Refrigeration Cycles Enhanced VCR

Chem. E 260 Vapor-Compression Refrigeration Cycles & Enhanced VCR Cycles Dr. William Baratuci Senior Lecturer Chemical Engineering Department University of Washington TCD 10: A & B SO 8: 8 May 27, 2005

Applications • V-C Refrigeration Cycle – Food preservation – Air conditioning – Industrial cooling processes • Gas Refrigeration Cycle – Air conditioning on passenger aircraft – Cryogenic refrigeration (very low temperatures, < 150 K) • Thermoelectric Refrigeration Systems – Peltier Effect – Very small and lightweight – Very low efficiency Baratuci Chem. E 260 May 27, 2005 2

Ammonia Absorption Refrigeration • Ammonia Absorption Refrigeration – No moving parts – No work input – Well-suited to remote and portable applications • Locations where electric power is not available • RV’s and campers • Invented by a student in Sweden • See external flow diagram or visit: http: //www. nh 3 tech. org/absorption. html for more details. Baratuci Chem. E 260 May 27, 2005 3

NH 3 Absorption Refrigeration Diagram Baratuci Chem. E 260 May 27, 2005 4

Ideal V-C Refrigeration Cycle • The Ideal V-C Refrigeration Cycle – – Baratuci Chem. E 260 May 27, 2005 Step 1 -2: Step 2 -3 Step 3 -4: Step 4 -1: Evaporator Compressor Condenser Expansion Valve Heat absorbed at constant pressure Isentropic compression Heat rejected at constant pressure Isenthalpic expansion 5

TS Diagram Baratuci Chem. E 260 May 27, 2005 6

Irreversibilities in V-C Refrig. Cycle Baratuci Chem. E 260 May 27, 2005 7

Refrigerant Selection • Selection Criteria – – – – Baratuci Chem. E 260 May 27, 2005 Cost P* vs T Relationship DHvap Chemical Stability Corrosiveness Toxicity Flammability 8

P*– T Relationship • Condenser – Tsat(Phigh) > Thot res because the working fluid must reject heat to the hot reservoir in the condenser. – Rule of thumb is at least: Tsat(Phigh) - Thot res = 10 o. C • Evaporator – Tsat(Plow) < Tcold res because the working fluid must absorb heat from the cold reservoir in the evaporator. – Rule of thumb is at least: Tcold res - Tsat(Plow) = 10 o. C • Vapor Pressure (Saturation Pressure) – Not too high: makes equipment expensive and dangerous. – Not too low: must be greater than atmospheric pressure • Air leaks into the system drastically reduce COPR. Baratuci Chem. E 260 May 27, 2005 9

Cascade VCR Cycles • Cascade V-C Refrigeration Cycles • • Baratuci Chem. E 260 May 27, 2005 Two separate refrigeration cycles Analogous to Binary Vapor Power Cycles One provides cooling to the other Usually two different refrigerants High COPR High cost Can reach very low temperatures 10

Cascade VCR Flow Diagram Baratuci Chem. E 260 May 27, 2005 11

Cascade VCR TS Diagram Baratuci Chem. E 260 May 27, 2005 12

Multi-Stage VCR Cycles • Similar to Cascade V-C Refrigeration • Two cycles use the same refrigerant • Instead of exchanging heat between two cycles, the refrigerant streams are mixed. • This is more efficient than heat exchange. • COPR increases (slightly better than Cascade VCR) • Cost increases (but not as expensive as Cascade VCR) • Can reach very low temperatures • Cannot reach as temperatures as cold as Cascade VCR Baratuci Chem. E 260 May 27, 2005 13

Multistage VCR Flow Diagram Baratuci Chem. E 260 May 27, 2005 14

Multistage VCR TS Diagram Baratuci Chem. E 260 May 27, 2005 15

Next Class … • Heat Pumps – Almost identical to VCR Cycles – The objective is QH not QC – Home heat pumps function as both heating and airconditioning systems • The key is the Reversing Valve • Gas Refrigeration Systems – Less efficient, but very lightweight • Passenger aircraft air-conditioning – Brayton Cycle operating in reverse • All of the improvements to the Brayton Cycle can be used. – Capable of reaching very low, even cryogenic temperatures • Can be used to liquefy many gases. Baratuci Chem. E 260 May 27, 2005 16