cheme 260 vapor-compression refrigeration cycles & enhanced vcr cycles may 27, 2005 dr. william...
TRANSCRIPT
ChemE 260 Vapor-Compression Refrigeration Cycles
& Enhanced VCR Cycles
May 27, 2005
Dr. William BaratuciSenior Lecturer
Chemical Engineering Department
University of Washington
TCD 10: A & BSO 8: 8
2
ApplicationsApplications
• 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
BaratuciChemE 260May 27, 2005
3
Ammonia Absorption RefrigerationAmmonia 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.nh3tech.org/absorption.htmlfor more details.
BaratuciChemE 260May 27, 2005
4
NH3 Absorption Refrigeration Diagram
BaratuciChemE 260May 27, 2005
5
Ideal V-C Refrigeration CycleIdeal V-C Refrigeration Cycle• The Ideal V-C Refrigeration Cycle
– Step 1-2: Evaporator Heat absorbed at constant pressure
– Step 2-3 Compressor Isentropic compression– Step 3-4: Condenser Heat rejected at constant
pressure– Step 4-1: Expansion Valve Isenthalpic expansion
C 2 1R
cycle 3 2
ˆ ˆH HQCOP
ˆ ˆH HW
BaratuciChemE 260May 27, 2005
6
TS DiagramTS Diagram
BaratuciChemE 260May 27, 2005
7
Irreversibilities in V-C Refrig. CycleIrreversibilities in V-C Refrig. Cycle
BaratuciChemE 260May 27, 2005
8
Refrigerant SelectionRefrigerant Selection
• Selection Criteria– Cost
– P* vs T Relationship
– Hvap
– Chemical Stability
– Corrosiveness
– Toxicity
– Flammability
BaratuciChemE 260May 27, 2005
9
P*– T RelationshipP*– 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 = 10oC
• 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) = 10oC
• 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.
BaratuciChemE 260May 27, 2005
10
Cascade VCR CyclesCascade VCR Cycles
• Cascade V-C Refrigeration Cycles• 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 temperaturesC HT T
BaratuciChemE 260May 27, 2005
11
Cascade VCR Flow DiagramCascade VCR Flow Diagram
BaratuciChemE 260May 27, 2005
12
Cascade VCR TS DiagramCascade VCR TS Diagram
BaratuciChemE 260May 27, 2005
13
Multi-Stage VCR CyclesMulti-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
C HT T
BaratuciChemE 260May 27, 2005
14
Multistage VCR Flow DiagramMultistage VCR Flow Diagram
BaratuciChemE 260May 27, 2005
15
Multistage VCR TS DiagramMultistage VCR TS Diagram
BaratuciChemE 260May 27, 2005
16
Next Class …• Heat Pumps
– Almost identical to VCR Cycles– The objective is QH not QC
– Home heat pumps function as both heating and air-conditioning 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.
BaratuciChemE 260May 27, 2005