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Refrigeration Cycles -

ME 200 – Thermodynamics 1

Chapter 10 In-Class Notesfor Spring 2004

Lectures 42 and 43

Vapor Refrigeration Cycles

Refrigeration Cycles

• Carnot Vapor Refrigeration Cycles

• Vapor Compression Cycle

• Working fluid is vaporized & condensed

during cycle

• Working fluid is termed the refrigerant

• Refrigeration cycle can provide cooling

(air conditioning and refrigeration) or heating (heat pump)

• Refrigeration cycle can be “powered” by

work or heat input, but we’ll focus on

work driven cycles

Refrigeration Cycles - Page 2

Cooling vs. Heating

Some Definitions

• Cooling Capacity: maximum rate of heat removal

from the refrigerated space by refrigerator

• Heating Capacity: maximum rate of heat addition

to heated space by heat pump• 1 ton of Refrigeration: capacity of a refrigerator

that can freeze 1 ton of water in 24 hours (12,000

Btu/hr, 211 KJ/min)

innet

L R

W

Q

COP ,

=

innet

H HP

W

QCOP

,

=

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Carnot Vapor Refrigeration Cycle

Implementation Issues

• Difficult to compressor and expand a 2-phase

mixture

• Need temperature differences between source

and evaporator and between condenser and sink

All processes aretotally reversible


Ideal Vapor Compression Refrigeration

• Replace turbine with throttling device

• Compressor operates with superheated vapor

• Ideal compressor is reversible & adiabatic

• Irreversibilities associated with throttle and finite

temperature differences for heat transfer

• Basis for most air conditioners, refrigerators,dehumidifiers, and heat pumps

• Common refrigerants are R134a for refrigerators,

R22 and R410a for air conditioners

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Domestic Refrigerator

12

41

hh

hh

W

QCOP

in

L R

−

−==

T

s

2

1

3

4

Tcond

Tevap

TH

TL

subcooling

∆Tsc

superheat: ∆Tsh

More Realistic Behavior


Refrigeration Cycle Analysis

Typical Assumptions

• specified evaporating (Tevap) and condensing (Tcond)

temperatures

• specified superheat into compressor (0 for ideal cycle)

• specified subcooling out of condenser (0 for ideal cycle)

• constant pressure throughout heat exchangers

• negligible ke and pe changes for all components

• adiabatic throttling valve

• adiabatic compressor with specified isentropic efficiency

(100% efficient for ideal cycle)

Compressor Inlet State

P1 = Pevap h1 = hg @ Tevap

Pevap = Psat @ Tevap

Compressor Outlet State

P2 = Pcond

Pcond = Psat @ Tcond

h2s

= h @ P2

& s2

= s1

C

s hhhh

η

)( 12

12

−+=

s

T 2

Tcond

Tevap

P

1

2s

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Refrigeration Cycle Analysis

Condenser Outlet State

P3 = Pcond

h3 = hf @ Tcond

Evaporator Inlet State

P4 = Pevap h4 = h3

Coefficient of Performance

P

h

3

4

Pcond

Pevap

P

h

23

4 1

Pcond

PevapCOP h h

h h=

refrigerating effect

specific work

1 4

2 1

−

−

678

123

T

s

2

Tcond

Tevap

TH

TL

3

4 1

Comparison to Carnot


Vapor Compression Cycle Example

Given: Household Freezer Tinside = 0 F, Troom = 80 F

Tevap = -15 F, Tcond = 95 F

Find: COPR for a) Carnot Cycle, b) Ideal Vapor

Compression Cycle, c) Vapor Compression

Cycle with ηC = 0.8, 5 F of superheat, 5 F of

subcooling

Refrigerant flow rate and compressor power

for part (c) with a cooling capacity of 0.25

tons

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Refrigeration Cycles - Page 9 Refrigeration Cycles - Page 10

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Refrigeration Cycles - Page 11 Refrigeration Cycles - Page 12

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