Department of Mechanical EngineeringME 322 – Mechanical Engineering

Thermodynamics

Lecture 29

The Vapor Compression Refrigeration (VCR) Cycle

Refrigerator used for Cooling

CCOPinE

cycle

QW

energy sought

energy that costs

The concept of an efficiency being greater than 100% makes people uneasy. Therefore, the conversion efficiency for a refrigerator is called the Cooling Coefficient of Performance (COPC). A refrigeration system that is used for cooling is called a refrigerator.

Observation: may be >1 100%E E R

HT

LT

2

Refrigerator used for Heating

HCOPoutE

cycle

QW

energy sought

energy that costs

The concept of an efficiency being greater than 100% makes people uneasy. Therefore, the conversion efficiency for a refrigerator is called the Heating Coefficient of Performance (COPH). A refrigeration system that is used for cooling is called a heat pump.

Observation: may be >1 100%E E R

HT

LT

3

Analysis of the Carnot Refrigerator

R1COP

/ 1in in

thcycle out in out in

Q QW Q Q Q Q

For the Refrigeration cycle …

H

H, H,

1COP1 /

1 1COP COP1 / 1 /

out outth

cycle out in in out

HCarnot Carnot

in out L H H Lrev

Q QW Q Q Q Q

TQ Q T T T T

For the Heat Pump cycle …R

HT

LT

R,1 1COP

/ 1 / 1Carnotout in H LrevQ Q T T

R,COP L

CarnotH L

TT T

4

Terminology• Refrigeration cycle

– The cycle is operating in a refrigeration cycle• The goal is to keep the cold space cold

– Transfer heat from a low-temperature source to the cycle

• Heat pump cycle– The cycle is operating in a refrigeration cycle

• The goal is to keep the hot space hot– Transfer heat to a high-temperature sink from the cycle

The words refrigeration or heat pump define the goal of the cycle.

5

The Vapor Compression Refrigeration Cycle

6

R

HT

LT

The Vapor Compression Refrigeration Cycle• Two phase changes

– Boiling (evaporator)– Condensing (condenser)

• Low temperature boiling fluids

Working fluid = Refrigerant

7

RefrigerantsMost refrigerants are halogenated hydrocarbons. The naming convention adopted by ASHRAE is,

R(a-1)(b+1)d = CaHbClcFd c = 2(a – 1) – b – d

1 0 11 2 1

22 1

2 1 1 1 2 1

a ab bdc a b d

c

Example: R22 (R022)

C

H

F

Cl

F chlorodifluoromethane

8

The Pressure-Enthalpy Diagram

9

Vapor Compression Refrigeration Analysis

2 1m h h

2 3m h h

1 4m h h

3 4h h

Performance

CCOP in

c

QW

10

P

h

The Ideal VCR Cycle on the P-h Diagram

2 1m h h

2 3m h h

1 4m h h

3 4h h1

23

4

1-2: Isentropic compression2-3: Isobaric heat rejection3-4: Isenthalpic expansion4-1: Isobaric heat addition

/in inq Q m /c cw W m

/out outq Q m

11

Refrigeration Effect and Capacity

1 4inq h h Refrigeration Effect:

Refrigeration Capacity: 1 4inQ m h h

Refrigeration capacity is often expressed in tons of refrigeration. Definition …

1 ton of refrigeration is the steady state heat transfer rate required to melt 1 ton of ice at 32°F in 24 hours.

1 ton = 12,000 Btu/hr = 3.516 kW

12

VCR Cycle IrreversibilitiesP

h

1

22s

3

4Isentropic efficiency of the compressor

Pressure drop through the condenser

Pressure drop through the evaporator

13

Practical VCR CycleP

h

1

22s

3

4

3T SCT

1T

SET

SCT = Saturated Condensing TemperatureDSC = Degrees of Subcooling = SCT – T3

SET = Saturated Evaporating TemperatureDSH = Degrees of Superheat = T1 – SET

Subcooling increases the refrigeration capacity

Superheating provides a dry vapor at the compressor inlet

14

Example

15

Given: A vapor compression refrigeration cycle is operating with a saturated evaporating temperature of -20°F and a saturated condensing temperature of 80°F. The refrigerant, R22, leaves the condenser as a saturated liquid and enters the compressor with 5 degrees of superheat. The pressure drops through the evaporator and condenser can be considered negligible. The compressor has an isentropic efficiency of 85%. The cycle has a capacity of 15 tons.

Find: (a) the mass flow rate of the R22 (lbm/hr)(b) the power requirement of the compressor (hp)(c) the coefficient of performance of the cycle

Example

16

P

h

1

23

420 FSET

80 FSCT

15 tons1 5 FT SET

1 5 FT SET

4 20 FT

3 80 FT

2s

Example

17

15 tons

1 5 FT SET

4 20 FT

3 80 FT

P

h

1

23

420 FSET

80 FSCT

1 5 FT SET

2s

Example

18

15 tons

1 5 FT SET

4 20 FT

3 80 FT

Strategy: Build the property table then do the thermodynamics!

The high and low pressures in the cycle can be found since the saturation temperatures are given.

The pressures at all four states are known!

P

h

1

23

420 FSET

80 FSCT

1 5 FT SET

2s

Example

19

15 tons

1 5 FT SET

4 20 FT

3 80 FT

P

h

1

23

420 FSET

80 FSCT

1 5 FT SET

2s

Example

20

15 tons

1 5 FT SET

4 20 FT

3 80 FT

P

h

1

23

420 FSET

80 FSCT

1 5 FT SET

2s

The property table is complete!

Example

21

15 tons

1 5 FT SET

4 20 FT

3 80 FT

P

h

1

23

420 FSET

80 FSCT

1 5 FT SET

2s

EES Solution (Key Variables)

12,000 Btu Btu15 tons 180,000ton-hr hrinQ

2545 Btu Btu24.45 hp 62, 225hp-hr hrcW

Comparison (same units) …