Principles of Vapour Compression Refrigeration

 Learning Outcomes At the end of this lecture you should be able to1. Describe the p-h chart and be able to represent the

cycle on this2. Describe the terms refrigeration duty, heat of rejection,

refrigeration effect, work input and coefficient of performance

3. Determine the refrigeration effect and coefficient of performance, work input of a simple refrigeration cycle on a pressure – enthalpy chart

The cycle recapped

Condenser Evaporator Cooled medium

Cooling medium

Expansion device

Compressor

High pressure Low pressure

1

4 3

2

Compressors

• Pump • Compress

Reciprocating compressor

Scroll compressor

Screw compressor

Plate Heat exchanger Evaporators

•

                                                                         

Shell and tube Evaporators

•

Plate fin evaporator

•

                                                                         

Plate Heat exchanger condensers

•

                                                                         

Shell and tube condensers

•

Air cooled condensers

•

                                                                         

Evaporative Condensers

Expansion Device

• To expand the fluid• To control superheat

at compressor inlet

The (reversible) Cycle

pressure

Enthalpy

High pressure

Low pressure

Inlet to compressor

- superheated

Isentropic compression

CondensingSome subcooling

Isenthalpic

expansion

Evaporation

Something for you

Condenser Evaporator

P=40oC P=5oCT=15oCT=85oC

Cooled medium

Cooling medium

T=A

T=F

T=B T=C

T=13 oC

D

E

Something for you1. What pressure is the SUCTION pressure gauge reading?

……………………. bar2. What pressure is the DISCHARGE pressure gauge reading?

…………………. bar3. What is the degree of Suction SUPERHEAT?…………………. K4. What is the degree of Discharge SUPERHEAT?…………………. K5. What do you consider is a reasonable temperature for the condenser

cooling medium 'A'? 0, 10, 25, 40 oC6. What reading would you expect at thermometer 'B'? 42, 38, 21, 5 oC7. What reading would you expect at thermometer 'C'?-5, 5, 35, 13 oC  8. What would you see in sight glass 'D'?a. Gas only, b. Liquid and a few bubbles, c. Mainly bubbles (foam)d. Liquid only,  9. What would you see in sight glass 'E'?

The 1st law of thermodynamics

• Energy is not destroyed or created• Conservation of energy

The thermodynamic boundary

Condenser Evaporator Cooled medium

Cooling medium

Expansion device

Compressor

High pressure Low pressure

1

4 3

2

The First Law of Thermodynamics

Pump

inout

m hout m hin

W

Q - W= m (hout – hin) + KE + PE

Q

The 1st law of thermodynamics

• Q is +tive if heat is transferred to the system ie evaporator

• Q is -tive if heat is transferred from the system ie condenser

• W is +tive if work done by the system is ie turbine

• W is -tive if work done on the system is ie compressor

Applying the first law to the Compressor

Compressor

12

m h2 m h1

W

Q= m (h2 – h1) +W

W is the power absorbed, or work input

First Law to the Condenser

Condenser

3

2

m h2

m h3

Q

Q= m (h3 – h2) +W

Q is the total heat of rejection

1st law to the expansion device

Expansion device

4 3 m h3

m h4

Q= m (h3 – h2) +W

4

1

First Law to the Evaporator

Evaporator

m h1

m h4

Q

Q= m (h1 – h4) +W

Q is the refrigeration duty or cooling capacity

(h1 – h4) is the refrigeration effect

COP – coefficient of performance

Q= cooling duty divided by power absorbed

COP is normally between 4 and 1

A worked example A vapour compression cycle operating with R134a is

used to chill water from 12 to 6oC. The condenser rejects heat to ambient air at 27oC.

Choose realistic values for following cycle properties. Assume the compressor operates reversibly.

• Evaporating temperature•     Evaporating pressure•     The superheat at the compressor inlet •     Condensing temperature •     Condensing pressure•     The subcooling at the condenser outlet.

A worked example Plot the cycle on p-h chart and determine the enthalpy at the         compressor inlet         compressor outlet,         evaporator inlet,         condenser outletAssuming that the refrigerant is flowing at 5kg/s and determine        refrigeration effect       work input       heat of rejection       coefficient of performance       water mass flow rate (Cp = 4.18 kJ/kgK)       air mass flow rate (Cp = 1.02 kJ/kgK

Can you Learning Outcomes At the end of this lecture you should be able to1. Describe the p-h chart and be able to represent the

cycle on this2. Describe the terms refrigeration duty, heat of rejection,

refrigeration effect, work input and coefficient of performance

3. Determine the refrigeration effect and coefficient of performance, work input of a simple refrigeration cycle on a pressure – enthalpy chart