RAC_Lect2-1

Week-2

Refrigeration Cycle

Refrigeration cycles can be categorized

Gas cycle or air cycle

Vapors cycle

Vapor Cycle In a vapor cycle the working fluid undergoes phase change and refrigeration effect is due to the vaporization of refrigerant liquid. The required mass flow rates for a given refrigeration capacity will be much smaller compared to a gas cycle.

subdivided into vapor compression systems, vapor absorption systems, vapor jet systems

Air Cycle In a typical gas cycle, the working fluid (a gas) does not undergo phase change In gas cycles, heat rejection and refrigeration take place as the gas undergoes sensible cooling and heating.

Applications: Passenger air craft, jet aircraft Missiles

Air Refrigerator working on Reversed Carnot Cycle

Carnot Cycle is the ideal cycle with 100% efficiency.

It is the ideal Engine and if we reverse it, it will be the ideal Refrigeration Cycle.

Carnot heat engine

T-s Diagram

T

s

1

2 3 QH

4

QL

Cooling space at TL

QL

Atmosphere at TH

QH

Win

Condenser

Evaporator

Compressor Turbine

3 2

4 1

1-2 : Isentropic Compression

2-3 : Isothermal Heat rejection

3-4 : Isentropic Expansion

4-1 : Isothermal Heat addition

Carnot refrigeration cycle

Carnot cycle consists of reversible processes which make its

efficiency higher than could be achieved in an actual cycle

Why? Discuss the Carnot cycle if it is unattainable idea?

There are two reasons:

I. It serves as a standard of comparison

II. It provides a convenient guide to the temperatures that should

be maintained to achieve maximum effectiveness

Coefficient of Performance (COP)

Unlike Heat engine, we use COP in refrigeration system because the output in process 2-3 is usually wasted.

Net work= Heat absorbed in evaporator-Heat rejected in condenser

The heat transferred during isothermal processes 2-3 and 4-1 are given by:

Net Work output =Heat rejected- Heat absorbed

Wnet=Q2-3 Q4-1 Wnet=Th(S2 S3) TL(S1-S4)

Lh

L

Lh

LCarnot

netCarnot

TTT

SSTSSTSSTCOP

WQ

inputWorkabsorbedHeatCOP

=

=

==

)()()(

..

4132

41

14

COP can be maximized by maximizing TL (target temperature)

and minimizing Th (ambient temperature)

In summer performance of refrigerator decreases because Th increases.

In Refrigerator the value of TL is less than for AC, so COP of AC is higher than COP of Refrigerator.

Lh

LCarnot TT

TCOP

=

1. Higher temperature should be more than the temperature of

cooling air to which heat is to be rejected

2. Lower temperature should be less than the temperature of

substance to be cooled

Temperature Limitation

What control do we have? We can concentrate on keeping the T as small as possible. Reduction of T can be accomplished by increasing A or U in the equation

In order to decrease T to zero, either U or A would have to be infinite. Since infinite values of U and A would also require an infinite cost; the actual selection of equipment always stops short of reducing T to zero.

Difficulty of achieving isothermal heat transfer during processes 2-3 and 4-1. For a gas to have heat transfer isothermally, it is essential to carry out work transfer from or to the system when heat is transferred to the system (process 4-1) or from the system (process 2-3). This is difficult to achieve in practice. Frictional effects in compressor leads to irreversibility, hence completely isentropic compression is not possible to achieve. Perfect insulation cannot be made practically

Limitations of Carnot cycle:

Bell-Coleman or Reversed Brayton Cycle

Process 1-2: Reversible, adiabatic compression in a compressor

Process 2-3: Reversible, isobaric heat rejection in a heat

exchanger

Process 3-4: Reversible, adiabatic expansion in a turbine

Process 4-1: Reversible, isobaric heat absorption in a heat

exchanger

Process 1-2: Gas at low pressure is compressed isentropically from state 1 to state 2.

Process 2-3: Hot and high pressure gas flows through a heat exchanger and rejects heat sensibly and isobarically to a heat sink.

Process 3-4: High pressure gas from the heat exchanger flows through a turbine, undergoes isentropic expansion and delivers net work output.

Process 4-1: Cold and low pressure gas from turbine flows through the low temperature heat exchanger and extracts heat sensibly and isobarically from a heat source

]1[]1[

]1[

)()(

)()()(

..

4

14

3

23

4

14

4132

41

4132

41

=

=

==

TTT

TTT

TTT

TTTTTTCOP

TTCTTCTTC

doneWorkabsorbedHeatCOP

PP

P

4132

1

4

3

1

1

2

.,4

3..1

2

,21...

PPandPP

TTand

TT

processisentropicFor

PP

PP

==

==

Therefore COP of the cycle can be written as

Comparison of reverse Carnot and reverse Brayton cycle

COP of reverse Brayton cycle decreases as the pressure ratio rp increases

Week-2Slide Number 2Slide Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18