Refrigeration Cycle COP Calculation

1
ATDCHB2
Learning Unit 6: Refrigeration

Introduction
• In practice, heat flows in direction
of decreasing temperature i.e.
from high-T regions to low-T
regions.
• Flow of heat from hot to cold
occurs in nature (spontaneously)
without requiring any devices.
• However, heat will not flow
spontaneously from a low-T
region (cold) to a high-T region
(hot).
• The transfer of heat from a low-T
region to a high-T region by itself
as it requires special devices
called refrigerators.
2
Spontaneous
Nonspontaneous

Refrigerators
3
• Refrigerators are cyclic devices and the
working fluid used in the refrigeration cycle is
called a refrigerant (next slide).
• Common refrigerants include
chlorofluorocarbons (CFCs), ammonia,
hydrocarbons (propane, ethane, ethylene,
etc.), carbon dioxide, air (in the air-
conditioning of aircraft) and even water (in
applications above the freezing point).

Refrigerators
4
•Refrigeration implies the maintenance of a temperature
below that of the surroundings
•Uses of refrigeration include:
• air conditioning, domestic fridges
• manufacture of ice (industrially)
• dehydration of gases
• lubricating oil purification
• low temperature reactions
• gas liquefaction, etc.

Different Methods of Refrigeration
1. By evaporation – refrigeration effect can be realized by evaporation of liquid.
2. By ice – ice is kept in between the walls of an insulated cabinet.
3. By expansion of air – expansion of air is accompanied by the cooling air due
to reduction in its temperature.
4. By throttling process – throttling of certain gases show the reduction in
temperature of gas after throttling.
5. By dry ice – CO2 in solid phase is termed as dry ice. Typical property of getting
transformed directly from solid phase to vapour phase at its sublimation
temperature of –78oC at atmospheric pressure.
6. Using liquid gases - liquid gases such as liquid N2, liquid CO2 are used for
refrigeration in refrigerated cargo vehicles. Liquid gases are expanded thereby,
causing its transformation from liquid to gas accompanied by lowering of
temperature of the refrigerated space.
7. Vapour Refrigeration System – based on vapour compression refrigeration
system.
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Refrigerants
• ‘Refrigerant’ is the working fluid used in
refrigeration systems or air conditioning
equipment having the capability of carrying
heat/ rejecting heat in the form of sensible
heat (or latent heat).
• Refrigerants possess the following
properties that enable the transformation of
liquid to gas (and vice-versa):
• High latent heats
• Low condensation points
• Chemically inert (safe to operate)
• During selection of refrigerant its chemical,
physical and other general properties are
being looked into along with refrigeration
cycle requirements and application.
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Refrigerants
• Refrigerants can be classified into 2 main categories:
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• Used directly as working fluids
• Undergo phase change, therefore primary
refrigerants are used in vapour
compression systems eg. R134a, R404a
Primary refrigerants Secondary refrigerants
• Are first cooled by primary refrigerants and
then used for imparting refrigeration
• Used for transporting low-T heat energy
from one place (the substance being
cooled) to another (evaporator) Eg. ‘brine’,
anti-freeze agents etc.
• Undergo change in their temperature by
absorbing heat and rejecting it at the
evaporator without any phase change.
• Offer advantages of ease of handling and
control, ease of maintaining temperature of
large buildings by controlling flow of
secondary refrigerants such as brine etc.
• They also eliminate requirement of long
primary refrigerant lines.
• All refrigerants are assigned with an internationally acceptable number such as R–12, R–
717 etc.

9
Example of Refrigerants (cont.)

10
Examples of Refrigerants (cont.)

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12

The Carnot Refrigerator &
Coefficient of Performance (COP)
• Heat engine cycles are absorption of heat into the system at a high
temperature, rejection of heat to the surroundings at a lower
temperature, and the production of work.
• In a continuous refrigeration process, the heat absorbed at a lower
temperature is continuously rejected to the surroundings at a higher
temperature
• So basically, a refrigeration cycle is a reversed heat engine cycle.
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14

• The Carnot cycle is an ideal cycle, therefore it is reversible
• For the cycle, ∆U of the working fluid is zero
• Therefore from 1st law: W = QH – QC
• Also: QH = TH ∆SH & QC = TC ∆SC
• The measure of the effectiveness of a refrigerator is its coefficient of
performance (COP) defined as:
Therefore
for a Carnot Cycle
15
W
Q
work
net
T
lower
the
at
absorbed
heat
COP C


C
H
C
C
C
H
H
C
C
C
H
C
T
T
T
S
T
S
T
S
T
Q
Q
Q
COP










 Best coefficient of performance (COP) is given by a cycle approaching
Carnot cycle operating between given temperature conditions.
 Such a cycle using wet vapour as working fluid is shown ((a) below)
including a T-S diagram (b).
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Coefficient of Performance (COP) (cont.)

Example
A refrigerator has working temperatures in the evaporator and
condenser coils of -30 and 32oC, respectively.
What is the maximum COP possible?
If the actual refrigerator has a COP of 0.75 of the maximum, calculate
the required power input for a refrigerating effect of 5 kW.
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Vapour-compression cycles
• ‘Liquefiable’ vapour is the most common type of refrigerant.
• Cycles making use of liquefiable vapour are used in most domestic and
large industrial applications.
• Evaporation and condensation processes take place when fluid is
receiving and rejecting latent heat at constant P and T inline with
reversed Carnot engine.
• Practical considerations have led to several modifications to the ideal
cycle as follows:
1. Replacing expansion engine with a throttle valve,
2. Modifying conditions at compressor inlet and
3. Undercooling condensed vapour
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•In practice, to transfer heat to a fluid at constant temperature
(as in a Carnot cycle) requires either
• infinite fluid flow rate
• isothermal work transfer
•It is easier to use evaporation and condensation
•In principle, one can use an air refrigeration cycle, however
the heat transfer coefficient for air is very low and Cp is small,
implying the need for a large flow rate
•One can use an expansion engine (delivering work) or an
expansion across a valve
•For pure fluids, condensation and evaporation is
approximately a constant temperature process.
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The Vapour Compression Cycle

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Steps
•1 – vapour compression, A  B (~ isentropic)
(A  B’ = irreversible ≠ constant S)
•2 – cooling of superheated vapour, B  E (~ constant P)
•3 – constant T condensation, E  C
•4 – liquid expansion:
a) through engine, C  D (~ isentropic)
or b) through valve, C  D’ (~ isenthalpic)
•5 – liquid evaporation in evaporator, D or D’  A, the entropy
increases producing saturated vapour at A
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23
Pressure – Enthalpy Diagram

•An expansion engine (eg. turbine) capable of handling a two
phase mixture (V+L) is expensive and not easily renewable
as a simple expansion valve or capillary.
•Work obtained from the expansion engine is small, therefore
domestic refrigerators use a valve or capillary.
•An expansion engine is feasible for large installations
24

25

But flow process across evaporator and condenser:
Therefore for condenser and evaporator, ∆H = Q
Hence
But
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)
0
,
( 



 U
cycle
for
Q
Q
Q
W C
H
net
S
W
Q
H
z
g
u







2
2
W
H
H
W
Q
COP D
A
C 


   
D
A
C
B
C
H
net H
H
H
H
H
Q
Q
Q
W 









Therefore
For an expansion valve: HC = HD’ (isenthalpic)
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   
D
A
C
B
D
A
H
H
H
H
H
H
COP





A
B
D
A
H
H
H
H
COP


 

A refrigerator uses refrigerant-134a as the working fluid and
operates on an ideal vapour-compression refrigeration cycle
between 0.14 and 0.8 MPa. If the mass flow rate of the
refrigerant is 0.05 kg/s, determine:
a) the rate of heat removal from the refrigerated space and
the power input to the compressor
b) the rate of heat rejection to the environment, and
c) the COP of the refrigerator
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Refrigeration Example

•A heat pump is a reversed heat engine used for heating
houses and commercial buildings during winter and cooling
them during the summer.
•It differs from a refrigerator only in that the high temperature
rejected heat is desired for heating rather than the low
temperature input heat being desired for cooling.
•In winter it operates so as to absorb heat from the
surroundings and reject heat into the building. Refrigerant
evaporates in coils placed underground or in the outside air.
Vapour compression is followed by condensation, heat being
transferred to air or water, which is used to heat the building.
•In summer, the heat pump serves for air conditioning. The
flow of refrigerant is simply reversed and heat is absorbed
from the buildings and rejected through underground coils or
to the air outside.
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Heat Pump

Refrigeration Cycle COP Calculation

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