refrigration system, types of refrigrants, their properties, Vapour compression cycle, superheating , subcooling, cascade refrigratioin systems, vapour absorbtion system , problems

1. ME 6404 – THERMAL ENGINEERING
UNIT – V – REFRIGERATION
Presented by
A. SANKARA NARAYANA MURTHY,
Assistant Professor,
Dept. of Mechanical Engineering,
Kamaraj College of Engineeriechnology,
Virudhunagar

2. REFRIGERATOR
• The transfer of heat from a low-temperature
region to a high-temperature one requires
special devices called refrigerators.
• The objective of a refrigerator is to remove
heat (QL) from the cold medium; the objective
of a heat pump is to supply heat (QH) to a
warm medium.

3. HEAT PUMP
• Heat pump supplies heat to the higher-temperature
region from lower temperature by giving work as
input.
• Heat pumps and refrigerators are essentially the
same devices; they differ in their objectives only.
for fixed values of QL and QH

4. TYPES OF REFRIGERATION
SYSTEM
• Vapour Compression Refrigeration
(VCR): uses mechanical energy
• Vapour Absorption Refrigeration (VAR):
uses thermal energy
• Steam Jet Refrigeration system
• Liquid N2 bath
• Ice bunk cooling system
• Etc…

5. THE PRESSURE-ENTHALPY DIAGRAM

The process of the vapor
compression refrigeration cycle may
conveniently be displayed on a
diagram having pressure and specific
enthalpy as coordinates.

Below the critical point (CP) the
saturated liquid line (SL) and
saturated vapor line (SV) enclose a
two-phase (wet) region between them.

To the left of the saturated liquid line
lie states which have lower
temperature than the saturation
temperature at a given pressure.

These are states of sub-cooled liquid.

6. THE PRESSURE-ENTHALPY DIAGRAM

To the right of the saturated vapor line lie states which have higher
temperature than the saturation temperature at a given pressure.

These are states of superheated vapor.

The area to the left of the liquid line is called the subcooled liquid region,
and the area to the right of the vapor line is called the superheated vapor
region.

Within the two-phase region the horizontal lines of constant pressure are
also lines of constant temperature

In the superheat region the lines of constant temperature leave the
saturation line as indicated.

As the pressure diminishes in the superheat region, the lines of constant
temperature tend to become lines of constant enthalpy, i.e. vertical on
the diagram, indicating that the vapor is beginning to behave like an ideal
gas with its enthalpy independent of pressure.

Lines of constant specific entropy and lines of constant specific volume ,
are shown in the superheat region.

7. REFRIGERANT IN VAPOR
COMPRESSION REFRIGERATION

The working substance in a refrigeration system is called the refrigerant.

There are lots of refrigerants, including gas, liquid and solid refrigerants.

There are many natural and artificial substances have been used in mechanical
driven and thermal driven vapor compression refrigeration systems.
 In lithium bromide vapor absorption refrigeration system, H2O is used as a
refrigerant and LiBr is an absorbent ; in NH3 vapor absorption refrigeration
system, NH3 is a refrigerant; is an absorbent.
 Water H2O is also used as a refrigerant both in vapor adsorption and in vapor
jet refrigeration cycles. In mechanical driven vapor compression refrigeration,
NH3, CO2, chlorofluorocarbons (CFCs), hydro chloro fluoro carbons (HCFCs),
hydro fluoro carbons (HFCs), azeotropic and zeotropic mixtures, inorganic
compounds, hydrocarbons, and others are used as refrigerants.

8. REFRIGERATION CHARACTERISTICS
OF REFRIGERANTS

The pressure- enthalpy diagram
is the usual graphic means of
presenting refrigerant properties
and its cycles.

A typical vapor compression
refrigeration cycle has been
shown in figure

9. REFRIGERANT PROPERTIES
1. Appropriate temperature and pressure characteristics

The saturated pressure with temperature is an important property
of refrigerant.
1) It is desired for the pressure at evaporating temperature to be
above atmospheric, to avoid inward leakage of air.
2)The pressure at the corresponding condensing temperature
should not be excessive, so that extra strength high-side equipment
is not required.
3) Low compression ratio is desirable, because the degree of
complication and difficulty of a compressor increases directly with
the compression ratio.
4) Discharge temperature of compressor should not be excessive,
to avoid problems as breakdown or dilution of the lubricating oil,
decomposition of the refrigerant, or formation of contaminants
such as sludge or acids. All of these can lead to compressor damage.

10. REFRIGERANT PROPERTIES
2. High latent heat of vaporization and low specific
volume of the refrigerant at the entry to compressor

A high latent heat of vaporization and a low specific volume of
the refrigerant at entry to the compressor are desirable for
smaller equipment and pipe size at given cooling capacity.

High latent heat means there is a high refrigeration effect.

For example, R11 has a much larger specific volume of suction
vapor of compressor than those of refrigerants of R22, R502 and
R717.

That means it requires a higher volumetric flow rate to produce
the same amount of cooling capacity.

Therefore, R11 is usually used with centrifugal compressors
because they are good at handing large volumetric flow rate.

11. REFRIGERANT PROPERTIES
3. Lower compression work

In order to get high COP, both high refrigeration effect and
low compression work must be considered in combination.
 For example, R717 (ammonia ) has a refrigerating effect q1 much
larger than other refrigerants, but its compression work w is
also high, as a result, COP of ammonia has the same order of
magnitude as that of the other refrigerants.

12. REFRIGERANT PROPERTIES
4. Some Important Physical/Chemical Properties of
Refrigerants

Any substance which has appropriate thermal properties can be
used as a refrigerant, but in practice the choice is limited by
many factors such as toxicity, flammability, chemical stability,
and the behaviors of the refrigerant with lubricating oil, water
and construction materials.

13. VAPOR-COMPRESSION REFRIGERATION
(VCR) CYCLE
The vapor-compression refrigeration
cycle has four components:
1. Evaporator,
2. Compressor,
3. Condenser, and
4. Expansion (or throttle) valve

In a basic vapor-compression
refrigeration cycle, the refrigerant
enters the compressor as a saturated
vapor and is cooled to the saturated
liquid state in the condenser.

It is then throttled to the evaporator
pressure and vaporizes as it absorbs
heat from the refrigerated space
VCR - Cycle

14. VCR - Cycle

The principal work and heat transfer that
occurs in the system are shown below,
these quantities being taken as positive in
the directions indicated by the arrows in
the Fig. 6.4.

In the analyses, each component is first
separately considered.

The evaporator, in which the desired
refrigeration effect is achieved, will be
considered first.

Considering a control volume enclosing
the refrigerant side of the evaporator,
conservation of mass and energy applied
to this control volume together give the
rate of heat transfer per unit mass of
refrigerant flow in the evaporator as:
1 4
e
e
Q
q h h
m
= = −

15. VCR - Cycle

Next consider the compressor.

It is usually adequate to assume that there is no heat transfer to or
from the compressor.

Conservation of mass and energy rate applied to a control volume
enclosing the compressor then give:

For a control volume enclosing the refrigerant side of the condenser,
the rate of heat transfer from the refrigerant per unit mass of
refrigerant is:
2 1
i
i
W
w h h
m
= = −
2 3
c
c
Q
q h h
m
= = −

16. VCR - Cycle

Finally, the refrigerant at state 3 enters the expansion valve and
expands to the evaporator pressure.

This process is usually modeled as a throttling process in which there
is no heat transfer, i.e., for which

In the vapor-compression system, the net power input is equal to the
compressor power, the expansion valve involving no power input or
output.

Using the quantities and expressions introduced above, the coefficient of
performance, COP, of the vapor- compression refrigeration system is
given by:
34 hh =
1 4
2 1
/
/
e e
i i
q Q m h h
COP
w W m h h
−
= = =
−

17. LIQUID SUBCOOING

In practice some degree of subcooling
may be acquired, and the point 3 moves
to the left of the saturated liquid on the
pressure-enthalpy diagram, as shown in
Figure.

Subcooling is the process of cooling
condensed gas beyond what is required
for the condensation process.

Subcooling is sensible heat and is
measured in degrees.

If it was possible to further cool down the
liquid to some lower value, say upto 3’,
then the net refrigeration effect will be
increased by
1 4' 1 4 4 4' 3 3'( ) ( )h h h h h h h h− − − = − = −
Subcooling of the liquid and superheating
of the vapor

18. LIQUID SUBCOOING

The volume refrigerating effect is of course increased by subcooling in
the same way as the specific refrigerating effect.

Since the specific work of compression remains the same, the
coefficient of performance is improved.

The subcooling may be achieved by any of the following methods:

(i) By passing the liquid refrigerant from condenser through a heat
exchanger through which the cold vapor at suction from the
evaporator is allowed to flow in the reversed direction.

(ii) By making use of enough quantity of cooling water so that the
liquid refrigerant is further cooled below the temperature of
saturation.

19. VAPOR SUPERHEATING

If the vapor at the compressor entry is in the
superheated state 1’, which is produced due to
higher heat absorption in the evaporator, then
the refrigerating effect is increased as

The specific work of reversible adiabatic
compression is increased by superheat. This is
indicated in Figure by the decreased gradient of
the line of constant entropy though point 1’ in
the superheat region compared with the
gradient of the line though point 1.

Although the specific work of reversible
adiabatic compression is increased by superheat,
so is the specific refrigerating effect, however,
their ratio, COP, may increase, decrease or
remain unchanged depending upon the range of
pressure of the cycle.
Subcooling of the liquid and
superheating of the vapor
1' 4 1 4 1' 1( ) ( )h h h h h h− − − = −

20. VAPOR SUPERHEATING

Here, it is necessary to consider the effect of heat transfer to the
refrigerant vapor in the suction pipe from the evaporator to the
compressor.

The suction line usually passes though warm surroundings, then heat
transfer to the vapor can take place, which will cause the temperature
increase.

This will cause a part of refrigerating capacity loss to refrigerate
engine room, so it will be called “useless refrigerant capacity”.

And the capacity used to refrigerate things which need refrigeration
will be called “useful refrigerant capacity”.

In practice, useless refrigerant capacity loss should be decreased, and
should increase useful capacity as possible.

21. CASCADE REFRIGERATION SYSTEMS
• Very low temperatures can be achieved by operating two or more vapor-
compression systems in series, called cascading. The COP of a refrigeration
system also increases as a result of cascading.

22. MULTISTAGE COMPRESSION
REFRIGERATION SYSTEMS
•When the fluid used throughout the cascade refrigeration system is the same, the heat
exchanger between the stages can be replaced by a mixing chamber (called a flash
chamber) since it has better heat transfer characteristics.

23. VAPOUR ABSORPTION
REFRIGERATION (VAR) SYSTEM
• When there is a source of
inexpensive thermal energy at a
temperature of 100 to 200°C is
absorption refrigeration.
• Some examples include
geothermal energy, solar energy,
and waste heat from cogeneration
or process steam plants, and even
natural gas when it is at a
relatively low price.
• Vapour Absorption refrigeration
(VAR) systems involve the
absorption of a refrigerant by a
transport medium.
• The most widely used system is
the ammonia–water system, where
ammonia (NH3) serves as the
refrigerant and water (H2O) as the
transport medium.

24. VAR SYSTEM
• Other systems include water–lithium bromide and water–lithium chloride
systems, where water serves as the refrigerant. These systems are limited to
applications such as A-C where the minimum temperature is above the
freezing point of water.
• Compared with vapor-compression systems, ARS have one major advantage: A
liquid is compressed instead of a vapor and as a result the work input is very
small (on the order of one percent of the heat supplied to the generator) and
often neglected in the cycle analysis.
• ARS are often classified as heat-driven systems.
• ARS are much more expensive than the vapor-compression refrigeration
systems. They are more complex and occupy more space, they are much less
efficient thus requiring much larger cooling towers to reject the waste heat,
and they are more difficult to service since they are less common.
• Therefore, ARS should be considered only when the unit cost of thermal
energy is low and is projected to remain low relative to electricity.
• ARS are primarily used in large commercial and industrial installations.

25. VAR SYSTEM

26. VAR SYSTEM
The COP of actual absorption refrigeration
systems is usually less than 1.
Air-conditioning systems based on
absorption refrigeration, called absorption
chillers, perform best when the heat source
can supply heat at a high temperature with
little temperature drop.

27. VCR CYCLE COMPONENTS
• Refrigerant
• Evaporator/Chiller
• Compressor
• Condenser
• Receiver
• Thermostatic expansion valve
(TXV)

28. REFRIGERANT
• Desirable properties:
– High latent heat of vaporization - max cooling
– Non-toxicity (no health hazard)
– Desirable saturation temp (for operating pressure)
– Chemical stability (non-flammable/non-explosive)
– Ease of leak detection
– Low cost
– Readily available

29. 29
LOW SIDE OPERATION
• Refrigerants have low boiling points
• When liquid boils, it absorbs large amounts of heat
• Amount of heat absorbed in evaporator is proportional to
amount of refrigerant boiled
• High side Components
• Expansion device
• Evaporator
• Accumulator (if equipped)

30. 30
EXPANSION DEVICES
• The expansion device separates the high side from the low
side and provides a restriction for the compressor to pump
against.
• There are two styles of expansion devices:
• The TXV can open or close to change flow. It is controlled
by the superheat spring, thermal bulb that senses
evaporator outlet temperature, and evaporator pressure
• Most OTs have a fixed diameter orifice

31. A TXV controls the
refrigerant flow from
the high pressure side
to the evaporator. A
receiver dryer is
mounted in the liquid
line of all TXV systems.
TXV SYSTEM

32. An OT controls the
refrigerant flow from the high
pressure side to the
evaporator. An accumulator is
mounted in the suction line of
all OT systems.
OT SYSTEM

33. 33
THERMAL EXPANSION VALVES,
TXVs
•The three major types of expansion valves:
•Internally balanced TXVs are the most
common.
•Externally balanced TXVs are used on some
larger evaporators.
•Block valves route the refrigerant leaving
the evaporator past the thermal sensing
diaphragm so a thermal bulb is not needed.
Internally
Balanced
Externally
Balanced
Block Valve

34. 34
THERMAL EXPANSION VALVES,
TXVS
• Variable valve that can change size of opening in response to
system load
• Opens or closes depending on evaporator pressure and
temperature

35. 35
EVAPORATOR OPERATION
Hot, liquid refrigerant flows through
the expansion device in the low side to
become a fine mist.
Refrigerant boils or evaporates to
become a gas inside the evaporator.
The boiling refrigerant absorbs heat
from the air during this change of state.

36. EVAPORATOR/CHILLER
• Located in space to be refrigerated
• Cooling coil acts as an indirect heat exchanger
• Absorbs heat from surroundings and vaporizes
– Latent Heat of Vaporization
– Sensible Heat of surroundings
• Slightly superheated (10°F) ensures no liquid carryover into compressor

37. 37
ACCUMULATORS
Accumulators are used in the suction line of all OT
systems.
The accumulator:
•separates liquid refrigerant so only gas flows to
the compressor.
•Allows oil in the bottom of the accumulator to
return to the compressor.
•provides storage for a refrigerant reserve.
•contains the desiccant bag for water removal.
•provides a place to mount low pressure switches
and sensors.

38. 38
HIGH SIDE OPERATION
• Takes low pressure vapor from evaporator and returns high
pressure liquid to expansion device
• Must increase vapor temperature above ambient temperature for
heat transfer to occur resulting in change of state from vapor to
liquid
• High side Components
• High begins at compressor and ends at expansion device
• Compressor
• Condenser
• Receiver-drier (if equipped)

39. 39
COMPRESSORS
There is a large variety of compressors.
Some of variations are:
The compressor manufacturer
Piston, vane, or scroll type
The piston and cylinder arrangement
How the compressor is mounted
Style and position of ports
Type and number of drive belts
Compressor displacement
Fixed or variable displacement

40. 40
SCOTCH YOKE COMPRESSORS
A Scotch yoke compressor has two pairs of
pistons that are driven by a slider block on
the crankshaft. The pistons are connected
by a yoke.
Pistons Yoke
Discharge Reed
Suction Reed

41. 41
CONDENSER OPERATION
Hot, high pressure gas is pumped from the compressor to enter the condenser.
The gas gives up its
heat to the air passing through the condenser. Removing heat from the hot gas causes it
to change state and become liquid.

42. 42
CONDENSER TYPES
Condensers A and C are round
tube, serpentine condensers.
Condenser B is an oval/flat tube,
serpentine condenser.
Condenser D is an oval/flat tube,
parallel flow condenser.
Flat tube condensers are more
efficient.

43. 43
SERPENTINE CONDENSER
Refrigerant flows from the
upper inlet to the bottom
outlet through two tubes.
These tubes wind back and
forth though the condenser.

44. 44
PARALLEL FLOW CONDENSER
Refrigerant flows from the upper inlet to the
bottom outlet through groups of parallel
tubes. Some carry refrigerant from the right
to the left, and others move it back to the
right side.

45. 45
HEAT EXCHANGERS
 Condensers have to move heat from the refrigerant to the air.
 Evaporators must move heat from air to the refrigerant.
 Both require a lot of contact area for both air and refrigerant.
 Both require free movement of air and refrigerant.

46. 46
RECEIVER DRYERS
A receiver dryer is mounted in the liquid line of a
TXV system. It is used to:
•to store a reserve of refrigerant.
•hold the desiccant bag that removes water from
the refrigerant.
•filter the refrigerant and remove debris particles.
•provide a sight glass so refrigerant flow can be
observed.
•provide a location for switch mounting.
Barb
Connections,
Note Sight
Glass
Male Flare
Connections
Male O-ring
Connections,
Note Switch

47. PROBLEM-1
An ideal vapor-compression refrigeration cycle operates at steady state
with Refrigerant 134a as the working fluid. Saturated vapor enters the
compressor at 2 bar, and saturated liquid exits the condenser at 8 bar.
The mass flow rate of refrigerant is 7 kg/min. Determine
a)the compressor power, in kW
b)the refrigerating capacity, in tons
c)the coefficient of performance

48. Analyzing vapor-Compression
Refrigeration Systems

49. Ideal Refrigeration Cycle
An ideal cycle has no irreversibilities within the evaporator and
condenser, and there are no frictional pressure drops. Compression
is isentropic. The T-s diagram is shown on the next slide.
Process 1-2s: Isentropic compression of the refrigerant;
Process 2s-3: Heat transfer from refrigerant to outside air, at
constant pressure;
Process 3-4: Throttling process to a two-phase mixture at lower
pressure;
Process 4-1: Heat transfer to the refrigerant as it flows at constant
pressure through the evaporator;

50. Analyzing vapor-Compression
Refrigeration Systems
As the refrigerant passes through the evaporator, the heat
transfer per unit mass of refrigerant flowing is:
A ton of refrigeration is equal to 200 Btu/min or 211 kJ/min.
Work done by compressor per unit mass flow of refrigerant is
flowmasstrefrigeranismhh
m
Qin



);( 41 −=
);( 12 hh
m
Wc
−=



51. Analyzing vapor-Compression
Refrigeration Systems
Heat rejected by the refrigerant:
Expansion valve:
Coefficient of performance:
);( 32 hh
m
Qout
−=


34 hh =
;
)(
)(
12
41
hh
hh
m
W
m
Q
c
in
−
−
==




β

52. SOLUTION
Let us first get the properties at each state in the cycle.
State 1: p1 = 2 bar, sat vapor. h1 = 241.30 kJ/kg, s1 = 0.9253
kJ/kg.K
State 2: p2 = 8 bar, s2 = s1, h2 = 269.92 kJ/kg
State 3: p3 = 8 bar, sat. liquid, h3 = 93.42 kJ/kg
State 4: Throttling process, h4 = h3 = 93.42 kJ/kg
a)The compressor power is:
CW m h h kg s kJ kg kW( ) ( / )( . . ) / .= − = − =2 1
7
269 92 241 30 3 34
60
 

53. Solution
b) The refrigerating capacity is
c)The coefficient of performance is
inQ m h h kg kJ kg tons kJ
tons
( ) ( /min)( . . ) / / / /min
.
= − = −
=
1 4 7 241 30 93 42 211
4 91
 
h h
h h
( ) ( . . )
.
( ) ( . . )
− −
β = = =
− −
1 4
2 1
241 30 93 42
5 17
269 92 241 30

54. • Problem-2
• A refrigerator uses refrigerant-134a as the working fluid and
operates on an ideal vapor-compression refrigeration cycle
between 0.12 and 0.7 MPa. The mass flow rate of the refrigerant
is 0.05 kg/s. Show the cycle on a T-s diagram with respect to
saturation lines. Determine:
• a) the rate of heat removal from the refrigerated space,
• b) the power input to the compressor,
• c) the rate of heat rejection to the environment, and
• d) the coefficient of performance.

55. Solution
Answers: (a) 7.41 kW, 1.83 kW, (b) 9.23 kW, (c)
4.06

56. Problem-3
Consider an ideal refrigeration cycle which uses R-12 as
the working fluid. The temperature of the refrigerant in
the evaporator is –20°C and in the condenser it is 40°C.
The refrigerant is circulated at the rate of 0.03kg/s.
Determine the coefficient of performance and the
capacity of the plant in rate of refrigeration.
For each control volume analyzed, the thermodynamic
model is the R-12 tables. Each process is SSSF with no
change in kinetic or potential energy.

57. Control volume: Compressor.
Inlet state: T1 known, saturated vapor; state fixed.
Exit state: P2 known(saturation pressure at T3).
At T3=40°C
2 1
2 1
cw h h
s s
= −
=
2
1
1 2
2
2
2 1
0.9607
178.61 /
0.7082
50.8
211.38 /
32.77 /
g
o
c
P P MPa
h kJ kg
s s
T C
h kJ kg
w h h kJ kg
= =
=
= =
=
=
= − =

58. Control volume: Expansion valve.
Inlet state: T3 known, saturated liquid; state fixed.
Exit state: T4 known.
Control volume: Evaporator.
Inlet state: State 4 known.
Exit state: State 1 known.
3 4 74.53 /h h kJ kg= =
1 4 104.08 /
3.18
3.12
L
L
c
q h h kJ kg
q
w
Capacity kW
β
= − =
= =
=

59. • Problem-3
Consider a 300 kJ/min refrigeration system that operates on an ideal
vapor-compression refrigeration cycle with refrigerant-134a as the
working fluid. The refrigerant enters the compressor as saturated
vapor at 140 kPa and is compressed to 800 kPa. Show the cycle on a T-
s diagram with respect to saturation lines, and determine the:
a) quality of the refrigerant at evaporator inlet,
b) coefficient of performance, and
c) power input to the compressor.