MEC 351 REFRIGERATION &

AIR CONDITIONG
CHAPTER 1: INTRODUCTION TO REFRIGERATION
Prepared by: Ahmad Najmie Rusli
Faculty of Mechanical Engineering,
UiTM Kampus Pasir Gudang.
Email: ahmad7586@johor.uitm.edu.my
 Introduction to Refrigeration & Air
Conditioning
Refrigeration applications:
Air conditioning
Food preservation, storage and
supply chain
Frozen food products
Water coolers
Industrial dehumidifiers
Ice making industries
Ice skating rinks (egDubai Mall)
Construction industries
Gas liquefactions

Figure: The cold chain (Production Users)

 Refrigerators and Heat Pumps
Refrigeration:
The process of transferring of heat from a lower temperature region to a higher
temperature one.

Refrigerators:
Cyclic devices that transfer heat from a low-temperature region to a high-
temperature one.

Refrigerants:
The working fluids used in the refrigeration cycles.

Heat Pump:
Cyclic devices that transfer heat from a low-temperature region to a high-
temperature one.
 Refrigeration System Components

To circulate the refrigerant

Compressor

Transfer heat to ambient air

Condenser

To control amount of refrigerant entering

Expansion
device
evaporator
 Refrigeration System Components
Other name is an accumulator
To store reserve refrigerant, contain the desiccant, and filter refrigerant
Receiver
dryer (accessory)

To transfer heat from in-room air to the refrigerant

Evaporator

To connect these parts together

Hoses
 Refrigeration System Components

Compressor Condenser Receiver dryer /

Accumulator
 Refrigeration System Components

Evaporator
 Refrigeration System Components

Hoses Expansion Devices

Refrigeration System Components

Figure: Connection of basic TXV (Type Expansion Valve)System

 Refrigerants Non-
flammable

Refrigerant define as the

working fluids used in the Good heat
Non-toxic
transfer
refrigeration cycles.

Refrigerant
Characteristics
High latent
Inertness &
heat of
stable
vaporization

Low freezing
Low cost
temperature
 Refrigerants
Refrigerants Examples Comments
Very high ODP & GWP
Chlorofluorocarbons R-11 Not applicable anymore
(CFCs) R-12
R 114
Medium ODP & GWP
Hydrochlorofluorocarbons R-22 Phasing out via Montreal
(HCFCs) R-123 Protocol
Zero ODP & low GWP
Hydrofluorocarbons R-134a
(HFCs) R-125
Zero ODP & low GWP
Inorganic Refrigerants R-717 (Ammonia)
R-747(CO2)
1. Refrigerators and heat pumps are essentially the same
devices BUT differ in their objectives.

Refrigerator is to remove heat(QL) from the cold space.

Heat pump is to supply heat(QH) to a warm space.
2. Both devices must have the required input.

MUST ALWAYS REMEMBER!

3. QL is the magnitude of the heat removed from the
refrigerated space at temperature TL .

4. QH is the magnitude of the heat rejected to the warm

space at temperature TH

5. Wnet,in is the net work input to the refrigerator.

6.QL and QH represent magnitudes and thus are positive

quantities.
 Performance of Refrigerators & Heat
Pumps
The performance of refrigerators and heat pumps is expressed in
terms of the coefficient of performance (COP), defined as

* These relations can also be expressed in the rate form by replacing the
quantities QL, QH, and Wnet,in by rate of QL, QH, and net,in
Performance of Refrigerators & Heat
Pumps
This relation implies that COPHP 1 since COPR is a positive quantity.
The Reversed Carnot Cycle

A refrigerator or heat pump that operates on the reversed Carnot

cycle is called a Carnot refrigerator or a Carnot heat pump.
Carnot cycle is a totally reversible cycle that consists of two
reversible isothermal and two isentropic processes.
Reversing the cycle does also reverse the directions of any heat
and work interactions.
The result is a cycle that operates in the counter clockwise direction
on a T-s diagram, which is called the reversed Carnot cycle.
 The Reversed Carnot Cycle
1-2: constant temp heat
absorption (QL) in the
evaporator.
2-3: isentropic compression in
a compressor.
3-4: constant temp heat
rejection (QH)in the
condenser.
4-1: isentropic expansion in
the expander.

Figure: Schematic of a Carnot refrigerator and T-s diagram of

the reversed Carnot cycle.
The Reversed Carnot Cycle

The coefficients of performance of Carnot refrigerators and heat

pumps are expressed in terms of temperatures as;
The Reversed Carnot Cycle

The reversed Carnot cycle is the most efficient refrigeration cycle

operating between two specified temperature levels.
However, the reversed Carnot cycle is not a suitable model for
refrigeration cycles.
1. Processes 2-3 and 4-1 cannot be approximated closely in
practice.
2. Process 2-3 involves the compression of a liquidvapor mixture,
which requires a compressor that will handle two phases.
3. Process 4-1 involves the expansion of high-moisture-content
refrigerant in a turbine.
4. Difficulty in maintaining isothermal conditions during the heat-
absorption and heat-rejection processes.
The Ideal Vapor-Compression Refrigeration Cycle

The vapor-compression refrigeration cycle is the most widely used

cycle for refrigerators, air-conditioning systems, and heat pumps.
Impracticalities reversed Carnot cycle can be eliminated by;
1. Vaporizing the refrigerant completely before it is compressed
2. Replacing the turbine with a throttling device, such as an
expansion valve or capillary tube.
The cycle that results is called the ideal vapor-compression
refrigeration cycle.
 1-2: Isentropic compression
in a compressor.
2-3: Constant-pressure heat
rejection in a condenser.
3-4: Throttling in an
expansion device.
4-1: Constant-pressure heat
absorption in an
evaporator.

Note: If the throttling device were

replaced by an isentropic turbine, the
refrigerant would enter the evaporator
at state 4 instead of state 4.

Figure: Schematic and T-s diagram for the ideal vapor-compression All four components are steady-flow
refrigeration cycle. devices, thus all four processes that
make up the cycle can be analyzed as
steady-flow processes.
The Ideal Vapor-Compression Refrigeration Cycle
Process in an ideal vapor-compression refrigeration cycle;
1. The refrigerant enters the compressor at state 1 as saturated
vapor and is compressed isentropically to the condenser
pressure.
The temperature of the refrigerant increases during this
isentropic compression process to well above the
temperature of the surrounding medium.
2. The refrigerant then enters the condenser as superheated vapor
at state 2 and leaves as saturated liquid at state 3 as a result of
heat rejection to the surroundings.
The temperature of the refrigerant at this state is still above
the temperature of the surroundings
The Ideal Vapor-Compression Refrigeration Cycle
Process in an ideal vapor-compression refrigeration cycle;
3. The saturated liquid refrigerant at state 3 is throttled to the
evaporator pressure by passing it through an expansion valve or
capillary tube.
The temperature of the refrigerant drops below the
temperature of the refrigerated space during this process.
4. The refrigerant enters the evaporator at state 4 as a low-quality
saturated mixture, and it completely evaporates by absorbing
heat from the refrigerated space.
5. The refrigerant leaves the evaporator as saturated vapor and
reenters the compressor, completing the cycle.
The condenser and the evaporator do not involve any work,
and the compressor can be approximated as adiabatic.
 The Ideal Vapor-Compression Refrigeration Cycle

Then the COPs of refrigerators and heat

pumps operating on the vapor-compression
refrigeration cycle can be expressed as;

The steady-flow energy equation:

Figure: The P-h diagram of an ideal
vapor-compression refrigeration cycle
The Ideal Vapor-Compression Refrigeration Cycle

Refrigerating Effect(RE):
the quantity of heat a unit of mass of refrigerant absorbs from
the refrigerated space.
RE = qL=(h1h4) [kJ/kg]
Refrigerating Load(RL):
the rate at which heat energy must be removed from the
refrigerated space to maintain the desired temperature.
RL = QL= m x (h1h4) or RL = m x RE [kJ/s]

The widely used unit for RL is in Refrigeration Ton (RT)

1 ton = 200 Btu/min
Or 1 ton = 200 x 60 = 12,000 Btu/h
And 1 watt = 3.41 Btu/h
Exercises 1:
1. Draw a complete system components of reversible Carnot Cycle,
then explains the process cycle based on T-s diagram.
2. Draw a complete system components of the Ideal vapor-
compression refrigeration cycle, then explains the process cycle
based on T-s and P-h diagram.
3. What is the differences between reversible Carnot Cycle and Ideal
Vapor-Compression?
4. Sketch and labels a household refrigerator with completed of
system components.
5. What is COPR and COPHP ?
Exercises 1:
6. A refrigerator uses refrigerant-134a as the working fluid and
operates on an ideal vapor-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 [7.18 kW]
(b)the power input to the compressor [1.81 kW]
(c)the rate of heat rejection to the environment [9.0 kW]
(d)the COP of the refrigerator [3.97]
 Actual Vapor-Compression Refrigeration Cycle
Introduction:
In the ideal cycle, the refrigerant leaves the evaporator and enters
the compressor as saturated vapor.
In practice, it may not be possible to control the state of the
refrigerant so precisely.
It is easier to design the system so that the refrigerant is slightly
superheated at the compressor inlet.
This ensures that the refrigerant is completely vaporized when it
enters the compressor.
The line connecting the evaporator to the compressor is usually very
long, thus the pressure drop caused by fluid friction.
Actual Vapor-Compression Refrigeration Cycle

An actual vapour-compression differs from the ideal vapour

compression due to irreversibility that occur in various components.
This situation created;
1. Fluid friction leaded to pressure drop
2. Heat transfer to or from surrounding
3. COP decrease
To overcome these situation;
1. Refrigerant is allowed to be slightly superheated at the inlet
compressor
2. Refrigerant is allowed to sub-cool slightly before it enters the
throttling valve
 Actual Vapor-Compression Refrigeration Cycle

Explanations:
Non isentropic
compression.
Superheated vapor
at evaporator exit.
Sub cooled liquid at
condenser exit.
Pressure drop in
condenser and
evaporator.

Figure: Schematic and T-s diagram for the actual vapor-

compression refrigeration cycle
Exercise 2:
1. Refrigerant-134a enters the compressor of a refrigerator as
superheated vapor at 0.14 MPa and -10C at a rate of 0.05 kg/s
and leaves at 0.8 MPa and 50C. The refrigerant is cooled in the
condenser to 26C and 0.72 MPa and is throttled to 0.15 MPa.
Disregarding any heat transfer and pressure drops in the connecting
lines between the components, determine;
a) The rate of heat removal from the refrigerated space [7.93 kW]
b) The power input to the compressor [2.02 kW]
c) The isentropic efficiency of the compressor [93.9%]
d) The coefficient of performance of the refrigerator [3.93]
 Heat Pump Systems
Heat pumps and air conditioners have the same mechanical components.
One system can be used as a heat pump in winter and an air conditioner in summer.
This is accomplished by adding a reversing valve to the cycle.

Figure: A heat pump can be used to heat a house

in winter and to cool it in summer.
INNOVATIVE VAPOR-COMPRESSION
REFRIGERATION SYSTEMS

Cascade Refrigeration Systems

Two or more vapor-compression systems in stages (series), called
cascading.
The COP of a refrigeration system may also increases as a result of
cascading.
Suitable for :
1. Very low temperatures applications
2. Too large temperature range between the evaporator & the
condenser
Different refrigerants may be used since there is no mixing of flow in
both cycles.
Up to 4 cascades can be used.
INNOVATIVE VAPOR-COMPRESSION
REFRIGERATION SYSTEMS

Cascade Refrigeration Systems

The heat exchanger (h-x)
serves as evaporator for
cycle A and condenser
for cycle B.
If heat exchanger is well
insulated, the heat
transfer from the fluid in
the cycle B should be
equal to the heat transfer
to the fluid in the cycle A

Figure: A two-stage cascade

refrigeration system with the
same refrigerant in both stages
INNOVATIVE VAPOR-COMPRESSION
REFRIGERATION SYSTEMS

Cascade Refrigeration Systems

At heat exchanger:
INNOVATIVE VAPOR-COMPRESSION
REFRIGERATION SYSTEMS

Exercise 3:
Consider a two-stage cascade refrigeration system operating between the
pressure limits of 0.8 and 0.14 MPa. Each stage operates on an ideal vapor
compression refrigeration cycle with refrigerant-134a as the working fluid. Heat
rejection from the lower cycle to the upper cycle takes place in an adiabatic
counterflow heat exchanger where both streams enter at about 0.32 MPa. If the
mass flow rate of the refrigerant through the upper cycle is 0.05 kg/s, determine:

1. The mass flow rate of the refrigerant through the lower

cycle [0.0390 kg/s]
2. The rate of heat removal from the refrigerated space
[7.18 kW]
3. The power input to the compressor [1.61 kW]
4. The coefficient of performance of this cascade
refrigerator [4.46]
INNOVATIVE VAPOR-COMPRESSION
REFRIGERATION SYSTEMS

Multistage Compression Refrigeration Systems

In multistage compression, the h-x between the stages were replaced by a mixing
chamber (called a flash chamber)
 Exercise 4:
A two-stage compression refrigeration system operating between the pressure limits of
0.8 and 0.14 MPa. The working fluid is refrigerant-134a. The refrigerant leaves the
condenser as a saturated liquid and is throttled to a flash chamber operating at 0.32
MPa. Part of the refrigerant evaporates during this flashing process, and this vapor is
mixed with the refrigerant leaving the low-pressure compressor. The mixture is then
compressed to the condenser pressure by the high-pressure compressor. The liquid in the
flash chamber is throttled to the evaporator pressure and cools the refrigerated space as
it vaporizes in the evaporator. Assuming the refrigerant leaves the evaporator as a
saturated vapor and both compressors are isentropic, determine;
1. The fraction of the refrigerant that evaporates as it is throttled to the flash
chamber. [0.2049]
2. The amount of heat removed from the refrigerated space. [146.3 kJ/kg]
3. The compressor work per unit mass of refrigerant flowing through the condenser.
[32.71 kJ/kg]
4. the coefficient of performance. [4.47]
Exercise 4: (Cont)

Figure: T-s diagram of the two-stage compression

refrigeration cycle
INNOVATIVE VAPOR-COMPRESSION
REFRIGERATION SYSTEMS

Multipurpose Refrigeration Systems with a Single Compressor

For applications that required refrigeration at more than one temperatures.( eg
refrigerator & freezer separately)
All exit streams from the evaporators supplied to a single compressor and it
handles the compression process for the entire system.

Figure: Schematic and T-sdiagram for a refrigeratorfreezer unit with one compressor.
 Liquefaction of Gases
Many important scientific and engineering processes at cryogenic temperatures
(below about -100C) depend on liquefied gases.

Figure: Linde-Hampson system for liquefying

gases
 ABSORPTION REFRIGERATION SYSTEMS
Suitable when there is a source of inexpensive thermal energy at a temperature of
100to 200.

Figure: Ammonia absorption refrigeration cycle

References

1. Thermodynamics An Engineering Approach 5th Edition - Gengel, Boles