Refrigeration and Air Conditioning Engineering (Lecture 01)

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1
FET 411, Refrigeration and Air
Conditioning Engineering
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Course outline
• Chapter 1: Introduction to Refrigeration
• Chapter 2: Refrigeration cycle
• Chapter 3: Equipments of Refrigerator
• Chapter 4: Properties of Refrigerant
Reference books
• Refrigeration and Air Conditioning – P. L. Ballaney
• A text book of Refrigeration and Air Conditioning – R.S.
Khurmi
Chapter 1: Introduction to Refrigeration
• Refrigeration may be defined as the process of removing heat
or heat energy from a body, that is below the temperature of
its surroundings.
Refrigeration
Natural
Mechanical
Compression
Absorption: Ammonia and
water vapor
Steam jetMd Rahmatuzzaman Rana

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Objectives
• To understand why the storage life of foods and agricultural
products can be extended by refrigeration
• To learn the adverse effects of low temperature for certain
commodities
• To refresh our knowledge about thermodynamics, in particular the
first law of thermodynamics, the phase diagram and the Mollier
diagram
Purpose of refrigeration of foods
- reduces postharvest losses
- preserves foods
- makes it possible to provide safe, wholesome food all consumers
have the right to expect
- by refrigerated transport, allows countries to export perishable
produce
How does it work?
Fig. Gas production rate as a function of developmental stage

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Q10 (reduction in reaction rate if temperature decreases by 10°C) typically
2.5 for biochemical reactions
• Respiration
• Texture change
• Microbial growth
Example
- Predictive microbiology: prediction of microbial growth/inactivation in
various conditions of temperature, gas concentration, salt etc.
- E.g. Pathogen Modeling Program
Negative effects of refrigeration: Chilling injury
- Variety of symptoms: lesions, brown heart in pineaple, mealiness in
tomato, gery discoloration in banana
- Most tropical fruits are affected
- Typically below 12°C, depending on cultivar
- Believed to be caused by imbalance between saturated and unsaturated
lipids

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Importance of refrigeration
- Number of farmers has gradually increased: <5% of world population
- Cold chains are vital to ensure safety, quality and commercial value of perishable
foodstuffs
- 75% of foods are processed, the majority involves one or more refrigeration steps
- Foods or ingredients are transported 2.5 times
- Approximately 70% of foods in developed countries are chilled or frozen when
produced, and 50% of all foods sold require refrigerated display at retailer level
A few numbers
- Annual purchase of chilled and quick-frozen foods is around 1200 billion
USD
- Refrigeration generates jobs: 1 out of 1000 jobs in industrialized
countries are in the equipment manufacturing field
- 300000 jobs in the USA and 70000 in Europe in the installation and
sectors
- Annual investment in refrigerating equipment totals 170 billion USD
- More than 1 billion domestic refrigerators/freezers in the world
- 300 m3 of cold storage worldwide
Simple cooling systems
• Pot – in – Pot (M. Bah Abba)
- Two ceramic pots
- Smallest pot inside largest pot
- Wet soil in between
- Cooling via evaporation: latent heat (enthalpy removed from pot
- Rolex award
- Simple design, local manufacturing possible

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Thermodynamics of phase change
Temperature Vs Total Heat
State diagram
Units of Refrigeration
• Unit of refrigeration: Tonne of refrigeration (TR)
• A tonne of refrigeration is defined as the amount of refrigeration effect
produced by the uniform melting of one tonne (1000kg) of ice from and at
0 oC in 24 hrs.
• Latent heat of ice is 335 KJ/Kg, hence,
• 1TR= 1000 Kg * 335 KJ/Kg in 24 hrs
= (1000*335)/ (24*60)
= 232.6 KJ/min (in actual 210 KJ/min or 3.5 KW)

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Coefficient of performance
• COP = Amount of heat extracted in the refrigerator (Q)/ amount of
work done (W)
= Q/W
• It is reciprocal of the efficiency of a heat engine
Difference between Heat engine, Refrigerator and heat pump
COP (E) = Work done
(W)/ Heat supplied (Q2)
=(Q2 – Q1)/ Q2
COP (R) = Heat taken
(Q2)/ Work done (W)
=Q1/ (Q2- Q1)
COP (P) = Heat delivered
(Q1)/ Work done (W)
= Q1/ (Q2- Q1)
= COP (R) + 1
Simple vapor compression refrigeration system
• It is named as vapor compression, due to low pressure vapor
refrigerant from the evaporator is changed to high pressure
vapor in the compressor.
• Refrigerant: NH3, CO2, SO2
• Advantage: small, less running cost, can be run over large
range of temperatures, COP is high.
• Disadvantage: Initial cost high, leakage problem

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Mechanism of a simple vapor compression refrigeration
system
1. Compressor: Low pressure, temp vapor refrigerant from evaporator drawn into
compressor at high temp through suction valve. The highly pressed and temp
vapor refrigerant is discharged into the condenser through discharge valve.
2. Condenser (cooler): Refrigerant cooled and condensed here. At this stage,
refrigerant gives up its latent heat to the surroundings condensing medium
(air/water).
3. Receiver: the condensed refrigerant stored here in a vessel.
4. Expansion valve (refrigerant control valve): allow the liquid high pressure
refrigerant in a controlled reducing rate. Some portion of refrigerant evaporates
here but greater are evaporate in evaporator.
5. Evaporator: The liquid-vapor refrigerant are passed through evaporator
(consists of coils of pipe) at low pressure and temperature. Here, the
liquid refrigerant absorbs its latent heatof vaporisation from the medium
(air, water or brine) which is to be cooled.
High pressure side: discharge line, condenser, receiver and expansion valve.
Low pressure side: evaporator, piping from expansion valve to evaporator and
the suction line.
Check the link for more: https://www.mobileair.com/refrigeration-cycle

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Pressure- enthalpy diagram
• Red line: temperature
• Green line: constant volume curve
• Black line: quality curves or mass fraction of vapor

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• COP = Refrigeration effect/ Work done
Factors influencing COP
1. There is no pressure loss in condenser, evaporator, connecting pipes and
compressor valves.
2. There is no heat transfer form or to the system at any point except the
condenser and evaporator
3. Compression is isentropic.
4. Transmission efficiency is from motor to compressor is 100%
5. Condenser temperature increase then COP is decrease due to
decreasing refrigeration effect.
6. Subcooling
7. Superheating
Actual refrigeration cycle
1. The liquid refrigerant in the condenser may be undercooled,
i.e. cooled below the condensing temperature before
passing through the expansion valve. This can be obtained
by large quantities of circulating cooling water which should
be at a temperature much lower than the condensing
temperatures.
2. Normally the gas leaves the evaporator in superheated condition
before it enters the compressor. This superheating may occur in
the evaporator pipes leaving the evaporator but still in the cooled
space. Thus the refrigerating effect will increase per kg of
refrigerant flow. This superheating may occur in the pipes
connecting the evaporator delivery and the compressor suction, in
spite of these pipes being insulated. Thus this is a loss or an
additional load on the compressor and condenser without any
useful cooling effect obtained. Thus these pipes should be
properly insulated.

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3) Compression has been assumed isentropic, but in actual practice it
may be very complex. The cylinder wall may be stabilize at a
temperature between the cool suction gas and hot discharge gas.
Thus during the first part of compression heat may flow from the
walls to the gas and in the later part of the stroke the reversal in the
direction of flow of heat may take place.
4) The refrigerant has to flow through pipes in the evaporator,
condenser and other connecting pipes. In actual practice due to
frictional resistance to flow, there is pressure fall in the connecting
pipes.
Effect of Subcooling
Effect of Superheating
A-B: isentropic compression
B-C: removal of superheat
C-D: removal of heat of condensation (isothermal)
D-E: expansion in expansion valve (isenthalpyl)
E-A: evaporation at the evaporator (isothermal)

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Pressure Enthalpy charts

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• Cooling load
o Rate of heat energy removal
o “Ton of refrigeration”: latent heat of fusion of 1 ton of ice per day = 3.52
kW
• Heat to be removed
o Heat of respiration
o Losses through ceiling, floor and wall
o Heat production of fans, lights, fork lifts, personnel
• Condenser
o Heat rejected to environment:
3 1( )cq m H H 
1
2
3
H Enthalphy of refrigerant at exit from
the condonsor (kJ/kg)
H ethalphy of refrigerant at the beginning
of compression of stroke (kJ/kg refrigerant)
H ethalphy of refrigerant at the


 end
of compression of stroke (kJ/kg refrigerant)
m refrigerant mass flow rate (kg/s)Md Rahmatuzzaman Rana
Mathematical expressions: Design and
analysis
• Evaporator
• Compressor
• Coefficient of performance
o Indicator of efficiency of system
o Ratio of useful effect to the work expended
o Coefficient Of Performance: heat absorbed by refrigerant / energy supplied
to compressor
o Refrigerant flow rate (q is heat to be removed)
2 1( )eq m H H 
3 2( )wq m H H 
2 1
3 2
H H
COP
H H


 2 1
q
m
H H



c
e
w
q rate of heat exchanged in the condensor (kW)
q rate of heat exchanged from the evaporator (kW)
q rate of work done on the refrigerant (kW)



Charging of refrigeration system
• After completely assemble of refrigeration system, the whole system is
tested for leaks, dried and evacuation, it is ready for charging.
• The refrigerant to be used is stored in liquid form in suitable containers.
• The usual practice is to feed refrigerant vapor from the top of the
container into the suction line of the system with the compressor running
and condenser in operation.
• The rate of removal of refrigerant from container, is accelerated by
immersing in hot water. It is important to supply the correct amount of
refrigerant into the system.
Common troubles in refrigeration system
• A large portion of service trouble with refrigeration system is
caused by MOISTURE.
• One small drop of liquid water can stop the circulation of
refrigerant by freezing in the expansion device.
• Other troubles such as corrosion, copper plating and
chemical damage to insulation may result if moisture is
present in the system.

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How water may appear in refrigeration system
1. Incomplete drying of the equipment before charging,
2. Low sides leaks admitting air which contains moisture,
3. Leakage in water-cooled condenser,
4. Water formation through oxidation of certain hydrocarbons in
the oil
5. Water dissolved in either oil or liquid refrigerant when it is
placed in the system and
6. Decomposition of the motor insulation in hermetically sealed
units.
Remedies of moisture removal
By using dessicant (drying agent) or absorbent and
chemical driers to remove water from the
refrigeration system.
1. Absorbents: Granular aluminium oxide (activated
alumina), silicon dioxide (silica gel),
2. Chemical driers: anhydrous calcium sulfate (drierite),
calcium oxide or quiklime and calcium chloride.
Troubles with non-condensable gases
• They are also service troubles with refrigerating system. They increase
operating pressures and temperatures and decrease efficiency. They
present in refrigeration system from one or more of the following sources:
1. Incomplete removal of the gas used for pressure testing the system prior
to charging,
2. Formation of non-condensable gas by chemical reactions within the
system.
3. Air and othe rnon-condensable gases dissolved in the refrigerant when
charged and
4. Leaks in the piping.
Remedies for non-condensable gases
• Small unit: it is removed by wasting the charge, evacuating
the system and recharging with fresh refrigerant.
• Large unit: it can be removed from the top of the
condenser and from the top of the evaporator with an
accompanying loss of only a small portion of the refrigerant
charge. The process of removing non-condensable gases
from a refrigerating system is called purging.

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Tests for leakage of the refrigerant
1. Leakage of SO2: detected by the white smoke which
appears if a swab soaked with a 25% solution of aqueous
NH3 is brought near it.
2. Leakage of NH3: may be detected with burning sulfur
candle.
3. Leakage of freons (halogenated hydrocarbons): may be
detected by the use of halde torch. (The halide torch
burns alcohol and the flame turns green in the presence
of freon vapours).
Methods of defrosting
• In many compression refrigeration systems the evaporator is a direct expansion
coil used to cool and dehumidifying the air passes over it. If the condition of
operation require metal surface below 32 oF, frost will collect on the tubes and
fins and periodic defrosting is essential.
• The methods of collecting frosts are
i. Interrupted service
ii. Water spray
iii. Hot gas and hot oil
iv. Electric resistance heater adjacent coil or arrangement for heating coil
by passing low voltage current through the metal.
Control compression refrigeration system
• Small systems (household refrigerator and home AC): the compressor is
stopped when the cooling thermostat has been satisfied and it is started
again when further refrigeration is needed.
• Large systems: The centrifugal compressor’s capacity may be varied to
match a changing load by
i. Varying the speed
ii. Adjusting vanes all the inlet to the impellers
iii. Regulators which throttle the suction gas so as to keep the evaporator
pressure constant and control the volume
iv. Varying condenser pressure.

Refrigeration and Air Conditioning Engineering (Lecture 01)

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