refrigeration engineering and cold chain - acharya ng ranga

101 Pages·2012·2.2 MB·English

For Class use only
ACHARYA N. G. RANGA AGRICULTURAL UNIVERSITY
B. Tech (Food Technology)
Course No.: FDEN 222
Credit Hours: 3 (2+1)
REFRIGERATION ENGINEERING
AND
COLD CHAIN
STUDY MATERIAL
Prepared by
Dr. S.Kaleemullah
College of Food Science and Technology
Pulivendula - 516 390
Department of Food Engineering
Course No : FDEN-222
Title : Refrigeration Engineering and Cold Chain
Credit hours : 3 (2+1)
Theory Lecture Outlines
L.No. Lecture outline P.No.
1 Refrigeration and air conditioning- Definitions - Necessity of refrigeration and air 4
conditioning – Factors affecting comfort air conditioning
2 Definition of Refrigerant - History of refrigerants - Classification of Refrigerants – 6
Primary refrigerants – Secondary Refrigerants – Halo carbon refrigerants
3 Azeotrope refrigerants - Inorganic refrigerants - Ammomia - Air - Carbon dioxide 11
- Sulphur dioxide - Water
4 Hydro carbon refrigerant – Designation system for refrigerants – Designation 15
sytem for Dichloro-tetrafluoro-ethane
5 Designation system for Dichloro-difluoro-methane - Selection of a refrigerant 17
6 Thermodynamic properties of refrigerants 19
7 Chemical requirements of refrigerants – Physical properties of refrigerants 21
8 Secondary refrigerants – Brines- Applications of various brines 25
9 Types of Refrigerators – Air Refrigerator – Vapour refrigerator – Advantages and 27
Disadvantage of vapour compression refrigeration system over air refrigeration
system
10 Mechanism of a simple vapour compression refrigeration systems 29
11 Evaporators – Capacity of an evaporator – Factors affecting heat transfer 31
capacity of an evaporator – Types of evaporator
12 Bare tube coil evaporator – Finned evaporator – Plate evaporator – Shell and 34
tube evaporator
13 Shell and coil evaporator – Tube in tube – Flooded evaporator – Dry expansion 38
evaporator
14 Natural convection evaporator – Forced convection evaporator – Frosting 42
evaporator – Non Frosting evaporators – Defrosting evaporators
3
15 Compressor – Classification – Suction pressure, Discharge pressure, 45
Compression ratio, Suction volume, Stroke volume, Clearance factor,
Compressor capacity, Volumetric efficiency
16 Reciprocating compressor - Parts of a reciprocating compressor - Cycle of a 48
reciprocating compressor
17 Rotary compressor – Centrifugal compressor – Advantages and disadvantages 52
of a centrifugal compressor
18 Condensers – Working of a condenser – Factors affecting the condenser 57
capacity – Heat rejection factor
19 Classification of condensers –Air cooled condensers – Water cooled condensers 61
– Tube in condenser – Shell and coil condenser
20 Fouling factor – Difference between air cooled and water cooled condensers – 66
Evaporative condenser
21 Expansion Devices – Types of expansion devices – Capillary tube 69
22 Hand operated expansion valve – Low side float valve – High side float valve 71
23 Ice manufacturing – principle of ice production 74
24 Application of refrigeration in different food products 78
25 Examples of Food processing by refrigeration and storage –Fruits and 79
vegetables
26 Examples of Food processing by refrigeration and storage - Meat products – fish 80
– poultry products – dairy products
27 Food Freezing – Freezing systems – Indirect contact systems 83
28 Plate Freezers – air blast Freezers – Freezers for liquid foods 85
29 Direct contact systems – Air blast - Immersion 89
30 Frozen food properties – Density – Thermal conductivity – Enthalpy – Apparent 91
specific heat – Apparent thermal diffusivity
31 Freezing time – Factors influencing freezing time – Freezing rate – Thawing time 96
32 Quality Changes in Foods during Frozen Storage 99
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Lecture No. 1
Refrigeration and air conditioning- Definitions - Necessity of refrigeration and air conditioning –
Factors affecting comfort air conditioning
Introduction:
Refrigeration is the process of removing heat from an enclosed space, or from a
substance and moving it to a place where it is Unobjectionable. It also includes the
process of reducing and maintaining the temperature of a body below the general
temperature of its surroundings.
For example, if some space (say in cold storage) is to be kept at - 2°C (271K),
we must continuously extract heat which flows into it due to leakage through the walls
and also the heat which is brought into it with the articles stored after the temperature is
once reduced to - 2°C (271K). Thus in a refrigerator, heat is virtually being pumped from
a lower temperature to a higher temperature. According to Second Law of
Thermodynamics, this process can only be performed with the aid of some external
work. It is thus obvious that supply of power (say electric motor) is regularly required to
drive a refrigerator.
Theoretically, a refrigerator is a reversed heat engine or a heat pump which
pumps heat from a cold body and delivers it to a hot body. The substance which works
in a heat pump to extract heat from a cold body and to deliver it to a hot body is called a
“refrigerant”.
Necessity of Refrigeration and Air Conditioning:
Refrigeration system is used for the manufacture of ice and similar products. It is
also widely used for the cooling of storage chambers in which perishable foods, drinks
and medicines are stored. The refrigeration has also wide applications in submarine
ships, aircraft and rockets.
Air conditioning refers to the treatment of air so as to simultaneously control its
temperature, moisture content, cleanliness, odour and circulation, as required by
occupants, a process or products in the space. The subject of refrigeration and air
conditioning has evolved out of human need for food and comfort. It also deals with the
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conditioning of air for industrial purposes, food processing, storage of food and other
materials.
Factors Affecting Comfort Air Conditioning:
1. Temperature of air: In air conditioning, the control of temperature means
maintenance of any desired temperature within an enclosed space even though the
temperature of the outside air is above or below the desired room temperature. This is
accomplished either, addition or removal of heat from the enclosed space as and when
demanded. It may be noted' a human being feels comfortable when the air is at 21°C
with 56% relative humidity.
2. Humidity of air: The control of humidity of air means the decreasing or
increasing moisture contents of air during summer or winter respectively in order to
produce comfortable healthy conditions. The control of humidity is not only necessary for
human comfort but it increases the efficiency of the workers. In general, for summer air
conditioning, the relative humidity should not be less than 60% whereas for winter air
conditioning it should not be than 40%.
3. Purity of air: It is an important factor for the comfort of a human body. It has
noticed that people do not feel comfortable when breathing contaminated air, even if it is
within acceptable temperature and humidity ranges. It is thus obvious that proper
filtration, cleaning, purification of air is essential to keep it free from dust and other
impurities.
4. Motion of air: The motion or circulation of air should be controlled, in order to
keep constant temperature throughout the conditioned space. Hence, there should be
equi-distribution of air throughout the space to be air conditioned.
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Lecture No. 2
Definition of Refrigerant - History of refrigerants - Classification of Refrigerants – Primary
refrigerants – Secondary Refrigerants – Halo carbon refrigerants
Introduction:
The development of refrigeration and air conditioning industry depends to a large
extent on the development of refrigerants to suit various applications and the
development of various system components. At present the industry is dominated by
the vapour compression refrigeration systems, even though the vapour absorption
systems have also been developed commercially. The success of vapour compression
refrigeration systems owes a lot to the development of suitable refrigerants and
compressors. The theoretical thermodynamic efficiency of a vapour compression system
depends mainly on the operating costs, safety, reliability and serviceability etc. depend
very much on the type of refrigerant and compressor selected for a given application.
The refrigerant is a heat carrying medium which during their cycle (i.e.
Compression, condensation, expansion and evaporation) in the refrigeration system
absorbs heat from a low temperature system and discards the heat so absorbed to a
higher temperature system.
History of Refrigerants:
The first refrigerant used was ether employed by Perkins in hand operated
vapour compression machine.
In the earlier days, ethyl chloride (C H Cl) was used as a refrigerant which soon
2 5
gave way to ammonia as early as in 1875. At about the same time, sulphur dioxide (SO )
2
in 1874, methyl chloride (CH Cl) in 1878 and carbon dioxide (CO ) in 1881, formed
3 2
application as refrigerants.
During 1910–30 many new refrigerants, such as N O , CH , C H , C H , C H
2 3 4 2 6 2 4 3 8
were employed for low temperature refrigeration. Hydrocarbons were, however, found
extremely inflammable, dichloro–methane (CH Cl ), dichloro–ethylene (C H Cl ) and
2 2 2 2 2
monobromomethane (CH Br) were also used as refrigerants for centrifugal machines.
3
A great break through occurred in the field of refrigeration with the development
of freons by E. I. du pont de Nemours and Co. Freons are a series of fluorinated
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hydrocarbons, generally know as fluorocarbons derived from methane, ethane etc. as
bases. Although Freon is a trade name, it has come to be commonly used for these
compounds. With fluorine, chlorine and sometimes bromine in their molecule, Freons
form a series of refrigerants with wide range of normal boiling points (boiling point or
saturation temperatures at one atmosphere pressure) to satisfy the varied requirements
of different refrigerating machines. The presence of fluorine in the molecule makes the
compound non–toxic and imparts other desirable physical and physiological
characteristics.
Desirable Properties of an Ideal Refrigerant:
There is no ideal refrigerant which can be used for all type of applications i.e.
there is no ideal refrigerant. A refrigerant is said to be ideal if it has all of the following
properties.
1. Low boiling point
2. High critical temperature
3. High latent heat of vaporisation
4. Low specific heat of liquid
5. Low specific volume of vapour
6. Non-corrosive to metal
7. Non-flammable and non-explosive
8. Non-toxic
9. Low cost,
10. Easy to liquify at moderate pressure and temperature
11. Easy of locating leaks by odour or suitable indicator, and
12. Mixes well with oil
The standard comparison of refrigerants, as used in the refrigeration industry is
based on evaporating temperature of - 15°C and a condensing temperature of + 30°C.
Classification of Refrigerants:
The refrigerants may, broadly, be classified into the following two groups.
1. Primary refrigerants
2. Secondary refrigerants.
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The refrigerants which directly take part in the refrigeration system are called
primary refrigerants whereas the refrigerants which are first cooled by primary
refrigerants and then used for cooling purposes, are known as secondary refrigerants.
The primary refrigerants are further classified into the following four groups.
1. Halo-carbon refrigerants,
2. Azeotrope refrigerants,
3. Inorganic refrigerants,
4. Hydro-carbon refrigerants.
Halo-carbon Refrigerants:
The American Society of Heating, Refrigeration and Air-conditioning Engineers
(ASHRAE) identify 42 halo-carbon compounds as refrigerants, but only a few of them
are commonly used. The following table 1 gives some of the commonly used
halo-carbon refrigerants.
Table. 1. Commonly used halo-carbon refrigernats
Refrigerant
Chemical name Chemical formula
Number
R – 11 Trichloromono fluoromethane CCl F
3
R – 12 Dichlorodifluoromethane CCl F
2 2
R – 13 Monochlorotrifluoromethane CCl F
3
R – 14 Carbonatetrafluoromethane CF
4
R – 21 Dichloromonofluoromethane CHCl F
2
R – 22 Monochlorodifluoromethane CHClF
2
R – 30 Methylene chloride CH Cl
2 2
R – 40 Methyl chloride CH Cl
3
R – 100 Ethyl chloride C H Cl
2 5
R – 113 Trichlorofluroethane CCl FCClF or C Cl F
2 2 2 3 3
R – 114 Dichlorotetrafluoroethane CClF CClF or C Cl F
2 2 2 2 4
R - 115 Monochloropentafluoroethane CClF CF
2 3
The halo-carbon compounds are all synthetically produced and were developed
as Freon family of refrigerants. Freon is a registered trade mark of E. I. Du Pont de
Nemours and Co., America. Most of the halo-carbon refrigerants, are now available from
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other manufactures under various trade names such as Genetron, Isotron etc. The first
of the halo-carbon refrigerant i.e. R–12 was developed in 1930 by Thomas Midgley.
1. R-11, Trichloromonofluoromethane (CCl F): The R-11 is a synthetic
3
chemical product which can be used as a refrigerant. It is stable, non-flammable and
non-toxic. It is considered be a low-pressure refrigerant. It has a low side pressure of
0.202 bar at -15°C and high side pressure of 1.2606 bar at 30°C. The latent heat at
-15°C is 195 kJ/kg. The boiling point at atmospheric pressure is 23.77°C. Due to its low
operating pressures, the refrigerant is exclusively used in large centrifugal compressor
systems of 200 TR and above. The leaks may be detected using a soap solution, a
halide torch or by using an electronic detector.
R-11 is often used by service technician as a flushing agent for cleaning the
internal parts of a refrigerator compressor when overhauling systems. It is useful after a
system had a motor burn out or after it has a great deal of moisture in the system. By
flushing moisture from the system with R-11, evacuation time is shortened. R-11 is one
of the safest cleaning solvent that can be used for this purpose. The cylinder colour code
for R-l1 is orange.
2. R-l2, Dichlorodifluoromethane (CCl F ): The R-12 is a very popular
2 2
refrigerant. It is colourless, almost odourless liquid with boiling point of -29°C at
atmospheric pressure. It is non toxic, non-corrosive, non-irritating and non-flammable. It
has a relatively low latent heat value which is an advantage in small refrigerating
machines. The large amount of refrigerant circulated will permit the use of less sensitive
and more positive operating and regulating mechanisms. The refrigerant is used in many
different types of industrial and commercial applications such as refrigerators, freezers,
water coolers, room and window air conditioning units etc. Its principal use is found in
reciprocating and rotary compressors, but its use in centrifugal compressors for large
commercial air conditioning is increasing.
R-12 has a pressure of 0.82 bar at -15°C and a pressure of 6.4 bar at 30°C. The
latent heat of R-12 at - 15°C is 159 kJ/kg. The leak may be detected by soap solution,
halide torch or an electric leak detector. Water is only slighted soluble in R-12. The
Solution formed is very slightly corrosive to any of the common metals used in
refrigerator construction. The addition of mineral oil to the refrigerant has no effect upon
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the corrosive action. The refrigerant is available in a variety of cylinder sizes and the
cylinder colour code is white.
6. R-22, Monochlorodifluoromethane (CHClF ): The R-22 is a man-made
2
refrigerant developed for refrigeration installations that need a low evaporating
temperature, as in fast freezing units which maintain a temperature of - 29°C to - 40°C. It
has also been successfully used in air conditioning units and in household refrigerators.
It is used with reciprocating and centrifugal compressors. It is also used for low
temperatures refrigeration applications, cold storage, food freezing and storage etc.
The boiling point of R-22 is - 41°C at atmospheric pressure. It has a latent heat of
216.5 kJ/kg at -15°C. The refrigerant is stable and is non-toxic, non-corrosive, non-
irritating and non-flammable. The evaporator pressure of this refrigerant at -15°C is
1.92 bar. This refrigerant has good solubility in oil down to -9°C. The leaks may be
detected with an electronic leak detector. The cylinder colour code for R-22 is green.