ABSTRACT Refrigeration has made the milestone in human life . Since from very long time the human being has developed so many ways of refrigeration for preserving food, cooling water, etc. but yet the techniques are developing. The science of refrigeration has developed so many ways to improve our lifestyle. Our project considers law of conservation of energy maximum utilization of the fossil fuel and saving energy. This project has done LPG as refrigerant, instead of using closed cycle as it is used in normal practice (VCR),the open cycle has been used. Before burning the LPG in burner, it is passed through the expansion device and evaporator and produce the refrigeration effect without using electric energy. LPG is cheap and posses environmental friendly nature with no ozone depletion potential. 1
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ABSTRACT
Refrigeration has made the milestone in human life . Since from very long
time the human being has developed so many ways of refrigeration for preserving
food, cooling water, etc. but yet the techniques are developing. The science of
refrigeration has developed so many ways to improve our lifestyle.
Our project considers law of conservation of energy maximum utilization of
the fossil fuel and saving energy. This project has done LPG as refrigerant, instead of
using closed cycle as it is used in normal practice (VCR),the open cycle has been used.
Before burning the LPG in burner, it is passed through the expansion device and
evaporator and produce the refrigeration effect without using electric energy.
LPG is cheap and posses environmental friendly nature with no ozone depletion
potential.
1
CHAPTER – 2
LITERATURE SURVEY
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2. LITERATURE SURVEY
COOLING METHODS-
From the ancient age man always prefers to have cold water for drinking purpose. Also
in India, our country the weather is too hot. Particularly in summer season, the normal
water temperature in open space is 35 to380 C. The water is not suitable for drinking
purpose. The required temperature is at the most 250 C. Hence lowering down of the
temperature is an essential task.
In old age there was ample space available to keep the earthen container
also the houses were sheltered by the trees and bushes which were enough to lower
down the water container temperature. Because in most of the situations the
temperature was depending on the temperature of the flowing air.
OTHER WATER COOLING PROCESSES:-
The different water cooling processes incorporated are since old age are as
Follows:-
1. Earthen pots
2. Using ice
3. Using vapour compression refrigeration cycles
4. Using vapour absorption cycle
Cooling of water in earthen pot is one of the earliest methods
employed by men for cooling the water in their houses. Only in recent years, it has
been put on sound footing thermodynamically. It is a process of adiabatic saturation of
air when cooling of surface water on container is made to evaporate to cool t with
transfer of heat from water to the surroundings. The initial investment cost of such a
system is low & the operation is simple & cheap. Simple evaporative cooling is
achieved by direct contact of water particles & a moving air stream. The water may be
sufficiently cooled by evaporative process to results a considerable degree of drinking
comfort in climates of high dry-bulb temperatures associated with low relative
humidity. The minimum outdoor temperature required for successful evaporative
cooling is above 350 c & another requirement is a relatively low. Wet bulb temperature.
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The cooling effect given by the evaporative cooling always depends upon the outdoor
temperature although the evaporative cooling does not perform all the function of true
water-conditioning but it provides coolness by filtering heat & circulating the cooled
air.
LIMITATION OF SIMPLE EVAPORATIVE COOLING SYSTEM:
Its capacity is limited by WBT of the ambient air. Maximum cooling achieved is
the wet bulb temperature. It is not useful for high D.B.T. & high summer weather
conditions. However, even with all this limitations, there are many regions in India as
the part of Rajasthan; part of Bihar Vidharbha in Maharashtra & some hot spot in
North where evaporative cooling will produce a condition well within the summer
comfort zone. There is increasing demand of evaporative earthen pot coolers in these
regions, as they are quite inexpensive compared with refrigerated water-cooling
system. Hence quick operating and reliable Vapour compression system, which is
having the massive capacity of cooling, is used.
Generally refrigeration is defined as any process of heat removal. More
specially, refrigeration is defined as the branch of science that deals with process
of reducing and maintaining the temperature of a space or material bellow the
temperature of the surroundings. Since ancient times, human being searched for
warmth in cold weather and cooling in hot weather. The ordinary people used lakes
and rivers to get relief from hot weather. Others brought the lake water to houses and
circulated it inside the house in the form of channels or fountains. But in modern time
this search led to refrigeration systems. The system maintained at the lower
temperature than surrounding atmosphere temperature is known as refrigeration system
while the equipment used to maintain this lower temperature is known as refrigerating
machines.
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Fundamentals of refrigeration-
The refrigeration may be defined as the process of removing heat from
substance under controlled conditions. It also includes the process of reducing &
maintaining of temperature of body below the general temperature of its surrounding
in other words, the refrigeration means continue extraction of heat from a body whose
temperature is already below the temperature of surrounding.
Types of refrigeration system-
1. Air refrigeration system –
In air refrigeration cycle, air is used as refrigerant. In olden days, air was
widely used in commercial application because of its availability at free of cost. Since
air does not change its phase ,therefore the heat carrying capacity per kg of air is very
small as compare to vapour absorption system.
2. Vapour compression system-
Vapour compression refrigeration system is an improved type of air
refrigeration system in which a suitable working substance, termed as refrigerant. It
condense and evaporates at temperature & pressure close to atm. Conditions.
Refrigerant used for this purpose is ammonia, carbon dioxide & sulpher dioxide.
3. Vapour absorption system-
The vapour absorption refrigeration system is one of the methods of producing
refrigerating plants. Refrigerant commonly used in vapour absorption system is
ammonia.
4. Steam jet refrigeration system-
In this application, however in the system jet have been supplanted generally
by system using centrifugal compressor. Now the system jet unit is remaining
important especially for industrial uses such as chilling of water to moderate
temperature in process industries.
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Cold storage –
In country like India the problem of storage of vegetables, fruits, and fishes
is accomplished by the use of cold storage. It is the best known method of
perseveration of food & other things. From very longer time the method of cold
storage is known to human beings.
The temperature designed for the storage is approximately -20±2ºc for most of
the foods. Essential requirement of cold storage is to design a refrigerating system that
would be energy efficient, less costly, and flexible with high reliability & ease of
maintenance. There are various methods of cold storage but as far as medium
temperature is taken while designing a cold storage system 0ºc to 8ºc temperature
range is taken. In case4 of frozen food storage the temperature rang is -18ºc to -22ºc is
taken.
The following table gives different values for different products-
PRODUCT STORAGE TEMP.
(ºC)
PRODUCT LIFE
(WEEKS)
STORAGE SEASON
Tomatoes 7 4-5 April-May, sep-Oct
Lime 8 6 Mar-April, Aug-Sep
Ginger 2-4 14 Feb-April
Orange 7-8 8 Feb-April
Mosambi 6-8 8 Apr-July , Nov-Feb
Apples 0-15 16-20 Oct-Mar
Eggs 1-5 20-40 May-Dec
Mango 8-10 4-6 May-June
Frozen foods -18 to -20 24 & more Year round
Figure 1: Schematic of vapour compression refrigeration cycle.
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LIQUID VAPOUR
HIGH PRESSURE
VALVE COMPRESSOR
LOW PRESSURE
Win
VAPOUR & Qout VAPOUR
LIQUID
Where,
R.E = Refrigerating Effect
Win= Work Input
The above diagram shows the simple vapour compression system. It includes
following components,
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CONDENSER
EVAPORATOR
COP = R.E/Win
1. Evaporators –
It is an important device used in low pressure side of refrigerating system. The
liquid refrigerant from the expansion valve enters in to the evaporator where it is boils
& changes in to vapour. The function of evaporator is to absorb heat from surrounding
location or medium. Evaporator becomes cold & remains cold due to following two
reasons –
1. Temperature of evaporator coil is low due to the low temperature of the refrigerant
inside the coil.
2. The low temperature of the refrigerant remains unchanged because any heat absorbs
is converted to latent heat as boiling proceeds.
Compressor-
Compressors are the heart of vapour compression system. It performs two
functions; one is to draw vapour from evaporator & lowers the pressure of refrigerant
in evaporator to desired level. And other is it rises pressure of refrigerating vapour in
condenser enough so that saturation temperature is higher than the temperature of
cooling medium.
In order to secure satisfactory performance, long life, and to protect the
compressor against overload, certain design criteria should be observed. Compressor
application notes and data should always be consulted when designing a system.
Ensure compressors are clearly labelled to indicate that refrigerants are being used in
the system. The use of crankcase heaters should be considered to avoid excessive oil
solubility.
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Condenser-
This is another important part of refrigeration system, which needs more
considerations its selection. The condenser removes the heat from refrigerant carried
from evaporator & added by compressor & convert the vapour refrigerant into liquid
refrigerant.
It is heat exchanger in which heat transfer takes place from high temperature
vapour to low temperature air or water, which is used as cooling medium.The types of
cooling media available air water. Both can be used separately combines as per the
availability and requirement. The condensers are classified according to cooling
medium used.
Expansion valve-
It is an important device used in refrigeration. It performs two functions, one
is it reduces high pressure liquid refrigerant to low pressure liquid refrigerant before
being fed to evaporator. And second it maintains desired pressure difference between
high & low pressure side & it also controls the flow of refrigerant.
Design and selection criteria are the same as those for conventional
fluorinated refrigerants. Capillary tube length is specific to each refrigerant. Computer
programmers and tables are available for determining capillary tube size and length,
although trial and error is generally the preferred route. Thermostatic Expansion
Valves (TEVs) for hydrocarbons are available. Alternatively TEVs for other
refrigerants that operate with similar pressure-temperature relationships can be used.
Electronic Expansion Valves (EEV) may also be used. EEV’s used in hydrocarbon
systems must conform to the requirements of electrical components.
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Refrigerants-
Definition-
Any substance capable of absorbing heat from another required substance can be used as
refrigerant.
Desirable properties of refrigerants-
Low boiling point.
High critical temperature.
High latent heat of vaporization.
Low specific heat of liquid.
Low specific volume of vapour.
Easy to liquify at moderate pressure & temperature.
Easy of locating leaks by odour or suitable indicator.
It should be nonpoisonous.
It should be non explosive
It should be non corrosive
It must be nonflammable
Leaks should be easy to detect.
Leaks should be easy to locate.
It should operate under low pressure.
Part moving in the fluid should be easy to lubricate.
It should be nontoxic (not harmful if inhaled or if spilled on skin).
It should be a stable gas.
Classification of refrigerants-
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Halo carbon refrigerants –
1. R11(CCL3F) -
Synthetic, non flammable, non toxic, low pressure refrigerant
0.2 bar at -15ºc
1.2606 bar at 30ºc
B.P=23.77ºc at atm. Temp.
Used in centrifugal compressor system of 200 TR,
Leak detected by soap solution halide torch,
Cylinder colour code is orange
R12 (CCL2 F2) -
Popular, colourless, almost odourless, non toxic, non corrosive, non flammable
B.P= -29ºc at atm. Pressure
Low latent heat value,
Require less sensitive & more positive,
Low but positive head & good volumetric efficiency.
Refrigerator, freezer, water coolers, room & window Arcs , etc.
0.82 bar at -15 ºc & 6.4 bar at 30ºc
Latent heat of 159KJ/Kg at -15ºc
Leak can be detected by soap solution, halide torch & electronic detector
R22 (CHClF2)-
Man made, to maintain temp. of -29ºc to-40ºc
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A/C units & household refrigerators
Reciprocating ¢rifugal compressors
B.P= -41ºc at atm. Pressure
Latent heat is 216.5KJ/Kg at -15ºc
Non toxic, non corrosive, no irritating, non flammable
Water mixes well with R22 than R12 by ratio as 3:1 hence drier has to be used
Colour code is green
Azeotrope refrigerants –
R500 -
Non flammable, non corrosive, less toxic
Reciprocating compressor
20% greater refrigerating capacity than R12
Condensing pressure 7.78 bar at -30ºc & evaporating pressure 1.37 bar at -15ºc
Latent heat 198KJ/Kg at -15ºc
Highly soluble in oil
Cylinder colour code is yellow
Inorganic refrigerants –
R717 ( NH 3) -
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Oldest, used in large & commercial plants, where toxicity is secondary
Absorption system
Under normal condition it is colourless gas
B.P= -33.33ºc at atm. Pressure
M.P= -78ºc from solid
Latent heat of vaporisation is 1315KJ/Kg at -15ºc
Large refrigerating effects with small sized machinery
Condensers pressure at 30ºc is 10.78 bar & water cooled type
Poisonous & toxic, irritating to eyes, nose & throat
Flammable, with 16% to 25% air by volume
Lighter than oil hence oil can be easily separated
Large reciprocating compressors
Ice manufacturing; bear mfg. Food freezing, cold storage, etc.
Hydrocarbons-
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Code Chemical name Triple (°c) Boil (°c) Critical(°c)