Fig 5.1 Content of this post |
Refrigerant :
Refrigerant is a media which absorbs heat energy from lower body temperature and it rejects to higher body temperature(surrounding)by changing its phase.
Eg. Air, Water, CO2, SO2,NH3, freon group(R12, R22,R502)
Classification of Refrigerant :
1. According to use :
a) Primary Refrigerant
b) Secondary Refrigerant
2. Refrigerant are also classified into four groups:
a) Halo carbon Refrigerant
b) Azeotropic Refrigerant
c) Inorganic Refrigerant
d) Hydrocarbon Refrigerant
a) Primary Refrigerant:
In primary refrigerant, refrigerant absorb directly from the product which is to be cooled in the evaporator.
Fig 5.2 Primary refrigerant |
b) Secondary Refrigerant
above fig shows the example of Ice-Plant. In this case NH3 is used as primary refrigerant. Primary refrigerant flows through compressor, condenser, expansion device, and evaporating coils. Evaporating coils are dipped in the brine solution and water cans are immerged in the brine solution. The product obtained is Ice.
Fig 5.3 Secondary Refrigerant |
In this case brine solution absorb heat energy from the product and primary refrigerant absorb heat from brine solution.
The brine solution act here as media between the product and primary refrigerant such media knows as secondary refrigerant.
Brine solution = water + NaCl.
Other Types details are as follows:
a) Halo carbon refrigerant:
In olden days initially air, water, SO2, CO2 and NH3 are widely used as refrigerant in refrigeration and Air conditioning field but they are having some limitations.
To remove this limitations American companies synthetically produce 42 compounds out of them a very few used as refrigerant like R12.
The above compound is treated freon family refrigerant. This group is widely used up to 1995.
It is responsible for ozone layer depletion. There is totally ban from 2005 and alternative refrigerant are known as eco-friendly refrigerant Eg. 134a.
India’s contribution for ozone layer depletion is about 1.5%.
ASHRAE – American society for Heating, Refrigeration and Air conditioning Engineers.
Eg. R12, R34 etc.
First Freon family R12 in 1930.
b) Azeotropic Refrigerant:
Azeotropic Refrigerant is mixture of two Freon family Refrigerant in definite proportion and then third component obtain is Azeotropic refrigerant.
Its liquid vapor phases are remain constant for a wide range of temperature. Azeotropic refrigerant having totally different properties than its parent refrigerant.
Azeotropic Refrigerant
|
Parent
Refrigerant
|
R500
|
73.2%= R12 &
26.8% = R152
|
R502
|
48.8% = R22
& 51.2 = R115
|
R503
|
40.1% = R23
& 59.9% = R13
|
R504
|
48.2% =R32 &
51.8% = R115
|
c) Inorganic Refrigerant:
This refrigerant are widely used from the beginning of the Refrigerant field.
Eg. Air, Water, CO2, NH3 etc.
Freon family refrigerant invented in 1930, to replace this group is widely use till today.NH3 having following inherent property:
1. Boiling point = 33.3 at atmospheric pressure.
2. Latent heat of vaporisation = 1315 kJ/kg at -150 C.
d) Hydrocarbon Refrigerant:
From this group Butene, Propene are having good property but this refrigerant are not used RAC field because they are highly explosive compound’s.
Properties of Ideal Refrigerant:
Refrigerant are compared at -150 C evaporator temperature and 300C at condenser temperature. There is no single compound can be treated as ideal refrigerant, but ideally refrigerant should be having following properties.
1.It should be having low boiling point at atmospheric pressure. If it is high, the compressor works increases, and capacity of of compressor decreases and then operating cost of system.
Eg. Boiling point of NH3 -33.30C.
2.Refrigerant should have low freezing point. It should be well below operating evaporator temperature. Most of the refrigerant are having freezing point temperature below -350C, hence it is not consider while selecting refrigerant.
3.High latent heat of vaporisation.
Eg. Latent heat of vaporisation of NH3 is 1315kJ/kg to obtain 1 TOM refrigerant supplied is less.
4.It should be having low specific heat of liquid because, High specific heat decreases refrigerant effect per Kg of refrigerant.
5.The pressure required to maintain evaporator and condenser should be low enough to reduce material cost must be positive to avoid leakage of air into the system.
6.It must be having high critical pressure and high critical temperature. High critical temperature is the highest temperature at which it can be condensed to liquid. If critical temperature too near to the condensing of liquid . If critical temperature too near to the condensing temperature, then excessive power consumption take place.
7.It should be having low specific volume of vapor to reduce size of compressor.
8.It should have non-flammable, non-toxic, non-corrosive.
9. It should not bad effect on sored material of food. When any leakage develops in the system.
10. It must be high thermal conductivity, to reduce material.
11. It must have high duplicity with lubricating oil and it should not have any reactive property with lubricating oil.
12. It should give high COP in working temperature range
13. It should be easily available at lower cost.
In general chemical formula is given below: CmHnClpFq
Where,
m = No of carbon atoms
n = No of hydrogen atoms
p = No of chlorine atoms
then it should satisfy the equation n+p+q = 2m+2
We have to find out number of carbons, hydrogen, Florine atoms and then designation given to Refrigerant is followed by R. If it having two digits then it indicates it is derived from methane. If it is having three digits then is derived from ethene.
The nomenclature given from left side is following way :
1. First digit is carbon atom subtracted by one.
2. Second digit from left side no of hydrogen atom added by one.
3.Chorine is not considered in nomenclature of refrigerant.
4. Third digit of Florine atom from left side as it is.
Example 1. Dichlorodifluoromethane.
Example 2. Dichlorotetrafluoroethane.
F
|
Cl - C – Cl
|
F
From above m = 1,
n = 0,
p = 2,
q = 2,
Hence nomenclature
given below:
= R(m-1)(n+1)q = R(1-1)(0+1)2 = R12
F
F
| |
Cl - C – C - Cl
|
|
F F
From above m= 2,
n = 0,
p = 2,
q = 4,
Hence nomenclature
given below:
= R(m-1)(n+1)q = R(2-1)(0+1)4 = R114
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