Any substance that absorbs heat through expansion or vaporization may be termed a refrigerant. In broad sense, refrigerant is also applied to secondary cooling medium as brine solution or cold water. As commonly interpreted, however, refrigerant includes only those working medium which passes through cycle of evaporation, recovery, compression and liquefaction. Thus circulating cold mediums, ice and solid carbon dioxide are not primary refrigerants. The primary refrigerants directly take part in refrigeration system whereas secondary refrigerants are first cooled with the help of primary refrigerant and are further used for cooling purposes.
Some common refrigerants:
1. Air: Air was one of the earliest refrigerants. Although air is free of cost and completely safe. Its low coefficient of performance (COP) makes it unable to compete with present day non-toxic refrigerants.
2. Ammonia: Ammonia is one of the oldest and most widely used of all refrigerants. It is highly toxic and flammable. It has boiling point of -28.0°F at atmospheric pressure. Its greatest application has been in large industrial and commercial reciprocating compression system where high toxicity is secondary. It is also widely used as refrigerant in absorption system.
3. Carbon dioxide: This colorless and odorless gas is heavier than air. It has boiling point of -109.3°F at atmospheric pressure. It is non-toxic and non-flammable but has extremely high operating pressures. At present time, its use is limited primarily to manufacture of dry ice (solid CO2).
4. Freon and Genetron Refrigerants: These are the series of non-toxic and non-flammable compounds having ethane and methane as bases. These fluorinated hydrocarbon are commonly known by trade name Freon and Genetron. These are the most important group of refrigerant used today. The most common of this group are
|Chemical name||Trade name|
|Trichloromonofluro methane (CCl3F)||Freon-11, Genetron-11|
|Dichlorodifluoromethane (CCl2F2)||Freon-12, Genetron-12|
|Monochlorotrifluro methane (CCl3F3)||Freon-13|
The entire group of this refrigerant is clear and water white in color and has somewhat ether odor similar to that of carbon tetrachloride. They are all non-flammable and are non-toxic for all practical purposes.
5. Methyl chloride (CH3Cl): It is colorless liquid with faint sweet non-irritating odor. It has boiling point of -10.6°F at atmospheric pressure. It is flammable and toxic to certain degree.
6. Sulfur dioxide (SO2): It is colorless gas or liquid and is extremely toxic and has pungent irritating odor. It is non explosive and non-inflammable and has boiling point of 13.8°F.
7. Water vapor: water vapor is the cheapest and probably the safest of all refrigerants. However. Because of its high freezing temperature of 32°F, it is limited in application to high temperature refrigeration.
8. Hydrocarbon refrigerants: many of the hydrocarbons are used as refrigerants in industrial installations, where they are frequently available at low cost. These include butane (C4H10), Isobutane (C4H10), propane (C3H8), propylene (C3H6), ethane (C2H6) and ethylene (C2H4). These refrigerants are highly flammable and explosive. Therefore, their use has been limited principally to the chemical and refining industries where similar hazard already exists.
9. Halogenated hydrocarbons and other refrigerants: These are the groups of chemical compounds formed from methane (CH4) and ethane (C2H6) by the substitution of chlorine, fluorine or bromine for part of their hydrogen content. Example includes methyl chloride, Freon and Genetron, Dichloroethylene (Dielene – C2H2Cl2) and ethyl chloride (C2H5Cl).
Mixture of refrigerants which do not separate into their components with pressure or temperature changes and have fixed thermodynamics unlike those of their components are called azeotropes. Several are known but only one is in common use. This is Carrene-7. This particular azeotropic mixture is important since it has theoretical refrigeration capacity is 18 % greater than that of Frean-12.
Under many circumstances, it is not desirable to carry the heat from the heat generating source directly by refrigerant. In such case, heat is carried away by using secondary refrigerant such as water and brine. The heat carried by secondary refrigerant is given to refrigerant in the evaporator and recirculated continuously. The indirect refrigeration system, in which vapor cools a circulating medium has the following advantages over direct cooling system.
1. The indirect system is easy to control and easy to handle compared to primary refrigerant.
2. The pipeline used for carrying heat by secondary refrigerant from the source is considerably smaller compared with the pipeline used with direct expansion refrigeration system. This is because specific volume of chilled water or brine is considerably low compared with specific volume of refrigerant vapor. Therefore, pipeline diameter required for secondary refrigerant is considered lower than the refrigerant pipeline diameter.
3. The indirect system keeps coil and pipes containing toxic refrigerant away from the load placed.
The commonly used secondary refrigerant are water, sodium chloride brine, calcium chloride brine and propylene glycol. When the required temperature to be achieved is above freezing point of water, then water is universally used as secondary refrigerant.
Brine: Brine is a solution containing the salt in dissolved condition in water. When the temperature required to be maintained is below freezing point of water, then brine solution is used. The freezing temperature of brine is lower than the temperature of water and it decreases with increase in salt concentration. If the concentration is increased beyond certain point, the freezing temperature of brine increases instead of decreasing. The solution at that concentration is called eutectic solution and the lowest freezing temperature is achieved at eutectic point.
|CaCl2 (30 % by wt. of salt)||-55° C|
|NaCl (23 % by wt. of salt)||-21°C|
Brine to be suitable as simple refrigeration carrying medium should
1. Remain liquid under all temperature to which they are subjected.
2. Be essentially non-corrosive when in contact with metal
3. Sufficiently high specific heat
4. Undergo no changes when in contact with refrigerants.