Refrigerants

A refrigerant is a fluid used in refrigeration system to transfer energy by its phase change from liquid to gas and vice versa. The refrigerants are used in Domestic Refrigerators, Air Conditioners and Central Chilling Plants. The best suitability of a refrigerant for a particular system is based on its thermodynamic properties, chemical properties, physical properties, safety and economic criteria.

The critical temperature should be as high as possible above the condensing temperature in order to have larger range of isothermal energy transfer. The specific heat of the fluid should be as small as possible. The thermal conductivity should be as large as possible so that size of evaporator and condenser becomes smaller. The Freezing point of refrigerant should be as low as possible.

The density of vapour refrigerant should be as large as possible. The refrigerant should be inert and should not react with the materials of refrigeration system. It should be chemically stable for the operating conditions. The refrigerant should be non toxic and non flammability in safety concern.

Here a few refrigerants which are in the common commercial use.

R11 – Trichlorofluromethane

R12 – Dichlorofluromethane

R13 – Chlorotrifluromethane

R14 – Tetrafluromethane

R21 – Dichlorofluromethane

R22- Chlorodifluromethane

R113 – Trichlorotrifluromethane

R123 – Dichlorotrifluroethane

R134a – Tetrafluroethane

As chemical names of refrigerants are long and complex, a method of referring refrigerants by numbers was formed. The method of designating a refrigerant by number as follows.

1^st Digit on the Right    - No. of Fluorine atoms

2^nd Digit from the Right    - No of Hydrogen atoms plus one

3^rd Digit from the Right    - No of Carbon atoms minus one (not used when equal to zero)

Example:

R 123 CHCl₂CF₂ Dichlorotrifluroethane

No of Fluorine atoms = 3

No of Hydrogen atoms + 1 = 2

No of Carbon atoms – 1 = 1

Thus it forms R 123

The lower case letter that follows the refrigeration designation refers to the form of the molecule when different forms (isomer) are possible. The number alone will be indicated if the form is symmetrical. If form becomes unsymmetrical, the letters a, b, and c are appended such as R134a.

The refrigerants can also be abbreviated as follows

Chloroflurocarbons (CFC)

Hydrochloroflurocarbons (HCFC)

Hydroflurocarbons (HFC)

Hydrocarbons (HC)

Refrigerant blends are formed by mixing two or more refrigerants. Blends can have as many as four refrigerants mixed together to give properties and efficiencies similar to the refrigerants. The blends are referred in 400 and 500 series in R Numbers. If a blend has R 407 designation, it is a near azeotropic blend. The 7 indicates that it is the seventh one produced in the series in the market.

ODP and GWP Is the most important terms used in refrigerants. ODP means Ozone Depletion Potential. Ozone is a gas layer found in stratosphere and troposphere to prevent harmful UV radiation from reaching the earth. Now it is rapidly being depleted by manmade chemicals containing chlorine, including refrigerants such as CFC and HCFC. An index called ODP is used for regulatory purposes. The higher the ODP, the more damaging chemicals to the ozone layers. The heat energy radiated by earth back into the atmosphere is absorped, reflected or refracted by gases such as CFC, HCFC and CO₂ and prevented from escaping to atmosphere. This warming process caused by atmospheric absorption is called Greenhouse effect or Global Warming. This is measured by an index Global Warming Potential (GWP). Refrigerants leaking from the system contributes to global warming. This is measured by comparing them to carbon dioxide which has a GWP of 1. CO₂ is the number one contributor to global warming.

There are many applications which restrict the direct use of refrigerants due to safety consideration. Under such circumstances the cheaper grade cooling media such as water, brine solution of sodium chloride or calcium chloride is selected.