Refrigerants are the “working fluids” in a refrigeration cycle that absorb heat at low temperature/pressure and reject it at high temperature/pressure.
1. Ideal Properties of a Refrigerant
To be effective and safe, a refrigerant should ideally possess:
-
Thermodynamic: Low boiling point, high Latent Heat of Vaporization (to carry more heat), and low specific volume.
-
Chemical: Non-toxic, non-flammable, non-corrosive, and chemically stable.
-
Physical: Low viscosity (for easy flow) and high thermal conductivity.
-
Environmental: Zero ODP (Ozone Depletion Potential) and low GWP (Global Warming Potential).
2. Classification of Refrigerants
Primary Refrigerants
These take part directly in the refrigeration cycle by undergoing a phase change (e.g., $R-22$, $R-134a$).
-
Halocarbons: Synthetics containing Chlorine, Fluorine, or Carbon.
-
CFCs (Chlorofluorocarbons): High ODP. Ex: $R-11$, $R-12$. (Banned/Phased out).
-
HCFCs (Hydrochlorofluorocarbons): Lower ODP. Ex: $R-22$. (Being phased out).
-
HFCs (Hydrofluorocarbons): Zero ODP, but high GWP. Ex: $R-134a$, $R-32$, $R-410A$.
-
-
Azeotropes: Mixtures of two refrigerants that behave like a single pure substance (boil at one constant temperature). Ex: $R-502$.
-
Zeotropes: Mixtures where components evaporate/condense at different temperatures (Temperature Glide). Ex: $R-407C$, $R-410A$ (near-azeotropic).
-
Inorganic/Natural Refrigerants: * Ammonia ($NH_3$ / $R-717$): Highest refrigerating effect; used in cold storage. Toxic/Flammable.
-
$CO_2$ ($R-744$): High pressure, low GWP.
-
Water ($R-718$): Used in steam jet refrigeration.
-
Secondary Refrigerants
These are cooled by the primary refrigerant and then transported to the cooling load.
-
Examples: Water, Brine (Salt + Water), Glycols.
3. Important Refrigerant Codes & Facts
| Refrigerant | Chemical Name | Common Use | Key Property |
| $R-11$ | Trichloromonofluoromethane | Large Central AC | Low pressure |
| $R-12$ | Dichlorodifluoromethane | Domestic Fridge | Phased out (High ODP) |
| $R-22$ | Monochlorodifluoromethane | Window/Split AC | Widely used in India |
| $R-134a$ | Tetrafluoroethane | Car AC / New Fridge | Chlorine-free (HFC) |
| $R-717$ | Ammonia | Ice Plants / Cold Storage | Attacks Copper (use Steel) |
| $R-290$ | Propane | Eco-fridges | Highly Flammable |
| $R-600a$ | Isobutane | Modern Fridge | Green refrigerant |
4. Numerical Designation (Naming Rules)
Formula for $C_mH_nF_pCl_q$ (where $n+p+q = 2m+2$):
-
Refrigerant Number = $R-(m-1)(n+1)p$
-
Example for $R-22$ ($CHClF_2$): $m=1, n=1, p=2$. Calculation: $(1-1)(1+1)2 = 022$ $\rightarrow$ $R-22$.
-
Inorganic compounds: $R-700$ + Molecular Weight. (Ammonia $17 \rightarrow R-717$).
5. Safety & Environmental Facts (Exam Special)
-
Color Codes: $R-12$ (White), $R-22$ (Green), $R-134a$ (Light Blue), $R-410A$ (Rose/Pink).
-
Leak Detection: * Halide Torch: Flame turns Green (for Freons).
-
Soap Bubble: Universal method.
-
Sulphur Stick: Produces white smoke (for Ammonia).
-
-
Moisture: Lead to “Sludge” formation or expansion valve “Freeze-up.” Use Driers/Silica Gel.
-
Lubrication: $R-134a$ requires PAG oil (Synthetic), whereas $R-22$ uses Mineral oil.