Properties of Refrigerants :-

R-11 — Ccl₂F- Trichloro-monofluoro carbonf methane) - has ozone depletion potential (ODP) of 1 and greenhouse potential of 3300(GP). GP is also called as Global worming potential (GWP) take as datum i.e. 3300 as 1. it is a very low pressure refrigerant suitable for large air conditioning installation in cruise ships and for cleaning out massive refrigeration machinery.

R11.R12 &R502- phased out since 1997 as per the international agreement Montreal Protocol signed in 1987.

CFC's released to the atmosphere are broken down by photolysis

to chlorine atoms which catalytically destroy ozone. CFC's also inhibit the release of heat radiated from earth thereby contributing to global warming.

R-12:- Ccl₂F₂ - Dichloro-difluoro carbon- ODP of 0.98 and GP of 10000 and GWP of 3.05. phased out as per Montreal protocol.

R-22:- CHcIF₂ - Hydro-monochloro-difluro carbon- ODP is 0.05, GP 1100, GWP 0.365. - replacement for R-l 1 and R12 till year 2020.

R502 :- Azeotropic mixture composed of 48.8% R-22 and 51.2% of R115. chemical formula of R115 is CclF₂CF₃ and has ODP of 0.6 and a GP of 25000 and included in Montreal convention. ODP of R502 is 0.23 and GWP of 5.1. It is particularly suited in hermitic sealed compressor.

R-134a :- best replacement of R-12, chemical formula CF3CH2F with an ODP of O and a GPof900.

R-123:- replacement for R-l 1. It is an HCFC with chemical formula of CHCI2CF3 with an ODP of 0.02 and a GP of 50 only.

R-125:- replacement for R-22, it is an HFC of chemical formula CHF₂CF₃ with an ODP ofOandaGPof 1900.

R11,R12,R22 & R502 do not react with steel, copper, aluminium or brass, but attack lead,tin,zinc, magnesium and their alloys. They also attack natural rubber, some elastomers and polytetrafluoroethane (PTFS), so correct materials, gaskets, seals to be chosen.

R717(Ammonia):- it has an ODP of O and GWP of O. It is flammable in air at concentrations on 16% to 27% and may form an explosive mixture. It is extremely toxic, has a pungent smell with a long term threshold limit of 35ppm (TLV). With TLV exceeding 1500ppm can damage body tissues and exceeding 2500ppm can cause death. However leaks can be instantly detected at less than 10 ppm concentration by the pungent odour which is a safety feature. It has high COP valve. NH3 reacts with copper, zinc and their alloys (brass & bronze) if water present. Only steels to be used in plant constructions. NH₃ is highly soluble in water forming ammonium hydroxide which is a weakbase. About 1300 volume of NH3 can be dissolved in 1 volume of water at low temp. However it is expelled by boiling. This action makes the vapour absorption refrigeration possible, which does not require a compressor for operation only a heat source. Its boiling point -33°C and Sp.volume 0.51 mVkg at -15°C .

R744 carbon dioxide :- Not explosive or flammable but leakage is potentially dangerous in a confined space, it is also toxic, working pressure is 70 bar at compressure discharge and 20 bar at compr,suction, so construction is heavy. Critical temp, is low (31°C ) and cause problem in condenser if sea-water temp, is high. It has low COP valve. Poor miscibility with oil requires a drain system at the evoporator to drain oil continuously. Gas is stored as liquid at a high pressure in a steel press vessel in a cold space. Temp, rise cause a press.rise and released by a safety valve or bursting disc. It has low boiling point(-78°C at 1.013bar) and low sp.volume 0.017 m³/kg at-15°C.

Comparison:- Each kg of CO2 has sp. Enthalpy of vaporization    275 KJ
"   "   NH₃ "    "       "       "        "       1314 KJ

"   "   R-22 "  ,"      "       "       "       158.7 KJ

whereas

5kg of CO2 which extracts 1375 KJ has a volume of 0.085 m3
lkgofNH₃"                                          1314"   "    "        "      "   0.51 m3

8 Vi kg of R-22 "        '"     1350"       "         "       "   0.79 m3

Desirable properties of Refrigerants.

(1) It should be non-flammable, non-explosive and non-toxic and it should not contaminate foods or damage the environment in the event of a leak.

(2) It should  be  con-corrosive  and  should  not  react  unfavourably  with lubricants, moisture and materials in plant construction.

(3)It should have moderate condensing pressure. The condensing pressure should be as low as possible in order to keep down the mechanical strength required in the compressor, condenser and high pressure side
pipings and to reduce leakage risk.

(4)The evaporating pressure should be as high a possible because press below atm. result in air and moisture being drawn into the system in the event of a leak.

(5)It should have high COP valve.

(6)High latent of evaporation so that less refrigerant mass flow for a given duty.

(7)Low sp.heat  of the  liquid  as  it is desirable during throttling at the exp.valve, causes liq. refrigrant to be cooled at the expense of partial evaporation.

(8)High critical temp, as it is impossible to condense at a temp above the critical temp, no matter how much the pressure is increased.

(9)Should have low moderate temp, after compression such that risk of oil decomposition and overheating of the compressor is less.

(10) Easy leak detection.

(11) Low cost, readily available and easily stored.

(12) It should not cause problem of oil-returning back to the compressor.

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