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This is because the convection rise due to combustion creates a higher velocity than the burning velocity . This effectively means that a Class 2L refrigerant is not explosive if ignited because the flame only propagates in an upwards direction from the ignition point and not rapidly outwards in all directions .
The burning velocity of R32 is at 6.7 cm / s ( 0.24 km / h ). In comparison , the burning velocity of ammonia is 7.2 cm / s , butane is 46 cm / s and hydrogen is 317 cm / s . The burning velocity of a gas is the speed of the flame front relative to motionless gas . The actual flame speed can be several times higher due to the expansion of combusting gas in combination with the burning velocity . This is especially applicable to A2 and A3 refrigerants as their higher heat of combustion generates rapid expansion and turbulence to dramatically increase the flame speed . For a gas mixture to ignite , three conditions must be met . The concentration of the flammable gas must lie between the Lower and Upper Flammability Limit ( LFL and UFL ) for the particular gas . For R32 this is between 14 % volume ( 300 grams / m3 ) and 29 % volume ( 620 grams / m ³). It should also be noted that 14 % concentration of any foreign gas in air is the accepted oxygen deprivation safety limit .
The second requirement is that the flammable gas mixture must have a velocity lower than 3 to 4 times its burning velocity ( 6.7 cm / sec for R32 ). In the case of a wall mounted split system , because R32 is heavier than air any leaked refrigerant leaving the unit will exceed 4 times its burning velocity due to gravity within 40cm . Furthermore , measurements and computational fluid dynamic models has shown that even a rapid R32 leak of 1 000 grams in one minute will not present a flammable mixture outside of the wall unit due to dilution and the falling velocity of the refrigerant .
The third requirement for ignition to take place is an ignition source of sufficient energy . R32 cannot be ignited by the usual static electricity we experience . Tests by independent laboratories in Japan and the US have demonstrated that sparks from light switches or contactors in residential appliances do not generally have sufficient energy to ignite R32 . Therefore , the most likely source of ignition in a residential application is an open flame such as a candle , combustion heater or gas cooker top .
Consequently , if an accidental release of R32 refrigerant occurs from a cylinder or piping , the velocity will be too high to ignite near the release point and the concentration will be too low where the velocity becomes low enough . So , ignition of R32 is difficult even if it is attempted intentionally .
Johann , what do we get out of this ? Well , yes , R32 is mildly flammable and precautions need to be taken . Yes , by law , persons working with refrigerants in South Africa do need training and need to be registered to use R32 and any other refrigerant for that matter .
Thanks for the question Johann , In the next issue I shall look into some of the practical aspects for working with R32 refrigerant .
REFERENCES
1 . ACRA 2 . JSRAE 3 . AIHRI 4 . CESA / AREMA
Thank you for all your questions . Send your problems ( and sometimes your creative solutions ) to acra @ netactive . co . za with ‘ Solutions Page ’ in the subject line . You may include pictures . RACA
www . hvacronline . co . za RACA Journal I April 2021 39