hvac basic---------------hvac means heating ventilation air conditioner system is very important these days invented in usa in between 18-19 century initially it was invented because for preventing food stuff at summer from geting roted initiaaly ice cube used for this system but in late phase refrigarent was invented it was a basically compostion of sulphur+methyl+ammonia gas but due to their undesired properties gas was stop to used.after many phase at present clorine flourine carbone has been use for refrigarent gas

,ok just for check