a portable oil field heater heats a liquid water - petroleum mixture by passing it through a single continuous path primary coil , and then a single continuous path secondary coil . the continuous path primary coil extends along the walls of the heater , and the secondary coil is a bundle above an lpg burner . the air for combustion is introduced from the top of the heater through false walls to beneath a false floor beneath the burner . thus , it is not necessary to have insulation along the sides of the heater ; thereby greatly reducing weight and maintenance . the safety of the unit is greatly improved by introducing the combustion air from a height well above the level of the burner .