Air heaters, or Convection Heaters, heat a gas which transfers heat to the target material.
Common uses for air heaters include drying, laminating, sealing, shrinking, softening, annealing, bonding, activating, melting and welding.
Application determines the construction of the air heater. Variables are: air source; exit and inlet temperature; air pressure; volume of air; air velocity; installation space; temperature response requirements and required operating temperature range; quality (cleanliness) of air; and installation or plumbing connections.
Electric air heater elements can be sheathed heaters, or bare resistance wire (open coil heaters). Each has advantages and disadvantages.
Sheathed Elem
ents are durable, are electrically insulated from gas flow, and are more durable than open coil. Sheathed elements require larger installation space and are respond more slowly to process change.
Upper limit exit air temperatures approach 1000 Deg F for sheathed heaters. Applications requiring high temperatures or recirculation must closely consider element watt densities.
Sheathed elements are ideal for: duct heaters; applications not requiring fast temperature response; when space is not a consideration; or for contaminated inlet air. Sheathed elements can be tubular heaters, finned heaters, or cartridge heaters.
Open Coil Heaters use bare resistance wire to heat air. 
Because resistance wire can operate at greater watt densities, greater heat transfer occurs in smaller space, resulting in higher exit temperatures and / or smaller physical heater size.
Because of low element mass, open coil heaters respond faster to changes in power levels. Fast response allows precise temperature control. Open coil heaters require less space than sheathed elements. Process connections and plumbing requirements are also simplified.
Higher watt densities cause open coil heaters to be more susceptible to catastrophic failure when heat cannot be transferred to the gas. Control improvements have helped to reduce or eliminate the potential for failure due to rapid changes in air flow. Open coil heaters are more sensitive to contaminated air. Temperatures of 1600 Deg F are possible with careful control.