Coil Heaters
Open coil heaters are coiled resistance wire (usually Ni-Chrome or Kanthal, but also others) fixed onto a supporting element.
The selection of the correct wire gauge, wire type and coil diameter requires quite some experience. There are standard elements available on the market, but quit often they need to be custom built. Open coil air heaters work best below air velocities of 80 FPM. Higher air velocities could cause the coils to touch each other and short out. For higher velocities, select a tubular air heater or strip heater. its recommended to use a flow switch in the airstream to avoid any overheating of the system.
Advantages
The big advantage of open coil heating elements is the very quick response time.
There are standard open coil heating elements available on the market. Most of these elements need a constant airflow over the resistance wire, but if the watt densities are low enough they may not burn out in still air.