Open Coil:
Pro:
Much cheaper
Could be easier to replace heating elements if required
Con:
-Only good for clean, dry air
-Heating elements may not last as long (particularly if clean, dry air is not supplied)
-During maintenance, energized elements can cause electrical shock
Fin-Tubbed:
Pro:
-Can be supplied with dirty or wet air
-Elements pose no electrical shock hazzard (but circuitry can; also, burn hazzard can exist)
Con:
-More expensive
-May or may not be as easy to replace damaged or burned-out elements
Could be more. These are off of the top of my head. As for the replacing of the elements goes, this will depened largely on the manufacturer and the overall design of the heater in question.
Open coil=fast responce to control. High watt density. Say Hot,Hot,hot...Like radient.
Sheathed or fin coil heater have a lag to control. i.e. mass/inerta. Say drag...
Fin heaters have a great advantage where lots of heat is needed but low watt density (surface heat)is required.
Sheath or finned are good in low pressure applications (altitude/vacuum) and humidity situations.
Humidity applications require very a good seal at terminations. One alternative is to have the seals/connections (terminals) outside the wet environment. Through the wall so to speak.
