Well not knowing the "installed air flow" kinda limits what you can
figure. But in any case try to keep the air temperature rise to
something reasonable, say 60F over the incoming air, try to avoid dead
spots with low flow, and look at the coil to make sure it's not more
than a dull red, assuming it's a nichcrome wire type coil. An air flow
sail switch wired to a relay for the coil wold not be a bad idea in
case the flow drops or the fan quits for some reason. A duct heater in
this situation would probably ignite any combustible vapor or material
drawn into the air inlet on the coils...