band / cylinder style heating element wattage calculations
The wattage is easy. First convert to SI units to make life simpler, then it's simply heat capacity (J/k/kg) * temperature increase (K) * mass flow rate (kg/s) - giving J/s, namely, Watts.

The heat transfer, however, is not so simple. You have a very small contact area (the wall of a 10" cylinder) and poor contact between the gas and the wall - some texts would use the tube diameter, 4", as the characteristic length. I suspect you're going to have to massively rethink the heat exchanger to increase the contact area and reduce the characteristic length (distance the gas is away from the wall). For example, many small diameter tubes (shell and tube HX) or  given it's electric, directly heated wire matrix inside the tube.

Which is a much bigger topic, and not EE.