Optimum coverage of surface area on heating element/wire
This is more of a mechanical problem than an electrical one. 

However, when heating air, the trick is to have the air in contact with the heating surface for as long as you can. As such, having the coils run in line with the airflow is better. 

Also, the more turbulent the air is the better. If the air is laminar, only a small percentage of it will collide with the heater coil. 

As such, making your coil 3 dimensional, going in and out of the image above, instead of flat, and having a stationary "fan like" plate on the inlet side of the coil to make the forced air spin around inside the heater coil would increase your heat transfer efficiency.

It does however, look like a dangerous contraption, also, a heater alone will not "dry" air, though it may feel less "damp".