Helical model is most commonly used in air heaters

You can use insertes inside tubes in order to increase the heat tranfer rate. Moreover, you can use tubes with more rought surface in order to create turbulence in flow. In any case, the possible pressure drop have to be taken into acount

Analogous to automobile radiator design, I believe a "zig-zag" section tubes might have better effectiveness. Also from material standpoint, copper can be easily fabricated to suite that style.

according to the shape,

Circular, Rectange and Square,

According to the design,

curved tube, Riser tube