Question

Comparison between horizontal and vertical condensation

Answer 1

Tubes are smooth. And for smooth surface the condensation will be of film condensation type i.e. it will form a film over the surface during condensation. The film thus formed will act as an insulation and therefore will decrease the effective heat transfer coefficient between the tube surface and air. Now due to gravity the thickness of the film will increase as we go downward. More the thickness of the film less will be the heat transfer coefficient. In horizontal position of the tube the thickness will be less as compared to vertical position ( as the vertical height in case of horizontal position will be less). Therefore in case of horizontal position the effective heat transfer coefficient will be comparatively higher. Thus the tubes are placed in horizontal position for faster condensation.

Answer 2

main difference b/w horizontal and vertical heat exchanger 
is "residence time" fluid which flow in horizontal heat
exchanger gets more residence time as compared to vertical 
heat exchanger.

construction wise both r same but functionally they differ.vertical is mainly used when less floor area availible.and vertical required higher pumping cost also good for handling heat sensible material as resident time is much lower in comparision to the horizontal one.