The orientation of the exchanger is of little consequence to efficiency when compared to the thermal design characteristics and the choice may be left to mechanical and space limitations. If a solvent is boiling off of a reactor and must be condensed and removed from the system then the overhead room may dictate whether vertical or horizontal is desired. If, however, the solvent is boiling off of a reactor and needs to be condensed and returned to the reactor, then a dephlegmator style condenser may be most advantageous because of the mechanical simplicity and lack of connecting piping. See Process Heat Transfer by Hewitt, Shires & Bott or Perry’s Chemical Engineers Handbook for references.