In classic warm air curtain installations air is expelled from bottom to top. A horizontal ar sideways air stream, one-sided or two-sided, is also possible and usually finds application where ceiling space is limited.
A bottom to top air stream is the most effective in thermodynamic terms, but is mainly found in industrial doorway applications. This type of air stream is also found in industry (for instance, in screening off welding fumes) although its use is problematic in areas where there is considerable public pedestrian traffic, for obvious reasons.