To provide an apparatus for forming a dry gas curtain on a low temperature operating surface to avoid condensation and frosting on the operating surface.

The apparatus is to provide a laminar flow curtain of dry gas on a low temperature operating surface 18 having a housing duct 26 extended in the longitudinal direction from the surface 18. A honeycomb member 30 satisfies the zone 31 of a housing duct adjacent to the operating surface, and each honeycomb cell 32 has a flowing passage in the longitudinal direction. Porous foam 34 having the passage satisfies the zone 33 of the duct adjacent to the member 30 at the opposite side to the operating surface. A gas plenum 36 is disposed adjacent to the porous member 34 at the opposite side to the member 30. In order that the laminar flow curtain is formed by passing the foam 34 and the member 34, dry gas is supplied to the plenum 36.