Abstract: In a fluid bed apparatus in which a layer of pulverulent or particulate material is fluidized by means of fluidizing gas which is introduced through openings in a bed plate in a first plurality of streams of flows of gas, means are provided for introducing a second plurality of streams of flows of gas at a site or sites of introduction and is directed from said site or sites along the surfaces of at least some of the walls of the housing to form a gas barrier covering at least a substantial part of said wall surfaces and the ceiling of the housing thus reducing detrimental effects, in particular deposition of products, condensation and corrosion, on the surfaces of the walls and ceiling of the housing.