Abstract: Spiral or helical counterflow heat exchanger (9, 9?) consisting of two adjoining chambers (10,11), in which a fluid at a high temperature flows in one chamber in one direction, and in which a fluid at a low temperature flows in the opposite direction in the other chamber, characterized in that both chambers are separated by one separating plate (6?) of flat monolithic double-sided enamelled steel annealed at temperatures above 500° C., and whereby the separating plate (6?) is held by its edges in a corrosion-resistant spacer (8,8?) that imposes a fixed distance to two other flat monolithic double-sided enamelled steel plates that each define one chamber at the side that is opposite the separating plate (6?), and which prevents corrosion of the edges of the separating plate and of the two other enamelled steel plates.

Abstract: Disclosed is a projection and communication board, having a double-sided enamelled steel plate with an enamel undercoat, on which a second white enamel coat is laid on the writing side, which after annealing presents a roughness defined by Ra =0.6-1.2 ?m, Rz =3.0-8.0 ?m, and R3z =3.0-8.0 ?m. On this a third thin finishing coat is applied consisting of enamel powder, more than 90% of which consists of grains of less than 15 ?m, and which after annealing presents a roughness defined by Ra =0.4-1.4 ?m, Rz =2.0-6.0 ?m, and R3z =2.0-6.0 ?m.

Abstract: Projection and communication board, characterised in that it consists of a double-sided enamelled steel plate with an enamel undercoat, on which a second white enamel coat is laid on the writing side, which after annealing presents a roughness of: Ra = 0.6-1.2 ?m Rz = 3.0-8.0 ?m R3z = 3.0-8.0 ?m and on which a third thin finishing coat is applied consisting of enamel powder, more than 90% of which consists of grains of less than 15 ?m, and which after annealing presents a roughness of: Ra = 0.4-1.4 ?m Rz = 2.0-6.0 ?m R3z = 2.0-6.

Abstract: Spiral or helical counterflow heat exchanger (9, 9?) consisting of two adjoining chambers (10,11), in which a fluid at a high temperature flows in one chamber in one direction, and in which a fluid at a low temperature flows in the opposite direction in the other chamber, characterised in that both chambers are separated by one separating plate (6?) of flat monolithic double-sided enamelled steel annealed at temperatures above 500° C., and whereby the separating plate (6?) is held by its edges in a corrosion-resistant spacer (8,8?) that imposes a fixed distance to two other flat monolithic double-sided enamelled steel plates that each define one chamber at the side that is opposite the separating plate (6?), and which prevents corrosion of the edges of the separating plate and of the two other enamelled steel plates.