or gas produces burnt gas at a temperature of the order or 2000 u00b0 c which in the heater space 2 of limited size produces heat which is initially transferred to the heater tubes 3 for heating for heating the working gas for instance helium flowing there through to the working temperature such burnt gas then also gives up heat in the air preheater 1 9 before being finally led off from the heater system 1 6 by means of an exhaust gas pipe the heater tubes 3 are connected in a conventional manner with manifold ducts which are not shown and in the design shown in fig1 and 2 extend from a lower plane marked by the broken line 4 with parallel sections 3 1 so as to extend into the heater space 2 the heater tubes 3 are bent in v shape and form where there are u like spaced and parallel sections 3 2 such sections forming a heater tube wall perpendicular to the direction of the burnt gas flow in the case of fig1 and there are two such heater tube wall arrays due to the v like arrangement of two cylinders 1 3 and the associated cooler units 1 5 the heater space 2 or chamber having the burnt gases flowing through it is closed on all sides to form a burnt gas duct that is to say at the bottom by a thermally insulating layer 5 not shown in detail at the top by a device for ducting.