The hot exhaust gases are deflected at the end of the combustion chamber on the baffle plate of the flame tube (9) and flow back between the flame tube of the burner and the inner wall of the heat exchanger.

According to the invention, the fuel is fed centrally into a fire tube (3), where it is mixed with combustion air in a combustion zone.

The spacing between the smoke tubes, as well as from the smoke tubes to the boiler shell and to the flame tube, substantially exceed requirements.

The lateral arrangement of the flame tube results in a highly compact boiler design that meets all the requirements for efficient steam generation.

The lateral arrangement of the flame tube resulted in a highly compact boiler design that meets all the requirements for efficient steam generation.

The shearing force on the end face floors caused by different linear expansion in the smoke tubes and the flame tube is therefore extremely low.

The air exits through first and second air line nozzles (4 or 5) which are arranged in two directly adjoining rows (6, 7) and inclined in the direction of the counterflow at an angle of 60° in relation to the axis of the fire tube.

At least one opening (5, 6) is in each case provided in the area of the mixing zone (15) and in the area of the post-primary combustion zone (18), in order to pass compressed air into the flame tube (3).

The first gas part stream comprises additional air and re-circulated, uncleaned flue gas from the combustor (10), said gases being combusted together with fuel inside the flame tube (40).