The picture shows an apparatus which uses a pellet catalyst. The catalyst needs a temperature of about 250 °C. The heating is achieved with earth gas. The hot air which leaves the catalyst is used to preheat the inflowing air. Since the pellet catalyst is mechanically not very stable, the flow direction of the gas cannot be changed. As a consequence a metal heat exchanger must be used which can only recover about 70 % of the heat in the off-gases.