flows axially through the ducts a number of these according to the embodiment illustrated those in every third vertical column are at their ends provided with magnetrons 8 supplying microwave energy via boxes 9 each of these boxes is air tight connected to one of the ducts at the inlet end thereof as has been shown the boxes are perforated to permit air inlet however those perforations are so small that the microwaves cannot exit through them accordingly there does inside each magnetron equipped duct appear two media hot drying air and microwaves the duct outlet ends are sealingly connected to an outlet chamber which in the same way as at the inlet end is formed by the provision of a partition 10 inside the casing 1 from that outlet chamber the air exits through conduit 3 after having first passed through openings in the duct bottoms and tops towards the sheets 4 for the purpose of drying them the top and bottom duct in each magnetron equipped column has a closed top and a closed bottom respectively and their height is just about half of the height of the intervening ducts because they do each serve just one of sheets 4 inside the ducts the microwave energy appears in the form of standing waves this resonance phenomenon arises thanks to a suitable dimensioning of the ducts to which will be reverted below fig2 does only to exemplify show an arrangement of the openings 11 for air and microwaves in this case a herringbone pattern that arrangement which does per se belong to the prior art has the advantage that.