15 of the plates the plates 5 and 2 as well as 6 and 3 as well as 7 and 4 only are fluidum tightly joined while the rest of the plates are just in contact with each other since which was previously mentioned the heat transferring plates are provided with corrugations in the form of ridges 9 and valleys 10 which together form a herring bone pattern and since which also was mentioned earlier every other double wall unit in the core of plates is turned 180 degrees in the planes of the respective plates in relation to the rest of the double wall units the corrugations on a double wall unit will be in contact with the corrugations on adjacent double wall units in the core of plates tin a plurality of points when brazing the core of plates together also these points are brazed together if the double wall unit according to the prior art is thereby used i e such double wall units where the ridges and valleys on each one of the alternating plates in all of their areal extensions respectively are adapted to and in near contact with the corresponding ridges and valleys on the respective intermediate plates the result is often that which is evident from the fig3 and 5 which show a cross section through such a brazed together point of contact and a cross section through two such points respectively from the fig3 and 5 it is evident that in addition to the wanted brazing joint 16 between the two double wall units shown in each one of the figures the double wall units consisting of on one hand the alternating.