transportation or if it is desired to use heat of a low temperature further an increased refractoriness in the lower part of the apparatus is achieved yet another possibility is that a smaller amount of external dry air is brought to the lower part of the shaft where the material transfers heat and moisture to the gas meanwhile the dried material is cooled if this air thereafter is brought to the upper part of the apparatus heat is recovered to a significant extent to the input cold material parallel or alternatively air from the upper zone can be brought back instead of air from the surroundings which improves the heat economizing further this gas flow can consist of dry gas from the following sources the surroundings the upper part of the apparatus dehumidified gas from the gas treatment compared to a horizontal structure a large effective area in the apparatus is attained on a very limited ground surface the screw line ramps combine several functions to maintain the porosity through the friction that is created at rest to abrogate friction and secures a controlled material flow at movements to give the wall a desired rigidity through the horizontal profile of the ramp the spiral shape and placement of the ramp make the stiffening profile self cleaning of remaining material that otherwise constitutes a fire hazard the spiral shape can create the desired turning of the wall from an axial movement.