An excessively low humidity in a room will create problems for the people, furniture fittings and production equipment present in the room.  This problem is enhanced with more effective insulation of buildings.  There are several known methods of increasing the humidity of air.  According to one such known method air is brought into contact with the surface of water with the aid of an evaporator, therewith vaporizing the water.  It is normal to heat the water and to increase the surface contact area of the water with the air, so as to increase vaporization. 
One problem encountered with water evaporators is that the heating devices used (electrical resistors, piezo-electric elements and the like) tend to become coated with contaminants present in the water, e.g. calcium compounds.  Another problem resides in the necessity of enlarging the contact surface between water and air.  Irrespective of whether enlargement is effected by generating turbulence in the water or by dispersing the water in fine-droplet form, small water droplets are unavoidably entrained by the departing water vapor, therewith reducing the efficiency of the humidifier and engendering the risk that bacteria, lime etc.  will accompany the droplets. 
There are several known methods for cleansing liquids and for separating liquids at low temperatures close to atmospheric.  These methods have been developed primarily for separating sea-water distillates, although they can, of course, be used in conjunction with other liquids.  One simple example of such distillation processes is that effected by the sun, in which water is vaporized from a flat surface by heat from the sun and the subsequent water vapor is condensed on an air-cooled or a water-cooled surface located in the same room or in an adjacent room. 
In accordance with another known method, designated "humidification-dehumidification", water vapor is carried away by a stream of air and is condensed by a more or less complicated process; c.f.  for instance U.S. Pat.  No.  4,350,570 and 4,383,703. 
The aforesaid methods and conventional distillation processes require the provision of large vapor chambers, and hence the yield is low in relation to the necessary volumetric capacity of the apparatus used and the capital costs incurred. 
In accordance with another known method of the aforementioned kind, designated membrane distillation, vapor is allowed to condense directly into a colder flow on the opposite side of a membrane which is placed adjacent an evaporation surface.  Examples of this known method are found described in U.S. Pat.  No.  3,340,186 and 4,476,024.  The theoretical yields of these techniques (membrane distillation) and the yields actually obtained therewith in practice are recited in the article entitled Low Energy Cost Desalination Processes Using Hydrophobic Membranes and included in the Proceedings of the Second World Congress on Desalination and Water Re-use, November, 17-21, volume 3, pages 277-286. 
A similar method which employs the use of a separate chamber for distillate between a membrane and a cooling flow is found described in U.S. Pat.  No.  3,563,870 and European Patent Application No.  0088315. 
The yield obtained with systems employed in the aforementioned membrane distillation techniques is restricted by the fact that the presence of a flow on the condensation side of the membrane impedes the transportation of both air and heat through the membrane.  The yield of such systems can be improved to some extent, by arranging a stationary air gap on the condensation side of the membrane, for example in accordance with Swedish Patent Specification 419699. 
The object of the present invention is to provide an apparatus of the kind defined in the introduction which has a markedly higher efficiency than prior art apparatus of a similar kind.