A METHOD OF AIR DRYING

Abstract: A flowing mixture (1) of air and vapour is dryed by dew deposition to a cooling surface (2) with the mixture separated from the cooling surface by a heat insulating, semi permeable insulation layer (3) and one between insulation layer and cooling surface enclosed vapour/air mass (4) with essentially lower temperature and vapour pressure than the mixture.

Description: A method of air drying. The present invention relates to a method and devices for drying a flowing mixture of air and vapour by means of dew deposition to a cooling surface. The object of the invention is to obtain an utterly energy efficient air drying process, especially for a meagre air/vapour mixture, and further to facilitate a recapture of the vapour condensation heaLThe invention is specially applicable in vapour separation from enclosures with a high moisture production, such as launderies, baths and drying chambers of differant kinds. In known manner enclosed air may be dried either by an air exchange, in that warm, humid air is exchanged to colder, dryer out door air or by cooling the air against a cooling surface thus affecting a dew deposition. In both cases vapour separation is coupled to a heat "transport loss" in form of the cooling/reheating of an air flow. In drying an already relatively dry air, as the case may be in the drying end phase in a drying chamber, this transport loss may be in the same range as the vaporization heat itself. With the invention vapour diffusion from a warm, rich or meagre air/vapour mixture to a cold wall is arranged without any appreciable temperature drop in the gas mixture. A separation of vapour from the mixture , even a meagre one, may be done with a very small transport loss implying a radically diminished energy cost for the separation. As with other known condensation processes the condensation heat of the vapour is recovered at a lower temperature level and may be reused, f.i. in preheating ventilation air or to an enforced cold air drying of a drying goods. Mentioned objects are met by a method and devices futher characterized in that the cooling surface is separated from the mixture by a heat insulated, semi permeable insulation layer and by one between cooling surface and insulation layer enclosed vapour/air mass having an essentially lower temperature and vapour pressure than the mixture, whereby vapour will diffuse from the mixture through the insulation layer and vapour/air mass and condense on the cooling surface. The invention is described as follows in reference to the figures, where figure 1 in a device section will illustrate the method. Figures 2 and 3 illustrate typical temperature and vapour pressure gradients. Figures 4 and 5 show in section and plan a first device. Figure 6 shows in section a second device. In referance to figures 1-3 is denoted by 1 a flowing mixture of air and vapour 11 with a dry bulb temp. = Tl and a wet bulb temp. = T2 being dryed through dew deposition on a cooling surface 2 with a surface temp. = T4, that is essentially lower than T1/T2. The cooling surface is in its turn cooled by a cooling medium 21, that may be air, water or the cooling agent in a cooling machine. With the invention the mixture 1 is separated from the cooling surface 2 by a heat insulating, semi permeable insulation layer3 and one between insulation layer and cooling surface enclosed vapour/air mass 4 having an essentially lower vapour pressure P2 and temp. T3 than the corresponding vapour pressure PI and temperatures T1/T2 in the mixture 1. Typical temperature and vapour pressure gradients through insulation layer and cooling surface are illustrated in figures 2 and 3. The gradients and a vapour flow 11 over the layers is determined by parametres such as temperatures, vapour pressure and mass flow of the mixture 1, by temperature T5 in the cooling medium 21, by area of the cooling surface and vapour diffusion and heat flow resistance in the insulation layer 3. A very high energy efficiency in separation of vapour from the mixture 1 is reached in that the latent heat in vapour 11 diffusing through the insulation layer is very large compared to sensible heat loss in the mixture due to heat flow in the layer. In one in practice executed test the following connections applied. The mixture 1 has a dry bulb temp. Tl = + 60° C and a wet bulb temp. T2 = +40° C and RH = 32% and is representative for drying grain or a finishing stage in lumber drying. The insulation layer comprices 50 mm loosely packed mineral wool covered on both sides by thin fibre cloth. The cooling surface area is double the size of the insulation layer and chilled by out door air with temp. T5 = +5°C. With the mixture flowing over the insulation layer a continuity in the temperature and vapour pressure gradients is aquired, whereby a dry and wet bulb temp. T3 =+26°C is found in the vapour/air mass 4. Vapour flow rates 1,0 kg/h,sqm insulation layer driven by a vapour pressure differance P1-P2 = 24 mbar. The latent heat flow in the vapour flow per sqm = 595 kcal/h The sensible heat loss in the mixture per sqm = k(Tl-T3) = 0.8x34 27 kcal/h = the "transport loss" in the vapour separation. The process efficiency = 595/595+27 = 0.98 The corresponding efficiency with a conventional exchange of mixture 1 with outdoor air appears from the following: The mixture +60° RH=32% vapour content 41.5 g/cbm Outdoor air +5° RH=80% vapour content 5.5 g/cbm exchange 36 g/cbm The corresponding transport of 1.0 kg vapour requires 1000/36 =28 cbm exchange air, that shall be warmed from T5 to Tl. The transport loss will be 0.3x 28 (60-5)= 460 kcal/kg vapour and the process efficiency wiii amount to 595/595+460 =0.56. A conventional condensation process with cooling the mixture under saturation and a following reheating will result into a similar process efficiency. A suitable construction of the insulation layer 3 may be mineral wool covered on both sides with a perforated cloth or board material. With insulation thickness, compression of the wool and varied perforation rate of the cover material, vapour diffusion and heat flow resistances may be varied within wide limits and be optimized for varied applications of the invention. It has been found suitable , especially by air cooling of the cooling surface 2, to have it enlarged in area in relation to the insulation layer 3 in an order of 2 to3. In consequence surface temperature T4 and temp. T3 and vapour pressure P2 in the vapour/air mass 4 will sink, increasing a driving force P1-P2 for vapour diffusion through the insulation layer. Heat absorbed by the cooling medium 21 is preferably recovered for instance for heating premises or for an enforced cold air drying of a drying goods. The cooling surface is preferably arranged vertical or inclining so that precipitated condensate follows the surface downwards towards a drain 6 without dripping and wetting the insulation layer. In referance to figures 4 and 5 is as follows described a first device for air drying according to described method. One by air chilled cooling surface 2 is formed according to my Swedish patent 9202247-4 as an inflated film cover in the form of a vertically or inclining extended film tube 22 with its lower end attached to a drain 6 and inlet tube 5. Within the the film cover is arranged a cage 7 covered with mentioned insulation later 3. An air/ vapour mixture 1 is driven through the cage, applied via the tube and drawn off via a conduit 8 coupled via the tube or alternatively via the upper end of the film tube. Between cage and cooling surface is retained said vapour/air mass 4. The cooling surface 2 is in the device essentially enlarged in area in relation to the insulation layer 3 . In above described manner vapour 11 will diffuse from the mixture 1, through the insulation layer 3 and via the air mass 4 condense on the inside of the film cover and further be collected in and drawn off the drain 6. The vapour separation is driven with neglible heat loss in the mixture. In referance to figure 5 the device is formed as a column, standardized in size and capacity, that may be coupled in any number via air channels 12,13 to f.i . a drying chamber 9. A mixture 1 is driven from the chamber through the columns and dehydrated back via channel 13 to the chamber. The columns are placed in plan fairly tight in zigzag, forming an efficient tube exchanger for a good heat exchange to a flow of outdoor air 21. The flow is driven further to an enforced cold air drying of a drying goods 10. One in practice suitable column size may for instance be dimensioned as follows. Area of cage and insulation layer appr. 10 sqm, cooling surface appr. 20 sqm in form of a film tube 22 with diameter 1.3 m and height 5 m, inflated by inner overpressure via choking in return conduits 8 and 13, laced in its upper end and laced to the tube 5 in its lower end. With a wet bulb temperature T2 = +40° in the mixture 1, suitable for a fairly slow drying of hard wood, drying capacity of one column will be as mentioned appr. 10 kg/h corresponding to a drying capacity of 10 cbm hard wood. Soft wood is dryed with a higher wet bulb temperature T2 and vapour pressure PI, wich will increase vapour flow in the column and correspond with a higher drying rate of the same volume (10 cbm) of soft wood. By mentioned dimensioning a drying chamber 9 with f.i. 80 cbm soft or hardwood is dehydrated by 8 columns, as illustrated in figure 5. Any wood dryer using a conventional vapour transport by means of air exchange with outdoor air may be converted to a vapour transport with appropriate number of columns. Combined with a second drying step 10, used for predrying fresh lumber,energy consumption may as test show be reduced to a third. In referance to figure 6 is further described a second device for air drying according to the method. A cooling surface 2 in the form of the evaporator 23 of a cooling machine is surrounded by a cage 7, covered by said insulation layer 3. Cage and cooling surface are separated by mentioned vapour/air mass 4. An air/vapour mixture 1 is driven along the exterior surface of the cage, and vapour 11 will in described manner diffuse through the insulation layer to the air mass 4 and condense on the cooling surface 23. After passage of the cage 7 the dryed mixture is driven over the condensor of the cooling machine. The device is well suited for dehydrating cool and moist premises. The cooling machine only need to be dimensioned for the con densation heat in condensed vapour,not for a transport effort to cool and reheat air. Other devices for drying air according to the method may occur within the scope of the invention.

Claims: Claims 1. A method for drying a flowing mixture (1) of air and vapour by means of dew deposition to a cooling surface (2) characterized in that the cooling surface (2) is separated from the mixture ( 1 ) by a heat insulated, semi permeable insulation layer (3) and one between cooling surface and insulation layer enclosed vapour/air mass (4) having an essentially lower temperature and vapour pressure than the mixture, whereby vapour will diffuse from the mixture through the insulation layer and vapour/air mass and condense on the cooling surface. 2. A method according to claim 1 characterized in that said insulation layer (3) comprises mineral wool covered by perforated cloth or sheet material. 3. A method according to claims 1 and 2 characterized i n that the cooling surface is essentially enlarged in area in relation to the insulation layer. 4. A device for air drying with the method according to claims 1,2 and 3 comprising one by air (21) chilled cooling surface (2) in the form of a number of vertically or inclining extended and inflated film tubes (22), each in its lower end attached to a tube (5) and a drain (6) characterized in that comprises within each film tube (22) a cage (7) covered by said insulation layer (3), further comprises one between said cage and film tube enclosed vapour/air mass (4) and an inlet (5) to and an outlet (8) from the cage, coupled to distribution channels (12,13) for said flowing mixture (1). 5. A device for air drying with the method according to claims 1,2 and 3 comprising said cooling surface in the form of the evaporator (23 ) of a cooling machine characterized in that it comprises the evaporator enclosed by a cage (7) covered with said insulation layer (3), further comprises one between evaporator and cage enclosed said vapour/air mass (4) and said mixture ( 1 ) driven along the outer surface of the cage and subsequently in contact to the condensor of the cooling machine.

Inventors: ZEILON STEN OLOF, ZEILON, STEN, OLOF
Applicants: ZEILON STEN OLOF, ZEILON, STEN, OLOF
Application year: 1999
Main classification: B01D5
IPC codes: B01D53, F24F3