August i, 1906.] 



THE INDIA RUBBER ^VORLD 



363 



AIR DRYING versus VACUUM DRYING. 



THE usual method of drjing rubber is by means of hot 

 air, passed through drying rooms bj' means of fans 

 or similar devices. It is an established fact that 

 this type of drying is most uneconomical, as only about one' 

 third of the heat carried by the air is given up to the mate- 

 rial that is to be dried and consequently about two-thirds of 

 the fuel consumed is wasted. Tlic reason is that, though the 

 charging capacity of hot air with moisture is naturally greater 

 than one-third, still if any evaporation at all on a commercial 

 basis is to be achieved, the air has to travel rapidly through 

 the drying rooms and consequently is unable to utilize all 

 of its carrying capacity. If the air is allowed to travel at a 

 slower rate and thereby given o])portunity to charge itself 

 with a higher percentage of moisture, the evaporation be- 

 comes sluggish and the results are the same. 



It should also be remembered that the large volume of air 

 required for effective drying absorbs much motive power for 

 the driving of a fan and thus adds to the cost of the process. 



Another thing, the materials to be dried have to be placed 

 sufficiently far apart to allow a free passage of the heated air 

 so that the buildings occupy considerable space, and the pro- 

 cess is necessarily of long duration, thus tying up capital 

 and causing loss of interest and the payment of high insur- 

 ance premiums. 



A further disadvantage of the hot air system is that it un- 

 avoidably carries quantities of dust with the hot air, which 

 is deposited on the wet and adhesive surfaces of the drying 

 maiertals, in many cases destroying their value. This dust 

 nuisance maybe avoided only by installing large and expen- 

 sive dust filters. 



The dust drawback cannot take place under the vacuum 

 system, as the vessels used for this purpose are hermetically 

 sealed. Further, an advantage of the drying in such vessels 

 is that the process is entirely independent of any climatic 

 conditions, which largely influence all drying under atmos- 

 pheric pressure. 



Moreover the hot air drying has a tendency of hardening 

 some materials, as for instance, is shown by the considerable 

 shrinkage and wrinkling of the air dried leather. 



If any porous fabric is treated under vacuum instead of 

 closing up its pores, the very reverse is obtained by rapid 

 boiling, the water being removed from the interior almost as 

 rapidly as from the surface. This will be particularly re- 

 ferred to later on in a specification of a new impregnation 

 process. 



The above drawbacks attached to the drying methods at 

 present used have led to a verj' general use of a vacuum in 

 the place of hot air, with most satisfactory results, and 

 among the most suitable vacuum apparatus with condensers 

 and vacuum pumps, are those of the Kmil I'assburg 

 System. 



By this it is possible not only to remove moisture eco- 

 nomically, but aksoto remove it at so low a temperature as to 

 absolutely exclude deterioration in quality. It is also the 

 only means (it quickly and absolutely removing all mois- 

 ture. This latter item is one of the most important features 

 where insulating material for electrical apparatus is con- 

 cerned. In this it is not the gain of time or application of a 



low temperature which is of the greatest importance, but the 

 dryness of the insulating material, as only an absolutely 

 dry insulator can insure a perfect conduction of electricity. 



With drying b\' means of hot air, the drj-ing material can 

 never be brought to a higher degree of dryness than the 

 heated air itself, the latter naturally always carrying a small 

 percentage of moisture, and even could the drying material 

 go below this ])ercentage, it would naturall}' re-absorb the 

 moisture from hot air. 



Under vacuum the slow evaporation such as exists under 

 atmospheric pressure is turned into a rapid boiling of the 

 water, not only not raising but lowering the temperature, 

 as the boiling point is reduced from 212° to about 95° F. 

 This also e.xplains the enormous saving of time in removing 

 the water and the saving of heat or fuel, as naturally much 

 fewer units of heat have to be transmitted to be evaporated. 



Certain materials for certain purposes have to be impreg- 

 nated with various kinds of colors, dyes, or compounds, for 

 water repelling, insulating, acid resisting purposes, etc., ac- 

 cording to the purpose they are to be used for, but notwith- 

 standing the perfect state of dryness of materials treated 

 under vacuum, defects will appear after such impregnation 

 if done under atmospheric pressure. The reason for this is 

 that the impregnation is not thorough. In other words, the 

 inner parts of the materials were not at all or only insuffi- 

 cientlj' impregnated. It was found that when these com- 

 pounds were of a very heavy nature, even at a high tempera- 

 ture they could not penetrate closely woven fabric or the 

 pores of materials on account of the air contained within. 

 Then, too, such liquid moved sluggishlj- and with great fric- 

 tion. 



Now it has been found that removing all air from the 

 pores in vacuo that the impregnation was complete. This is 

 done in a specially constructed apparatus. To explain this 

 improvement, it must be borne in mind that by such evac- 

 uation not only the air is removed from the material that is 

 to be impregnated, but in addition to this the assistance of 

 the atmospheric outer pressure under which the impregnat- 

 ing compound was entering the insulating material was ob- 

 tained, a pressure varying from 14 to 15 pounds. Even by 

 means of such improvement, the modern requirements for 

 insulating material used in the electrical trade could not be 

 satisfied. The electrical current of high tension required 

 still better insulation. 



To obtain this, the following means were adopted ; After 

 having allowed the impregnation compound to enter the va- 

 cuum impregnation vessel by means of the outer atmospheric 

 pressure, this was increased by adding artificial pressure 

 (compressed air) to the required extent, so that not a parti- 

 cle of even the innermost insulating material, was left free 

 from compound, no matter how it was protected by outer 

 lavers of covered wire. Naturally, the vessel in which this 

 impregnation takes place must be made strong enough to 

 stand an outer pressure and also afterwards an inner pres- 

 sure. 



The enforced impregnating or combined impregnating 

 process under vacuum, together with artificial pressure, is 

 only necessary for materials which are of a very close texture 



