JLLV 1, 1913.] 



THE INDIA RUBBER WORLD 



509 



Then too, the drying process taking place under a high 

 vacuum in the absence of oxygen, oxidation cannot take place. 

 The rubber not being overheated nor impaired by oxidation has 

 greater elasticity and tensile strength, and accomplishes that 

 which is the aim of every manufacturer — to obtain the greatest 

 yield when being made into final product. 



As diflfercnt grades of rubber must be subjected to varying 

 temperatures, the temperature of the heating medium is easily 

 regulated without over-heating or materially affecting the drying 

 time ; so that the vacuum process offers many advantages over 

 the old method of drying rubber, whether it be fine Para or 

 Pontianak. 



Another very great advantage of the vacuum process lies in 

 the fact that as the drying period is only a few hours, varying 

 according to grade of rubber, the crude rubber can be washed, 

 dried and processed in a fraction of a day ; consequently there 

 can be no deterioration after drying and before using — as it is 

 well known that washed rubber oxidizes very rapidly — and 

 there is no danger of the final manufactured pmduct being 

 porous or spongy, due to the presence of moisture. 



The rubber industry has expanded tremendously in recent 

 years, owing to an ever increasing variety of products in which 

 rubber is used. To meet the daily factory demands for large 

 quantities of crude rubber of different grades, the use of the 

 old method of drying is disadvantageous and unsuitable, as well 

 as excessively expensive. 



In an average establishment of to-day, making a general line 

 of rubber goods, two tons of crude rubber is a conservative 

 estimate of its consumption. If the old hot-air method is used, 

 in order to properly and thoroughly dry the washed and sheeted 

 rubber, six weeks are consumed in the drying process. Seventy- 

 two tons of rubber would be hanging in the drying lofts, which 

 at $1 per pound, would represent an idle investment of $144,- 

 000 on raw material, the carrying charge at 5 per cent, amount- 

 ing to upward of $20 per day ; and should the carrying charges 

 for instance, factory space, etc.. be included, the above sum 

 would be greatly increased. 



The same quantity of rubber could be more thoroughly and 

 permanently dried by one or two vacuum chambers in a day of 

 ten liours, so as to "work up" whatever grade may be required 

 for each day's output, and the initial cost of such an installation 

 would be less than the cost of the old-fashioned drying rooms 

 for the same quantity. So that the vacuum chamber pays for 

 itself in the savings on investment, carrying, insurance and 

 other fixed charges on raw material, as well as gives a flexibility 

 to the factory for its daily production that cannot be obtained 

 by any hot-air method. 



To illustrate the great saving in factory space, a vacuum 

 drying chamber having a capacity of approximately two tons 

 of dry sheeted rubber per 10 hours, occupies a space of 85^2 

 feet high, IS feet wide by 9 feet long; and its auxiliaries, the 

 condenser and the pump, can be conveniently located at any 

 place in the factory in proximity to the dryer. 



In addition, the vacuum drying process offers further ad- 

 vantages over the hot-air system because of the other factory 

 economies derived by its use. The drying expenses for a hot-air 

 system for steam consumption and attendance are enormous. 

 It is an established fact that by any hot-air system not more 

 than one-third of the heat units supplied are utilized in the 

 evaporation of moisture; whereas by the vacuum process practi- 

 cally every heat unit is transmitted to and comes in direct con- 

 tact with the material being dried. The saving of fuel is con- 

 sequently most considerable not only on account of the very 

 much shorter drying time — reduced from weeks to hours — but 

 because of the much higher efficiency of the heating medium 

 used in the vacuum drying process. 



The vacuum drying apparatus is constructed of cast iron and 

 is practically indestructible ; there are no appreciable mainten- 

 ance charges and fire hazards are eliminated. 



As to attendance, only one operator is required to operate the 

 vacuum dryer, who, while one charge is being dried, is engaged 

 in filling the second set of trays with rubber for the next charge, 

 and in recharging the apparatus replaces each tray of dried 

 rubber with one of wet. 



A vacuum drying apparatus consists of a vacuum drying cham- 

 ber, which should be constructed of a special grade of close-grain 

 homogeneous cast iron, equipped with heating shelves made of 

 hydraulically straightened sheet steel plates. I emphasize that 

 the plates should be hydraulically straightened, as by this means 

 only can a uniform even and flat surface be secured on which 

 the trays are placed and which will withstand the strain of 

 the varying steam pressures used during the drying process. 

 It is important that the heating shelves are so constructed as 

 to insure an even surface, as any distortion of the heating shelves 

 will cause the trays to rest uneven and thereby permit an unequal 

 transmission of the heat to the material being dried. 



Connected with the vacuum chamber is a condenser, and it is 

 most important that this auxiliary be of proper condensing 

 capacity for the vapors given out during the drying process. 

 In the determination of the size of the condenser, somewhat 

 complex problems arise, for not only must the volume and speed 

 of such vapors be determined, but the proportion to be con- 

 densed during the early stages of the drying process must be 

 determined, when the free or mechanical moisture is driven off; 

 and in this connection, there must also be considered the tem- 

 perature of the cooling water available for condensing purposes. 

 These elements cannot be determined by any "Rule of Thumb 

 Method," but must be carefully calculated. The size and 

 capacity of the dry vacuum pump is of equal importance to the 

 well balancing of a vacuum apparatus ; for in order to secure 

 the highest efficiency it is necessary to create the highest obtain- 

 able vacuum as quickly as possible, and when obtained to main- 

 tain a uniform vacuum throughout the drying period. 



Complaints have occasionally been made by some people who 

 have attenipted to dry rubber under vacuum that such vacuum- 

 dried rubber was not satisfactory, and the blame was promptly 

 put on the vacuum drying apparatus and process. I may state 

 that though these complaints were, of course, bona fide, the 

 cause for such complaints was misplaced. The vacuum ap- 

 paratus and process have the advantage of working almost auto- 

 matically and require scarcely any attendance ; but these ad- 

 vantages have in the above mentioned cases been abused, and 

 of course not without detrimental effects to the vacuum-dried 

 rubber. 



To illustrate what I mean, a certain size of a vacuum appa- 

 ratus is intended to receive a certain quantity of rubber per 

 charge, and naturally the heating surface of the dryer, which 

 is equivalent to its drying capacity, has, as I have stated, to be 

 most carefully calculated for this purpose, and if the instructions 

 for operating such apparatus, which are most simple, are only 

 followed, it is a physical certainty, I might say, that the vacuum- 

 dried rubber will be satisfactory and far superior to air-dried 

 rubber, because with a vacuum apparatus and process nothing is 

 left to chance or climatic conditions. 



In cases, however, where complaints have arisen, the well- 

 meaning people who were using such a dryer, being surprised at 

 the capacity of the apparatus far exceeding their expectations, 

 thought it right to go a little further by further increasing the 

 charge of rubber and ultimately loading the apparatus with a 

 nmch larger quantity than their apparatus was intended for. 

 Of course, it was soon found that the increased charge could 

 not be dried in the stated time, nor with the stated temperature 

 of heating steam. As it is only human not to decrease one's 

 desires, the natural human remedy was resorted to, that is, an 

 increased temperature of heating steam and also a prolonged 

 drying time. If you consider that the heating surface at a cer- 

 tain temperature within the dryer is intended for a layer of 

 rubber of a uniform and certain thickness, its capacity, or rather 



