Vacuum Driers.—The vacuum drier is so well known that only a brief description need be given. It consists of a chamber heated by steam pipes and capable of having the contained air and moisture withdrawn by a pump. This description sounds very simple, and in practice the operation of vacuum drying is really a simple one, and can well be entrusted to an intelligent coolie under efficient supervision. Indicators are fitted which show the vacuum pressure and the pressure of steam in the heating pipes which travel underneath horizontal slabs upon which trays may be placed. Still, in spite of the apparent simplicity of the process, there would appear to be a number of little details which, if overlooked, prove to be factors influencing the result to a considerable degree. Thus it is not uncommon to find complaints that the rubber is not dry when packed. The writers have seen rubber taken from a vacuum drier still containing a visible quantity of moisture. One would have imagined that continuous working of the drier would give the experience necessary to obtain dry rubber, but, apparently, such is not the case in a number of instances. Elaborate instructions are given by the makers, but often they are more honoured in the breach than in the observance. Really, there are only two points to bear in mind:
(1) That the rubber must be fairly thin.
(2) That the temperature be not allowed to rise too high. Some makers advise 140° F. as a maximum, but no harm results from a temperature of 150° to 160° as long as the interval is not prolonged.[11]
[11] These figures refer to temperatures recorded by thermometers placed in the folds of the rubber.
These two points presume that the vacuum drier is true to its name, and that one can obtain a maximum steady pressure. The machines are so well made now that no drier should be taken over from those responsible for its erection unless it can show a vacuum pressure of 28 inches within fifteen minutes of starting the pump; and with the pump stopped, there should not be a greater fall in pressure than 1 inch within ten minutes after stopping the pump.
One of the most frequent sources of error is the control of steam pressure which is responsible for the temperature of the drier. It is quite unnecessary and unwise to maintain any steam pressure once the drying is well under way. All that is necessary is to heat the chamber well, with a steam pressure of 5 lbs., before inserting the rubber. As soon as the maximum vacuum pressure has been obtained, steam should be shut off from the heating pipes, and it will be found that the temperature is well maintained throughout the operation with a rise of ten to twenty degrees at the end. If the drier is working at a vacuum pressure of 28 inches, and if the crepe has been prepared thin enough, the rubber should be quite dry within two hours. Should the operation have to be extended to two and a half hours at 28 inches vacuum pressure, it is a sign that the crepe is too thick. On such occasions it is often noticed that these thicker crepes are not thoroughly dry, having moist spots enclosed in them. On re-rolling, these moist patches become easily visible, and are a source of great annoyance, inasmuch as they take quite a long time to dry out.
As mentioned before, the crepe for vacuum drying should be thin. There is no necessity to give it a superfine finish, and the presence of small holes is quite permissible, as they disappear on subsequent re-rolling. The thin crepe may be folded loosely to the length (or breadth) of the tray several times, but in no other way can the drier be expected to perform its work satisfactorily. A case was noted in which thin crepe was excellently prepared, and four or five layers were rolled together for vacuum drying. Naturally this mode of procedure does not give the drier a fair chance, and it would be ridiculous to judge vacuum drying on the results. After two and a half hours at a temperature of 145° F. the rubber appeared to be only about three parts dry, and the subsequent air-drying extended well into a fortnight.
It is the common practice to screw up the door of the chamber as tightly as possible. As a rule it is found in course of time that the obtainable maximum vacuum pressure decreases. This may be attributed solely to the forcible screwing up of the door. Around the inside edges of the door are strips of rubber compound, the function of which is to form a tight joint. Should the door be screwed up too tightly, these strips become deformed in course of time, and slight leaks occur. It should be pointed out that it is only necessary to screw up the door at the beginning of the operation. When the vacuum has been obtained, the screw pressure may be released, as the atmospheric pressure outside the chamber is more than ample to keep the door in a close fitting position. This will be obvious from the fact that the difference in pressure between the inside and the outside of the door amounts to, say, 28 inches atmospheric pressure—i.e., nearly 14 lbs. per square foot. By slackening the screw handles, therefore, as soon as the indicator shows the maximum working vacuum pressure, the life of the door joints may be prolonged, and the drier will remain efficient for a longer time.
A careful consideration of the question of temperature leads one to the conclusion that the practice of placing a thermometer through the roof of the chamber does not enable one to determine the temperature correctly. In the same way a thermometer suspended behind the observation window cannot indicate the temperature of the rubber, as in both of these positions the thermometer must be influenced by radiation from the walls of the chamber. The only position in which the correct temperature could be indicated is between the folds of crepe. This can be arranged easily so as to enable one to read the temperature from the observation window.
Colombo Commercial Company’s Drier.—The drier of the Colombo Commercial Company consists in principle of a number of small chambers or units in which crepe rubber is placed, and through which hot air is passed. As in the case of vacuum drying, a great deal depends upon the preliminary treatment of the rubber. If the crepe is not rolled thin enough drying will be unduly prolonged, with a possibility that the rubber will become tacky. The temperature usually obtained is about 150° F., and if the rubber is thin the production of an installation of two chambers should be at the rate of 1 lb. of dry rubber per minute. The usual period of drying is under two hours. One advantage which this drier has over the vacuum drier is that the chamber can be opened at any time for a short period to withdraw or insert trays. The thin crepe is folded several times, as in the case of vacuum-drying.