CHAPTER XVIII

SMOKE FILTERS

The first types of the Standard Box Respirator contained cotton pads, which sufficed to remove the ordinary smoke of the battlefield and even that from the earlier toxic materials. Improved methods of producing toxic smokes, by means of which smaller particles were obtained, led, early in 1918, to the recognition of the need of improved protection against these smokes. The first attempts to meet this need consisted in improving the filtering qualities of these pads. It was soon found, however, that to make better filter pads would greatly increase the total resistance of the canister. This was highly undesirable, since the resistance of the ordinary canister was already so high as to be very uncomfortable. To overcome this objection, some of the early designs of filter canisters were provided with a mechanical valve, which could be operated by hand, to by-pass the air around the filter when the canister was used against gas alone, or so set as to make the air pass through the filter when smoke was feared. This introduced a factor of uncertainty among the men during a gas attack, since each man must decide for himself whether smoke was present. This reason alone was sufficient for discarding this design.

A preliminary study of the situation indicated that any filter for fine smoke particles must have a high resistance per unit of area, but that the total resistance must be comparatively low. In order to secure the large area necessary to bring the total resistance within reason, the experimental work was developed along three lines: The formation of a filter into a bag, cup, or jacket to surround the outside of the canister; the use of an arrangement sufficiently compact to go inside the canister; and the use of a filter as a separate unit, to be attached to the canister by an air connection.

A survey of the possible filtering materials indicated that only two offered promise, namely, paper and felt.

PAPER FILTERS

Reports that the British had developed thin, creped, sulfite-cellulose wood pulp paper for filters led to an intensive study of this material by the Chemical Warfare Service.

[Illustration: FIG. 104.—Crepe Paper Doughnut Filter Canister.]

In general we may say that the development of paper filters (in sheet form) met with little success. Papers affording the required protection did not live up to the resistance specifications. The reason for this probably is in the method of making paper. The pulp is fed onto the screen of a Fourdrinier machine under conditions that do not permit of uniformity in the distribution of the fibers and consequently there is no uniformity in the size of pores. In order to eliminate the large holes, which allow the smoke to pass readily, the paper must be pressed to reduce these pores to the proper magnitude. This naturally results in an approximately equal decrease in the size of the small pores, with a consequent increase in the final resistance out of all proportion to the protection gained. A very satisfactory paper was finally produced, but the resistance was too high and it was necessary to increase the total available filtering area, which resulted in the accordion type of filter. This filter was incapable of development on a large scale because of the large amount of hand work required in assembling. The lack of uniformity in a single sheet has been overcome with some success by making up a filter from 40 to 80 layers of tissue or crepe paper, trusting that the law of chance would bring the large pores in some successive layer. Such a filter was adopted by the British, but since it did not give protection comparable with that afforded by felt filters, it was rejected in the United States.

In the so-called “doughnut” filter use was made of tissue paper. Instead of seeking for uniformity in a vertical direction through a block of tissues, it was sought along the axis horizontal with the sheet. The effectiveness of such a filter was less than that of felt. In addition, serious difficulty was met in cutting the pile of tissue paper into the proper shape so that eventually it was abandoned as a production possibility.

FELT FILTERS

Work on the felt filters started about June, 1918. Great difficulties were met in the beginning, as a felt satisfactory for this purpose must be made under carefully controlled conditions and production conditions during the war did not readily lend themselves to such control. However, the opportunities afforded in felt making for uniform packing and arranging of the fibers (the whole process of making a felt, is one of gradual packing of fibers into a relatively small volume) are such as to assure a greater degree of success than is the case in paper making.

Very successful filters have been obtained with the use of felt. There are two serious objections to its use, however. The first is the great cost of the filter (this was above one dollar per filter at the close of the War); the second is that felt is a valuable industrial commodity. It is thus very desirable that a cheaper and a less important industrial material be found.

THE 1919 CANISTER

Just before the Armistice, the Gas Defense Long Island Laboratory brought out the so-called “1919 Canister,” which consisted of an oval section, perforated metal, war gas material container with a central, flat, perforated breathing tube connected to a nozzle at one end. (See also page 228.) After this inner container is packed with the war gas chemicals, a filter jacket is slipped over it and the top edge sealed to the inner container.

[Illustration: FIG. 105.—1919 Felt Filter Canister.]

Attempts were made to put paper filters on this canister by wrapping it with layers of paper. In some cases, layers of coarse burlap or mosquito netting were applied between the layers of paper to give mechanical strength and air space. The fact that many filters gave good protection showed that a filter of this type and material is possible, but the operations of wrapping and sealing require careful work in production and inspection and even with the greatest skill and care, imperfections are almost impossible to avoid. This chance of defects, together with the labor involved, makes the process undesirable.

A THEORY OF SMOKE FILTERS

Tolman, Wells and Gerke, during the course of their work on toxic smokes, developed the following theory of smoke filters.

The phenomena occurring in the filtration of smoke are exceedingly complicated, but the general nature of the process may be simply described in terms of the kinetic properties of the small particles comprising the smoke.

A filter may be regarded as a series of minute capillaries through which the smoke slowly flows. In order that filtration may take place, it is not necessary to assume that the capillaries of the filter are smaller than the particle, for the particles may diffuse to the walls of the capillaries and it is believed that with typical filters this is the actual method of smoke removal for particles less than 10⁻⁴ cm. in diameter.

In accordance with this view as to the nature of smoke filtration, the important factors involved are (1) the Brownian motion of the smoke particles, (2) the area and arrangement of the internal surface presented by the filter, (3) the flow of the smoke as a whole, and (4) the attractive forces between the filter surfaces and the smoke

## particles. The first three of these factors determine how many

## particles come within the range of the mutual forces of the particle

and filter surface, and the fourth factor determines the chance or expectation that the particle will permanently adhere to the surface of the filter.

TESTING SMOKE FILTERS

All the early tests made on smoke filters used diphenylchloroarsine, because it was felt that the filter must be tested against a toxic smoke. A man test was developed as representative as possible of actual conditions in the field, and the time necessary for a man to detect diphenylchloroarsine smoke in the effluent stream when breathing at a normal rate, using a carefully controlled concentration of smoke produced by detonation, was used as the criterion of the protection offered by the canister. This test was subject to extensive individual variations, due to the varying physiological resistances of different men to diphenylchloroarsine smoke. Further, it was quite inadequate for rapid testing on a large scale. A testing machine was then developed, which gave results comparable with those obtained in the man test. The method used in detecting the gas was physiological, that is, by smell or by its irritating action towards the membranes of the eye. While these are purely qualitative tests, they are much more sensitive than any possible chemical tests.

Because of the desirability of having a method which could be controlled chemically, other methods were developed.

Ammonium chloride is a solid smoke, consisting of particles of quite variable sizes. It is sensitive to dilution and clogs the pores of the filtering medium quite rapidly. For this reason it was used in the study of the rate of plugging or clogging of the filter (the closing of the pores of the fabric or other material to the passage of air).

The smoke is produced by the reaction of ammonia and hydrogen chloride-air streams. The smoke thus generated is passed from the mixing chamber to a larger distribution box and from there through the filter, at a standard rate. The concentration of the smoke may be accurately determined by chemical means or photometrically, using a Hess-Ives Tint Photometer, the Marten Photometer, or a special photometer developed by the Chemical Warfare Service.

A comparison of a large number of tests with those of other smokes would indicate that ammonium chloride smoke offers accurate information as to protection sought, but is hardly a desirable smoke for testing on a large scale.

The third method developed was the sulfuric acid smoke. This smoke was produced by passing dry air through a tower of solid pieces of sulfur trioxide and then mixing the vapor with a large volume of air at 50 per cent relative humidity. It is not a clogging smoke and the filtering efficiency does not change materially in the time of exposure required for a test. The smoke lends itself easily to chemical analysis and offers data as to exact particulate cloud concentrations which will penetrate canisters; photometric measurements are also applicable.

[Illustration: FIG. 106.—Tobacco Smoke Apparatus for Testing Canisters.]

The fourth method consists in the use of tobacco smoke. This is generated by passing air over ignited sticks of a mixture of tobacco (63 per cent), rosin (30 per cent) and potassium nitrate (7 per cent). This smoke is composed of particles of extreme uniformity in size; chemically it is relatively inert. It is not a clogging smoke and is not sensitive to moisture and dilution. The density of the effluent smoke is compared with that of the entering smoke in a Tyndall beam, and the filtering capacity of the material determined in terms of the amount of air necessary to dilute the entering air to the same concentration of the effluent air. The method is simple in manipulation and the test is a rapid one (50 canisters per day). Because of the apparent superiority of tobacco smoke as a testing smoke, the accompanying disadvantages are possibly outweighed.

From the standpoint of inherent chemical properties, the general desirability of a suitable testing smoke would decrease in the following order: tobacco, sulfuric acid, ammonium chloride.

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