United States Patent Office
DIFFUSION TYPE PROTECTIVE ENCLOSURE Saul Hormats, Pikesville, Eugene Sovinsky, Baltimore, and Ella S. Bwaayer, Magnolia, Md., assignors to the g United States of America as represented by the Secretary of the Army
Filed Feb. 4, 1958, Ser. No. 713,273 6 Claims. (CI. 128—140) (Granted under Title 35, U.S. Code (1952), sec. 266)
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment to us of any royalty thereon.
This is a continuation in part of appliaction Serial Num- 15 ber 423,258, filed April 14, 1954, now abandoned.
This invention relates to diffusion type protective enclosures for protecting human beings or animals from the effects of atmospheric pollution in the form of gases, aerosols, smokes, or bacteria. 20
In order to provide protection from war gases and other types of atmospheric contamination, it is the practice to draw air through filtering materials which by physical entrainment, adsorption or chemical reaction eliminate the undesirable material. 25
To provide individual protection, the filter material is mounted on a gas mask and at each breath the wearer must physically draw the air through the mass of filtering material. In collective protectors, such as shelters or protectors for tanks, pumping means is provided to pass the 30 air through the filtering material.
Gas masks of the type described above are obviously not well suited to use by small children or by animals. The necessity for pumps in connection with the collective protectors greatly increases the complexity of the installa- 35 tion.
We have devised a protector operating on a different principle, namely diffusion. We utilize a relatively thin fibrous material of such area that the removal of carbon dioxide and its replacement by oxygen takes place by dif- 40 fusion, with no necessity for a net flow of air through the filter. Thus, infants or animals can simply be placed in the protector and need exert no more effort than is involved in normal breathing. Similarly a group of persons whose breathing will not, of course, be synchronized and 45 who therefore will not produce definite pressure change within the enclosure, may occupy a single enclosure without the necessity for using pumping equipment.
Diffusion is governed by Graham's law. A statement of this law is given in Walker, Lewis, McAdams and Gilli- *>0 land, Principles of Chemical Engineering (1937), pages 317 and 318.
The "coefficient of proportionality" referred to by Walker et al. may- be expressed as a value termed "diffusivity," "diffusion coefficient" or "diffusion constant" for &5 a specific gas in a given material. The diffusion of one gas through another is discussed in Perry, Chemical Engineers Handbook, 3rd Edition, pages 538-540 and the diffusivities of various gases in air are given, the diffusivity of carbon dioxide in air being .164 sq.cm./sec. 60 When diffusion is through a porous medium the same basis equations hold, but the diffusivity of a given gas has a different numerical value than when the diffusion is through air. We employ the carbon dioxide diffusivity to characterize out material. 65
Under the condition prevailing in our enclosures, i.e., a carbon dioxide of one or two percent within the enclosure and negligible (for design purposes) carbon dioxide in the ambient atmosphere, the equation for the diffusion of carbon dioxide through the wall becomes 70
where Q is the volume of carbon dioxide measured at atmospheric pressure and ambient temperature, diffusing through the wall in one second, in cubic centimeters, A is the area over which diffusion takes place in square centimeters, L is the wall thickness in centimeters, V is the fraction by volume of carbon dioxide in the air within the enclosure and D is the carbon dioxide diffusivity of the material in sq.cm./sec.
We utilize material comprising one or more fibrous sheets impregnated with activated carbon. This material may be of various physical forms. One form consists of filter paper into which finely divided activated carbon has been brushed in an amount of up to about 26% by weight, several sheets being employed. Another form which we have found highly suitable consists of a fibrous pad of about 4.0 to 4.5 mm. thickness impregnated with about 30% to 70% by weight of carobn in the minus 50 mesh size range and stabilized by a suitable adhesive.
Still another form, particularly suitable for rooms and sheltsrs is a carbon-impregnated fiber board about lA in. thick and containing from 15 to 50 percent activated carbon.
These materials effect a purification in several different ways. There is, of course, a physical exclusion of airborne particles. The activated carbon serves to selectively adsorb certain liquids and gases. Moreover, since the filter is a microporus structure and is operating under conditions of diffusion, the phenomenon of selective diffusion comes into play. As is well known, see Walker et al. supra, the rate of diffusion depends on the molecular weight of a gas, being greater for a gas of low molecular weight. Many of the highly toxic agents used as war gases have molecular weight which are much higher than the atmospheric gases and therefore diffuse at a much slower rate.
The slow penetration by diffusion of these high-molecular weight gases provides optimum conditions for their adsorption by the activated carbon.
We obtain our desired mode of operation by the use of a suitably large area of filter materiall. The area required is governed by several factors. The carbon dioxide concentration permissible in the enclosure is one governing factor. This permissible concentration varies with the individual. An adult can tolerate a higher concentration than an infant. The tolerance for animals varies from that of humans. The amount of carbon dioxide produced also varies with the individual. For conditions of rest, the amount produced varies with the weight and the basic metabolism rate of the individual. For conditions of activity, the amount produced increased with the amount of activity. The amount of carbon dioxide to be disposed of (which is governed by the factors discussed above) and the diffusivity of the filter materials determine the area required.
The material should have a carbon dioxide diffusivity falling in the range 1.5 to 7.0X10-2 sq. cm./sec, i.e., roughly %o to Vi the diffusivity of carbon dioxide in air. The diffusivity is a measure of the porosity of the material. The fibrous pad described above, which is officially termed "gas-aerosol filter material," ordinarily has a diffusivity in the upper portion of the range while the carbon impregnated fiber board ordinarily has a diffusivity in the lower part of that range.
These values of diffusivity are of an entirely different order of magnitude than those for certain other materials that have been considered as diffusion membranes for gases, e.g. natural rubber. We have found by experiment that dentist's dam, about .010 in. in thickness, requires 250 times the area of gas-aerosol filter material to diffuse the same amount of carbon dioxide, giving a carbon dioxide diffusivity of about 9xl0~6 sq. cm./sec.