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by H. H. Fawcett and George Gardner General Electric Co.
SAFETY A
W O R K B O O K
F E A T U R E
A Small Particle Detector A stronger light will be thrown on small particles with the introduction of this new analyzer L I T T L E is really known about small particles, although they have been studied at length over the past 100 years. Part of the trouble is the few efficient tools with which to measure them ; much more arises because their behavior is so complex. Absence of steady-state conditions where freshly generated particles are present only aggravates the condi tion. When these particles begin to combine, the total mass may stay much the same but the number of particles per cubic centimeter may vary over wide limits. Small particles in the range of 10~3- to 10 _7 -cm. radius are of considerable interest to the chemist and chemical engineer because they have the properties of both gas and solid. They have tremendous areas in proportion to their mass, and, in appropriate materials, make fine catalysts. They are highly mobile and are eager to find chemical partners. This enables fast reactions and causes the particles to combine or stick to surfaces. In the case of carbonyls, they make superbly fine powders or beautiful plated surfaces. Like many things that are beneficial and useful, small particles in the wrong places can cause malfunction of a process, trouble in the human body, and difficulties in relations with one's neighbors. The safety engineer is more likely to consider these small particles as a nuisance than an intriguing phe nomenon. A tool with which to study their reactions, concentrations, and habits makes his job easier in controlling their inhibitions.
The Analyzer
A practical method for the de tection and measurement of small particles takes advantage of their ability to act as condensation centers for water vapor under suitable conditions. If the sample under
study is contained in a chamber whose atmosphere is saturated with water vapor and the pressure within the chamber is suddenly lowered, the sample will be cooled and supersaturation will result. Under this supersaturated condition tiny water drops will form on the individual particles, and they will become visible. One classic instrument which uses this principle is the Aitken nucleus counter. Basically, this uses a saturated blotter to provide the water vapor. A small pump is used to reduce the pressure, and the particle count is secured by estimating the number of droplets that settle on a squared glass slide. This instrument requires rather ex pert handling for best results. One approach which makes for easier manipulation is to make the sample chamber with its saturating blotter a long brass tube. By putting a light source at one end and a photoelectric cell at the other, the light transmission of the fog formed on sudden expansion can be meas ured. The amount of sudden expansion determines the relative humidity within the chamber and controls the lower range of particle size on which moisture will condense to form a water drop. For example, for a relative humidity of 100.1%, droplets will form on particles having a radius down to 1 0 - 4 cm. For a relative humidity of 112%, droplets will
INDUSTRIAL AND ENGINEER
ING CHEMISTRY has again in 1958 been one of the special ized magazines to receive the National Safety Council's Public Interest Award. T h e award is made each year in recognition of exceptional service to safety.
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This compact unit with its p r o b e can be used to sample and a n a l y z e areas that might have required extensive equip ment. On-the-spot analysis will reduce the time n e e d e d for corrective steps
form on particles having a radius down to 1 0 - 6 cm. Strictly, this refers to water drops; other ma terials will vary with hygroscopic properties, and, in larger particles, with porosity. An arrangement for effectively varying the amount of supersaturation will thus permit some idea of particle size. The right-hand cham ber is divided into two sections by means of an adjustable plunger. Positioning this plunger determines the initial expansion and thus the degree of initial supersaturation. An orifice located within the plunger allows the pressure to equalize in about one second. During the equalizing gas flow provided by the orifice, further expansion takes place, DRKBOOK
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but the rapid growth oi water drop lets, formed in the initial expansion, prevents further increase in the supersaturation. Applications
One interesting feature of small particle measurements methods is their surprising sensitivity in terms of the weight of the particles to the mass of the air sample under study. The ratio of particle weight to air sample weight may easily be 1 to 1000 trillion. Applications of such a measure ment technique are interesting. For example, a small room which is considered clean may have a particle concentration of 5000 or 10,000 particles per ce. A careless operator, dipping a hot soldering iron in a can of paste flux, may raise this to 20,000. A burning cigarette can raise it to 50,000 or higher. Most of the common types of air filters stop only the larger particles. The special filters de veloped during World War II and the higher pressure drop filters such as absorbent cotton are effective over the whole spectrum of sizes. They may fall off in efficiency until a certain minimum size is reached
Feature
Schematic d r a w ing shows ele ments of the p a r ticle size analyzer. Corrosion resistant materials should b e used in con struction to assure long service and eliminate possi bility of errone ous results due to scaling
and then, paradoxically, the ef ficiency may again increase. The nuclei meter is a useful tool for the study of filters such as those which are used in respirators where ef fective filtering is vital. With increased industrial con centration the effluent from stacks is receiving more study. Here nuclei measurement methods are of great value, since they can be undertaken on the spot while the sample is fresh. An interesting case of the
Common sources of small particles in industry are:
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Combustion. This may be the conventional burning of a hydro carbon fuel, such as coal or oil, or it may be due to high tempera ture vaporization of a metal such as occurs in the electric arc. The number of small particles varies tremendously with condi tions. Burning a small quantity of white gasoline produces far fewer particles than the same quantity of gasoline with a little lubricating oil added—such as one drop of oil per quart of gaso line. Heating. When some substances are heated, they release small particles. An embedded furnace winding may emit particles. This process often does not begin until some critical tempera ture is reached, following which large numbers of particles are released. Gas Reactions. A gas may react with other gases to produce solid particles. One of the more familiar is the reaction ob served between a stoppered bottle of ammonia and another stoppered bottle of hydrochloric acid on the reagent shelf. Of more practical importance are the particles formed by the photolysis of S 0 2 , which is a constituent of many industrial stack gases.
Most practical particle problems do not fall neatly in these simple categories. Particles in the exhaust of an internal combustion engine might be due to (1) combustion of the hydrocarbons, (2) heating of hydrocarbons and their surroundings, and (3) gas reactions such as the formation of iron carbonyls due to reaction of carbon monoxide with heated iron surfaces.
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consequences of stack effluents occurs when SO2 resulting from burned sulfur can pass through an elaborate filter into a clean room where the modern fluorescent lighting changes it to sulfuric acid by photolysis. Some toxic substances previously difficult to monitor effectively may be easily detected. When it was first decided to check nickel carbonyl with the nuclei meter, the group carrying out the experi ment had made a careful test setup before going to the storage area to get the newly purchased cylinder of nickel carbonyl. Before starting the experiment it was decided to sniff the room in the storage area with the nuclei meter. It indicated that the cylinder valve, considered tight enough for transit, was actually leaking gas. An important aspect of small particles which is now being recog nized is the synergistic effect of aerosols as carriers of toxic vapors. Formaldehyde is an example of a substance whose inhalation toxicity has been studied extensively. O n e model of the small particle detector described above is now commercially avail able from Gardner Assoc, 408 Charles St., Scotia, Ν . Υ.
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly.
