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Table 1.--Dust analyses-Cornering and light polishing shop, factory A.
Rate of sampling, 4.5 cubic feet per minute. Volume of samplo, between 275 and 301 cubic feet.
Table 3.-Comparative tabulation of number of small dust particles and weight of dust
in air with high and low suction heads.
Figure 4 is a graphical representation of the various dust counts and U-tube readings taken from Table 3.
An examination of Table 3 and figure 4 shows that in general, and other conditions being equal, a reduction in suction head is quickly followed by an increase in air dustiness.
The dust counts with low exhaust pressure are relatively high, varying, with two exceptions, from 165,000 to 2,980,000 particles per cubic foot of air. These two exceptions, samples 6252 and 7164, may be justly eliminated. The dust count on these, being less than 50,000 particles at low exhaust pressure, indicates that, relatively speaking, little dust was being created in the operation. It will be noted that while sampling 6252 the operator only worked 10 minutes of the sampling time, and in the case of sample 7164 the operation was very light. The average of all the dust counts is 853,000 particles, and with the two samples 6252 and 7164 omitted, the average becomes 990,000 or practically 1,000,000 particles per cubic foot of air.
With the application of the normal, higher exhaust pressure the dust content is considerably lower, an average of all the samples being 200,000 particles. In this case there are again two samples, 6261 and 7161, which greatly increase the value of the average. Disregarding these two values the general average becomes 120,000 particles per cubic foot.
The total weight of total solids per cubic foot of air is 0.069 milligrams with the low exhaust prossure (averaging 0.56 inch) and 0.023 milligrams with the high exhaust pressure (averaging 3.21 inches).
An examination of the corresponding observations in the two halves of Table 3 (the normal conditions being on the left, the reduced exhaust velocities on the right) shows more clearly than the general averages just what was happening. In one case (samples 7163 and 7164), where only a very light operation was going on, the reduction of the suction head from 3 to 0.63 inches had no effect on the dust content. In every other instance the dust count went up when the suction was reduced, sometimes only a little-from 720,000 to 1,000,000 (6261-2), or from 854,000 to 1,418,000 (7161-2) or
u Tube Reading in Inches from 130,700 to 165,000 (7181-2), but usually very markedly. In five instances the count of dust particles increased more than tenfold–from 22,400 to 1,773,000 (6241-2), from 45,200 to 894,000 (6243-4), from 44,500 to 654,000 (6263-4), from 56,800 to 2,980,000 (7165-6), and írom 52,200 to 638,000 (71934)
It is evident that with the type of wheels used in the two rooms studied, and with the processes carried on there, and with the design of hoods and exhaust ducts in use, a general relation between suction velocity and air dustiness can be deduced which is represented by the heavy line in figure 4. According to this curve we may expect on the average to find over 700,000 small dust particles per cubic foot in the air of such a shop with an exhaust suction head of an inch or less, some 500,000 with a 2-inch head, and some 300,000 with a 3-inch head. Obviously, however, variations in the process or in the construction of the exhaust system will make it quite impossible to extend our deductions as to such a relation beyond the conditions specified. Conclusions as to Reasonable Standards for Special Ventilation to Control Air
Dustiness in a Polishing Shop. The Massachusetts State Board of Health some years ago laid down the principle that in fixing standards of industrial hygiene it was reasonable to require that conditions should be maintained in any industry approximately equal to those already found in the best plants of that industry in actual operation. On such a basis we may perhaps fairly take the normal conditions in the two admirable polishing shops of factory A as a standard for establishments of a similar type.
It will be noted that in both the cornering and light polishing shop and the heavy polishing shop the suction head normally maintained in the exhaust pipes varied between 2.38 and 4.63 inches and averaged 3.21 inches. Only 5 out of 15 observations fell below 3 inches and only one below 2.5 inches. A fall to 1.25 or 1.50 inches (when the exhaust was reduced for experimental purposes) was at once followed by a marked increase in air dustiness. It would appear from these observations that the 5-inch suction head called for by the Wisconsin Code is unnecessarily severe, while the 2-inch head specified in the New York and New Jersey Codes is a trifle lenient. For an absolute lower limit it is perhaps scarcely possible to go much beyond this figure; but we would suggest that a fairer measure of actual performance would be obtained by specifying that the suction velocity in the exhaust pipes of a polishing shop should at no point fall below 2 inches, and should average 3 inches when measured at a number of different points.
The next point of interest is the linear velocity of suction maintained at the throat of the exhaust duct. This velocity, measured as described on page 433, varied for normal conditions in the cornering and light polishing shop from 1,470 to 4,905 feet per minute and averaged 2,409 feet per minute, while with the lowered exhausts the velocity varied from 418 to 1,340 feet per minute and averaged 845 feet. It would appear, then, that good conditions were main