stitute of Canada, Badger Paper Mills, Great Northern Paper Co., International Paper Co., Mosinee Paper Corp., and Rayonier Inc. for contributing pulp samples; T h e laboratories of American Smelting and Refining Company, Canadian Copper Refiners Ltd, Kennecott Copper Corp., and South Dakota State University for participating in t h e collaborative study; 0. E. Olson for advice and technical contribution; and A. Szell and D. J. Henrie for performing some of the analyses.
Literature Cited Chern. Eng. N e w 45, 12 (May 29. 1967). Johnson, H., Enuiron. Sci. Techno/., 4,850-3 (1970). Luke, C . L., Anal. C h e n . , 31,572-4 (1959). Olson, 0. E., J . A0.4C, 52,627-34 (1969). Olson, 0. E., Frost, D.V., Enciron. Sci. Techno/.,4, 686-7(1970). Parker, C. A , , Harvey, L.G., Anal., 87,558-65 (1962). Watkinson, J . H., Anal. Chern.. 38, 92-7 (1966). West, P. W., Cimerman, Ch., ibid., 36, 2013-16 (1964). ReceiL,ed f o r recieu July 16, 1973. Accepted January 7, 1974.
Feasibility of Automatic lsokinetic Stack Sampler Samuel
H. Dworetsky
Bell Laboratories, M u r r a y Hill, N.J. 07974 /
A technique is suggested for the isokinetic sampling of emissions from stack sources. Two capacitance manometers with a common pliable element are used to compare the stack and probe flows. STACU
Smoke stack effluent must be sampled a t the same rate as the stack flow (isokinetically) if errors are to be held within tolerable limits. This note suggests a novel technique for automatic isokinetic stack sampling through the use of a n electrical pressure transducer. No technical skills would be required by the personnel involved in the measurement. Present techniques require t h e initial measurement of the stack velocity profile using a Pitot tube, which measurement is followed by the actual sampling. The Pitot tube technique, besides being extremely cumbersome, also involves the poor assumption that t h e velocity profile is stable. The technique described here takes advantage of the pressure developed by a passing flow of gas, as described by Bernoulli's equation. An arrangement of two capacitors with a common pliable element is used as a pressure transducer to compare the probe and stack velocities. An electrical feedback circuit varies the probe flow motor, such t h a t the pressure difference is zeroed, ensuring the equality of t h e flow rates. Monitoring the speed of the probe motor gives a continuous record of t h e stack velocity at the point being sampled. A schematic representation of the device is shown in Figure 1. Here the lower portion of the probe forms two capacitors, A and B, and is electrically insulated from the remainder of the probe. Any difference between the flow velocities will result in a pressure difference as described by Bernoulli's equation. The pressure difference will displace t h e pliable membrane laterally, resulting in a difference between the capacitances of A and B . The resulting electrical signal is sent to the motor by means of an a m plifier feedback circuit varying its speed until the stack and probe flows are equal, thereby equalizing the capacitances of A and B. From Bernoulli's equation the pressure difference A p resulting from a velocity difference A Vis
s p
=
ApAl.'-' 2
(1)
where p is the density of the flowing medium. Assuming p to be t h e density of air and with the knowledge t h a t labo7, ratory devices can detect a pressure difference of we have 464
Environmental Science & Technology
PLIABLE MEMBRANE
PRCEE
\ Schematic representation of the proposed isokinetic stack sampler
Figure 1.
A\'.
=
-"Ap P
1.3 x X lofi dynicm1.3 X lo-. g/cmd 2 cm- sec-
=,x =
(2)
Here atmospheric pressure is assumed to be 106 dyn/cm2 a t m . The density of air is and Ap is 10-6 T or 1.3 x taken to be 1.5 x g/cm3. Considering the left side of Equation 2 , we have A\T. = "A\'
(3)
Then Equation 2 yields
A V 1 -=I1'Since typical stack velocities are of the order of hundreds of centimeters per second, it appears that if the membrane can respond to pressure differences of 10W6 7 , then fractional measurements of approximately may be obtainable. Since field measurements of 1 4 % are quite adequate, it is obvious that considerable leeway is allowed for any problem which may be encountered in the development of this device. The possibility then exists that automatic isokinetic stack sampling may be effected with this device. The scheme presented here appears to be sufficiently promising to warrant serious development effort. Received for reuieu Ju/>25, 1973. Accepted ,Vouember21, 1973.
