COURTESY WESTERN PRECIPITATION GORP
Figure 1.
I
Null-type nozzle assembly, probe B
RICHARD DENNIS, WILLIAM R. SAMPLES, DAVID M.ANDERSON, and LESLIE SILVERMAN Harvard School of Public Health, Department of Industrial Hygiene, Boston 15, Mass.
Isokinetic Sampling Probes Without particle-size data, isokinetic sampling of gas streams
b Involves no
unusual problems
b Assures reliability of T
H
E
AERODYNAMIC
CHARACTERISTICS
of five specially designed sampling probes have been investigated by this laboratory in conjunction with a general study of isokinetic sampling methods and their relationship to correct representation of particulate concentrations in high velocity (1000 to 6000 feet per minute) gas streams. S o specific references pertaining to theoretical aspects of probe design and only limited perform-
sample
ance data were found in the literature (6). Many investigators have stressed the importance of isokinetic sampling from the standpoint of particle fractionation or inertial segregation. I t has been generally recognized that nonisokinetic collection of particulate materials ( > l o microns) from high velocity gas streams may produce considerable error in estimated dust loadings. The magni-
Figure 2.
tude of error as reported in the litcrature (4, 6, 74) depends upon the particle size and ranges from 10 to 2.07, for a 207, deviation from isokinetic flow. Data obtained in this laboratory (5) on a suspension of Cottrell precipitated fly ash (count median 0.6 micron, mass median 14 microns) showed only a lOy, negative error in calculated dust loadings for sampling velocities 607, greater than isokinetic. Furthermore, a 400y0 varia-
Null-type stack sampler, probe A
1. 1 '/Z-inch 0.d. tube, No. 1 6 gage, 1,370-inch id.; 2. Z'/s-inch 0.d. tube, No. 1 8 gage, 2.027 inch id.; 3. Inside static, 81/pinch diameter holes equally spaced; 4. Outside static, 12*/~-inchdiameter ho!es equally spaced; 5. Static tubes, 3/~,j-incho.d., 0.1 17-inch i.d.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
tion from isokinetic flow produced no detectable concentration changes when sampling atmospheric dust (geometric mean diameter = 0.5 micron). Hence, it appears that isokinetic sampling is relatively unimportant for fine particles. In the absence of sizing data, however, samples should be collected isokinetically, for the procedure ordinarily entails no unusual problems and eliminates uncertainty about reliability of the sample. The simplest technique requires only a single tube with a streamlined entry and a knowledge of gas velocity distribution in the stack. There are situations, however-e.g., long-period sampling with variable stack velocity-where more elaborate devices are justified from the standpoint of ease in operation and accuracy. Experimental results reported here are based on tests of three null-type or static-balance probes and two streamlined probes designed to serve as modified Pitot-static tubes when not used for sampling. Duct velocities ranged from 900 to 7000 feet per minute and probe sampling rates from 1.5 to 65 cubic feet per minute. Description of Sampling Probes
Null-Type or Static Balance Probes. Three probes were designed on the null or static-balance principle, wherein a zero pressure differential (null reading) between static pressures measured on the inside and outside surfaces of the probe presumably indicates isokinetic sampling velocity. When designed and operated properly, this type of probe
Figure 3.
Schematic of experimental null-type sampling probe, probe C
enables the operator to maintain isokinetic sampling velocities throughout a test by manual or automatic control, regardless of variation in stack gas flow. Also, he can calculate directly the duct air flow from recorded data on sampling rate us. sampling time without using separate flowmetering instruments such as Pitot-static tubes. Since construction of null-type probes does not always permit placing of filter media-eg., Whatman filter thimbles-in the head of the probe (I), dust deposition may occur in the probe and sampling lines before the collection media. In small samples a significant weight error results if such deposition is neglected. Fabrication costs are higher for most null-type probes and their effective sampling ranges are usually less than those for probes employing interchangeable entry nozzles (7). Figures 1 through 3 illustrate the
Figure 4.
nulI-type probes employed in this investigation. Figure 1 shows a sampling device (probe B) designed by the Buell Engineering Co. for high flow rates (IO to 65 cubic feet per minute for stack velocities of 1000 to 6500 feet per minute), which was built and loaned for this study by the Bethlehem Steel Co. Dust samples are collected in clothbag filters attached to the downstream end of the probe. Static pressure within the probe is measured 0.875 inch (0.636 inner diameter) from the streamlined entry, while that outside (duct static pressure) is measured 5.81 inches (2.73 outer diameters) from the entry. General probe contours approach those of a Pitot-static tube, although the length of the nozzle section is considerably shorter with respect to probe diameter. Figure 2 shows probe A loaned by the Western Precipitation Corp. and used
Drawings of streamlined probe 1-1
1. Probe housing, rear portion; 2. Rear static tube, 0.1 25-inch i.d., 0.0625-inch i.d.; 3. Rear static extension, 0.1 875-inch a.d., 0.1 250-inch i.d.; 4. Sampling tube,0.375-incho.d.,0.325-inchi.d.; 5. Whatman extraction thimble, 19 X 61 mm.; 6. Thimble retaining collar; 7. Sampling head; 8. To flowmeter and pump VOL. 49, NO. 2
FEBRUARY 1957
295
Figure 5. Drawing showing contours of four streamlined sampling probes. Dimensions in inches
for long-period efficiency tests on dust collectors. Sampling volumes range from 1.5 to 8.2 cubic feet per minute for a duct velocity range of 1000 to 6000 feet per minute. Inside static pressure is measured 1.125 inches (approximately 2 inner diameters) downstream from the probe entry and outside static pressure 2.75 inches (1.83 outer diameters) from the inlet. Although the forward half of the nozzle section is streamlined, the rear portion of the probe consists of ordinary pipe fittings. Figure 3 shows an experimental probe (probe C) representing elementary nulltype design, where inside static pressure could be measured 2, 4: or 6 diameters downstream from the entry. A separate Figure 6. Schematic drawing showing apparatus for testing null-type probes 1. Duct flowmeter, annulus type, 6-, 8-, or 10inch diameter disk; 2. Null-type sampling probe; 3. Sampling probe flowmeter; 4. Fan; 5 . Venturi meter; 6. Static pressure probe. Manometers: A. Duct flow, negative pressure system; B. Inside static, probe; C. Probe differential, inside to outside static; D. Outside static, probe; €. Probe flow; F. Duct static, positive pressure system; G. Duct flow, positive pressure system
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INDUSTRIAL AND ENGINEERING CHEMISTRY
tube is used to determine duct static pressures. Streamlined Sampling and Flowmetering Probes. The second probe design investigated (Figures 4 and 5) consists of streamlined sampling heads (containing Whatman filter thimbles) which can be used alternately as Pitotstatic tubes. For the latter application, differential pressure between the probe entry (impact pressure) and static pressure recorded a t the rear tip of the probe indicates the velocity pressure in the air stream. Figure 4 illustrates the complete construction of one probe and Figure 5 shows four variations in shape produced by using two bodies and four interchangeable nozzle tips. Sampling rates for these devices (0.219-inch inlet diameter) range from 0.3 to 1.1 cubic feet per minute for duct velocities of 1000 to 6000 feet per minute. T h e primary application for this type of probe is in determining comparatively high dust loadings (>0.1 grain per cubic foot) during short sampling periods (
