Sampling Effectiveness of the Inlet to the Dichotomous Sampler James 6. Wedding* and Michael Weigand Aerosol Science Laboratory, Engineering Research Center, Colorado State University, Fort Collins, Colorado 80523
Walter John and Stephen Wall Air and Industrial Hygiene Laboratory, California Department of Health Services, 2151 Berkeley Way, Berkeley, California 94704
The sampling effectiveness of the inlet to the dichotomous virtual impactor has been measured in a wind tunnel at speeds from 5 to 40 km/h by using monodisperse oleic acid particles of 5-30 pm aerodynamic diameter. A static test was performed a t near-zero wind speed. Over this range of wind speeds the 50% cut point varied from 10 to 22 pm, with a maximum near 2.5 k m h . At a given wind speed the cutoff curve (effectiveness vs. particle diameter) is not sharp. Flow visualization studies revealed strong channeling of the flow inside the inlet. A simple theoretical model based on this flow pattern including inertial and gravitational forces on the particles accounts well for the observations. It is concluded that a better inlet is required for the dichotomous sampler. The present experience indicates the need for rigorous experimental validation of new inlet designs.
Background The inlet is a vital component of a sampler; unfortunately, this aspect of sampler design has not received adequate attention. Furthermore, few samplers are equipped with inlets which have been rigorously tested. The Hi-volume sampler ( I ) is typical of many samplers in that the inlet determines the upper particle size cutoff. The accuracy of measurements of suspended particulate with the standard Hi-Vol can be questioned because of the large variation in sampling effectiveness as a function of wind speed and direction as shown by Wedding (2, 3 ) . Several investigators have studied the particle and fluid mechanics related to the sampler inlet problem. Davies ( 4 , 5 ) investigated sampling biases of various sized tubes as a function of particle size, sampling rate, and wind velocity. Agarwal (6) performed a theoretical study of a cylindrical tube. May (5)looked at a variety of samplers in a field study using a portable wind tunnel. Errors in sampling at nonisokinetic conditions as well as probe design limitations were studied by the researchers in ref 7-14. Steen and co-workers (15-17) characterized those samplers used in Europe to collect aerosols. These results confirmed that it is difficult to design an inlet that has an efficiency independent of sampling conditions. The present study involved the determination of the variation with wind speed of the sampling effectiveness (transmission efficiency) of the inlet of the commercially available Sierra 2443 dichotomous sampler (Figure l a ) . This type of dichotomous virtual impactor has been developed by Loo et al. (18,19) in support of programs of the U S . Environmental Protection Agency. The dichotomous samplers are being deployed in a trial network by the EPA as part of a project directed toward the establishment of an inhalable particle standard based on a particle size cutoff a t 15 pm (20). Since the cutoff is determined by the inlet, it was important to assess the effect of wind speed on the cutoff. It was unnecessary to study the effect of wind direction because of the cylindrical symmetry of the inlet. Studies were carried out in a wind tunnel. The results motivated additional testing near zero 0013-936X/80/0914-1367$01.00/0
@ 1980 American Chemical Society
wind speed. A theoretical model was then developed to explain the observations. Experimental Section
Wind Tunnel Tests. The Sierra Model 2443 ambient aerosol sampling inlet was tested in the closed-loop Aerosol Science Laboratory Wind Tunnel at Colorado State University (shown schematically in Figure 2). The tunnel has crosssectional dimensions of 1.22 m square at the test section. The longitudinal component of turbulence intensity at the test location was found by hot wire anemometer measurements to be 4%. The tests utilized monodisperse aerosols with nominal aerodynamic diameters of 5-30 pm generated by a vibrating orifice atomizer operating in an inverted manner. Aerosol from the atomizer was injected through a 15.24-cm diameter pipe containing a Kr-85 charge neutralizer. The pipe diverged into the annular region between two cones. Six pipes spaced around the annular region led the aerosol into the tunnel. This injection system produced a particle concentration profile across the test section which was found to be independent of wind speed. Variation of the concentration across the width of the inlet was less than 10%. The sampling inlet was tested at wind speeds of 5,15, and 40 km/h, as measured upstream of the test section with a calibrated hot wire anemometer (Andersen air velocity meter). To determine the inlet effectiveness, the aerosol concentration was measured before and after each test by using a sampling manifold 0.90 m wide with six isokinetic sampling nozzles spaced at equal intervals (-15 cm) in the same horizontal plane as the inlet opening. Each nozzle led to a 47-mm Gelman AE glass fiber filter. The dichotomous sampler inlet was mounted on a vertical tube -1 m long sealed to a Milipore filter holder on the bottom. The sampling effectiveness of the inlet was determined by comparing the quantity of aerosol passed by the inlet and deposited on the filter to that collected by the isokinetic sampling system with appropriate corrections for differences in sampling volume and pressure drops as monitored in the lines. Particles utilized in the study were made from an oleic acid-ethanol mixture tagged with uranine. Collection substrates were washed in 50 mL of pure ethanol and then diluted 1:l with distilled deionized water. A 4-mL aliquot of each sample solution was measured for uranine content with a fluorometer (Turner Model 111). Other commercial models of the inlet differ in design slightly from the Sierra 2443 (see Figure la). The Sierra 244 inlet has no holes in the center pipe but instead ends -1 cm from the top. The Beckman inlet (Figure 1b) in contrast to the Sierra inlets has rain grooves in the bottom cone and minor differences in pipe dimensions. A check of several examples of each inlet showed that in each case the dimensions agreed closely with manufacturer’s specifications. Static Tests. In order to completely characterize the inlet, it is necessary to determine the sampling effectiveness at near-zero wind speed where impaction is absent and sedimentation is the only particle deposition mechanism. The arrangement used for the static measurements is shown in Figure 3. This work was carrried out at the Air and Industrial Volume 14, Number 11, November 1980
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Figure 1. Inlets for the dichotomous virtual impactor: (a) Sierra Model 244E, (b) Beckman automated dichotomous particulate sampling system.
TESTING FACILITY FOR A Y B L N l AEROSOL SANPLERS
I i IlWMl EXPERIMENTAL CONFIGLRATON AEROSOL SCIENCE LABORATORY TEST FACILITY
Figure 2. Schematic drawing of the wind tunnel test facility.
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Figure 3. Experimental arrangement for measurements of inlet sampling effectiveness at near-zero wind speed.
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Hygiene Laboratory in Berkeley. Monodisperse particles of oleic acid containing 1%uranine by volume were produced by a vibrating orifice aerosol generator (Berglund-Liu) and passed through a Kr-85 charge neutralizer. Optical particle counter No. 1 (Climet Model 201) continuously monitored the size and concentration of the particles. The aerosol entered the top of a plastic bag, flowed downward a t an average velocity approximately I/~O of the flow velocity into the inlet, and exited a t the bottom. Anemometer measurements showed a fairly uniform downward flow of less than 1 cm/s with occasional gusts up to several times the average velocity. The inlet was pumped by an electronically stabilized flow controller with 5.7 L/min of the flow split isokinetically into optical counter No. 2 (Climet Model 208). The total flow was the standard 16.7 L/min sampling rate of the inlet. The particle counting rate in optical counter No. 2 was first determined by sampling for 1min with the inlet removed from the sampling tube. Under these conditions, the sampling criteria of Davies (21) which consider the effects of impaction and sedimentation are satisfied. Then the inlet was replaced, and, after a delay of 1. min to allow the flow to stabilize, another 1-min count was taken. The sampling effectiveness was calculated from the ratio of inlet off to inlet on counts. The error was less than 1-2% from counting statistics. The optical counter data were taken only from the main particle peak in a multichannel analyzer so that multiplets from particle coagulation, etc., were automatically excluded. For each particle size the inlet was cycled through several on and off counts to obtain averages. As a reference method, aerosol from the inlet pipe was collected directly onto a Teflon membrane filter. The uranine was then extracted from the filter by sonication in alcoholwater (90%/10%)and quantitation on a fluorometer (Aminco Model 54-7439). Successive 20-min filter samples were taken with the inlet on and off the pipe. Flow Visualization. Understanding of the data on the sampling effectiveness is aided by knowledge of the airflow pattern within the inlet. To this end, the metal top of the inlet was replaced with one of transparent Lucite and smoke introduced to make the airflow visible. A fog of glycerol particles from an ultrasonic nebulizer was also used. Because the airflow was extremely sensitive to ambient air currents, consistent results were obtained only when the aerosol was injected a t a speed exceeding -20 cm/s (0.7 km/h).
T h e o r y of t h e Sampling Effectiveness
At zero wind speed the airflow is directed toward the axis of the inlet because of the cylindrical symmetry (refer to Figure la). For any appreciable (-1 km/h or greater) wind speed, the flow entered the inlet channels, following the vertical radius closely, as observed in the visualization tests. The horizontal width of the air parcel sampled is small compared to the inlet diameter as air enters virtually along the stagnation point streamline. The width can be calculated from the known volumetric flow rate (16.7 L/min), the 1-cm height of the inlet slit, and the assumed wind speed. The air not sampled simply flows around the inlet, entering and exiting the inlet slit. The above considerations lead to a simplified model of the sampling effectiveness. As illustrated in Figure 4,air enters the slit and flows in a channel of constant width along the vertical arc of the lower surface of the inlet. A particle traveling in the flow will drift outward along the radius under the influence of centrifugal and gravitational forces. The sum of these forces may be equated to the Stokes drag force 37rDp ; d r = mu2 mg cos o
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