A Sample Extraction Diluter for Ultrafine Aerosol Sampling

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Aerosol Science and Technology ISSN: 0278-6826 (Print) 1521-7388

com/loi/uast20 A Sample Extraction Diluter for Ultrafine Aerosol

McMurry (1984) A Sample Extraction Diluter for Ultrafine Aerosol

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: A Sample Extraction Diluter for Ultrafine Aerosol Sampling J. E. Brockmann, B. Y. H. Liu & P. H. McMurry To cite this article: J. E. Brockmann, B. Y. H. Liu & P. H. McMurry (1984) A Sample Extraction Diluter for Ultrafine Aerosol Sampling, Aerosol Science and Technology, 3:4, , DOI: / To link to this article: Published online: 06 Jun Submit your article to this journal Article views: 437 View related articles Citing articles: 15 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 23 November 2017, At: 14:02

2 A Sample Extraction Diluter for Ultrafine Aerosol Sampling J. E. Brockmann*, B. Y. H. Liu, and P. H. McMurry University of Minnesota, Particle Technology Laboratory, Department of Mechanical Engineering, 11 1 Church Street Southeast, Minneapolis, MN A diluter that was designed for sampling high concentrations (- 10" of ultrafine aerosols (particle diameter smaller than 0.02 pm) is described. Particular care was taken to minimize the effects of coagulation and diffusional deposition within the diluter. Furthermore, the diluter was designed so that the effects of these processes on the size distribution of the sampled aerosol can be calculated. Provision was made to monitor continuously Row rates of the aerosol sample and dilution INTRODUCTION When sampling gases and aerosols it is often necessary to reduce concentrations by dilution. Dilution may be required to bring concentrations within the range of measuring instruments or to quench chemical reactions or coagulation. Factors that may be important in selecting a diluter for a particular application include residence time of the sample within the diluter (residence times should be short if coagulation or chemical reactions are important), dilution ratios, flow rates, and deposition of the measured species on surfaces within the diluter. The method that is used to draw the sample into the diluter may also be an important consideration. With air entrainment diluters, for example, the diluted sample can be delivered to the measuring instruments with little or no overall pressure drop from the source. Several diluters that have been described in the literature are summarized in Table 1. *Present address: Sandia Laboratories, Division 9422, Albuquerque, NM air. With these flow measurements, dilution ratios can be calculated without recourse to empirical calibrations that may change with time. In its present configuration, it can achieve dilution ratios ranging from 300 to 4000; this range could be extended with minor modifications. In this paper the procedure that was used in designing the diluter is described, and the performance of the diluter is compared with design expectations. These diluters were designed for a variety of applications including stack sampling, measurement of cigarette smoke aerosols, and laboratory studies of fine and ultrafine condensation aerosols. A variety of dilution schemes was used with these devices. Ragland et al. (1978) used an external vacuum pump to draw a sample into a dilution chamber. The pump also served to recycle filtered air back into the chamber for dilution. The pinhole diluters of Fuchs and Sutugin (1965) and Delattre and Friedlander (1978) consist of capillary tubes arranged in parallel with an absolute filter. These devices can function either with an elevated source pressure or with an external vacuum pump. With the entrainment devices (Heinsohn et al. (1976), Scheutzle et al. (1975), Verrant and Kittelson (1978), McCusker et al. (1982)), flowing dilution air is used to create a low pressure region that draws the sample. The diluter that is described in this paper was designed specifically for sampling ultrafine aerosols (Brockmann, 1981). Air entrainment was used to draw the sample because the instrument that was used for measure- Aerosol Science and Technology (1984) Elsevicr Science Pubhshing Co., Inc.

4 Sample Extraction Diluter ment (a TSI Model 3020 condensation nucleus counter) cannot withstand a large pressure drop between the inlet and outlet. The diluter was designed so that the effects of diffusional deposition and coagulation (processes that tend to be important for ultrafine aerosols) were usually small. Also, because laminar flow tubes were used, the data can be corrected for diffusional deposition when it is significant. In regions where the particle free dilution air and aerosol sample were mixed, the flow was turbulent to ensure rapid and uniform mixing. All flows are routinely monitored so that dilution ratios can be determined directly. In the following sections, the method that was used to design the diluter is discussed and the actual performance of the diluter is compared with design expectations. DILUTER DESIGN A schematic diagram of the ultrafine aerosol diluter is shown in Figure 1. The design of the sample inlet and primary diluter is shown in more detail in Figure 2. The aerosol sample is extracted through a 5.29-cm-long capillary tube, and immediately mixes with the primary dilution air. This diluted aerosol then flows through the 10.6-cm-long transport tube to the throat of the Venturi where it mixes with the secondary dilution air. The Venturi is used to create the suction needed to draw the sample. Thls allows a sample to be extracted and the diluted sample to be delivered to measuring instruments at atmospheric pressure. The choice of a capillary tube for sample extraction was made primarily for the following reasons. First, the loss of particles in laminar tube flow can be calculated by the well-known equation for Brownian diffusion in laminar flow (Gormley and Kennedy, 1949). The second reason is that the sampling capillary acts as a flow meter which permits direct measurement of the sample flow. Pressure drop as a function of volumetric flow is well characterized for laminar flow in capillary tubes (Schlichting, 1955). Finally, the sample capillary tip permits sample extraction with a minimal disturbance of the flow from whch it is sampling. The sample flow mixes into the primary dilution air as a turbulent jet which promotes good mixing. The primary dilution serves to reduce concentration to retard any concentration dependent process during the laminar transport of the primarily diluted sample to the throat of the nozzle for final dilution. The flow rate of the primary dilution air is measured with a rotameter. Secondary dilution takes place at the throat of the Venturi. The flow rate of the secondary dilution air is also measured with a rotameter. The use of Venturi suction for extracting the sample (and for transporting the primarily diluted sample) permits sampling at or below ambient pressure and also permits the diluted sample to be delivered to the instruments at ambient pressure. Turbulence in the cm-diameter throat of the Venturi assures rapid and complete mixing. The flow is expanded gradually to recover dynamic pressure, and provision is made for instrument sampling from this expanded flow. The excess aerosol is dumped through an absolute filter. In the following sections the design procedure is discussed. The flow rate of aerosol through the sample capillary (and hence the dilution ratio) depends on the size of the capillary tube and on the flow rates of the primary and secondary dilution air. An equation that gives the relationship between these variables is presented. The purpose of this analysis is to present a rational design approach; in actual operation, dilution ratios are determined directly by flow rate measurements. The effects of diffusional deposition and coagulation on the size distribution of the sampled aerosol are also discussed. Designing For Nominal Dilution Ratios Using Bernoulli's equation, the pressure balance for the diluter shown in Figure l can be

5 361 crn - Ps AP ACROSS SAMPLE CAPILLARY PRESSURE TRANSPORT r EXPANSION OF FLOW TO RECOVER DYNAMIC PRESSURE [ TAPS \ PE CAPILLARY LINE AIR - ACCELERATED DILUTION FLOW, LOW FLOW I STATIC PRESSURE SECONDARY DILUTION FLOW FIGURE 1. A schematic diagram extraction diluter. of the sample I ABSOLUTE Fl LTER LOW APf DRAW DILUTED SAMPLE FOR INSTRUMENTS - DUMP UNUSED DILUTED SAMPLE

6 INLET FOR PRIMARY DILUTION DOWNSTREAM PRESSURE TAP 18go. HYPO NEEDLE TUBING PRESSURE TAPS AND INLET RADIALLY STAGG HYPO NEEDLE TUBING TRANSPORT PRIMARY DILUTED SAMPLE FIT TO MANIFOLD) 5.28 cm - "LUER LOK" TIP PRESS FIT TO TUBING 6.08 cm SAMPLING TIP AND UPSTREAM PRESSURE TAP-SLIDE FIT TO MANIFOLD FIGURE 2. Details of the primary dilution section and pressure taps. PRIMARY DILUTION AND PRESSURE TAP MANIFOLD WITH TUBES AND SAMPLING TIP

7 446 Brockmann, Liu, and McMurry expressed as where P, = static pressure at the sample point (dynes/cm2) PE = static pressure at the exhaust point (dynes/cm2) AP' = pressure drop across the filter (dynes/cm2) AP, = pressure drop across the sample capillary (dynes/cm2) AP2 = pressure drop across the transport capillary (dynes/cm2) = gas velocity (average) at Venturi throat (cm/sec) p = gas density (g/cm3) If the pressure at the sample and exhaust points are equal and if the pressure drop across the filter is small compared with other terms (assumptions that are often valid in practical situations), Eq. (1) reduces to AP, + AP2 = 1/2pT52. (2) The pressure drops in the sample (AP,) and transport (A P2) capillaries are functions of volumetric flow rates, Q, through the capillaries. In fully developed laminar tube flow, pressure drop is linearly related to volumetric flow rate. However, if the flow develops from a flat velocity profile, as is the case in the sample and transport capillaries, the pressure drop has an additional contribution equal to 0.58pu (Schlichting, 1955). Therefore, for flow through the capillary tubes, pressure drop AP (dynes/cm2) and volumetric flow rate Q (cm3/sec) are related by AP=UQ~+~Q (3) where b = 128pl/(77d4) (5) p = gas density (g/cm3) d = capillary diameter (cm) p = gas absolute viscosity (g/cm/sec) I = capillary length (cm). The overall dilution ratio R is given by where the subscripts 1, 2, and 3 refer to the flow rates through the sample capillary tube, the transport capillary tube, and the Venturi, respectively. The sample flow rate Q, depends on AP,, which in turn depends on Q,, Q,, and the dimensions of the capillary tubes and Venturi. The relationship between Q, and these parameters is determined by substituting both capillary tubes -into Eq. (2). Thus where d, is the throat diameter. Again, the subscripts 1 and 2 refer to the sample and transport capillaries, respectively. Stainless steel hypodermic tubing was used to construct the sample and transport capillaries. Three different sample capillaries were used to cover a range of dilution ratios. The dimensions of the capillaries and of the Venturi are given in Table 2. Because flow rate through the sample capillaries was determined by measurements of pressure drop from Eq. (3), it was necessary to determine a, and b, accurately. Each of these parameters depends on the inverse fourth power of the capillary inside diameter. These inside diameters were determined experimentally by measuring the pressure drop versus flow rate for each capillary and fitting Eq. (3) to the data with diameter as a parameter. Agree-

8 Sample Extraction Diluter TABLE 2. Dimensions of Capillaries and Venturi Transport Sample capillaries capillary 18 gauge 19 gauge 20 gauge 11 gauge Inside diameter, cm i i i (manufacturers' specifications) Inside diameter, cm (determined experimentally from Eq. (3)) length, cm Venturies Throat diameter Converging section Expanding section ment between experiment and Schlichting's equation was excellent, and the experimentally determined diameter values fell within the manufacturers' tolerance limits as shown in Table 2. The experimental values were subsequently used in determining flow rates. Because the expression for pressure drop is applicable only so long as the capillary length exceeds the entrance length, an expression for entrance length is helpful. This is supplied by Schlichting (1955): I,,% = 0.29 Red. (8) where I,,% is the length required for the flow centerline velocity to approach to within 10% of fully developed centerline velocity, and Re is the Reynolds number. The equations presented above provide the basis for the design of this diluter, and indeed for the design of any sample extraction diluter using Venturi suction and capillary flow metering. The equations permit performance evaluation of ths sample extraction dilution system (SEDS) for various capillary dimensions and volumetric flow rates and are necessary in specifying these dimensions and flows for construction. Diffusional Deposition Particle losses to the walls of the capillary tubes can be calculated from the equation given by Gormley and Kennedy (1949) cm 11" half-angle 7" half-angle P = / and P = 0.819exp( ) exp( ) exp( - 576) for 6 > 0.02 (9) where /3 for 5 < 0.02 P = aerosol penetration (exit concentration/entrance concentration) 6 = vdl/q D = particle diffusion coefficient (cm2/sec) L = tube length (cm) Q = volumetric flow rate in tube (cm3/sec) This result applies to particle losses in cylindrical tubes with fully developed laminar flow. In principle, entrance effects in the flow profiles for the capillaries used in the diluter could cause discrepancies between actual penetration efficiencies and those predicted by Gormley and Kennedy. An analysis of the results of Chen and Comparin (1976), who examined the effects of entrance regions on particle penetration efficiencies, indicates that this is not a significant source of error. Their results show that for pm-diameter particles, penetrations are overestimated by about 5% for a penetration efficiency of 10%. This should be a worst-case error for this diluter. Therefore, Eq. (9) provides

9 Brockmann, Liu, and McMurry SAMPLE FLOW RATE,Q,crn3/sec FIGLIRE 3. EKect of diffusional deposition on aerosol penetration through the sample capillaries. Calculations are shown for particles of several different sizes. Flow rates corresponding to pressure drops between 1 and 10 cm H,O are shown for each of three different sample capillary sizes. an acceptable means to calculate particle penetration through the capillaries. Diffusion losses in other parts of ths SEDS are negligible. Figure 3 summarizes aerosol penetration through the sample capillary. Curves of penetration for given particle sizes are plotted against capillary flow rates. The trade-off between dilution ratio and sample flow rates (increasing the sample flow rate decreases the dilution ratio) should be kept in mind. Coagulation Coagulation of the sample during sampling and dilution depends on the residence times in the sample and transport capillaries, the particle concentration, and the primary dilution ratio. It is desirable to keep the number change from coagulation low, so that corrections for diffusion loss may be made without the complication of simultaneous coagulation. For small coagulation effects, the relative change in number concentration is A N/N = KNt (10)

10 Sample Extraction Diluter TRANSPORT FIGURE 4. The dependence of residence time on fow rate for sample capillaries of three sizes and also for the transport capillary. Nominal dilution ratios (assuming a primary dilution flow rate of 45 cm3/s) are also shown. The effects of coagulation can be estimated from these residence times. where N = concentration (number/cm3) AN = change in concentration (number/cm3) K = coagulation coefficient (assumed to be constant) (cm3/sec) t = time (sec). Figure 4 illustrates the relationship between mean residence time and volumetric flow rates in the sample and transport capillaries. Nominal dilution ratios (calculated from Eqs. 6 and 7 with Q, = 45 cm3/sec) are also shown. These dilution ratios correspond to values of AP, ranging from 5 to 10 cm of H,O. As can be seen in Figure 4, decreasing the residence time in the sample capillaries has the adverse effect of decreasing the dilution ratio (and thus increasing the concentration in the transport capillary). Eq. (10) is used to evaluate the overall effect of coagulation for the system and to optimize trade-offs. When both coagulation and diffusional deposition are small, data can be corrected as if these effects were independent. In this SAMPLE FLOW RATE,Q cm3/sec case, corrections are easy to make. When both effects are large, however, correcting data is not straightforward; the two effects are coupled, and corrections for one effect cannot be made independently of the other. For these reasons it is preferable to operate with small coagulation and deposition effects. Experimental Evaluation Performance of the SEDS using the 20 ga. sample capillary in sampling and diluting an aerosol has been evaluated experimentally. Experiments have been performed with dioctyl phthalate (DOP) aerosols of 0.035,urn and pm mean size and geometric standard deviation of approximately 1.2. The aerosol has been generated by a condensation aerosol generator (Liu and Lee, 1975) and concentrations measured with an electrical aerosol detector (EAD) as described by Liu and Lee (1975). Calculations indicate that for these data coagulation is negligible and that penetration is at least 97%. Therefore, particle diffusion loss and coagulation would not be noticed. Figure 5 is

11 Brockmann, Liu, and McMurry CONDENSATION AEROSOL GENERATOR DRIED FILTERED LINE AIR OUTPUT AEROSOL ELECTRICAL AEROSOL ABSOLUTE FILTER SAMPLE EXTRACTION DILUTION SYSTEM FIGURE 5. A schematic diagram of the apparatus that was used to determine whether aerosol dilution was equal to flow dilution. a schematic diagram of the experimental test setup. Figure 6 shows a comparison between dilution ratios obtained by flow measurement and those obtained by EAD measurement. The standard deviation of these data from the 45" line is 8.4%. Two parallel lines have been drawn at +lo% from this line corresponding to estimated measurement uncertainty. Because flow measurements were more accurate than the aerosol measurements, it is likely that actual aerosol dilution was more accurate than f 10%. It is concluded that flow dilution is an accurate indicator of aerosol dilution in this device. The actual performance of the diluter has also been compared with design expectations. In practice, flow rates are measured and the dilution ratio R is calculated with Eq. (6). However, the design equations (Eqs. (2) to (5) and (7)) enable one to determine the expected sample flow rate Q, in terms of flow rates Q, and Q, and in terms of dimensions of the capillary tubes and Venturi. Using this calculated value of Q,, it was found that the expected dilution ratio R was typically within about 20% of the measured value. Deviations between actual and expected values of R can be explained in part if the pressure drop across the filter and FIGURE 6. A comparison of aerosol and flow dilution ratios for two particle sizes. Note that the two are consistent to within experimental uncertainty (about 10%). Because the flow measurements were more accurate than aerosol measurements, it is likely that the actual uncertainties in aerosol dilution ratio are smaller than 10%. DILUTION RATIO ( FLOW I

12 Sample Extraction Diluter 45 1 differences between upstream and downstream pressures are accounted for. CONCLUSIONS A sample extraction and dilution system specifically designed to sample coagulating ultrafine aerosols is presented. The principle of Venturi suction is employed in the twostage SEDS. It is capable of rapid (< 20 msec) dilution on the order of 1000 : 1 and is well characterized with respect to flows. Particle losses and coagulation within the diluter are minimized by design, and can be characterized with respect to established theory. The system is specifically suited to sampling high concentrations (1010/cm3) of ultrafine (particle diameter < pm) aerosols and delivers more than 60 L/min of diluted sample. It is stable and easy to use; all flows are measured and continuously monitored. The design principles are straightforward and may be applied to a number of situations where rapid high dilution ratios are required. This research was supported in part by EPA grant R and NSF grant CPE The contents of this paper do not necessarily reflect the views of these agencies. REFERENCES Brockmann, J. G. (1981). Coagulation and deposition of ultrafine aerosols in turbulent pipe flow. Ph.D. thesis, Univ. of Minnesota. Chen, R. Y., and Comparin, R. A. (1976). Deposition of aerosols in the entrance of a tube. J. Aerosol Sci. 7: Delattre, P., and Friedlander, S. K. (1978). Aerosol coagulation and diffusion in a turbulent jet. I und EC Fundumentals, Vol. 17: Fuchs, N. A,, and Sutugin, A. G. (1965). Coagulation rate of highly dispersed aerosols. J. CON. Sci. 20: Gormley, P. G., and Kennedy, M. (1949). Diffusion from a stream flowing through a cylindrical tube. Proc. Royal Irish Acad. 52-A: Heinsohn, R. J., Davis, J. W., Anderson, G. W., and Kopetz, E. A. Jr. (1976). The design and performance of a sack sampling system with dilution. In 69th Annual Meeting of the APCA, June-July, Liu, B. Y. H., and Lee, K. W. (1975). An aerosol generator of high stability. Am. Ind. Ifyglene As~oc. J. Dec. 1975: McCusker, K., Hiller, F. C., Wilson, J. D., McLeod, P., Sims, R., and Bone, R. C. (1982). Dilution of cigarette smoke for real time aerodynamic sizing with a SPART analyzer. J. Aerosol Sci. 13: Ragland, J. W., McCain, J. P., and Smith, W. B. (1978). (Draft Report to EPA Office of R and D, Research Triangle Park, NC, 27711) Design, construction and testing a field usable prototype system for sizing particles smaller than 0.5 pm diameter, SRI Report A-Dilution system. Scheutzle, D., Prater J. J., and Ruddel, S. R. (1975). Sampling and analysis of emissions from stationary sources I. Odor and total hydrocarbons. J. APCA 25:9, Schlichting, (1955). Boundary Layer Theory, 6th ed. McGraw-Hill, New York. Verrant, J. A,, and Kittelson, D. B. (1978). Sampling and physical characterization diesel exhaust aerosols. SAE Paper No Received 20 March 1984; accepted 17 May 1984

CHAPTER 6 : GAS SAMPLING SYSTEMS

CHAPTER 6 : GAS SAMPLING SYSTEMS 1 Scope : 1.1 This Chapter describes two types of gas sampling systems in paragraphs 2.1 and 2.2 meeting the requirements specified in para 4.2 of Chapter 3 of this Part.