Sampling concentrated aerosols diluter design for ultrafine particles

1. Aemsol Sci. Vol. 29. Suppl. 1, pp. S333-5334, 1998 0 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0021-850298 $19.00 +O.OO

Pergamon

SAMPLING CONCENTRATED AEROSOLS DILUTER DESIGN FOR ULTRAFINE PARTICLES M J LANCASTER and D R BOOKER Aerosol Science Centre, AEA Technology plc, 401.8 Harwell, Didcot, Oxfordshire, OX1 1 ORA Key Word Index:

ultratine, urban aerosol, dilution, vehicle

With the increasing health concerns about particulate air pollution, and more specifically, the potential association between traf?ic-related air pollution and diminished pulmonary function and/or increased respiratory symptoms, the measurement of concentrated ultrafine aerosols is becoming common practice. However, concern over the current measurement practices of, for example, vehicle emissions, has highlighted a need for improved practices and in particular sample conditioning of “raw” exhaust particles were peak particle concentrations in excess of 10’ particles crnb3 are typical. The objective of this study was simply to develop a generic spreadsheet based design package based on a review of diluter designs. In particular the work by Brockmann et al (1984) was found to be very relevant and of high quality. The design package has been evaluated by constructing and testing several diluter systems. Since a low residence time was required to minimise particle coagulation effects, the diluter was designed around a two-stage unit (See Figure). The design procedure utilises Bernoulli’s equation, since the pressure balance for these diluters can be simply expressed as P, - ( Pr+DPr - %pVt2 ) = DPI + DP2 where P, = static pressure at the sample point Ps = static pressure at the exhaust point DPr = pressure drop across the filter DPi = pressure drop across the sample capillary DP2 = pressure drop across the transport capillary Vt = gas velocity (average) at venturi throat p = gas density And under well defined conditions, a two-stage diluter utilising two capillary flows can be expressed as:

2% where Q is the volumetric flow rate, d is the venturi throat diameter, s333

Abstracts

s334

of the 5th International

a and b are capillary constants The subscripts

Aerosol Conference

based on Schlichting,

1 and 2 refer the sample and the transport

1998

1955.

capillaries,

respectively.

For the capillaries, agreement between experiment and schlichting’s equation was excellent, and the experimentally determined diameters values fell within the manufactures tolerance limits. The experimental results were subsequently used to determine flow rates. For a complete design package, the diluter design calculations were transport calculations thus providing both a complete design package.

coupled

with

aerosol

c Gilibrator

DP t Compressed

Air

MicroManometer

References Brockmann, J.E., Liu, B.Y.H., and McMurry, Sampling” Ultrafine Aerosol Schlichting,

(1955)

Bounard

P.H.,

1984

” A sample Extraction

Layer Theory. 6th ed, McGraw-Hill,

New York.

Diluter

For