DEKATI EJECTOR DILUTER IN EXHAUST MEASUREMENTS V2.11

Transcription

1 Dekati Ltd Technical Note DEKATI EJECTOR DILUTER IN EXHAUST MEASUREMENTS V2.11 Dekati Ltd. Osuusmyllynkatu 13 FIN Tampere Finland Author: HEI, Dekati Ltd. Date: Keywords: Dilution, Sampling Modified: , ENI Tel: Fax: Introduction Dekati diluter enables aerosol sampling from high particle concentrations, temperatures and humid environments. It is especially useful in vehicle exhaust and combustion aerosol measurements and suitable for any application where you can extract about 7-8 l/min exhaust sample for particle analyses. This sample can be extracted from anywhere in the exhaust system, as long as the pressure is practically stable and the temperature is under 450 ºC. Ejector diluter Figure 1. Dekati diluter. Operation principle The operation principle of Dekati diluter is based on under pressure caused by the dilution gas flowing through an annular nozzle. This under pressure draws a sample flow through a nozzle. If the dilution gas flow is increased the under pressure increases correspondingly and the sample flow is increased (Figure 2). Thus, the dilution ratio remains constant, if the pressure in exhaust pipe is constant (i.e. the pressure drop through the nozzle remains constant), which is practically the case in an end-pipe Dekati Ltd. - Technical Note - 1 -

2 measurement. The dilution air flow is controlled using a critical orifice at the pressurized air inlet. SAMPLE IN SAMPLE OUT dp DILUTER BODY DILUTION AIR SAMPLE NOZZLE Figure 2. Operating principle of the Dekati ejector diluter The strength of Dekati Diluter is that as long as your sample pressure remains constant the dilution ratio remains constant. In addition, each diluter is calibrated individually. In the calibration data, the dilution ratio is checked for inlet pressures mbar. In case of pressure fluctuations in the sampling site, the exhaust from the diluter is lead back to the sampling point to keep the pressure drop through the nozzle constant and thus prevent deviations in the dilution ratio. The dilution ratio check is recommended to be carried out during situations where the inlet pressure is varying significantly during the measurement period. This is the case e.g. if heavy duty engines are tested with transient cycles. The dilution ratio can be checked by measuring a tracer gas concentration in raw and diluted sample. Preventing nucleation and condensation In case of hot sampling, a two-stage dilution system is recommended to be used. In this setup, the first stage is heated. This way, the vapor pressures of volatile components are decreased and thus condensation and nucleation of these compounds, and in some cases fouling of the dilution unit, is prevented. In addition this allows the secondary dilution with cold dilution gas without condensing the volatile components. Moreover, the secondary dilution is carried out with cold air to cool the sample in a controlled manner. Otherwise, some particles may be lost onto the transport lines by thermophoresis. The dilution air for the first diluter as well as the first diluter itself should be heated approximately up to the temperature of the sample gas. The maximum operating Dekati Ltd. - Technical Note - 2 -

3 temperature for the Dekati diluters is 450 C. However, the dilution ratio during hot dilution needs to be calibrated separately. Measurement setup The recommended setup for tailpipe or stack measurement includes: two diluters one diluter heater + temperature controller pressurized air heater + temperature controller pressurized air regulator, filter and dryer Figure 3 represents the recommended measurement setup for two Dekati diluters. Isokinetic Sampling Air Heater Air, 2 bar Dekati Diluter (1/10) and Insulated Diluter Heater Diluter exhausts back to stack Air Dryer & Regulator Unit Dekati Diluter (1/10) Figure 3. Dekati diluter is an ejector type diluter, i.e. the pressure drop through the ejector nozzle controls the dilution ratio. If the exhaust pipe is connected to same pressure as the inlet then there are no pressure drop variations through the nozzle and the flow through the nozzle remains constant as does the dilution ratio. The dilution ratio of one Dekati diluter is 1:10 giving a total dilution ratio of about 1:100 to the double-diluter setup. Diluters are also available with dilution ratios of 1:7 and 1:15 (at NTP). Isokinetic sampling Isokinetic sampling means that the sample flow velocity in the sampling probe is equal to that of the sample flow in the exhaust tube. If isokinetic sampling is not achieved, the concentration of large particles can be over- or underestimated. To achieve isokinetic sampling, a correct sized probe has to be selected. Dekati Ltd. - Technical Note - 3 -

4 In vehicle exhaust measurements isokinetic sampling from the tailpipe is not so essential. It begins to affect the results only with larger particles, and since vehicle exhaust particles are mostly < 1μm the effect is negligible (Figure 4). For particles smaller than 1 μm flow velocities can vary from 0.05 to 10 times the optimal velocity with no practical effect on the particle size distribution. 100 % 80 % 60 % 40 % Effect of sample flow from DEKATI diluter, calculated at 150C for N2 & 10 % of water vapour according to Hinds, (1999) and Willeke and Baron (1993) C/Ctot, 2.8 slpm C/Ctot, 6.5 slpm C/Ctot, 13.2 slpm C/Ctot, % 20 % 0 % -20 % % -60 % -80 % -100 % Aerodynamic diameter, µm Figure 4. Effect of isokinetic sampling. Losses inside Dekati diluter The particle losses of the Dekati diluter have been determined with various flow rates using Electrical Low Pressure Impactor (ELPI TM ). For particles smaller than 1μm the losses proved to be negligible in all tests. For particles in the size range of 1 5 μm, in the worst case, the losses were less than 10%. The worst case was for liquid particles with a sample flow downstream of the first dilutor less than 10% of the design value. The tests were not carried out for particles larger than 5 μm. Specifications of the Dekati Diluter Maximum operating temperature 450 C Total length 360 mm Maximum diameter 120 mm Inlet 12 mm, male pipe Outlet 12 mm, male pipe Exhaust 12 mm, male pipe Pressurised air 8 mm swagelok, female Operation pressure 2 bar For more information, please contact Dekati Ltd. Dekati Ltd. - Technical Note - 4 -

5 References Ahlvik, P., Ntziachristos L., Keskinen J., Virtanen A., (1998). Real Time Measurements of Diesel Particle Size Distribution with an Electrical Low Pressure Impactor. SAE Technical Paper Series Maricq, M., Chase, R. E., Podsiadlik, D. H. and Vogt, R. (1999) Vehicle exhaust particle size distribution: a comparison of tailpipe and dilution tunnel measurements, SAE Technical Paper Series Willeke, K. & Baron, J Aerosol measurement: Principles, Techniques and Applications. Van Nostrand Reinhold, New York. Hinds, W.C. Aerosol Technology - properties, behavior, and measurement of airborne particles. Second edition. John Wiley & Sons, inc. pp Dekati Ltd. - Technical Note - 5 -