Minimum gas speed in heat exchangers to avoid particulate fouling

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3946 M.S. Abd-Elhady et al. / International Journal of Heat and Mass Transfer 47 (2004) 3943–3955Fig. 2. Particle size distributions (PSD, left) and scanning electron microscope (SEM, right) photos of glass (a,b), bronze (c,d) andcopper particles (e,f), respectively.The cooling airflow rate is also controlled through acritical nozzle and a pressure regulator.Fouling of the heat exchanger tube is monitoredusing a digital camera through a glass window installedin the heat exchanger section as shown in Fig. 1b. Thedigital camera is the Finepix S602 zoom, with a resolutionof 2832 pixels · 2128 pixels, i.e. 6.03 million pixels.The digital camera has a 6· optical zoom which comparedto small image format has a zoom area of 35–210mm. The fouling layer thickness is measured optically.Images are taken of the heat exchanger tubes before andduring fouling. The clean heat exchanger tube radius istaken as a reference. The tube radii in the camera imagesand hence the fouling layer thicknesses are determinedwith an accuracy of ±1 pixel which corresponds to ±0.04mm. The temperatures of the hot air and the cooling airbefore and after the heat exchanger are measured usingK type thermocouples with an accuracy of ±0.4 °C. Theair speed at the exit of the heat exchanger section ismeasured by a hot wire anemometer with an accuracy of±0.015 m/s. Air exhaust from the setup is filteredthrough a bag filter before it goes into the atmosphere.Each fouling experiment is carried out in the followingprocedure: Hot and cold airflow rates are adjustedto the desired flow rates at the beginning of theexperiment. The hot air temperature is adjusted throughthe air heater and the experiment is kept running till asteady state is reached. After a steady state has beenreached, particles are injected at the desired rate and thefouling process is monitored.3. Results and discussion3.1. Influence of flow speed on growth of fouling layerThe following experiments are performed to see theinfluence of flow speed on the growth of the fouling layerand to determine the flow speed at which fouling ceases.Two types of particles are used in the experiments, thenon-metallic glass particles and the metallic bronze andcopper particles defined in the previous section.In the first experiments, glass particles have beeninjected in the flow at different gas flow speeds and thefouling behavior is monitored. The average airflowspeed between the tubes has been changed from 2.7 to3.8 to 5 m/s and in each case the operating time was 9 h.The air temperature is 200 °C and the particles
M.S. Abd-Elhady et al. / International Journal of Heat and Mass Transfer 47 (2004) 3943–3955 3947Fig. 3. Fouling layer at the end of 9 h of operation for different flow speeds. The foulant is spherical glass particles of diameter 21 ± 16lm. (a) V ¼ 2:7 m/s; (b) V ¼ 3:8 m/s; (c) V ¼ 5 m/s.Table 1Comparison between the fouling behaviors for glass particles under different operating conditionsAverage airflow speed between the HE V ¼ 2:7 m/s V ¼ 3:8 m/s V ¼ 5 m/stubesFinal thickness (mm) 1 0.75 0.3Circumferential and radial growth Continues Starts very fast and thendecreases abruptlyStarts very fast and thendecreases abruptlyconcentration is 0.15 g/N m 3 of flowing air. Fig. 3 showsthe fouling layer at the end of operation for the abovementionedcases. Table 1 shows a comparison betweenthe three different cases. It is shown that as the speed ofthe flow increases, the fouling layer surface area andthickness are reduced. Increasing the average air speedbetween the heat exchanger (HE) tubes from 2.7 to 5 m/sreduces the final fouling layer thickness from 1 to 0.3mm after 9 h of operation. When the average flow speedbetween the HE tubes is 2.7 m/s, it is found that thecircumferential and radial growth of the fouling layercontinued from the beginning till the end of the experiment.Increasing the average flow speed between the HEtubes to 3.8 and 5 m/s showed a different foulingbehavior. The fouling rate was very high at the beginning,especially for the first 3 h. Then the fouling ratestarted to decrease abruptly for the next 6 h of operation,till hardly any radial or circumferential growth canbe seen.Figs. 4 and 5 show the growth of the fouling layer forglass particles over the heat exchanger tube as a functionof time. The average air speed between the tubes is 2.7m/s for Fig. 4 and 3.8 m/s for Fig. 5. Fig. 4a shows thatat the beginning of the fouling process, some particlesstart to deposit at intermittent and distant positions.These points of deposition start to grow radially, Fig.4b–e, till the whole surface of the heat exchanger tube iscovered completely with particles, Fig. 4f. The surficialgrowth of the fouling layer is emphasized by the encircledareas in Figs. 4a–f. The surficial growth of thefouling layer can be explained by the fact that fineparticles are most likely to stick first to the HE tubesbecause of their higher sticking velocity compared tocoarse particles [7]. Then larger particles, which depositon the heat exchanger tube and stand alone in theshearing flow, start to roll over the heat exchanger tubeif the shearing flow speed is above a certain limit. Theparticle rolling motion is stopped when it is blocked by astanding heap of particles, which leads to the surficialgrowth of the fouled areas till they cover the unfouledsurfaces. The flow speed required to roll a particleresting on a flat surface, and, the sticking velocity of aparticle, will respectively be discussed in detail in thefollowing subsequent sections.Fig. 5 shows that at the beginning of the foulingexperiment, Fig. 5b, the glass particles in the flow startedto deposit on the rear end of the HE tube and continuedto the circumference, Fig. 5c–e. The rear end of the heatexchanger, Fig. 5f, is subjected to the lowest shearingforces due to separation of flow from the tube surface,Achenbach [9]. The flow field for the examined HEshown in Fig. 1 is solved using the finite volume methodapplied by the commercial CFD-package CFX version4.2. The low Reynolds number k–e model is used to solvethe flow field around the HE tubes. For all but theadvection terms, use is made of a second-order centraldifferencing scheme. In the equations for the velocitycomponents, use is made of the third order QUICKinterpolation scheme for the advection terms. For the kand e equations the first order HYDRID-scheme is appliedwhich is more stable than the QUICK scheme andensures the k and e values to remain positive in theconverged solution. In the computations use is made ofthe SIMPLEC pressure-correction scheme in which thepressure field is indirectly specified via the continuityequation. For the u and the v velocity use is made of theStone’s method, for the turbulence quantities a linerelaxationmethod is used and an algebraic multi-gridmethod is applied for the pressure equation. The flowfield is solved for a turbulent flow, at Re ¼ 2 10 5 , which
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Minimum gas speed in heat exchangers to avoid particulate fouling

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