Minimum gas speed in heat exchangers to avoid particulate fouling

M.S. Abd-Elhady et al. / International Journal of Heat and Mass Transfer 47 (2004) 3943–3955 3951Air flow velocity (m/s) .1614121086420Copper particles0 10 20 30 40 50 60Particle's diameter (µm).of the copper particles smaller than 10 lm at a flowvelocity of 9.5 m/s. This could be due to the smallamount of the fine particles present and due to the removalof them by larger hitting particles. The removal offine particles by the large depositing particles leads to alower limiting fouling speed than the limiting foulingspeed, which corresponds to the smallest particle in thegas flow. The effect of the particle size distribution onthe fouling mechanism still has to be investigated.The critical flow velocity shown in Fig. 8 is calculatedfor a metallic non-charged spherical copper particleresting on a good conductor surface. In case of anelectrically charged particle resting on a bad conductorsurface like glass an electrostatic adhesion force isdeveloped which leads to a higher critical flow velocitythan in case of a non-charged particle. The critical flowvelocity then should be calculated based on the nature ofthe particles in the flow and the flow conditions.3.3. Sticking of particlesCritical flow velocity (Theoretical).No Fouling Condition (Experimental).Fouling Condition (Experimental).Bronze particlesFig. 8. The critical flow velocity and limiting fouling speedversus particle diameter.In practical applications of heat exchangers especiallyin waste incinerators, the flue gases contain a wide rangeof particle sizes. To prevent fouling of heat exchangersbased on the critical flow velocity, the critical flowvelocity should be selected based on the particle sizemost likely to stick first to the heat exchanger tubes. Thefollowing experiments have been done to determinewhich particle size deposits first from a batch of particlesflowing over a heat exchanger tube. A mixture of equalmasses of bronze and copper particles is injected into theairflow. The bronze and copper particles used are definedin the experimental setup section. The particlesconcentration is 2 g/m 3 of airflow. Hot air at 200 °C andflow rate of 13E)3 Nm 3 /s is passed to the heat exchangersection. This results in a Reynolds number of700 based on the HE tube diameter and the superficialflow velocity. An airflow rate of 1.5E)3 Nm 3 /s and atemperature of 20 °C was used for cooling the heat exchangertube internally. The heat exchanger has run for9 h. At the end of the fouling experiment samples aretaken from the fouling layer deposited on the heat exchangertube and these are examined by SEM. Twosamples of particles are taken from the fouling layerdeposited on the middle centered tube of the heat exchangersection: The first sample of particles was scratchedfrom the top of the fouling layer. The secondsample of particles was scratched from the first foulinglayer of particles, which had deposited on the tube of theheat exchanger. This sample was taken by blowing airthrough the HE at a very high speed such that most ofthe fouling layer was blown off and only a very thin layerof particles remained on the tube of the heat exchanger.Fig. 10 shows SEM pictures for the particles at thebottom (near the heat exchanger tube) and at the top ofthe fouling layer respectively. First the fine copper particleshave accumulated over the heat exchanger tubeand then afterwards the larger bronze particles. The topof the fouling layer consisted mainly of the large bronzeparticles surrounded by the fine copper particles.The same kind of experiment above is repeated usingspherical glass particles instead of using a mixture ofFig. 9. Photos for the examined heat exchanger tube at the end of operation under different airflow velocities: (a) V ¼ 2 m/s, a bronzelayer of thickness 0.3 mm is formed on the HE steel tube; (b) V ¼ 4:5 m/s, a very fine layer of bronze particles is formed; (c) V ¼ 5:5 m/s, no fouling.