Local heat transfer, solids concentration and erosion around membrane tubes in a cold model circulating fluidized bed (original) (raw)
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Heat transfer from a circulating fluidized bed to membrane waterwall surfaces
AIChE Journal, 1987
Average and local heat transfer coefficients were measured for transfer from circulating fluidized beds of sand particles (mean size 188 and 356 pm) to two water-cooled membrane wall surfaces located on one face of a 152 mm square by 7.3 m tall column. The measurements cover a range of superficial gas velocities from about 4 to 7 m/s, suspension densities from about 8 to 130 kg/m3, suspension temperatures from 150 to 400"C, and secondary-to-primary air ratios of 0 to 1.5. Heat transfer coefficients, averaged over a 1.53 m length of the membrane waterwall surfaces, depend strongly on suspension density, but show almost no separate dependence on gas velocity, bed temperature, or secondary-to-primary air ratio for the conditions studied. For the surface nearest the top of the unit, the coefficient decreases with distance measured downward from the top, suggesting that particles travel downward along the surface. As a result, averaged coefficients are lower and the influence of particle size is less than for previously reported circulating fluidized bed heat transfer measurements where miniature heat transfer surfaces were employed.
A study of heat transfer in a circulating fluidized bed
International Journal of Energy Research, 1993
An experimental investigation was carried out to study the effects of operating parameters on the local bed-to-wall heat transfer coefficient in a 4.5 m tall, 0.150 m diameter circulating fluidized bed with a bed temperature in the range of 65°C to 80°C, riser flow rate varying from 1400 litres/min to 2000 litres/min, bed inventory in the range of 15 kg to 25 kg of sand, and average sand sizes of 200 pm, 400 pm and 500 pm. A heat flux probe was attached to the riser wall at five different vertical locations for measuring the heat flux from the bed to the wall surface. From the present work, the heat transfer coefficient in the dilute phase was found to be in the range of 62 to 83 W/m2 K, 51 to 74 W/mZ K, and 50 to 59 W/mZ K for sand sizes of 200 pm, 400 pm and 500 pm, respectively. Relevant mathematical correlations were developed to predict local heat transfer coefficient based on the results of the practical work.
Effect of Fluidized Bed Particle Size on Heat Transfer Coefficient at Different Operating Conditions
Tikrit Journal of Engineering Sciences, 2018
The aim of this study is to investigate the effect of gas flow velocity, size of sand particles, and the distance between tubes immersed in a fluidized bed on heat transfer coefficient. Experimental tests were conducted on a bundle of copper tubes of (12.5 mm) diameter and (320 mm) length arranged in a matrix (17×9) and immersed in a fluidized bed inside a plastic container. One of the tubes was used as a hot tube with a capacity of (122 W). (25 kg) of sand with three different diameters of sand particles (0.15, 0.3 and 0.6 mm) was used in these tests at ten speeds for gas flow (from 0.16 m/s to 0.516 m/s). The results showed a significant inverse effect of fluidized bed particles diameter on the heat transfer coefficient. Accordingly, the heat transfer coefficient for (0.15mm) diameter sand was found to be higher than that of (0.3 mm) and (0.6 mm) sand by about (3.124) and (6.868) times respectively, in all tests. The results showed good agreement with results from other studies co...
International Journal of Heat and Mass Transfer, 2009
Experiments were conducted in a cold model circulating fluidized bed having riser cross-sectional area of 100 mm  100 mm, height of 4.8 m, bed temperature of 75°C and superficial velocity of 8 m s À1. Local sand having average diameter of 231 lm was used as bed material. The experiments were conducted for three tube configurations: membrane tube, membrane tube with a longitudinal fin at the tube crest and membrane tube with two longitudinal fins at 45°on both sides of the tube crest. The results show that membrane tubes with one and two longitudinal fins have higher heat transfer than membrane tubes and the heat is mainly transferred in the combination portion of tube and membrane fins. In addition, the membrane tube has the highest heat transfer coefficient.
Determination of particle and gas convective heat transfer components in a circulating fluidized bed
Chemical Engineering Science, 1993
Ahatract--In order to study the role of particles in augmenting heat transfer from the wall of a circulating fluidized bed (CFB), simultaneous heat and mass transfer experiments were carried out. For heat transfer, particles are important hydrodynamically, augmenting gas convection, and are a source of internal energy, i.e. particle convection. For mass transfer, only the former occurs. Simultaneous heat and mass transfer experiments using naphthalene as a sublimation material were performed in a 20 cm diameter circulating bed operating at atmospheric conditions. The presence of particles in the circulating bed causes an order of magnitude increase in the bed to wall heat transfer in comparison to single-phase turbulent gas flow. In contrast, the mass transfer is increased by 50% over single-phase gas flow. The gas convection component of the total heat transfer, found from the mass transfer experiments, varied from 10 to 20% of the total heat transfer. In this range of solids concentration between 12 and 80 kg/ m', particle convection dominates. The superficial gas velocity has little influence on the particle convection or on the gas convection component. The particle convection varies with the density of particles in the core, probably due to variations in the wall fraction covered by particle clusters. Gas convection is insensitive to the density of particles in the core.
Heat transfer in a high temperature fluidized bed
Korean Journal of Chemical Engineering, 1999
The heat transfer characteristics between the bed and immersed tube in a high temperature fluidized bed (7.5 cm I.D.• cm H) were investigated with sand and iron ore particles. The heat transfer coefficients were measured at operating temperatures of 200-600~ and gas velocities of 1-10 Umy. The bed emissivity measured by the radiation probe was found to be 0.8-0.9. The experimentally obtained radiative heat transfer coefficient was in the range of 30-80 W/m2K for the operating temperature of 400-800~ and the contribution of radiation to total heat transfer was about 13% and 18% for the operating temperatures of 400~ and 600~ respectively.
Heat transfer in a membrane assisted fluidized bed with immersed horizontal tubes
The effect of gas permeation through horizontally immersed membrane tubes on the heat transfer characteristics in a membrane assisted fluidized bed operated in the bubbling fluidization regime was investigated experimentally. Local time-averaged heat transfer coefficients from copper tubes arranged in a staggered formation with the membrane tubes to the fluidized bed were measured in a square bed (0.15 m x 0.15 m x 0.95 m). Glass particles (75-110 micrometer) were fluidized with air distributed via a porous plate, where the ratio of gas fed or removed through the membrane bundles and the porous plate distributor was varied. The experimental results revealed that high gas permeation rates through the membranes strongly decreased the heat transfer coefficient at high superficial gas velocities for tubes at the top of the tube bundle, which was attributed to the reduced mobility and increased bubble hold up and/or dilution of the emulsion phase, reducing overall heat capacity. In the d...
Bed-to-tube heat exchange coefficients in a small, fine-particled, cold fluidized bed
International Communications in Heat and Mass Transfer, 1986
The paper describes an experimental study of the bed-to-tube heat tran£ fer coefficient in a laboratory-slze fluidized bed with uniform, homogeneous, fine, almost spherical particles. The bed size is 0.2 x 0.2 meters, the dense phase is composed of glass "microspheres" wltha min i mum sphericity ~ ffi 0.85 and diameter of 0.5 millimeters; the fluidizing medium is air at about room temperature. The "cold" fluid (a water solution) flows inside of small tubes arranged in several parallel arrays, and the local surface temperatures of the pipes as well as the overall heat exchanged are concurrently mea measured. The results cover a range of bed heights from very shallow (height to bed diameter ratio equal to 0.I) to deep (height to diameter ratio equal to 3), with the ratio superficial veloclty/minlmum fluldlzation velocity ranging from 1 to about 4. The results are compared with soma of the existing correlations, and both the qualitative and the quantitative agreement of predicted and experimental values are discussed in detail.
Heat transfer studies in fine particle fluidized beds
1986
Experiments were carried out to determine the heat transfer coefficient between fluidized beds and immersed fine, freely moving wires. The wires varied in diameter from 50-810 ..mu..m, and were made of Alumel, while the bed material consisted on uniformly sized sand, aluminum, polyethylene and glass beads, from 105 to 754 ..mu..m. Each wire was approximately 20 cm long, was totally immersed in a bed 14 cm ID, and was heated by an electric current. The current and voltage drop across the wire were measured independently and from these measurements the heat transfer coefficient and wire temperature were determined. Results indicate that for wires and bed particles of the same diameter the heat transfer coefficient is greater than for wires in air alone. This result supports the argument than a limiting Nusselt number exists for gas-particle heat transfer in fluidized beds.
Heat transfer between a fluidized bed and a horizontal tube
Chemical Engineering Science, 1958
Measurements of coefficients of heat transfer from fluidized beds to horizontal watercooled tubes are discussed. The bed diameter was 0.565 m. The following quantitities were varied : (1) bed temperature ; (2) mass velocity of the fluidizing air ; (3) mean particle diameter ; (4) particle shape; (5) particle density; (6) water-tube diameter. The second, third and fifth quantities were varied down to much lower values than before [l] ; the sixth quantity had not previously been varied.