Limiting factors in gas-fluidized bed heat transfer

Abstract

Factors controlling the maximum achievable rates of heat transfer between a wall surface and a fluidized bed have been further studied. Particle packing density and movements close to the heat transfer surface are of overriding importance. Experiments were carried out in a pressurized flowing packed bed to vary the gas kinematic viscosity and to control the particle residence times more precisely than is possible in a fluidized bed. Despite the interpretation of an earlier experiment, the kinematic viscosity was found to have a negligible effect on the rates of heat transfer for beds of smaller particles (< 500 μm). For such materials, maximum coefficients obtained in a pressurized fluidized bed were consistently lower than those in the flowing packed bed apart from one test when the particles had become contaminated with oil. The static pressure had an important effect on the heat transfer rates to fluidized beds of larger, closely sized particles. The quality of fluidization of beds of large dense particles appeared to improve as the static operating pressure was raised and it is also thought that conditions were being reached under which significant interphase heat transfer by fluid convection could occur. Heat transfer coefficients to beds of 625-μm copper shot nearly doubled as the operating pressure was increased from 1 to 10 atmospheres. With smaller particles the pressure effect was negligible. These results show that it is possible to achieve higher rates of heat transfer to beds of large particles than small when working at higher static pressures.