Design and characterisation of fluidised bed cooling towers

This thesis discusses the operating characteristics and design of fluidised bed cooling towers (FBCT), which may be used to cool hot water for industrial purposes. Limited data exist for such a three-phase fluidised bed acting as a cooling tower. This motivated some early workers to investigate its usefulness in cooling tower applications and they showed that the FBCT produces heat and mass transfer rates much higher than in conventional fixed-bed towers. Despite this advantage, the FBCT has not been commercially exploited to date. An extensive experimental study is presented using up-to-date instrumentation to determine the thermal and hydraulic characteristics with a view to establishing a design criteria for full-scale FBCTs. Experimental tests were performed to account for the effect of the plenum chamber and the spray zone region upon the thermal performance of the FBCT. Data analysis was performed so that the effect of the fluidised bed alone as well as the plenum chamber could be known. A prototype was designed and built incorporating nine calibrated Platinum Resistance Thermometers for fluid temperature measurements with one located just below the fluidized bed itself while another was positioned below the plenum chamber to measure outlet water temperatures. Two differential pressure transducers and an electronic water flowmeter were used to measure air pressures and water flow rates respectively. All instruments were connected to a data-logger linked to a personal computer. Two different software packages were written and installed on the computer, to automatically retrieve experimental data from the rig during test runs and to automatically process the retrieved variables for analysis. Nine independent variables were measured in order to determine the tower thermal-hydraulic performance. Water and air flow rates ranged from 0.5 - 5 and 0.5 - 4 kg/s m2 respectively giving liquid/gas mass flux ratios that ranged from about 0.1 - 6. The inlet hot water temperature ranged from about 25 - 55°C while the inlet air wet-bulb temperature averaged about 18°C. Four different spherical packing arrangements were studied at static bed heights that ranged from about 25 to 400 mm. The spray nozzle height from the distributor grid ranged from 400 - 1500 mm. Data analysis was performed for thermal-hydraulic performance using both dimensional analysis and the Merkel approach. A least-square multiple regression analysis carried out on dimensionless and dimensional groups that resulted from this analysis showed that correlations derived are in good agreement with other experimental data. Correlations were derived for the prediction of the bed air pressure drop and hence the power requirement, the tower thermal performance, the minimum fluidisation velocity, and the expanded bed height. Correlations used to design a full-scale FBCT are presented. Novel work included measurements of local radial and axial temperature variations within the fluidised bed. Thermal performance decreased as the liquid/gas mass flux ratio was increased while it increased as the particle size was decreased. High density particles gave a higher bed air pressure, and hence a higher power requirement than low density ones. Minimum fluidization gas velocity was independent of the static bed height. Expanded bed height increased as the liquid and gas mass fluxes were increased. Thermal performance was found to increase when the effect of the plenum chamber was included in the analysis as compared to the fluidised bed itself. Methodological criteria for the design of a full scale FBCT have been developed. Design analysis suggests that FBCTs can be several times smaller in size than conventional cooling towers, and that they may operate with a similar or lower power requirement than the latter.

A thesis submitted to Middlesex University in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering.