Heat rejection from horizontal tubes to shallow fluidized beds

Abstract

As fluidized beds can result in an order of magnitude increase in the heat transfer coefficinet for a surface, they can potentially be coupled with dry cooling towers for power plant heat rejection. On such a large scale, economic considerations necessitate the use of an inexpensive particle and shallow bed depths. Existing heat transfer mechanism models for fluidized beds are discussed, and a new model presented. Heat transfer coefficients from a horizontal tube in a row of dummy tubes to a shallow fluidized bed were experimentally measured. Coefficients from banks of horizontal tubes are lower than coefficients from vertical walls primarily because of particle stagnation on the tube tops and particle recirculation problems. Experimentally, different tube and distributer geometries were tried in order to reduce stagnations and enhance particle recirculation. Experimental data for these different geometries is compared to exising horizontal tube correlations and the new model. The RMS deviation of data from the model is less than 17%. The best correlation was obtained by modifying the Vreedenberg correlation to include a dependency on the particle fraction of the bed. The RMS deviation of data from the modified Vreedenberg correlation was 13.8%. Using both experimental data and the modified Vreedenberg correlation, economic optimizations were performed to compare fluidized bed dry cooling towers to a finned tube tower. For a 1000 mw plant, heat exchanger costs are 13% lower for fluidized beds, but the fluidized bed is severely penalized by the cost for the power needed to keep the bed fluidized. The incremental cost of the fluidized bed is 16% higher than that for a finned surface, but rapidly approaches the finned tube incremental cost as the particle size and bed depth are reduced.