Date of Award
Doctor of Philosophy
Agricultural and Biosystems Engineering
In cold climates it was determined that the heat loss from the hot effluent of a beef manure anaerobic digester represented a sizeable portion of the energy production of the digester. This suggested an effluent to influent heat exchanger was necessary. Available heat exchangers were unsuitable;A different concept heat exchanger using a vertical flat plate for the heat transfer surface with the hot and cold flows passing on either side was then considered. The exchanger would be divided into a number of tanks with an overall counterflow arrangement. The extra tanks and the counterflow arrangement would increase the effectiveness. Gravity flow would allow the introduction of gas agitation to keep the solids from settling, prevent plugging, and enhance the heat transfer coefficient. The concept was called a multipass mixed tank exchanger;Design equations and curves for the multipass mixed tank exchanger were developed using the number of transfer units, N(,tu). These equations were suitable for design when the overall thermal conductance, U, was known;A 1.18 m by 2.30 m prototype water to water heat exchanger was constructed and used to verify the design equations. The prototype was to have been used on an anaerobic digester at the ISU Beef Nutrition farm, but the digester was not yet completed at the time of this writing;To determine the convective heat transfer coefficient, h, (from which U can be estimated), and to consider the effects of air agitation, laboratory equipment was constructed. The equipment consisted of a large vertical cylinder with a closed bottom and open top, containing a fixed sample. Heat was applied at a known rate to the outer cylinder. In the center of the sample, a small, water cooled, pipe acted as a heat sink. The h value was determined from the known heat rate and the temperature drop between the sample and the center pipe;The effects of air agitation on h were determined by supplying varying amounts of air at the sample bottom. Sufficient air agitation to completely blend the sample was observed to greatly increase the h value compared to lower air rates. Additional air flow rates had a lesser effect;The experimental results indicated that the h value for a specific manure sample could be predicted by knowing the total solids and the air agitation rate. The h value for effluent could only be partly predicted by knowing the air agitation rate. It was hypothesized that other parameters, such as viscosity and Reynolds number, were necessary. These were not evaluated;The multipass mixed tank exchanger is a possible method to recover lost heat. Further studies are necessary to verify that the concept can be put to practical use.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Thomas Hobson Greiner
Greiner, Thomas Hobson, "Heat transfer in a beef manure anaerobic digester " (1980). Retrospective Theses and Dissertations. 7374.