DESIGN OF PASSIVELY AERATED COMPOST PILES - VERTICAL AIR VELOCITIES BETWEEN THE PIPES

Passively aerated compost piles are built on a base of porous material s, such as straw or wood chips, in which perforated air supply pipes a re distributed. The piles are not turned during composting, nor is for ced-aeration equipment used, which significantly reduces the operating and capital expenses associated with these piles. Currently, pile con figurations and materials are worked out by trial and error. Fundament ally based design procedures are difficult to develop because the natu ral convection air flow rate is not explicitly known, but rather is cl osely coupled with the pile temperature. This paper develops a mathema tical model to analytically determine the maximum upward air flow velo city over an air supply pipe and the drop in vertical velocity away fr om the pipe. This model has one dimensionless number, dependent on the pile and base properties, which fully characterizes the velocity prof ile between the pipes.