OCR Text |
Show The material to be dried is injected directly into the exhaust of the pulse combustor where it is subjected to the extreme turbulence of the hot combustion gases as well as rapidly changing positive and negative pressures. The material is dispersed as noted above and the initial evaporation takes place in this heated exhaust stream. The material flows into a drying chamber where further evaporation takes place, then through a conveying duct to the cyclone where the material is separated from the airstream. The material then passes through an airlock. The material can be separated into a dry product stream and a back-mix stream; the latter being recycled to be mixed with the dryer feed in order to reduce the moisture content of the material entering the dryer. Control of the dryer can be done in two ways. The rate at which the material is injected into the dryer is determined by a thermocouple sensor, which regulates the speed of the feed auger using a variable speed drive motor. Hence injection is slower for wetter materials and faster for drier materials. Alternatively, the gas input into the combustor could be modulated to suit the evaporative load of the dryer. Safety systems on the pilot plant were designed to be fail-safe. These include shutdown interlocks for combustion air failure, combustion failure, and main blower failure. For example, when combustion fails, pressure in the combustion chamber decreases dramatically. This is detected by a pressure switch which then signals the control panel to shut down all material and fuel feeds, leaving only the blowers on for cooling. A data logging system is used to record approximately 20 points of temperature, flow and pressure in the pilot plant. Feed Material Tests have been conducted with spent coffee grounds, wet saw dust, and pressed paper sludge. The moisture content of these material was measured to be 60 to 6 5 % on a wet basis. Results and Discussion The results of the drying tests on coffee grounds is given in Table 1. The results show the performance for single pass operation of both material and air. The product from the dryer was very uniform in texture; ie. there were no clumps. Product carry-over from the high efficiency cyclone was minimal. The overall efficiency was good; 1680 BTU/lb of water evaporated. Calculations for the coffee grounds show that 5 4 % of the energy input was used to evaporate the water in the feed. A further 2 0 % was used to heat all of the water in the feed to the boiling point as it went through the dryer. 19% was used to heat the intake air to the dryer. The remaining 7 % was heat losses through conduction and heating the "bone dry" coffee 7 |