Second Stage Dryers
A second stage dryer is a two-stage dryer system where the first stage is a direct fired hot air rotary dryer which dries sludge to 40-45 % dry solids. The partially dried product from the first stage dryer is passed to the final drying in the second stage dryer. The second stage is a belt-type dryer with multiple belts and drying chambers.
Heated air blows down through the product and out at the bottom of the belt, where it is recirculated through the heat exchanger to the top again. The heat exchanger uses hot water from the first stage dryer as heat source. The water is recirculated between the first and second stage dryers for the required heat transfer. Energy costs for a two stage system can be reduced by more than 20 % compared to a conventional system. Product slowly passes through the chambers and belts to dry fully.
The product is dried gently and minimises dust, which makes it a safe process. The two stage process eliminates the need to recycle the product.