Steam Advantage
STEAMTUBE VS. DIRECT HEAT  Rotary direct heat drying, also involves a rotary drum or some other means of conveying material. But instead of steam, the dryer utilizes a burner combustion system. The burner produces hot gas, which travels through the dryer. Process material is fed into one end of the dryer and discharged from the opposite end. The drum is fitted with flights or lifters, which pick the material up and veil it into the hot gas stream. The contact made by the material coming into intimate contact with the hot gas causes a transfer of heat which heats the water vapor from the process material. This type of direct heat drying is accomplished primarily by convective heat transfer.
Both dryer technologies are ultimately dependent on a combustion system.
• The direct heat dryer utilizes a burner system to produce the hot process gas which carries the thermal energy. • The steam tube dryer is dependent on a boiler, which has a combustion system for producing the steam. The steam carries the thermal energy. While some applications are established, many engineers struggle with the question of which technology to use for a particular drying project. Direct heat drying works well with most minerals and inorganic materials. This would include mined ores, sand, aggregates, clay, cement mix, fertilizers, glass, etc. These products have little sensitivity to heat. So the gas entering the dryer can be very hot. With this type application, VOC generation is usually not a consideration. Steam tube dryers work well with organic or biomass products which tend to be sensitive to higher temperatures. STD is the primary type dryer for soy bean and oil seed conditioning, corn germ, gluten feed, wood flour, rendering bi-products and many others which have sensitivity to high temperature processing. The steam tube dryer is also considered best available technology in processing soda ash and a variety of petrochemical processes where the drying process includes vaporization of substance other than water or vapors prone to deflagration.
Direct heat dryers typically work best when the burner system can operate with a minimal amount of excess air or near stoichiometric balance of the gas and air. This allows the dryer to minimize the air required to carry the thermal energy. With heat sensitive materials such as biomass and organic products, the approach temperature of the gas is limited to the temperature the material can withstand without either combusting inside the dryer or giving off excessive VOC from oils being driven off. This is where the steam tube dryer technology is the best fit. For practicality and in order to illustrate the difference in processing organic and biomass products, we chose to illustrate one specific process, which is fairly evenly split between direct heat and steam tube dryer technology: Dried Distillers Grain with Solubles (DDGS). Other material processes can be compared with this model. See our DDGS case study CASE STUDY
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Louisville Dryer Company | 1100 Industrial Blvd | Louisville, KY 40219 Toll Free: (800)735-3163 Fax: (502) 969-9028