Dryers are equipments used to obtain products with very low water content. The feed can be either a concentrated liquid or a solid with a high moisture content, but the end product will always be a dry solid with a moisture content that will depend on each product, but usually below 10 % and as low as under 1%.
There is a wide range of dryer types, and it will be necessary to determine the adequate type for each product. As an example we can name all types of spray and flash dryers, rotary and tube bundle dryers, fluid bed dryers, belt and tray dryers.
Dryers can be of direct or indirect type depending if the heating medium is in direct contact with the product. Tube bundle dryers are of the indirect type because the steam used as heating medium flows through the tubes and these are in contact with the product, so the steam and product are never in contact.
However, most dryers are direct type ones, using air as heating and drying medium, transporting the evaporated water from the product out of the dryer. The same principle is used to classify the way the drying air is heated, and it also can be directly or indirectly if combustion gases are mixed with the drying air or not. In certain industries it is very common the use of indirect hot air generators, such is the case of the dairy industry when using hot air for spray and fluid bed dryers.
In some products such as corn bran resulting from the corn wet milling process, it is recommended to mix part of the product exiting the dryer with the feed, in order to achieve adequate flow rates and water content levels to insure the correct performance of the equipment. This process is called recirculation of the product and the use of special mixers design for each product in order to obtain the correct product entering the dryer.
In other cases the main feed must be concentrated previously in an evaporator before entering the dryer. This is because the concentration process uses steam, which is thermically much more efficient the air as heating medium. Besides, the possibility of using multiple effects evaporators is even more energy efficient. Another factor for using a concentrator before a dryer is that if the feed should enter the dryer with its original water content the required equipment would have to be much bigger due to the causes named above, thus increasing the overall investment.
Among the auxiliary equipment for dryers that our company provides you can find: