Vacuum belt / band driers consist of a housing with built-in conveyer belts for the product running over heating plates. Automatic belt control ensures precision belt operation. The belts run parallel to one another and a metering pump with oscillating application nozzle is assigned to each individually controlled belt.
The shift in the boiling point under vacuum provides for a low evaporation temperature and therefore gentle product temperature.
Pumpable products usually run through a highly viscous, frequently sticky phase during the drying process. This formation of vapor bubbles in the product results in a dry cake on the belt toward the end of the drying process. The temperature to which the product is subjected during the drying process has a significant effect on the achievable product quality. The belts run over a number of heating zones and over a cooling zone at the end. Steam, pressurized water or thermal oil can be used for heating; each zone can be controlled separately, allowing the temperature of the heating surfaces to be adapted to the specific requirements of the product.
The entire vacuum belt / band drying system is controlled by a stored program control (SPC) and operated over a control panel or PC. All parameters and errors are logged. Recipe mode, hand-over protocol, supervisor, server link, and operating and maintenance instructions via PC are possible.
The dry cake is cooled to a brittle breakage consistency in the final zone, then chopped up with a guillotine and crushed up in a breaker or granulator..
For products containing moisture only in the form of water we generally recommend a combination of steam ejectors followed by a condenser and water ring vacuum pump for generating the vacuum.
The advantage of this arrangement is the vapor compression following the steam ejector. The increase in pressure makes it possible to use cooling water at a temperature of approx. 30°C for the condenser. In the case of wet product moisture containing solvents, for example, or when the use of a steam ejector type vacuum pump is not possible, it is necessary to connect a condenser with correspondingly low cooling water temperature in front of the vacuum pump An automatic washing system is used for cleaning the drier.
Nozzles and spray heads installed at precision locations inside the drier and subdivided into a number of cleaning zones ensure good cleaning results. The operator can select between preliminary cleaning, main cleaning and flushing on the control. The times can be set separately for each cleaning zone. Hot water is taken from a steam heated cleaning reservoir by a cleaning pump. As a rule the preliminary cleaning and flushing cycles are accomplished with fresh water, while the water for the main cleaning cycle is recirculated.