When dealing with unplanned shutdowns, there are many mechanical and process-related causes of shutdowns, one of the common causes of hollow paddle dryer shutdowns is blockage of cyclones, chamber outlets, conveying lines and similar equipment; blockages have two main Causes: mechanical problems and moisture. Mechanical problems—whether bridging or reduced flow—are easier to deal with than moisture. Use mechanical devices such as pneumatic hammers, bridge crushers, or increased outlet sizes to deal with these problems.
More difficult problems include moisture and condensation. By definition, the air inside a dry processing vessel—whether it’s a chamber, cyclone, or baghouse—is humid and warm, while the temperature outside the vessel is cooler. Condensation occurs if the inner surfaces of the container are allowed to cool below the dew point of the moist air.
For processes with dryer outlet temperatures of 160°-180°F, the dew point can be in the range of 120°-130°F. If the temperature in which the container is sitting is 80°F, there is enough temperature difference to cause heat loss and condensation, causing clogging or bridging of the product. These problems are usually eliminated if the vessel is insulated and jacketed or heated.
A key component of the cyclone tip or product exit. In the cyclone tip, the air is spinning at a high velocity. Conversely, due to the high air velocity, the air pressure is low. For example, in a high efficiency cyclone separator with a 6 inch water column pressure differential between the air inlet and outlet, the static pressure in the tip could be -20 inches water column. This low pressure is the same as that which moves ocean water from sea level to 1,400 feet above sea level. This low pressure in the cyclone tip area causes fog or condensation in the air.
