|
Definition: Drying: to make dry, free from moisture. Tip: to inform
Hopper design for drying is critical to the air flow inside the hopper. Per some manufactures of dryers there exist a ratio of hopper height to diameter of the hopper and this arrangement is ideal at a minimum of 3:1 height to diameter. It is said that a minimum may be 2:1 in the ratio of height to diameter. This actuality can be translated to be the height of our material inside the hopper to achieve dry materials. This is the height above the midpoint of the cone section of the hopper, thus from a processing point it might be advantageous to create some reference points either outside or inside the hopper dependant on how it is filled.
For example if we were to take the diameter of the hopper at 36 inches (ID) and then settle on two marks up the side wall of the hopper height, one would be at 72 inches and the other at 108 inches, (6 feet and 9 feet respectively). The question then becomes are the current hoppers designed correctly?
It has been stated that if the height of the material is less than the desired ratio that the airflow well rat hole though the center of the resin which in fact is the least resistant to the air flow thus the rest of the material and especially material below the cone shall not see proper airflow or temperature and will not dry.
The design of the cone end of the hopper is critical for flow of materials inside the hopper. This angle can be calculated by pouring a pile of the material you are processing onto a clean surface and than by measuring the angle that forms. This is the angle that the hopper discharge end should be constructed to. This angle insures a plug flow and all others result in a non plug flow and a hopper with many hammer marks.
Monitor the air temperature:
Ideally it is to place a thermocouple or thermometer at the air inlet into the hopper itself and that this point / location now becomes the reference point for the air temperature set point on the actual unit. With the newer drying system out there it appears this is common but with older models out there this is not the case at all. If in the plant the temperature set is not being governed by this location than the air temperature may be lower than what it has to been set at, thus the temperature is too low to dry the material. This should be inspected and corrected especially when using longer hoses to supply the air flow. Thus if the set temperature produces only 230F inlet temperature at the hopper than by raising the set temperature to achieve a result of 250F at inlet is desirable (given that 250F is require for that material)
Another point for temperature monitoring is the outlet temperature of the air flow back to the dryer. It should be understood that this is usually lower than the inlet temperature due to the BTU lost and radiant heat lost via the hopper etc. Dependant on residence time in the hopper some suggestion may be that this is also a monitor of air flow, in that if there is excessive airflow the temperature well be close to the inlet temperature. Additionally if the temperature is extremely low as compared to the inlet temperature there may be a blockage or restriction to air flow that needs to be investigated.
Another point of monitoring air temperature may be at the inlet to the dryer unit itself. In desiccant type system it is known that the return air temperature must be below 150F ( 65.5C) or the air entering the desiccant well not give up its moisture to the desiccant and may actually start to pull or dry the desiccant bed now returning a wetter air to the hopper and creating issues with not drying the material.
SLSILVEY
360-882-3183 | 
