Material is fed into a negative pressure system (fan downstream of feeder and separator) using one of several methods. Mechanical methods may be used, such as in the positive pressure setup. Alternatively, a simple feeder, such as a duct opening, may be sufficient if the material is light enough for the amount of vacuum the fan creates. Hopper fed elbows can be useful as any extra material not collected in the air stream will fall down the elbow and can be re-used.
The negative pressure system is well suited for applications such as unloading rail cars. Another application would be for handling toxic materials so that any leakage would be into the system. Grains, seeds, granular chemicals, and pellets have been successfully transported using this method. In this type of system, since the fan is downstream of the separator, the amount of material going through the fan is minimized. Therefore, the wear and tear on the fan is limited. Another advantage to this type of system is that any leakage is into the system, thereby eliminating dust problems. The disadvantage to this type of system is that if the loading is high or the length of the system is large, the components must be designed for high vacuum. This adds cost to the components and must be considered when comparing methods of transport.
The material loading is the ratio of the weight flow of the material to that of the air. For any given material, there is a minimum transport velocity required to convey the material. Therefore, the airflow rate will depend several different pipe / duct sizes may be used, but only one will be the most economical. There is quite a range of suggested material loadings for a particular setup, so it is recommended to consult with a pneumatic conveying expert prior to making any calculations. Typically, a material loading of anywhere from 2:1 to 1:1 or less is acceptable for standard industrial fans. Pressure blowers may be used up to material loadings of 6:1. Over that, more specialized designs, such as multiple fans or multiple stage blowers, may be required. Once a pipe size has been selected, the airflow rate can be calculated based on the minimum transport velocity of the material being handled.
Once the calculations for the system have been performed and the appropriate pipe size has been selected based on material being conveyed and system resistance, the fan operating point is a simple calculation. The required flow rate of the fan is simply the velocity multiplied by the area. If the material being conveyed will be going through the fan, special considerations must be given to the fan design. Several factors must be considered in order to select the appropriate fan for the application being considered. The fan blade type selection is very important because one does not want to select a blade type that is prone to collecting material. Backward curved and airfoil blades are efficient, but are better suited for clean air applications because they often collect material on the blades. Radial blades are better suited for material handling applications. Backward inclined blades and blades that have a radial tip have also been used successfully in certain material handling applications. Fan speed is also important in selecting a fan for material handling. The operating speed should be minimized as much as possible. High-speed fans with high tip speeds create higher velocities that correspond directly to the level of erosion and impact on the fan and system components.
The fan should be selected with the critical speed significantly higher than the operating speed. A good rule of thumb for material handling fans is to keep the rigid support critical speed at least 1.5 times greater than the operating speed.
Special materials may be required to resist corrosion, abrasion, and impact depending on the material being handled. In some applications, liners are added to the fan wheel at locations where the most abrasion will occur. These liners can then be replaced periodically without having to replace the entire wheel. The level of abrasion of the material will determine the level of erosion protection needed. Some applications may only require a special coating, while others may require special material liners to be installed on the fan wheel and / or housing. Special coatings may also be required to resist corrosion or to make cleaning easier. Oversized access doors may be used to make maintenance and cleaning easier to accomplish. Special construction of the housing, known as swing-out and clamshell, allow for easy access to the fan components for cleaning and maintenance. In swing-out construction, the fan wheel, shaft, bearings, and motor are mounted on a door. The door can be opened for easy cleaning of fan components without removing ductwork. Shaft seals may be required to resist materials from leaking out around the fan shaft.
If the material being handled is explosive or flammable, spark resistant construction is required. AMCA Standard 99 specifies Type A, B, and C spark construction, which are available for many fan designs. If materials such as coal are being transported, the National Fire Protection Association requires the fan housing design to withstand an explosion.
If high temperatures are present, such as an application where pneumatic conveying and drying are both being performed, high temperature construction may be required. This may include shaft seals, shaft coolers, motor heat shields, special materials, and/or insulated housings.
Special consideration may need to be given to bearing selection. The fan arrangement should be selected such that the bearings are out of the air stream. Also, higher capacity bearings may need to be used to allow for loads created by the material impacting on the impeller.
Fan orientation can also be important in material handling applications. In centrifugal fans, bottom horizontal or bottom-angular-up discharges are preferred. In other configurations, if material settles in the fan housing, it drops to the bottom and stays there. With bottom horizontal or angular up discharges, material tends not to settle due to high velocities at the bottom of the housing.
For additional information please refer to http://olegsystems.com.
Systems Design Engineer
Oleg Systems Co.