A: Jack Osborn, Airdusco, says:
Unfortunately, this problem and the related subsequent problems are common in industrial operations that use pneumatic material transfer. A filter-receiver’s limitations (often incorrectly called a dust collector) are frequently misunderstood. Both filter-receivers and dust collectors separate conveyed material from the airflow, but the actual material rates entering those devices are significantly different.
A true dust collector separates very low rates of incoming dust from a dust collection system. In fact, the calculations for a dust collection system don’t consider dust loading, only airflow. This means the physical amount of dust being collected comprises a much smaller mass than the mass airflow.
When the mass amount of material being conveyed by airflow increases to a level where the energy losses in the system because of that mass become significant, then the system is no longer a dust collection system. In this case, the system becomes a pneumatic transfer or conveying system that requires different engineering and design. This includes most milling equipment used in size reduction, planers used to trim lumber, and wood hog hammermills used to reduce scrap wood, and other related pneumatic transfer systems.
This same material-loading or mass-flow level directly affects the proper selection of the filter-receiver used to separate the airflow from the conveyed material. The air-material separator (AMS) used for a dust collection system is typically called a dust collector, while the same device for a pneumatic transfer or conveying system is called a filter-receiver. The differences are semantic, but it’s useful to emphasize the equipment terms for proper device selection.
Both devices use filter media, typically cartridge or bag filters, to physically separate the conveyed material from the airflow used to convey the material to the AMS. Inherently, both filter types have a maximum material loading that the filters can handle. Typical material rates are 5 to 10 grains (7,000 grains equals 1 pound) per cubic foot of conveying airflow for cartridge filters and 20 to 60 grains per cubic foot of conveying airflow for bag filters. Therefore, for 1,000 cfm of conveying airflow, the rate of actual material to the filters can’t exceed approximately 1.4 lb/min for cartridge filters and 8.6 lb/min for bag filters. This is fine for typical dust collection requirements but far below acceptable pneumatic conveying rates.
However, the key is to understand that this is the loading at the filters and not the rate of combined material and airflow into the filter-receiver. Thus, if sufficient material can be separated from the incoming airstream before it reaches the filters, high material flowrates can be accommodated.
This is why certain operations that are commonly called “dust collection” run into trouble. Using a true dust collector for an application that handles high material rates (in tons per hour) will result in poor filter performance and likely additional problems such as difficulty getting the material out of the dust collector. A true dust collector can’t handle high material rates.
This problem has led to AMS units that are designed specifically for receiving and filtering (thus the name filter-receiver) high incoming material rates. These units use tangential inlet sections with vortex breakers, inner cones, or both to allow cyclonic separation of the major portion of the incoming material before it reaches the filters. This reduces the tons-per-hour incoming rate to a level acceptable for proper filter separation. This also assists in the material discharge from the filter-receiver.
If you have a milling, planer, hog, or other system that handles high rates of material that requires filtered separation from the conveying air, make sure the actual material rate to the filters is within an acceptable level.
Don’t miss Jack Osborn’s upcoming webinar “Dust collection hood design for engineers and nonengineers.” To learn more and register, click here.
Jack Osborn is the engineering manager at Airdusco.