Designing high-velocity industrial dust collection systems requires a fundamentally different engineering approach than that used for conventional heating, ventilation, and air conditioning (HVAC) systems. Even experienced engineers can overlook critical factors that influence system performance and safety.

This article is the fifth in a series of articles highlighting the most common and costly design oversights I’ve encountered in real-world dust collection system engineering. In Part 1, I discussed useful design references, software, spreadsheets, and field-validated velocity criteria. In Part 2, I covered common duct system design errors that inhibit dust collection system performance. In Part 3, I discussed common mistakes that inhibit system fan performance. In Part 4, I described common dust collector configuration mistakes. In this installment, I’ll cover silo bin vents.

Not all silo bin vent dust collectors need a fan

Do not automatically add a fan to a silo bin vent collector. Silos that are filled mechanically by belts, bucket elevators, or screw conveyors require a fan to pull displacement air and maintain inward leakage at all silo openings. However, for silos that are filled by pneumatic conveying systems, the situation is more complex.

Airflow calculations determine bin vent performance

Several factors contribute to total air volume handled by the silo bin vent in a silo filled by pneumatic conveying. First, conveying airflow must include temperature correction to determine the actual cubic feet per minute (acfm). Second, silo material discharge aeration air must be considered. And third, the air displaced by the material entering the silo also contributes to the total airflow.

Surge airflow drives fan and filter media area selection

The true controlling factor in airflow handled is the surge airflow at the end of a conveying cycle, which can reach 200% of normal total flow. If the fan selected cannot handle the pneumatic conveying surge flow at the end of a conveying cycle, it becomes a restriction, pressurizing the silo and creating the potential for dust to escape.

In addition, pressure drop across the filter media increases with the square of airflow ratio (flow at surge/flow at steady state), so filter area and resulting pressure drop must be evaluated at both steady and surge airflow conditions. If the system designer doesn’t understand both the stable airflow and surge airflow values, as well as the corresponding filter pressure drops for each, the fan selected might not be able to keep up with the surge airflow.

Why fan-less silo bin vents often work better

For such pneumatically filled silo systems, it is usually better to avoid the fan all together. The pneumatic conveying system provides plenty of pressure (measured in psig) to push air through the bin vent filter media pressure resistance (measured in inches of water gauge). The bin vent pressure drops must be evaluated at both normal airflows and surge airflows, and the filter media must have enough surface area to handle the surge air volume, at an increased pressure drop acceptable to the pressure containment abilities of the silo.

Configure pulse-jet cleaning for fan-less operation

Finally, for fan-less silo bin vents, it is imperative to properly configure the bin vent’s compressed-air pulse-jet filter cleaning system. With such fan-less systems, it becomes necessary to keep the pulse-jet system operating for a few pulse cycles after pneumatic conveying is completed. Modern dust collector pulse-jet cleaning timers have the ability to maintain a few pulse cycles after the fan is de-energized.

In Part 6 of this article, I’ll discuss concepts related to grasshopper leg, or sawtooth, ducts.