BCR: Ensuring Effective Process Ventilation
To design an efficient control system, first identify each emission point and mark every location on a plant layout drawing. Next, determine the air volume required at each source by using the formula Q = VA, where Q is the volume (quantity) of air needed to produce a given in-draft velocity over a given area, V is the velocity (ft/minute) and A is the area (sq ft). You should also decide what type of collection hood to use based on your plant’s maintenance needs, operational considerations and the airflow volume required for capture. The two basic types of hoods are enclosing, which completely or partially enclose the process or contaminant generation point; and exterior, which are located adjacent to an emission source without enclosing it. Both are commonly used in the brick industry.
Next, look at the plan and elevation views for the plant and determine the ductwork routing. Develop a line sketch to ensure that the system will work as planned. The location of the equipment, how the ducting will run to those pieces of equipment, and the location of the abatement equipment should all be included in this initial sketch.
Once the routing has been determined, size the ductwork to the appropriate diameters and lengths so that it will transport the captured dust with the right amount of velocity. The ductwork should be sized using the same formula used to determine the required air volume (Q = VA). Make sure that the ductwork design also takes into consideration the type of dust being collected—extremely abrasive materials will require a heavier gauge of ductwork and wear-resistant construction materials.
After the initial design is complete, develop a detailed design specification to ensure that the plant’s air quality and construction goals will be achieved. Make sure that the proper piping techniques are used for branch entries, that heavy-duty abrasion-resistant elbows are specified where needed, that proper engineering practices are used in close areas, and that tapered transitions are used above enclosure hoods to minimize undesirable materials entrainment. Also make sure to calculate the ductwork pressure losses stemming from the acceleration of air to the transport velocity, hood entry losses, frictional losses in the ductwork, the collector differential pressure and stack losses, as well as other potential factors. These losses can affect your plant’s energy use, as well as the overall efficiency of the system, and should be minimized wherever possible.
To choose the correct system fan for your dust collection system, determine the system pressure loss curve, then identify the air chemistry (clean or contaminated) and select the fan type. Match the fan curve to the system curve to determine the optimum operating point.
Finally, develop engineering drawings showing the full system details, and prepare to install your new process ventilation system. If the right steps have been followed, your process ventilation system should meet your plant’s goals for improving your housekeeping efforts and employee operating environment, as well as achieving compliance to state and federal emission standards.