Airborne Particle Sizes

Duct System Design

In designing the ducting for a dust collection system, very careful attention must be paid to the selection and layout of the piping and interconnecting fittings. Understanding of the following nomograph is key to the understanding of the basic principals of good duct design.

For a given duct diameter, select the appropriate line labeled 1" to 16". Then on the left hand scale identify a horizontal line corresponding to the airflow for the given duct. One may then read the corresponding pressure loss and air velocity for the duct. The pressure losses are given in inches water gauge for 100 feet of duct. Since the pressure losses are linear with pipe length, simply adjust the pressure loss for pipe length (50 feet of pipe has one half the loss of 100 feet). For conversion of inches mercury to inches water please refer to the following conversion chart.

For a system design to be effective the airflow around the pickup nozzle must be sufficient to carry the material into the duct system. Additionally, the airflow must be adequate in each branch and trunk of the duct system to insure that the materials are carried to the dust collector and not left in the walls of the duct system. Two standards are commonly used in industry. For light and most airborne materials, 3500 feet per minute velocity (approximately 60 feet per second) is adequate as a minimum system design parameter. For other materials or heavier particulates 4500 feet per minute (approximately 75 feet per second) is the minimum standard. In our practice, we tend toward the conservative and like to design all systems for a minimum duct velocity of 80-100 feet per second, which is sufficient to carry metal chips vertically upward in a duct.

Use of high quality materials and fittings in the duct system design is also important. In medium-pressure dust collection systems, 22-gauge galvanized steel spiral pipe is the most common. Fittings such as Y's, T's and couplers are standard catalog items. These components are designed to slip together and are joined with tech screws, pop rivets, or spot welding. Sealing of all joints and seams in the duct-work is critical to good system performance. It is very easy to lose the bulk of your system pressure budget to leaks in the ducting system.

Pressure Conversions

The following chart may be used to convert the various pressure measurements.