SOURCE POINT CAPTURE SYSTEM

Source Point Capture: A very common type of dust collection uses what is termed source point capture. This type of system can take several forms, but usually consists of a central dust collection unit which is connected through a series of ducting to various remote locations. The ducting can be attached to stationary dust generating equipment such as saws, grinders, routers or mills or it can simply be connected to a collection hood mounted on a work table. The dust generating point on a piece of equipment is usually enclosed by some form of hood which connects to the duct. As the machine operates and dust is generated, the dust collector pulls air through the hood, capturing the dust before it can be dispersed into the air around the equipment. It then filters the dust and exhausts the filtered air, either back into the workspace or outside the building depending on the installation type.

When this type of system is connected to a hood on a work table, a worker will generally be performing some type of hand work in front of the collection hood or capture zone, allowing the contaminate generated to be pulled into the hood and into the dust collector. These are commonly used for such things as soldering or light cleaning of dust generating products. Capture zones, or the area within which the capture velocity exists, is usually very small. A typical capture zone will lose over 90% of its capture velocity within the distance of one conveying duct diameter from the face of the hood. This greatly limits the effectiveness of this type of system, especially when they are utilized for operations where an operator is moving parts back and forth in front of the hood. Once outside the capture zone, dust will simply migrate throughout a room.

A typical effective capture velocity for a soldering operation, where smoke is simply drifting from the tip of the soldering iron, is in the 200 to 300 FPM range. When grinding or sanding however, the contamination is being thrown off the tool head at such a high speed that a capture velocity of up to 2,000 FPM may be required. This greatly increases the volume and, consequently, the size of the fan required to generate this flow. Of even more concern is the resistance to flow this type of system encounters due to the friction of air flow through the ducting. This equates directly to the horsepower required for the fan.

The source point capture system offers advantages for operations where there are many different machines operating in a large space with no way to effectively group and enclose them for effective dust control. While they are not 100% effective, they can greatly reduce the amount of contaminate thrown into the air which, in the case of high speed automated tooling, can be significant. A secondary dust collection system for surrounding air is generally also required but can be significantly reduced in size with volumes calculated on air changes in a room per hour rather than on air flow velocities in FPM.

Another form of source point capture system is known as a down draft table. In this system, the dust collector and inlet hood are built into the work table with the table top being the inlet hood opening. The table is usually covered with a grating or mesh material which allows the air to be pulled through the table surface where it is directed into the filter system with the filtered air being exhausted back into the work area.

Down draft tables, while very effective for operations such as soldering or light sanding, are not commonly used for heavy grinding or sanding operations. This is due to the need for relatively high capture velocities or the velocity of the air as it enters the capture hood or table top opening, when dealing with grinding or sanding operations. A down draft table system can be very effective for a variety of applications but, as with other types of source point capture systems, it will not capture 100% of the transient dust. If the purpose is to significantly reduce air borne dust contamination they will be of value but if absolute control is necessary this type of system will require a secondary dust collection system to collect dust which will inevitably escape the table capture zone.

Re-Circulating:

The most cost effective dust collection system for areas that can be enclosed is what is known as a re-circulating system. In this type of system the dust collector, whether located outside the building or inside, will pull air from a work area into an inlet device where the air will then be directed through filter media. The contamination will be trapped in the filter media and the clean air will be exhausted back into the work area. This results in no pressure balance issues within the room and does not result in the loss of heated or cooled air. The contaminated air is simply circulated through a filter system and then back into the room. The re-circulating method of dust collection generally take one of two forms:

This entry was posted on Tuesday, April 22nd, 2008 at 11:40 pm and is filed under White papers. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.