Options for capturing wood dust.

Here are some ideas for improving dust collection efficiency based on several manufacturing variables.

A properly designed dust collection system can provide many benefits. Maintenance and heating costs will be reduced, the risk of fire will be decreased, and floors and machines will be cleaner. What's more, employees will be healthier and happier, which can contribute to increased productivity.

Typical dust collection systems consist of hood(s), dust collector(s), fan(s) and ductwork. The following article defines the basic components of a dust collection system and outlines the advantages and disadvantages of various dust collector options.

GOOD HOODS A MUST

For most machines and operations, effective control of wood dust begins with proper collections at the source. Hoods are used to create an airflow that transports wood dust away from the source and into a dust collection system.

The hood must be properly sized and located to ensure an adequate airflow into the hood to control the dust. The airflow must be high enough to capture the wood dust and overcome the air currents caused by machine motion and room air currents. To make the system economical, the hood should be placed as close to the dust source as possible.

The airflow required to move wood dust from a specific machine or operation depends on the size of the machine, the amount of dust generated and the method by which the dust is generated. An industrial hygienist, equipment manufacturer or engineer knowledgeable in dust control can assist in determining the required airflow for a machine.

The hood is connected to a duct that transports the wood dust to a collection point. Typically the duct is designed to have airflow velocities of 3,500 to 4,500 feet per minute. Lower velocities are insufficient to convey wood dust through a duct and can result in unsafe accumulations of dust inside the duct. Higher velocities cause excessive pressure drop in the duct resulting in higher operating costs. They also increase abrasion on the ductwork.

The fan generates the required airflow needed to transport the captured wood dust through the system ductwork. The fan is selected based on the required airflow, static pressure and noise requirements of the system.

TYPES OF COLLECTORS

Several types of dust collectors are suitable for capturing wood dust. The oldest and simplest dust collector is the cyclone. The spinning action of the airflow in the cyclone causes centrifugal separation of the heavier dust particles. The cyclone is less efficient on very fine, low-density particles. These are the particles that pose the most danger because they remain airborne and are easily inhaled into the lungs. Because of its low efficiency on free wood particles, the cyclone usually is used where the air is not recirculated into the building or as a precleaner for a more efficient dust collector.

Other dust collectors are often described by the type of filters used and the method of cleaning. One design still used for smaller applications 500 cubic feet per minute (cfm) to 8,000 cfm - has envelope-style bags. These bags often are woven of cotton, although other woven materials may be used. Dust collects on the outside of the envelopes. The envelope collector is the least efficient of the fabric filters because of the pore size created by the weaving process. Felt bags are commonly used as filters in baghouse dust collectors. Because of the small fiber size, tight packing of the fibers and the thickness of the media, these filter bags are more efficient than fabric envelope filters.

The initial efficiency of envelope or bag filters generally is too low to allow air recirculation until the filters become seasoned. As the wood dust collects on the surface of the filter, it builds a dust cake which adds to the filter's efficiency. Within a few hours or days of operation, the combined efficiency of the dust cake and filter reach levels high enough to allow recirculation.

The newest and most efficient filtration technology in many wood dust collection applications employ filter cartridges. Usually cylindrical in shape, cartridges use pleated cellulose or synthetic material as the filter media and allow immediate air recirculation with new cartridges. In addition, the pleated design provides cartridges with much more filter media than a same-sized envelope or bag. The increased surface area means wood dust collection systems using filter cartridges can be smaller in size.

CLEANING COLLECTORS

With envelopes, bags or cartridges, as the dust cake increases in thickness and density, its restriction to airflow increases as well. If the media is not cleaned, airflow will be reduced to the point that hoods can no longer effectively control dust close to the equipment.

Cleaning is typically accomplished by shaking or pneumatic cleaning. Because airflow must be stopped for the shaking to be effective, shaker units are used on smaller installations where continuous duty is not required. Shaker units are available with manual cleaning via a lever or crank or with motorized cleaning. Most larger wood dust collection systems use some form of pneumatic cleaning. Cleaning is accomplished by forcing a reverse flow of air through a filter cartridge bag, which blows the wood dust off the filter and into a collection hopper or dust drawer. This type of cleaning is practical because cleaning can be accomplished while the dust collector is operating.

Medium-pressure pneumatic cleaning commonly is used on large baghouse dust collectors with heavy dust loading. A rotating arm on the clean air side of the baghouse serves as a blowpipe and blasts medium-pressure air - 10 pounds per square inch (psi) to 15 psi - into a small percentage of the filter bags at a time. While these bags are cleaned, the remainder of the bags remain in normal operation. A pressure blower is used to generate the cleaning air, so expensive compressed air from the shop or plant is not required.

High-pressure pneumatic or pulse-get cleaning also is used on some baghouse dust collectors. With this cleaning method, multiple diaphragm valves use 90 to 100 psi of compressed air to clean a section of filter bags at a time. As with the medium-pressure cleaning, only a portion of the bags are pulsed at once, so cleaning occurs during continuous duty operation.

High-pressure pneumatic or pulse-jet baghouses typically are used for light to medium dust loading. The use of high-pressure cleaning allows these baghouses to operate at increased airflows compared to baghouses using medium-pressure cleaning. This means a smaller dust collector can be used, which saves valuable floor space and initial capital costs.

With either baghouse style, ease of maintenance is a concern. The filter bags need to be replaced periodically - every two to three years of normal operation. A design that allows bag moral from the clean air side is preferred because changeout is accomplished more quickly in a safe, clean area. Removing bags from the dirty side of a baghouse dust collector is often tedious and requires extra attention to safety by maintenance personnel. This is an important consideration for large baghouses that may have 500 or more filter bags.

Cartridge-style wood dust collectors using pleated media have been successfully applied to several wood dust applications in recent years. These cartridges boast high efficiencies over their operating life. Proper filter cartridge design must consider the special characteristics of wood dust, which is often quite fibrous compared to other manufacturing dusts. Special designs are available that ensure maximum release of fibrous wood particles during the cleaning operation.

Cartridge collectors that use shaking as a cleaning mechanism are available for relatively low airflows - 500 to 3,000 cfm. These systems, such as the other shaker discussed, are cleaned while the fan is off. Pulse-cleaned cartridge collectors use a high-pressure air jet to clean the cartridges. Cleaning can be done while the system is in operation. Cartridge dust collectors that have air entry at the top with airflow downward across the elements are especially efficient at cleaning, because the downward airflow assists gravity in removing the dust during the cleaning pulse. This design minimizes re-entrainment of pulsed dust back into the cartridge.

Cartridge dust collectors can be comparable to baghouses in initial capital costs. However, the decreased system size, ability to recirculate air at any time and simplified maintenance of cartridges compared to bags make cartridge dust collectors the choice for several woodworking applications.

HAND SANDING DUST

For some operations, such as hand sanding, source capture of wood is not accomplished easily. This can be due to the size of the workpiece or the need to manipulate the piece to reach all of the surfaces. This dust still can be a significant nuisance because it is often quite small in size and remains airborne. Such operations easily create enough dust to exceed the legal limits, forcing operators to wear uncomfortable masks. This wood dust can also jeopardize the quality of other processes, such as finishing.

Environmental control booths are one answer for hand sanding. These collectors create a pattern of airflow that draws wood dust away from the operator into a bank of filters, usually high-efficiency pleated cartridges.

These booths are available with integral fan, pulse cleaning, lighting and acoustically lined walls to provide comfortable and safe working conditions for the operator. Booths come in sizes and airflows to accommodate various-sized workpieces and operations.

The proper dust collector for specific applications depends on a number of factors. Engineers qualified in dust collection technology can design a system that can provide the benefits of a cleaner, safer and more productive operation.

RELATED ARTICLE: Selecting the Proper Ductwork and Flexhose

When selecting pipe, fittings and flexhose for woodworking dust collection there are several things that should be taken into consideration.

Pipe and fitting should be constructed of galvanized metal. If plastic material is used, static electricity will develop. On the other hand, stainless steel is extremely expensive and black iron pipe will rust quickly.

Air-tight seams are necessary to avoid leakage. Sealing of field joints is critical.

Proper gauges and construction are necessary to avoid collapsing of the duct. The spiral pipe's exterior rib provides tremendous support to withstand collapsing under vacuum. Stove pipe is designed for positive pressure and not vacuum systems (negative pressure).

All flexhose used in dust collection should have wire reinforcement. This gives the hose the required strength to withstand vacuum. There are four types of material used in flexhose recommended for dust collection: rubber, polyester neoprene, metal and urethane.

Rubber is cost effective, durable, and utilized for most general purpose hoses.

Polyester neoprene coated hose is the least likely to develop static electricity making it good for sanders. Polyester neoprene is a good general purpose hose which is flame retardant, but not good for outdoor use.

Metal flexhose is extremely durable and cost-effective but causes higher resistance against flow.

Finally, urethane, though costly, is one of the best materials used in manufacturing of flexhose because it has the best abrasion resistance, tear strength, and puncture resistance. Urethane is widely used for moulders, tenoners, planers, and CNC routers.

What is not good for dust collection is PVC hose. PVC is used to convey fumes and is not recommended for dust collection, because of the build up of static electricity.

Selecting improper pipe, fittings and flexhose will be detrimental to the performance of the dust collection system.

Editor's Note: Information in this article was supplied by Air Handling Systems of Woodbridge, CT. For additional information contact Curt Corum, sales manager, at [phone].

Faith Salchert is product manager, Torit Products, Donaldson Co. of Minneapolis, MN. Salchert can be reached at [phone].