What is the right dust extraction system for a small/medium factory with 5+ machines?
A quality dust collector is a critical piece of equipment in your manufacturing process. When performing well, it allows your machines to achieve maximum production rates, quality of finishes, long tool life, clean work environment and low running cost.
In this article, we assess the technologies and design parameters relevant for achieving quality, reliable low maintenance dust extraction. We assume you operate in the timber or joinery industry and need a full factory extraction system.
We discard technologies only suitable for single, or smaller, machine setups. These include:
- Shaker Systems
- Static Bag Units
- Cyclones; and
- All things Cartridges (cartridges are NOT suitable for wood dust).
Pulse Jet and Reverse Air Flow Dust Collector Systems
The relevant options for a small/medium factory are Pulse Jet & Reverse Air Flow (RAF) dust extraction systems.
Pulse Jet Dust Collectors – is a tried and tested cleaning technology for baghouses applied across many industries. Pulse jet cleaning uses compressed air and pulse valves to blast air down inside the filter bag providing good online cleaning. There are many applications where a pulse jet dust collector would be a great choice. However, in some cases good design principles are not applied on pulsejets, making them unfit for purpose, as often seen in the timber industry. That is why it is imperative that you get the correct advise if you decide upon a pulse-jet dust collector system.
Reverse Airflow (RAF) Dust Collectors – have become the industry standard for the timber industry. Although a newer technology than pulse jet cleaning, the RAF technology is a proven performer in the timber industry across world markets.
RAF systems utilise a high pressure fan in the top of the collector to momentarily force a reverse airflow stream down the filter bags. This causes the bags to release any dust they are holding. This cleaning method is effective without putting mechanical stress on the filter material, ensuring the longevity and effectiveness of filter bags.
Components of a Dust Extraction System
An effective cleaning system is imperative in maintaining system performance and longevity of bag life. For Reverse Pulse cleaning it is furthermore important that the pulse valves and reservoir are sized correctly. If not, the system is starved of air/pressure. The result is inferior cleaning and performance, a common occurrence in the Australian and New Zealand market.
The extraction fan is the engine of your system. It needs to overcome the total system pressure loss, while delivering the required air volume. The efficiency of the fan greatly impacts the running cost; the difference can be in the thousands per year. Understanding system pressure losses is crucial in optimal fan dimensioning.
Pulse jet dust collectors have a higher initial pressure drop; around 1kPa.This is a design characteristic irrespective of design and brand. RAF dust collectors have a lower initial pressure drop of around 300pa. The higher system pressure means you need a larger motor to achieve the same extraction volume. The difference in power requirement between a pulsejet and RAF system, for a 20,000m3/h application, is about 11kW/h.
Hint – Ask for the fan curve when buying a new system. Make sure you understand it, even when the industry salesman pitching it may not.
A pulsejet should have a dedicated inlet chamber to allow the dust to separate from the airstream. Lack of a dedicated inlet chamber is unfortunately a commonly seen shortcut, taken to cut initial cost.
Inlets located in the hopper section are not suitable for pulsejets, as they result in higher dust load reaching the filter bags. The can velocity disallows the dust to drop down and separate, causing significant reduction in filter bag life, cleaning efficiency and performance.
The large hopper of the positive pressure RAF filters allows for good dust separation and are, when combined with non-return valves, suitable options.
Timber dust is flammable and when combines with ample air (oxygen), highly explosive. A full factory dust collector should have appropriate and sufficient explosion relief and non-return valves installed. The non return valve prevents the fireball from an explosion travelling back into the factory via the duct work.
The filtration velocity is arguably the single most important design characteristic in quality dust control design. Incorrect filtration velocity by design is the most common reason for poor performance and premature bag life in the industry. Required filtration velocity is based on air volume, filter media type, dust loading and dust type. This should be assessed by a suitably experienced dust control engineer only.
How much does a does extraction system cost?
Here we consider the economics of running a dust extraction system. The true cost of your dust extraction system is the initial capital outlay and the actual running cost over its lifetime.
The low pressure drop on the RAF system along with its gentle cleaning characteristics results in low power consumption, long bag life and minimal maintenance.
Due to the additional mechanical parts, higher pressure drop and need for compressed air, the pulsejet system has significant higher running and maintenance cost. A saving of e.g. 11kW/h means a cost saving of $6,050.00 per year. The savings on less bag changes is significantly more.
What type of dust collection system is best for me?
Both Pulsejet and RAF technologies can be suitable, when applied correctly. However, as with most things engineering, the devil is in the details. A cheap pulsejet is rarely a bargain. A correctly dimensioned and well designed quality pulsejet is likely to work well, but comes at a higher capital cost. Watch out for inexpensive pulsejet system, as it is likely to have been subject to cutting corners and will yield subpar performance, high running costs and maintenance issues.
A correctly dimensioned Reverse Airflow Filter system is not likely to be the cheapest option in the market, but is often found to be the best value proposition, when considering its low running and maintenance cost.
That being said, each application must be assessed on its merits and any respectable industry professional should assess your particular application before making recommendations.
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