DUST COLLECTOR MONITORING… IS THERE A COST EFFECTIVE SOLUTION?
The retention of particulates, no matter if they are carried in an exhaust gas stream or are a valuable product in a drying process, is covered by modern fi ltration technology. Beside more exotic principles like electrostatic precipitators or wet scrubbers, a vast range of applications where particulates are carried by a dry gas stream are retained by so called ‘dust collectors’ these days. (Pic 1) These dust collectors, which are also called ‘bag houses’ or dust fi lters, have gone through an impressive evolution over the past decades and have reached incredibly effective retention rates and thus very low particle emission levels downstream.
operators of such units will damage their local reputation and credibility in their neighbourhoods if their production process covers the whole area with dust because his dust fi lter failed. In other words, there are a number of very good reasons why such dust collectors should be continuously monitored apart from any regulatory or other offi cial requirements.
Pic 1. Medium Size Bag Hoses Filter Source: Rippert
The dust collecting elements inside the fi lter houses are designed in different ways and tend to prioritise manufacturer preferences over technical necessities. Whether the fi lter element has the form of a hose, a bag or a pleated cartridge, the basic material is usually made of organic or synthetic fabric, woven or as a fl eece. Depending on the basic principle, the fi lter cloth is internally supported by a metallic frame, cage, grid or tube on the downstream side.
Most of the dust collectors are designed for continuous operation and feature a blow back system of various designs to clean the collected dust load off the fi lter elements periodically. According to local governmental regulations the complete fi lter installation may need an approval for certain applications and/ or is subject to regular inspections by offi cial bodies. In some industries the fi lter houses may reach enormous dimensions to be able to handle the individual exhaust gas stream and provide adequate operational conditions and dirt holding capacity.
Even more dust collectors can be found in less extreme environments, having ‘normal’ sizes but never the less must provide very low and stable emissions. There is a danger that
The fi lter elements however, need to perform a challenging task. During operation they collect the dust particles on the surface of the upstream side and quickly build up a so called fi lter cake. This fi lter cake improves the retention rate to the better because the surface structure gets even fi ner. On the other hand, the fi lter cake will increase the pressure loss across the fi lter element and consequently steadily reduces the exit fl ow. Modern dust collector installations provide delta p controller which trigger the blow back sequence. The blow back system basically consists of a capacitive vessel of suffi cient volume for the storage of pressurized air as well as a number of valves and pipe work to direct an air shot into the downstream side of the fi lter elements in order to brake the fi lter cake on the upstream side loose and to force the collected dust to fall towards the bunker room at the bottom of the house. The blow back takes place while the complete fi lter remains operable which consequently explains why the dust is falling towards the bunker in sequences. (Pic 2)
Considering the mechanical forces to which the fi lter cloth is exposed to during this blow back process, it is easy to imagine that the life time of the fi lter elements is limited. Even worse, the elements degrade over an unpredictable period of time just like a pair of socks which get frayed over time before the formation of real holes. This wear is caused primarily by the relative movements of the fi lter material on the support cage, the impact of the blow back and sometimes even through abrasive dust particles. If the material gets slightly worn it does not necessarily mean that the fi lter element will not function any more. The fi lter cake will build up again even on a thinner structure, but for a short while a higher amount of residual dust will slip through. This process will go on until the fabric structure fails completely, a hole breaks and the dust will fl ow freely causing all the hassle no one really wants.
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Pic 2. Function Principle Back Puls Air Purge The problem:
A major obstacle for a broadly acceptable method of monitoring non regulatory or small dust fi lters was the availability of affordable control devices. The usual downsizing of sophisticated optical equipment widely spread in the regulated applications could not meet an acceptable ratio adequate for the investment in the dust collector itself. Therefore common practice is to change the fi lter elements on a time schedule regardless of their condition. The way out of this dilemma could be the adoption of a very well established technology available as monitoring equipment for a low budget but with great potential to increase the operational effi ciency of the dust collectors.
The answer: Bühler Technologies, one of the leading global suppliers of
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