Unless acting passively to displace air that needs to be evacuated from a process and capturing the dust, most filtration equipment will either be under a high positive or negative pressure internally. If negative, pressure is generated by the fan on the exit of the filter, this is what draws the air from the process and via the filtration media to removal the contaminant or if under positive pressure this forces the air out through the filter media. In each case the filter media removes airbourne dusts from the air stream and collects them within the unit cabinet or hopper. As a consequence unless some form of isolation device is fitted between the collection bin/bag and the interior of the filter unit then the filtration unit’s collection facility can only be emptied when the plant is switched off. In many instances this is acceptable, however on continuous operation, heavy duty dust loads or processes that produce a high quantity of bulk materials, this may be impractical.
As a consequence some form of isolation is required that can allow continuous use of the filter system whilst evacuation of the collected materials can occur. This can be done in a number of ways. Simplistically a slide or butterfly valve can be placed between the collection point and the base of the unit to allow periodic emptying of the collected material by isolating the bin from the filter.

Alternatively should you require continuous flow out, some form of mechanical seal can be fitted to allow continuous evacuation of the materials. There are a number of ways to do this. Screw conveyors can be used to clear large hoppers but will still require an air lock valve at some point to prevent air leakage. To create this air seal we can utilise double dump valves, rotary type or rotary airlock valves or a twin valve system. On the double dump type, flap type or interlinked, sliding valves are placed at the discharge point in series and open alternately so that at any one time there is always one valve creating an airlock between the interior of the filter and atmosphere. On rotary type valves a series of blades, either metal or rubber continuously rotate on a central spindle so that there are always a number of blades in contact with the outer wall maintaining a seal between the two.

On applications handling smaller products, steel or short rubber tipped blades can be utilised. However on some applications e.g. on wood working processes, where larger pieces of material are present in the air stream the rubber section of the blade is extended to allow the blades to distort thus allowing larger product to pass through the valve should these lumps be drawn into the extraction system. Where the process is potentially handling explosive dusts, these valves can be manufactured to comply with the ATEX regulations, and then passage of the explosive force through the valve itself. Dustair are able to provide rotary and double lock valves for most applications details of which can be provided upon request. Valves can be operated utilising either single phase, three-phase or 110 V electrics or could use other alternative means for the driver such as pneumatic or hydraulic motors. Chain drives or direct drives can also be provided dependent upon the configuration of the equipment. We are also able to provide spares and servicing of the valves both on and off site. If you have any requirements for these or any form of isolation device please do not hesitate to contact Dustair Ltd via our contact page.

Case Studies

Case Study 1

In this application rotary valve were utilised as an airlock between the discharge from the collection Hopper and a transfer system that…

In this application rotary valves were utilised as an airlock between the discharge from the collection hopper and a transfer system that allowed the collected dust to be blown to a secondary collection. In this operation the interior of the hopper was at a low negative pressure whilst the ductwork utilised to convey the material is under positive pressure. The closed loop system of the transfer conveyor needs to operate independently of the filter unit and therefore an airlock is required between the two.

Case Study 2

Here we see a rotary valve fitted to the underside of the hopper on a tubular filter unit designed to evacuate continuously from the hopper…

Here we see a rotary valve fitted to the underside of the hopper on a tubular filter unit designed to evacuate continuously from the hopper whilst the plant is in operation. The valve discharged directly into a collection skip and has the facility to be isolated separately from the operation of the filtration plant. This allows the rotary valve to be isolated when the skip is full so that this can be changed over for a new empty skip whilst the plant is in operation. During this time a quantity of dust will accumulate in the hopper but is immediately evacuated once the valve is returned to operation. Alternatively we can provide switchover facilities discharging through diverter sections with automatic switchover dampers that can divert the material to either one of a number of collection skips.  This can also be controlled automatically if required.

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