The size 50 – 1100 range of envelope filters are designed to cater for the most common dust problems, which usually involve a ducted system. Larger systems utilise a number of units in a modular form, or other types of filter from our range.

To cater for the wide range of air volumes encountered in ducted systems we can offer a variety of fan options. The units can be supplied without fans for multiple banking fan applications. Our engineers are at hand to select the correct fan for your particular requirement.


The filtration of the air is achieved by passing it through cloth arranged in a multi-pocket, envelope form. Standard units are supplied with bags manufactured in polyester felt fabric, however, other media are available for greater efficiency. These include plain needle felt fabrics, felts with either surface treatments or membranes, or alternatively felts manufactured incorporating epitropic fibres to give anti-static properties.


Each filter unit incorporates an electro-mechanical shaking device which raps the filter element thoroughly following a delay after the main fan is switched off. This is all dealt with automatically by a control panel supplied free issue with each unit. Alternatively the smaller units in the range can be supplied for manual shake cleaning.


This range of fans from 0.7Kw up to 18.5Kw is designed to give high energy efficiency for the volume being handled, whilst still delivering sufficient external system pressure to cover most of the range of commonly encountered problems. The fan impellers are of a backward inclined, plate type and are mounted direct onto the motor shaft. These are mounted on top of the unit giving easy access for wiring and servicing, and in the event of fire ensures that the motor is outside the air-stream.


The unit(s) are manufactured with a valley type hopper to channel the shaken dust into round collection bin(s). Each has a quick release system allowing easy removal after shaking down to allow the bins to be taken away for emptying at intervals. Square bins are also an option, particularly for explosive dusts.

Bin liners may also be fitted with pressure balance pipes as an option.


The casings of all units are robustly constructed from heavy gauge sheet in welded panel form with stiffening where required. The unit door is of the quick release type to allow easy access.


All of the components are designed for minimal tool use when carrying out maintenance. All of the bearings on the mechanisms are life sealed, limiting the amount of lubrication required. A major advantage of the Dustair envelope design is the clean method of bag replacement. As the filter is fixed at the base and shaken from above, all of the hazardous dust is collected inside the bag, meaning clean side access for maintenance and bag changing.


The units are manufactured from Zintec mild steel, which is then primed and top coated, finishing in an 18 E 53 Ral Code Blue. Alternative colour finishes are available.


  • Silencers are offered as an extra item if required for areas where noise must be kept to a minimum.
  • Flange mounting units are offered for silo mounting
  • Explosion relief features, anti-spark fans, earthed filter elements, reinforced construction and blast relief doors are available for hazardous applications.
  • Manometer(s) provide visual indication of the pressure drop across the filter(s)
  • Secondary and absolute filters may be fitted where a higher degree of filtration than normal filters is required.
  • Alternative hopper discharges via flap or rotary valves.
  • Units may be manufactured in alternative materials to suit special recommendations.
  • After sales spares and/or maintenance contracts are available to ensure the equipment continues to work effectively in line with C.O.S.H.H. regulations.

Case Studies

Envelope Filters

Case Study 1

Envelope filters utilise a series of vertical bags sewn together at their base in order to compress a large surface area into a smaller volumetric space…

Envelope filters utilise a series of vertical bags sewn together at their base in order to compress a large surface area into a smaller volumetric space. Filter bags are fitted with a mechanical shaker system operated from above and designed to move the bags breaking free any collected dust to collect in the hopper beneath. These can be fitted with either integral fans or floor mounted fans. In the first image however, an existing fan had to be incorporated into the system and unfortunately was the wrong handing to give efficient entrance to the hopper. However, the fan selection was excessive for the application and therefore the pressure losses did not affect the operation of the equipment. It did however save the customer a considerable sum of money utilising the existing fan. Standard envelope filters can be built into various configurations in order to suit the customers application and in this instance (below), they were elevated from floor level on a custom-made steelwork to allow discharge into a suitable skip. The air return can also be directed either into the factory or to the exterior dependent upon the external air temperature and whether the heat was to be recuperated.

Case Study 2

Our standard range of envelope filters can be adapted to many applications dependent upon the site requirements. The larger filter unit was supplied…

Our standard range of envelope filters can be adapted to many applications dependent upon the site requirements. The larger filter unit was supplied to a white goods manufacturer exhausting from various parts of the process. These photographs were taken at decommissioning stage of the plant when the company entered liquidation. The plant had been operating effectively for over 15 years and was dismantled and relocated following servicing and continued to operate effectively for a number of years after. The smaller unit was installed on a sanding process and therefore had to be isolated from operator access due to the explosive nature of the dust. Explosion relief panels were fitted to the filter unit discharging into an area isolated from operators.

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