Belt Presses and Aerosols

Belt presses are sometimes criticized as a source of aerosols. We have found that often there are two reasons for this- poor maintenance practices, and confusion between condensation and aerosols.

Introduction

A well designed and maintained belt press generally will produce no measurable aerosols. Recently one of our customers reported hiring a consultant to measure if aerosols were present around one of our presses, and the result was none detected.

Another customer reported no visible aerosols, "but if you look inside the plant room on a cold and frosty morning, you can see a mist- so there must be aerosols". This is a clear confusion between condensation and aerosols. An aerosol has not undergone evaporation and condensation, and would not be more visible on a cold day. Condensation should not be considered a health hazard as it is "distilled" water.

Sources of Aerosols

The normal method of reducing aerosols emitted from the belt washing sprays is to enclose the sprays and the belt inside what are called washboxes. These washboxes have flexible seals which seal around the belt and should prevent the sprays and aerosols from escaping. A common problem is that these spray seals are not maintained, or poorly adjusted. This is unnecessary, as the required maintenance is relatively low cost and easy to do.

Seal Basics - The Soft Kiss

The first point to consider is that the washbox seals should remain flexible, and should be set up correctly.

The seals should form a smooth curve and therefore press with sufficient pressure on the belts to seal, but not excessive pressure such that too much friction is generated.

 

Many operators and maintenance personnel believe that thicker rubber or other elastomeric material will provide a longer life and better sealing, and mistakenly increase the seal material thickness. This is one of those engineering situations where bigger is not necessarily better. The thickness of the material is originally designed to allow a compromise between flexibility and wear life. Increasing the thickness reduces the fle,ibility and increases the pressure on the belt. In addition, whilst there is more material to wear, the wear rate in fact becomes greater. Adding to the problems created, is that the e,tra friction can cause excessive tensioning in the belts, excessive drive power requirements, and in some cases can set up a destructive vibration in the washbox, eventually causing premature failure of the box.

Adjustment

The belt washboxes are usually mounted on adjustable frames so that they can be set up so that the belt passes through the mid-point of the seals, and that 'soft kiss' obtained.

A common problem is that during installation the feed piping flexible connection is set up at the wrong orientation, so that it is not possible to adjust the washboxes seals correctly. Flexible hoses provided by some manufacturers are flexible in bending, but not in e,tension. These hoses need to be set up so that the hose is parallel to the belt, and this will allow the wash box to be adjusted properly in relation to the belt. Incorrect adjustment can allow aerosols to escape.

Conclusion

Simple maintenance and adjustment of belt press washbox, seals will virtually eliminate aerosol escape from a well designed belt press. Attention to this low cost detail will provide a well operated, safe, dewatering press or thickening gravity table, with benefits in overall process performance. As the operators feel more comfortable around the machine, they are more inclined to make the small process adjustments which turn an ordinary installation into a system working like a well tuned violin!

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