Cyclone Dust Collectors
An Underestimated Technology? by Malcolm Gale, Managing Director of MikroPul Ltd
uddenly cyclone dust collectors are all the rage. The phenomenal success of the bagless, dual cyclone based Dyson vacuum cleaner and its latest rival the “triple vortex” machine has brought all the merits of this apparently simple air/dust separation technique to housewives throughout the world. The benefits are similar to industrial applications - no more dirty filter bags to change, no more having to buy and hold spare bags, and no more finding you haven’t got a spare bag just when you most need one. And indeed the theory of cyclonic separation is relatively simple. It is based on the principle that the induced spiralling action of the gas / particle stream subjects particles to substantial centrifugal forces, which effectively adds G-force weight to the airborne solids. This alters the relationship between the particles and the minimum gas velocity required to keep them airborne (known as saltation velocity). As the effective mass of the particles increases, they will drop out of suspension and be discharged from the cyclone hopper, leaving the clean air to exit through the central outlet tube (see fig 1).
previously discounted. One example was a company which was having difficulty with a particular process where the residual dust load was fine (100% below 20 micron, 90% below 10 micron) and, although not excessive, it could not be released to atmosphere. The particulate arrestment was in the form of a bank of roughing HEPA filters, which needed to be changed every 8-10 weeks. To install a self cleaning primary filter was going to be expensive, quite apart from there being little room for additional equipment. Cyclone technology had been considered previously and discounted as the understanding was that such a light dust load of such small particles could not be effectively arrested by cyclones. Although this is true of many conventional cyclone designs, new cyclone technology can employ a geometry, which enables high collection efficiency of small particles. Further, in the case outlined above, computer modeling of the process scenario enabled a cyclone performance prediction to be made which would make significant improvements to the system, at a relatively low cost. In this particular case, even with 90% of the dust load being below 10 micron (the point at which most conventional cyclones become ineffective), the computer model gave an overall predicted efficiency of 83%. This represented a significant reduction in the dust burden to the HEPA filters. Since the cyclone’s installation, the company has only been changing HEPA filters once a year since.
Application Driven The size and range of cyclones available is vast with each design custom built for its duty. Applications vary from power stations, which use cyclones with an inlet scroll you can drive a bus through, to the pharmaceutical and nuclear industries with highly polished glove box and lab size applications. All manner of materials of construction can be
MikroPul Ltd Warrington Cheshire can be contacted on: Tel: 01925 849220 Fax: 01925 849221
However, in industrial applications cyclones have long been regarded as inefficient dust collectors with limited uses when compared to, say, fabric filters. But with the correct applications knowledge and experienced guidance, advanced cyclone technology can be more widely used in industrial powder handling and product recovery systems. It is usually the lack of knowledge regarding the correct applications of cyclones and the inability to accurately predict performances of such machines that has somewhat restricted their wider acceptance and use. To fully exploit the potential of cyclones, industrial filtration experts are able to computer model the saltation phenomenon resulting in very accurate performance predictions for their range of high efficiency cyclones. This ability has enabled the application of cyclones in situations where they may have been
used and refractory lined cyclones with hastalloy outlet tubes have been supplied. Construction codes to BS5500 and ASME VIII can be undertaken and there are cyclones operating at 32 barg in the petrochem industry. Also available are high temperature cyclones for applications in excess of 1,000°C. Removable wear liners are incorporated in most cyclones and explosion relief or 10 bar pressure shock resistant units can also be manufactured. Other applications where cyclones have an added advantage are pigments and flavourings production where clean down between batches is required. Installing a cyclone in front of a dust filter, and designing it with “quick release-easy strip” cyclone body parts and polished internals can cut downtime significantly and increase product collection where previously good product was disposed of in the filter. Other surprising advantages have been discovered as illustrated by a cyclone installation at a pharmaceutical manufacturing site. The manufacturer’s problem was one of personnel safety. Operations at the site surrounded the manufacture of a range of pharmaceutical products including a medicated shampoo, the active ingredient of which was Selenium Sulphide. As part of the manufacturing process, the Selenium Sulphide powder, which is harmful to the eyes and lungs, is drawn under suction from an existing mill through a modified knock-out chamber into a mixer/blender. The carry-overs from the knockout chamber pass through a second collection unit before ending up in the bag filter. In the past, the second collection unit was very labour intensive as well as inefficient (most of the carry-over dust ended up in the bag filter, which was only supposed to be a ‘back-stop’ device). This meant that the operators had to empty the drum beneath the filter at frequent intervals using breathing apparatus. In looking for a replacement method, acute space limitations added to the problem of equipment selection. The solution was to replace the second collection unit with a new specially computer-modelled cyclone on top of the mixer/blender vessel so that a higher proportion of the material could be put straight back into the process without manual handling. Advanced computer modelling was used to custom design a high efficiency compact cyclone. A compact model 18 (12” barrel diameter) with a rotary airlock fitted to the discharge was selected. Where as previously 100kg would have been carried over and manually handled, the quantity of dust has been reduced to approximately 3kg, a dramatic 97% reduction in carry-over dust going into the bag filter. This means the operators now only have to go into the area for a fraction of the time that they used to with the old system. The reports from site are that the operators are more than happy with the new set up. Installations such as this demonstrate that, with expert consultation, a small, correctly designed and applied, yet inexpensive cyclone can be easily used to overcome a serious problem, where a more complicated or expensive solution might otherwise have been sought.
PROCESS PRODUCTS - June/July 2003