For many years Dustair Limited has been manufacturing a full range of cyclones from low through to medium up to high efficiency in either single or multiple form for various applications. Commonly nowadays cyclones are used only as pre-filters before filtration equipment in order to reduce loadings, however on some applications cyclone may be suitable as a stand-alone unit. See selection guide for cyclone sizings.
Our full range of cyclones can be manufactured in various material utilizing either mild, galvanized or stainless steel and can be provided either free standing or incorporating additional support steel work. Where necessary they can be provided with either additional after filtration or exhaust fans dependent upon the requirements on site.
Additional equipment for the efficient function of the cyclones including surge bins, dust collection facilities, rotary discharge valves and lagging of the equipment can be provided as and when required.
Where multiple banks of cyclones are required interconnecting ductwork would normally be included within our scope of supply, however if required the cyclones can be supplied individually for assembling into equipment on site. They range of cyclones covers a broad spectrum of efficiencies to match both particle size and the percentage removal required. Efficiency curves can be provided for most of the range together with sizing charts and layout drawings when required.
The three main sizes are based upon low (G type), medium (M type) and high (H type) efficiencies.
|CYC Efficiency Tables||G Type Sizing||H Type Sizing||M Type Sizing|
Much has been written on the subject of passing dust through a unit having cyclone proportions. In analysing the numerous references which are available, the reader finds great difficulty in isolating basic facts on which he can begin to build his understanding.
The first rule which is generally accepted is the simple empirically proved fact that the end product, which can be termed as ‘collecting efficiency’, is related to energy expended. In more basic terms we set out the rough yardstick ‘collecting efficiency’ increases as the ‘water gauge’ goes up. There are exceptions to this rule as in the case of the multi-cellular cyclone, which uses a number of small cyclonets housed within a plenum chamber. The rule of thumb ‘CE is proportional to WG” is modified with formula too complicated and bulky to discuss in this text; our comment is confined to broad generalizations in the hope that reader may pass on to informative references, which deal with the subject in detail.
With all cyclone work it must be appreciated that to separate the particles from the airstream tangential entry and airspeed are necessary to create a vortex which does the spiralling action of downward and upward air movement; this is in essence all that the fabricated steel shell and inner thimble induces.
The density of the particle will detemine together with its shape and size what can be done with it once it enters the cyclone. It is impossible to supply momentum to an object if the object is not capable of receiving it. To explain this more fully let us consider two balls each measuring two inches diameter. The first ball can be a balloon; this is a plastic skin containing air which pressurizes to maintain spherical shape. The second can be more dense, say something like a cricket ball. Now we imagine the human arm throwing object 1 (the balloon), the thrower expends energy, say 200 ft and the ball travels no more than 5ft. In the case of object 2 (the cricket ball), the same amount of energy is expended and the ball travels 100 yards. This simply means certain physical characteristics are required before one can create momentum from air speed and the tangential entry on the cyclone in order that the dust may be centrifuged out.
After appreciating that density plays a part in the behaviour of the particle the next consideration is that of shape. The physical contours of the particle are of the utmost importance, they determine the sail cloth effect that resists natural settling in free air. Particles are never truly spherical; all shapes are involved, varying between two uniformed extremes, i.e., the ball bearing and the postage stamp, which has a very high sail effect with resultant slow settling rate.
The problems involved in calculating settling rates are relatively easy if the particle is of uniformed shape. In considering any dust sample it inevitably transpires that the shapes are anything but uniformed, this makes calculation labourious and in some cases well nigh impossible.
At this stage special equipment is employed to aid analysis. This equipment consists of a long thin transparent tube coupled at the base with an air connection and required auxillaries to meter airflow. The dust sample is placed in the tube and air is introduced to create a fluidizing action; the performance of the particles are noted, some will fall against the airflow, others will be blown up the tube to be captured in a container interconnected with the equipment.
Numerous tests are carried out at varying up-draft velocities and the physical performance of the dust recorded. This method known as elutriation or terminal velocity analysis automatically takes into account particle shape, weight and size. With the application of Stokes law, which expresses terminal velocity for spherical particles of known specific gravity and size the dust to be handled can be specified. The unit of measurement is the micron (1/1000th millimetre) the analysis will record the percentage weights through a range of micron sizes.
On completion of the analysis consideration is then given in order to place the dust to be collected into a category where it can be handled by a particular type ‘DUSTAIR ’ cyclone.
Having placed the dust into a category where a certain type of unit is best suited, the diameter of the cyclone (or cyclones) has to be investigated. In some instances several small units are desirable in preference to one big one. To appreciate this point more fully the happenings within the cyclone must be understood. The body of the unit in conjunction with the thimble induces a vortex. Air enters the inlet port and screws down the body of the cylinder in company with dust particles. The body diameter can be thought of as a deflector plate working in conjunction with the tangential inlet. See figure 1. If we consider Fig. 2 where we have the tangential inlet with two distinct body diameters superimposed it can be appreciated that for equal conditions of air speed, shape and S.G. the smaller body diameter will have contact with the particle in advance of its bigger counterpart.
The figures analysing types G, M, H and MC can now be understood.
In the case of the ‘G’ inlet speed increases as the body diameter goes up in order to counteract the angular distance travelled before the particle comes into contact with the cyclone body. As pressure consumption measured in inches water gauge is a function of inlet speed it can be observed that the W.G. increase with body diameter.
When considering type M, H and MC we have the reverse condition in the case of W.G. consumed; this is held constant throughout the range, this also means that the inlet port speed is standard for all sizes. As no attempt is made to counteract the efficiency drop with increasing size the maximum economical size is about 48”, although in some cases sizes up to 60” can be successfully applied in some cases.
These units have been applied extensively by Dustair . in the past, the largest at a rating of 30,000 cfm on a 15ft body diameter. In the past the ‘G’ has been used indiscriminately, but as time has progressed more informative information has come to hand, which has narrowed its suitability drastically. The cyclone was introduced approximately 70 years ago and the ‘G’ that we know today varies very little from the original design. It has, however, done much to propagate the thinking and development of the other four main groups of cyclones that are in use today.
HYBRIDS (HIGH THRO’PUT DUSTAIR EQUIV. G
FROM (MEDIUM EFF. “ “ M
G (HIGH EFF. “ “ H
(MULTI-CELLULAR “ “ MC
In passing it is worth noting that the ‘G’ can be used as a coarse pre-cleaner in series with cyclones of higher efficiency to give classification or to take the buttress load prior to passing on to a bag filter. Its singular filter role is new in most cases confined to woodwaste and the rare grain, dried sugar beet, polishing exhaust duties.
In some cases ‘Z’s are now used on woodwaste which is the last stronghold of the ‘G’ as the singular filter. It would be fair to say that as the Clean Air Acts become more stringent its role as the ‘only filter on line’ will be very rare indeed.
It is impossible to isolate applications for which the types G , M, H and MC are suitable. Each case is treated as an individual consideration at the project stage, when points of durability, flexibility, efficiency and running costs, to mention, but a few points, are evaluated. In instances where sufficient quantities of dust are available tests are carried out by us to ensure the suitability of the equipment offered.