Removing dust from process gases
with optimized cyclone systems

On account of their simple design, limited space requirement, high operational reliability, as well as their low operating and investment costs, cyclones are used in almost all branches of industry. The cyclone’s main applications today are in hot gas and high-pressure dedusting, and as a preliminary separator for raw gases with high dust loads or as a final separator for easy-to-remove dusts.

In mineral processing, the removal of dust particles from air/gas mixtures by means of cyclones in combination with or without settling chambers is widely applied as a classical supplementary or auxiliary process on account of the high separation rate and the low pressure loss. Usually they are installed immediately downstream of the comminution processes for the purpose of preliminary dedusting so as to reduce the load on sizing or filtering equipment. Another not unimportant application is the removal of abrasive minerals in order to protect pipelines, blower stations but also measurement and control equipment against wear.

A TEC cyclones with high separation efficiency or High-
Efficiency Cyclones (Fig. 1), A TEC-HEC for short, frequently differ from other cyclones on account of their comparatively high cylinder body. Another feature is the form of the inlet cross-section. The tapered form prevents the settling or caking of material in the inlet channel and, as a result, guarantees defined flow conditions at the inlet over a longer period.

The function of the dip tube of a cyclone is best described by the term “vortex finder”. It takes the still rotating solids-laden gas and leads it out of the cyclone. This results in highly unfavourable fluidic conditions, which are reflected in the high proportional pressure loss of the dip tube. Depending on the loading degree, this can account for up to 90  % of the total pressure loss.

Guide vanes are used to reduce the pressure loss at the dip tube. With their vanes, they “trap” the rotating clean gas flow. This is followed by the formation of a stabilized vortex tube in the centre of the cyclone, which precesses more or less. The flow conditions now generated result in an around 30 % reduction in the pressure loss at the dip tube.

A TEC has translated these findings into a patented product, the HURRIVANE^® (Fig. 2). It supplies HURRIVANE^® worldwide to customers as cost- and time-efficient option for retrofitting to their cyclones. The customers immediately notice a change in the operating conditions, as a much reduced pressure loss is noticeably reflected in the power consumption of the gas conveying equipment. Classical applications for ­HURRIVANE^® are the upper stages in the suspension heat exchangers. But also in mill circuits, energy savings can be achieved with these special dip tube guide vanes.

Naturally, the HURRIVANE^® is particularly suitable for installation in A TEC-HEC, as a result of which, depending on the situation, separation and pressure loss are both positively influenced.

As is widely known, a reduction in the pressure loss with a constant separation rate can only be realized with the parallel connection of more than one cyclone. A special design based on the principle of parallel connection of two cyclones is represented by the HURRICLON^® (Fig. 3). In the ­HURRICLON^®,
A TEC has succeeded in combining the particular advantages of parallel connection with the compactness of a single cyclone.

Like a classical cyclone, the HURRICLON^® has a spiral inlet, a cylindrical top section and a conical bottom section down to the material discharge. But in contrast to a classical cyclone, the HURRICLON^® has two dip tubes with openings lying opposite. The “upper dip tube” leaves the HURRICLON^® as in a classical cyclone, whereas the “lower dip tube” is led down and exits the conical section from the side. Only outside the cyclone are the two dip tubes brought together again. To additionally stabilize the flow conditions within the HURRICLON^®, there are two chiral HURRIVANES^® are fitted at the openings of the dip tubes.

The splitting of the volume flow inside the HURRICLON^® between two dip tubes reduces the diameter by a factor of
around 1.4. As a result, providing the usual diameter ratio of dip tube to cylinder casing, e. g. of 1: 2, is maintained, a much slimmer design is obtained. The smaller cylinder casing diameter has an indirectly proportional effect on the generated centrifugal forces, i.e. these increase with decreasing cylindrical casing diameter. ­Besides this crucial advantage in respect of the separation rate, thanks to the stabilized flow achieved with the ­HURRIVANE^®, the HURRICLON^® exhibits a ­comparatively low pressure loss, which, depending on the specific model and application, can be up to 50 % lower than in a classical cyclone with the same capacity.

On account of the known weaknesses of the cyclones in respect of the removal of very fine dusts, cyclones are practically no longer used as the last dedusting unit prior to the release of the gases into the environment. But in contrast to this, thanks to its ruggedness and cost efficiency, the cyclone is now indispensable a separating unit in gas circulation processes. One of these processes or its sub-processes is the cement pyroprocess, which is explained in detail in the following.

State of the art for pre-heating the raw meal in the cement ­pyroprocess is the application of a suspension heat exchanger (Fig. 4). The suspension heat exchanger is based on a discontinuous counterflow principle. For the process flow, this means that after every gas-solids heat exchange in the suspension, this is re-separated. Both the now cooler gas and the warmer solids go in to the next stage opposite.

In this cascade of 4, 5 or 6 cyclone stages (Fig. 5), the cyclones of the lower stages are mostly designed for a minimum pressure loss. Only the cyclone or cyclones of the upper stage are optimized for separation. The resulting pressure loss of this stage can account for up to 30  % of the total pressure loss of the heat exchanger tower. Naturally, this is a particular challenge, as the particle size distribution is shifted significantly in the direction of the ultrafine material sizes by the separation at the end of the lower stages. For that reason, in the top stage with a moderate pressure loss, despite A TEC-HEC, separation rates of “only” 93  % can be expected. For the cyclones of the lower stages, the separation rates lie between 80 % and 90 %.

For optimization of the pressure loss with the same separation rate, the installation of the HURRIVANE^® is recommended. Further, the installation of a HURRICLON^® as the final cyclone in the cascade is one possibility for reducing the overall pressure loss.

The application of grate coolers for the controlled cooling of cement clinker after the kiln brings with it a hot current of flue gas, which cannot be recirculated into the pyroprocess, e.g. in the form of secondary and tertiary air. The temperature of the this waste gas flow ranges between 150 ° and 400 °C, i. e. at temperatures which are ideally suitable for drying and/or milling processes. Naturally, the waste gas is laden with clinker dust when it exits the cooler. The removal of this dust serves two purposes, first recovery of the product and secondly the cleaning of the waste air. The latter is intended to minimize wear caused by clinker dust and contamination of the product to be dried with clinker dust.

Depending on the type of waste air filter, preliminary dedusting e.g. with a HURRICLON^® has various effects on the downstream filters. If electrostatic filter is used, then thanks to preliminary dust removal, the specific electrical resistance of the suspension and the filter rate increases or the power input per filter surface area unit decreases. With a cloth filter, it is particularly important to ensure the temperature does not exceed 230°C for long periods. Particles entrained in the gas flow can, however, have a much higher temperature. If these reach the cloth in the filters, they damage it on account of the sustained punctiform thermal load. In both cases, the upstream HURRICLON^® removes these particles, without burdening the air filter compressor with any significantly higher pressure loss.

It is known that the separation rate of a cyclone depends essentially on the centrifugal and inertia forces acting in it. These depend in turn on geometrical and fludic factors. This explains why a cyclone can only be optimally dimensioned for one single operating state. Any deviating operating states show an immediate effect on the pressure loss and the separation rate. For example, a reduction of the volume flow leads to the reduction of all velocity vectors and therefore to a reduction of the pressure loss but also a fall in the separation rate. But also the change in the gas temperature has certain effects as the density and viscosity of the gas and consequently the particle suspension change.

Now, to remain permanently within the optimal operating state for a given cyclone or HURRICLON ^® geometry, A TEC uses a special circulation system. In principle, a subflow of the clean gas is recirculated to the cyclone or HURRICLON^® inlet when required. For controlled conveying of the volume of gas and to overcome the pressure difference, another blower is used. The almost constant volume flow generated at the inlet guarantees acceptable flow conditions in the cyclone and therefore a uniform pressure loss and above all a uniform separation rate. Such systems
are used mainly in the dedusting of gases with low loads, where separation is performed mainly in the cyclone vortex by means of centrifugal forces. For this reason, it is important to maintain the velocity vectors. Especially since their directed contributions go to the second power into the centrifugal forces, which has an effect on the accelerated particles.

A TEC supplies such a system for the re-dedusting of suspension heat exchanger flue gas of cement pyrolines, which is used for drying and as a working gas in fuel mills. This is system is supplied under the name Opti-Coal^®. Heat exchanger waste gas is often used for fuel mills as it only contains low quantities of oxygen and consists mainly of nitrogen and carbon dioxide. But on account of the separation at the end of the cyclones or HURRICLON^®, this gas is still loaded with fine particles that have passed through the top cyclone stage of the suspension heat exchanger. Removal by means of a filter system would ensure effective dedusting, but besides the immense investment costs, this would also result in a comparatively high pressure loss. For this reason, here the economically efficient Opti-Coal^® system is used. Core of the Opti-Coal^® technology is the ­HURRICLON^®, which thanks to its especially slim shape enables high centrifugal forces in centrifugal separation. In addition it exhibits a comparatively low pressure loss, which as a positive effect on the power consumption of the circulating air blower on the one hand and the conveying blower on the other. The separation rate averages 60  %, depending on the load, sometimes even more than this. That is independent on the actual volume flow as the speed at the inlet to the ­HURRICLON^® is kept almost constant by the circulation system (Fig. 6).

Depending on whether the design is drawn up for a cyclone, e.g. A TEC-HEC, or a HURRICLON^®, key operating data are necessary such as:

– Gas volume flow

– Entry load of the gas-solid suspension

– Particle size distribution of the solids

– Mean particle density of the solids

– Thermodynamical gas data: temperature, viscosity, density, gas moisture content

The other geometry data needed are generally dictated by the application-specific model series. It is generally possible to estimate whether the selected size of the unit is appropriate on the calculated inlet velocity and the mean centrifugal acceleration. The inlet velocity should range between 18 and 20 m/s and at the same time the mean centrifugal velocity should by no means fall below 100 g. If this is not the case, the equipment must be covered over and if necessary, instead of one cyclone/HURRICLON^®, two parallel-connected units should be used. Such problems often occur in the scale-up of existing plants.

The special feature of the HURRICLON^® with its two dip tubes has an effect on its design. On account of the smaller cylinder diameter, the outer (spiral inlet) and the inner circumferential speed are higher by a factor of 1.1 to 1.25 than in a traditional cyclone with the same volume rate.

It has been repeatedly proven (G. Staudinger, J. ­Keuschnigg, and M. Klupak, TU Graz & VA-Krems, 1991) that for a cyclone with a central cylinder, which corresponds to the geometry of a HURRICLON^®, the usual calculation methods for u_i, Δp_i and d* are not applicable. For this reason, the design of the ­HURRICLON^® is only roughly based on the common cyclone design methods (e. g. VDI). With protracted tests, the individual values for pressure loss and the fraction separation efficiencies were determined empirically, from which semi-
empirical correction terms were derived. Without these correction terms, it is not possible to calculate the pressure loss and separation rate of a HURRICLON^®.