In today's cement plants, powdery materials such as raw meal and cement are treated in silos. Although from the outside the silos look similar, they all have different functions.

A blending silo must be capable of ensuring uniformity of the raw meal product selected by the proportioning control system. In the raw meal production system, the blending silo acts as insurance against upset conditions at the raw mill, ensuring that final clinker quality remains unaffected.

A silo must provide effective storage and reliable discharge with maximum clean out of the material. The task of every cement plant manager is to make certain that all materials are handled in the most cost-effective manner with regard to initial investment and operational costs, and that neither the performance nor reliability of the unit is compromised.

The Controlled Flow (CF) silo was developed to make this task easier by limiting the aeration within the silo and, in effect, only aerating one area of the container at a time (Figure 1). Aeration is used solely to control the flow, letting gravity do the rest.

Controlling the flow In the CF silo from F.L.Smidth, the entire volume of material must be kept moving and a suitable residence time distribution of the material in the silo must be established. This controlled flow is achieved by discharging the raw meal through a number of outlets-operated at different flow rates-in the silo bottom and mixing it from the individual outlets in a small mixing tank. The silo is effectively divided into a number of plug flow units, operated in parallel with different flow rates leading into a relatively small ideal mixer.

The material is supplied continuously to the silo, the top deck of which is fitted with manholes, over-and-under pressure valves, and level indicators. The silo bottom is divided into seven hexagonal sectors. In the center of each sector, there is a discharge opening covered by a pressure-relief cone.

Each of the sectors is further subdivided into six triangular segments, meaning that the silo bottom consists of 42 segments, all incorporating porous aeration boxes. Three separate rotary blowers, each equipped with a noise-reduction device on the blow-off pipe, mean that three segments can be aerated simultaneously.

The fluidization air is blown through this pipe when a pre-set pressure is reached. The air supply to each segment is filtered to prevent material entering the silo if a pipe breaks. The seven discharge openings are equipped with manually operated slide grates and pneumatically operated shut-off valves, which drop the material onto air slides to the centrally placed mixing tank below (Figure 2). Effective aeration fluidizes the raw meal in the mixing tank, which performs as an ideal mixer.

As the mixing tank is placed on load cells, the weighing signal from these cells starts and stops the total extraction process from the silo bottom, keeping the material level in the tanks within narrow limits. The minimum amount of material in the tank will typically correspond to 12 minutes consumption of kiln feed.

Operation of the silo is controlled by programmable logical controllers (PLCs) incorporated in a microprocessor-based system, which controls the extraction sequence from the seven outlets (via the shut-off valves) and the aeration of the segments.

The idea is that the silo gets divided into seven sub-silos with different flow rates. Each sub-silo corresponds to six triangular segments in the silo bottom. As part of the standard extraction sequence, three segments corresponding to different sub-silos are aerated simultaneously via the shut-off valves.

Within a typical cycle of about 12 minutes, every segment in the silo bottom should be aerated at least once. Distribution of flow rates is achieved by means of different opening times of the shut-off valves, 60 seconds for four of the outlets and 40 seconds for the other three. Also, different diameter throttle plates inserted between discharge boxes and shut-off valves are used.

Due to the distribution of flow rates among the sub-silos, the layers of raw meal in the silo will graduate towards the discharge opening with the fastest flow (Figure 3). This will add to the blending effect.

Controlled Flow Storage (CFS) At the time the first CF silos were being put into operation, there was a need for silo storage of fly ash for cement production. As a result, the first two CFS silos-each having a diameter of 10 meters- were installed.

The CFS silo (Figure 4) only has one extraction hole through which the material enters a feed box with several outlets. Above the extraction hole is a large cone-shaped steel structure that creates a flow inside the silo helping to maximize silo discharge.

The silo bottom is divided into six segments with aeration boxes with only one segment aerated at a time to ensure low power consumption. The raised bottom of the unit is fitted with a single, centrally positioned outlet and divided into six triangular segments inclined at 15 degrees towards the outlet. Each segment is covered with an aeration box, and a steel cone covers the outlet. This cone releases the pressure above the outlet and creates a flow within the silo helping to maximize emptying. In contrast to the cone in the CF silo, the one in the CFS unit is of much simpler construction based on three sections standing on vertical legs.

Each segment is aerated individually for approximately five minutes with an air supply of 0.6 m[superscript]3/m[superscript]2 of segment area.The air supply is cleaned by filter in an attempt to prevent the cement from moving backwards if pipe breakage occurs.

Each segment is aerated individually for approximately five minutes with an air supply of 0.6 m[superscript]3/m[superscript]2 of segment area. The air supply is cleaned by filter in an attempt to prevent the cement from moving backwards in the event of pipe breakage.

A pneumatically regulated valve is placed prior to each filter, and the air for the silo bottom is supplied by one rotary blower delivering a pressure of 0.8 bar. The air distribution system has a design velocity of 20 m/sec.

The squared extraction outlet is fitted with aeration pipes in the corners, and the construction allows for cleaning with air lances. The outlet has a manually operated slide gate and feed box. The box is fed with air at 1.0 m[superscript]3/m[superscript]2 of bottom area and equipped with a pneumatically regulated flow-control gate that can close quickly in case of a transportation stoppage.

Controlled Flow Inverted (CFI) An extension of the previously mentioned designs is the Controlled Flow Inverted (CFI) cone silo (Figure 5), which provides free space under the cone itself facilitating the construction of a ring foundation if it is deemed necessary.

The unit is designed around the principle of complete discharging while minimizing the specific power consumption for aeration. This is achieved by aerating only two sections at a time or, in some cases, each section individually depending on the production.

The silo bottom is divided into six, eight, or 10 sections depending on the diameter. Each section slopes 15 degrees toward its respective outlets in the cone and is fitted with an aeration box. They are aerated in groups of two for about five minutes with an air supply of around 0.6 m[superscript]3/m[superscript]2. Two rotary blowers supply filtered air to the aeration boxes at 0.8 bar, and the air is distributed via a pipe system with two-way solenoid valves, one for each silo bottom section. The design velocity of the pipe system is 20 m/s. Again, rotary blowers with noise-reduction apparatus combat any possible pressure rises.

Each extraction outlet is equipped with aeration boxes to allow manual cleaning, and they all have manually operated slide gates and pneumatically regulated shut-off gates.

From the silo outlet, the material passes via fan-assisted air slides to the feed tank, which is aerated by a single rotary blower operating at 0.8 bar. The tank has one or more outlets with pneumatically regulated flow gates that can open and close quickly to assist in packing or bulk loading operations.

FLS also has utilized these designs in multi-compartment silos, which require tailor-made modifications for each installation and have proven popular in blended cement applications. Another development is the CFBI blending silo, based on the CFI model but with additional features such as integral flow zones for controlled mixing.