Proportioning the air supply between the fluidizing and supplementary air lines controls the discharge rate of a blow tank. If insufficient product is being fed into a pipeline, the proportion of the total air supply that is directed to the blow tank should be increased. If too much product is being fed into a pipeline, this proportion should be reduced.
Figure 4. Blow, No Blow
For a given time-averaged mean value of product flow rate to be obtained from a blow tank, a very much higher value must be achieved during the steady-state portion of the cycle.
The discharge limit of the blow tank will be reached when all the air is directed to the tank. A further increase in product flow rate can be achieved by boosting the volumetric flow rate of the air, but this will have an adverse effect on the conveying of the product in the pipeline. The alternative is to increase the diameter of the blow-tank discharge pipe.
If a blow tank is to convey a product over a range of distances, the proportions of the air will change according to the distance conveyed. If this is not taken into account, the pipeline will be underutilized for shorter distances, and may block on longer distances. Here, a pressure gauge in the air supply line is particularly useful, for it can ensure the product flow rate through the pipeline is always the maximum that can be conveyed with the given air supply, regardless of conveying distance.
An automatic controller proportions the total air supply between the blow tank and the supplementary line. Control is usually based on pressure signals since, assuming that the blower or compressor operates within a prescribed range, any change in conveying rate, and hence phase density, will be reflected in a change of operating pressure.
If too much product is discharged into the conveying line, the pressure will increase and so the modulating valve will decrease the proportion of air passing to the blow tank to compensate. If the pressure drop across the conveying line is low, the modulating valve will reduce the supplementary air and so allow more air to pass to the blow tank and hence increase its discharge.
Air moisture is another concern. When air is compressed, its capacity for supporting water vapor decreases. Even relatively dry air will reach its saturation point and condensation will occur as the pressure is increased. Unless positive measures are taken to remove it, this moisture will be transported through the air supply lines with the conveying air. If a fluidizing membrane is used in a blow tank, this water will blind the membrane with certain products,"causing a gradual increase in pressure drop across the membrane and a resultant decrease in output of the system.
Most problems associated with moisture can be overcome by drying the air. If the product is hygroscopic, it will probably be necessary to incorporate a desiccant-type drier. If moisture and condensation are to be avoided, then a refrigerant drier should suffice.
Both the blinding of a fluidizing membrane and a restriction in the discharge pipe will add to the pressure drop across a blow tank. If the pressure drop across the product feeder increases, the pressure drop available for conveying the product in the pipeline will decrease and, so, cut conveying capacity.
Part of the blow tank pressure drop occurs in discharging the product from the blow tank. This is particularly a problem in top-discharge blow tanks requiring a long discharge line. The conveying air should be introduced as close to the tank as possible to minimize this pressure drop. In a tall blow tank it may be necessary to bring the discharge line out through the side to reduce its length.
The performance of a blow tank can be monitored quite easily by pressure gauges. A pressure gauge installed in the supplementary air supply line will effectively give a measure of the conveying-line pressure drop, and hence the use of the pipeline in conveying the product. A pressure gauge in the blow tank will then give an indication of the pressure drop across the blow-tank discharge line. If the blow tank has a fluidizing membrane, a pressure gauge in the air supply line to the blow tank will help monitor the state of the membrane.
With top-discharge blow tanks, there is also a minimum discharge limit, which relates to the fluidization air velocity in the discharge line.
If an attempt is made to convey a product at a low velocity from a top-discharge blow tank with only a small proportion of the air flow rate directed to the blow tank, the blow tank could "stall" and cease to discharge product into the conveying line. This is because the air velocity in the blow-tank discharge line will be very much lower than that at the product pick-up point. For a product having poor permeability and air retention properties, this could result in a blockage of the discharge pipe. If this occurs, use a smaller diameter discharge pipe.
Granular products can cause discharge problems from a top-discharge blow tank. Air permeates very easily through these products and it is possible that insufficient resistance will be built up to discharge the product. Bottom-discharge blow tanks are generally recommended for granular products.
Because of their high permeability, granular products require a higher proportion of air directed to the blow tank for a given rate of discharge than that needed for powders.
Granular products with a high percentage of fines are very much less permeable. They generally are not suitable for dense phase conveying in conventional systems. They will require very little air for their discharge from a blow tank, and so if the discharge line is unnecessarily long or has a long horizontal section, the discharge line is likely to block.