Lock hoppers provide a means of both allowing operation of many feeding devices that have only a low pressure capability, to operate at very much higher pressures, and allowing continuous conveying from a single blow tank feeder.
To illustrate the mode of operation the theme of blow tanks is continued, and a sys- tem often referred to as twin blow tanks operating in series is considered. The lock hopper is located between the supply hopper, which will generally be at atmospheric pressure to allow continuous loading of material, and the material feeding device, which can be at any pressure required, almost without limit. A typical layout with regard to a blow tank is illustrated in Figure 4.18.
The lock hopper, or pressure transfer vessel, is filled from the hopper above. The lock hopper is then pressurized to the same pressure as the blow tank, either by means of a pressure balance from the blow tank, which acts as a vent line for the blow tank while it is being filled, or by means of a direct line from the main air supply. With the transfer vessel at the same pressure as the blow tank, the blow tank can be topped up to maintain a continuous flow of material. The lock hopper will have to be pressurized slowly in order to prevent a loss in performance of the system while it is conveying material.
Once the material has been loaded into the blow tank the lock hopper will have to be vented to return it to atmospheric pressure. The lock hopper can then be loaded with another batch of material from the supply hopper.
The blow tank in Figure 4.18 is shown in a top discharge configuration, but with- out a fluidizing membrane. The air enters a plenum chamber at the base, to pressurize the blow tank and fluidize the material, and is discharged via an inverted cone into the conveying line. A vertically in-line arrangement of vessels, with one positioned above the other, does require a lot of headroom, and so the blow tank arrangement shown in Figure 4.18 is sometimes employed to minimize the head required.
Alternative feeding arrangements
If a lock hopper arrangement is used, as shown in Figure 4.18, the pipeline feeding device need not be a blow tank at all, despite the use of high pressure air. With the transfer pressure vessel separating the hopper and the pipeline feeding device, the feeding device can equally be a rotary valve or a screw feeder, for there is virtually no pressure drop across the feeder. Any pressure drop will, in fact, be in the direction of material flow and so there are no problems of air leakage across the device, as there are with conventional feeders of this type.
Rotary valves and screws
A rotary valve or screw may be used in this situation to guarantee the feed of a steady flow of material into a pipeline. If a rotary valve or screw is to be employed, designs to cater for high pressure differentials do not have to be used. Erosive wear problems associated with abrasive materials are also significantly reduced with this type of system. A sketch of a screw feeder based on this lock hopper principle is given in Figure 4.19.
Venturi feeders can equally be operated at high pressure when located in a vessel under a lock hopper. At high pressure the influence of changes in pressure on the com- pressibility of air are not so great, and the generation of very high velocities within the venturi are not as necessary and so the device is very much easier to design and operate. An analysis of venturi flows and operation is given in Chapter 10.
In cases where there is a need for a high air supply pressure, either to convey a material in dense phase or over a long distance, and continuous operation is essential, a twin blow tank system is ideal. Although these systems do require more headroom than
rotary valves, screw feeders and many single blow tank systems, this need not be excessive. It clearly depends upon the material flow rate to be achieved, but if a reasonable cycling frequency between the two pressure tanks is employed, the capacity of the vessels can be of a reasonable size and a compact system can be obtained.
A particular application of these systems is for the direct injection of pulverized coal (DIPC) into boilers and furnaces. In the case of furnaces the material often has to be delivered against a pressure. This, of course, presents no problem since high air supply pressures can be utilized. A general requirement of DIPC systems is that the material should be conveyed at a very uniform rate, and that it should also be capable of achieving a high turn-down ratio. An operating range of 10:1 on material flow rate is often requested in this respect. Blow tanks are capable of operating quite success- fully over this range and so they are ideally suited to this type of application.
Alternative vessel configuration
Due to the head-room required, particularly for high tonnage duties requiring blow tanks, a side by side arrangement of blow tanks was devised. The driving force for this development was the possibility of replacing screw pump feeding systems with such blow tanks. The lock hopper fits into the existing space beneath the hopper, vacated by the screw pump, and the blow tank is placed alongside. This requires the material in the lock hopper to be conveyed to the blow tank, but it does allow continuous operation. A sketch of such an arrangement is given in Figure 4.20.