Pressure drop
Material flow rate through a pipeline is primarily dependent upon the pressure drop available across the pipeline. A basic requirement of any feeding device, therefore, is that the pressure loss across the device should be as low as possible in low pressure systems, and as small a proportion of the total as possible in high pressure systems.
If the feeder takes an unnecessarily high proportion of the total pressure drop from the air source, less pressure will be available for conveying the material through the pipeline, and so the material flow rate will have to be reduced to compensate. Alterna- tively, if a higher air supply pressure is employed to compensate, more energy will be required, and hence the operating cost will be greater.
Maintenance
Maintenance of these items is another important factor. If air leakage has to be accepted with a particular feeding system, the rate of loss must not increase unduly with time, otherwise insufficient air may ultimately be supplied to a pipeline and a blockage may occur after a period of time.
Material properties
Material properties are particularly important and have to be taken into account in the selection of feeding devices. In feeding systems that have moving parts, care has to be taken with both abrasive and friable materials. Material flow properties need to be taken into account with feeding devices, and particle size must be considered in all cases, particularly the two extremes of large lumps and very fine particles.
Devices available
Many diverse devices have been developed for feeding pipelines. Some are specifically appropriate to a single type of system, such as suction nozzles for vacuum systems. Others, such as rotary valves, screws and gate valves, can be used for both vacuum and positive pressure systems. The approximate operating pressure ranges for various pipeline feeding devices is shown in Figure 3.2.
It will be seen that there is no scale on the vacuum side of Figure 3.2. This is because the pressure of operation is only atmospheric and there will be essentially no pressure difference across the feeder, regardless of the type of feeder. In some situations a small resistance may be built into the system but this is generally only to help promote flow into the feeding device.
Developments have been carried out on most types of feeding device, both to increase the range of materials that can be successfully handled, and to increase the oper- ating pressure range of the device. Each type of feeding device, therefore, can generally be used with a number of different types of conveying system, and there are usually many alternative arrangements of the feeding device itself.
Lock hoppers
It should be pointed out that Figure 3.2 is drawn for stand-alone feeding devices. The pressure capability of many of the positive pressure feeding devices listed can be improved significantly, with little further modification, if they are used in conjunction with lock hoppers. This puts the devices into the higher pressure rating and so this will be considered in the next chapter.
Blow tanks
For high pressure systems, and particularly where the material has to be fed into a sys- tem that is maintained at a high pressure, blow tanks are often employed. These are generally used for conveying batches, although they can quite easily be adapted for continuous conveying. This is the particular advantage of all the other feeding devices shown in Figure 3.2. Although blow tanks are generally associated with high pressure conveying, they are also used for low pressure conveying.
Blow tanks are being more widely used in industry and so information on blow tanks is included in this chapter as well as in Chapter 4, apart from the fact that they cover a very wide range of operation. Blow tanks have no moving parts and so have particular advantages with regard to the feeding of abrasive and friable materials.