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Determining the correct extraction flow rate for different dust types

Determining the right amount of extraction flow rate is the first step in solving any dust extraction project and is crucial to the proper function of the system. If a wrong determination of the flow rate occurs – in the worst case its under-dimensioning – the project is invaluable right from the start-up. Ductings and low-capacity baghouse technologies will be designed, constructed and installed, which will be immediately apparent when you start the system itself. The repair cam be very expensive, sometimes needing even a complete replacement of the entire technology. The design of the flow rate is best to be checked with more experienced designers or suppliers before you move into the realisation phase.

How to specify the correct flow rate

This step is crucial for the right funcion of the system. If the dust extraction system dedusts more sources of dust at a time, it may be neccessary to determine the right flow rate for each source alone. The total flow rate of the dust extraction system is then equal to the sum of the individual sources. There are no tables for the determination of the required flow rate, it is only the experience of the designer that comes into play.

In the case of extraction systems, the problem of under-dimensioning the necessary flow rate is immediately evident –  the dedusted source is still leaking dust. The opposite mistake can be too much of flow rate. The space is completely dust-free, but the cost of operating the extraction system is unnecessary high.

Another important factor influencing the choice of dust extraction flow rate is the extraction adaptor. In practice, the extraction covers, hoods, tables and nozzle attachments are the most used variants. The choice is always dependent on the specific application, but one rule applies to all cases.  The closer to the source the extraction occurs, the lower flow rate is needed and the higher the effectivity is. All of this applies only for situations where the hood doesnt obstruct the access to the working space of the machine.

We recommend inexperienced designers to consult the specific flow rate requirements with suppliers of the dust extraction system. This is one of the most important input parameters of the dust collector system design itself.

Determining the concurrence

On production lines where are more but not all dust sources extracted at the same time, it is reasonable to consider designing a control system. The extraction system is then equipped with automatically operated valves and a control system. The system controls the RPM of the suction fan with dependency on the individual workplaces use. This is a modern way of industrial dust extraction – energy consumption corresponds to the current production program. However this method of regulation must be designed with caution, as it also incorporates some technical constraints:

  • Decreasing the flow rate velocity in the ducting occurs when the fan speed is reduced. Subsequent lowering of the flow speed can lead to dust sedimenting in the pipeline – especially in the branches and elbows. This can lead to clogging.
  • The concurrence is usually described with a per cent number. Concurrence of 70% means that from 10 machines, only 7 is operating. Also with 50% it means only 1/2 of the machines are operating. The most frequent and recurrent mistake is that the designer or supplier determines the required flow rate in ratio. This logic could lead to situations where 10 000 m3/h of 60% concurrence equals 6 000 m3/h. This is obviously a mistake – it does not respect the required flow rate of the individual extraction points and the subsequent combination. In practice, there are situations in which the 60% concurrence of certain machines results in a sufficient extraction performance but insufficient flow rate for another combination of 60% machines.
  • The regulation of explosive dust extraction systems can be also difficult. It is always neccessary to keep the air speed at a level where minimal sediments will be formed when dealing with explosive dusts. The minimal air speed in the ducting pipeline should be at least 18 – 20 m/s. This value limits possibilities of flow rate regulation. The solution is to either use higher air speeds or split the ducting into several branches.