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Thread: SIZE OF WATER DROPLETS

  1. #1
    Mark Kilfoil Guest
    I am doing some research into pneumatic conveying. However, the products which have to be conveyed are wet, therefore, the conveying air has to transport the solid particles plus water droplets. In order to model the process, I need to determine the maximum size of water droplet which can exist can be blown along the pipe.

    None of the physics books which I have consulted were very helpful. Thus, I reasoned that the maximum size may be when the force(due to surface tension) holding a droplet together is equal to the force (due to pressure differences across the droplet) tending to split it. To get the inwards force I multiplied the surface tension by the circumference of the droplet. The separating force came from the equation for drag force, published drag coefficients and an assumption that in the Reynolds's number range corresponding to the air velocity (50 - 60 m/s) the pressure drag is 75 % of the total drag.

    This gave an average droplet diameter of approximately 3 mm, which seems about correct. The 3 mm is an average diameter because a droplet being blown along by fast moving air will probably have an ellipsoidal not spherical shape.

    Any comments or suggestions will be welcome.

    Regards
    Mark Kilfoil

    Mechanical Engineering : Technikon Witwatersrand
    Phone + 27 11 406 2300 Fax + 27 11 406 2770 E-mail : mkilfoil@mail.twr.ac

  2. #2
    Harald Wilms Guest

    Water Droplets

    I have difficulties to imagine droplets and solids to be conveyed through the conveying line independently/separately. With impacts at the wall the droplets will form a film which most likely will not break up again into droplets. Solid particles may stick to this film, causing either build-up and blockage or at least increased pressure drop. What is the purpose of conveying liquid and dry (?) solids separately through the same line? Do you feed them separately? Velocities of 50-60 m/s seem to be high for pneumatic conveying.

  3. #3
    Mark Kilfoil Guest
    Harald

    Thanks for the response.

    The products being conveyed are wet (approximately 15 % moisture by mass). Only about 1 - 2 % moisture can be on the surface or inside the particles (they have low porosity). Thus, I reasoned that at the feeding point the remainder of the moisture must be in droplet form.

    You are correct in stating that droplets will collide with the walls and then form a film on the inner surface of the pipe. I asked about the droplet size because I am trying to understand what is happening at the begining of conveying, as the wet product enters the pipe.

    The high velocity is because it is lean phase conveying. Also, the product includes some large particles (diameter of an equivalent sphere may be larger than 30 mm) and the density is approximately 2700 kg/m^3.

  4. Conveying Moist Materials

    I do not think that you will be able to convey material and water droplets separately in a pipeline. Even if you feed them separately they will soon mix together to give a damp or wet bulk solid, or slurry, depending upon the moisture content.

    I have conveyed minus 15 mm coal having a high surface moisture content. The coal was conveyed in dilute phase and the mean value of conveying air velocity was about 20 m/s. On impact with bends in the pipeline some of the moisture was transferred to the walls of the bends by centrifugal force. The fines in the material then adhered to the bend wall because of the moisture. This is a continuing process and the pipeline soon blocked. I had to dry the coal to achieve about 2% free moisture before I could convey without problems.

    In some other research work that I have carried out I was looking at the influence of the moisture content of silica sand on the erosive wear of bends in the pipeline. The moisture content was so high (up to 26%) that there was no possibility of feeding wet sand into the pipeline and so I fed dry sand into the pipeline and injected water into the pipeline about 3 m downstream. The conveying air velocity was about 50 m/s at the pipeline inlet, expanding to about 100 m/s at the pipeline exit and the wet sand conveyed without problems.
    David Mills

  5. #5
    Mark Kilfoil Guest
    Dr. Mills

    Thanks for the response.

    I my initial post I neglected to mention that the range of solid particle sizes in wet product being conveyed is usually very large (eg. + 25 mm to - 0.5 mm). Based on your response, it seems that I should expect the "free" moisture to mix with the fines. Thus instead of fines and moisture droplets travelling seperately at and downstream of the feeding tee there will be "blobs" that are a mixture of solid fines and water. Am I understanding it correctly ?

  6. Conveying Moist Materials - Part II

    Free moisture on large particles will tend to be centrifuged off on impact with the pipeline wall and particularly at bends. This will make the entire pipeline wet. The moisture will tend to cause the fines to agglomerate. Only if the free moisture content is very high are you likely to get particles of water being conveyed and these will lose their identity on impact with each bend. I can only imagine that it must be extremely difficult to feed very wet material into a pipeline. This is why I fed dry material and water separately. You may also need to take into account the fact that as the air expands through the pipeline its relative humidity will decrease and so its capability for drying the wet bulk solid will gradually increase along the length of the line.
    David Mills

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