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Homework Help: Another Stumpy Question

  1. Nov 3, 2006 #1
    maths for aquaculture
    How long would it take to fill a cylindrical grow-out tank (2m high, 8m diameter, 10% freeboard) from an inlet pump delivering 25 litres of water every minute through a 25mm pipe that has 4 spray outlets?

    first of all I know that the 4 spray outlets are irrelevant to the quetion and are a decoy. as well as this i know that the pipe is cylindrical

    tank volume with freeBoard =90.47786842
    25 L/min converts into 0.025 m3/min
    to get that into m3 per sec(delivery rate) I times it by 60=1.5m3/sec

    So to get the answer do I divide the delivery rate(1.5) into the tank volume i.e(90/1.5) OR do I use the delivery rate as a height(h) variable and work out volume of pipe i.e(pi*h*r*r)and then divide the volume of the pipe into the tank volume i.e(vt/vp) to get the amount in secs? any help will be greatly appreciated as i am confronted with alot of these types of queations
  2. jcsd
  3. Nov 3, 2006 #2


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    You have a volume and a rate at which water is being poured into it. But the dimensions(units) are incompatible as your volume is in m³ but the rate is in L/min. You will want to either convert the m³ in litres or the litres in m³.

    Once you've done that if you have doubt on wheter it is the volume that you must divide by the rate or the rate by the volume, just look at the units.

    1) If you divide rate by volume, you get an answer whose units are (volume/time)/(volume)=1/time. This is not quite what you want.

    2) If you divide volume by rate, you get an answer whose units are volume/(volume/time)=time. This is what you want.
  4. Nov 6, 2006 #3


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    No, you divide by 60! You surely are not going to get 1.5 cubic meters per second if you are only delivering 0.025 cubic meters per minute! But why convert from minutes to seconds anyway? Were you specifically asked to give the answer in seconds rather than minutes?

    The size of the pipe is irrelevant also. You were told that water is being delivered to the tank at 25 litres/minute. Divide the volume of the tank, in litres, by that. Or, if you really want to, convert 25 litres/minute into cubic metres per minute and divide the volume of the tank by that.
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