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Flow in network or manifold

  1. Nov 16, 2015 #1
    Hi, A chamber (manifold type cylinder) has 1 inlet and 8 outlets of 3 inch diameter each.

    8 suction blowers are connected to the chamber's outlet. Each blower's suction flow rate is 1000 cfm.

    what will be the flow rate through the inlet of the chamber (diameter = 3 inches).

    Will the inlet flow rate reduce because of reduced cross section? What should be the chamber's inlet diameter to receive 4000, 6000 and 8000 cfm ?
     
  2. jcsd
  3. Nov 16, 2015 #2

    DrClaude

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    Staff: Mentor

    Is this homework?
     
  4. Nov 16, 2015 #3
    Dear DrClaude,

    I am a individual and have been breaking my head for months to solve this. Appreciate some help
     
  5. Nov 16, 2015 #4

    russ_watters

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    Obviously, conservation of mass requires that if the outlet flow is 8,000 CFM, the inlet must also be 8,000 CFM (assuming the air density doesn't change much). But the flow numbers sound impossible for such a small inlet unless the blowers are extremely high static pressure. Are those just the ratings or have you actually measured the flow? Do you have the specs of the blowers? Do you have a sketch of the system you can post? What is the purpose/function of the system?

    I'm moving the thread to mechanical engineering.
     
  6. Nov 16, 2015 #5
    Dear Russ_watters,

    Each blower with a inlet port of 3 inches (area 12.56 sq inches) pulls 1000 CFM when open to atmosphere. when 8 blowers are connected in parallel in a chamber, they will require 12.56*8=100.48 sq inches of cross section inlet

    But the chamber inlet is 3 inches (12.56 sq inches) means the cross section area for the network of blowers has reduced. Will the 8 blowers be able to pull 8000 cfm through the 3 inch inlet or will the cfm of blower network reduce ?
     
  7. Nov 16, 2015 #6

    russ_watters

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    Is that 3" or 4" diameter? Check your numbers.
    The airflow will certainly be greatly reduced. How much depends on the exact specs of the fans and ducting. This is not something you can rule-of-thumb: a detailed analysis is required.
     
  8. Nov 17, 2015 #7

    JBA

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    If this problem is treated as a box with a 3" diameter orifice, with an orifice coefficient of 1.00, in one face and with the blowers inlets directly connected to the box; then, even if the blowers' were capable of reducing the box interior pressure to 0 psia, assuming an (atmospheric) 14.7 psia, 65°F air source, the maximum possible flow through that 3" orifice would be choked flow at 1894 scfm. (I have an orifice flow program for just this type of analysis)
     
    Last edited: Nov 18, 2015
  9. Dec 18, 2015 #8
    Sorry for reverting late. please allow me to redraft the question with some updated data

    I have 2 exhaust fans that can handle 1000 cfm each. Diameter of each fan is 12 inches. I need to fit these 2 inside a duct in parallel. Hence the duct cross section will be 12x24. But the suction side of the duct can only be 12x12. Hence we see that the duct from the inlet to where the 2 blowers sit, is a diverging shape. If I fit a bell mouth at the inlet that is 12x12, the shape looks like a venturi.

    Q1. In this given condition, what is the ideal cfm that i can expect ?
    Q2. What if there were 4 or 6 fans in parallel ?

    Regards
    Dilip
     
  10. Dec 18, 2015 #9

    russ_watters

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    Please provide a drawing of the entire system. I can't picture it from your description and can't tell if you've made changes or just added elements to it.
     
  11. Dec 19, 2015 #10
    Dear sir,
    please have a look at the rough drawing and advice
     

    Attached Files:

  12. Dec 19, 2015 #11

    JBA

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    I have run an air flow analysis program using a round orifice with an equivalent area to your 12" x 12" square restriction and an orifice factor of 1 (100% efficient) that shows that there will be a best case 1 psia pressure drop across that restricted entry at a 20,000 scfm flow rate. Unfortunately, I don't know how that result may be applied to determine its effect on the fans' performance.
     
  13. Dec 20, 2015 #12

    russ_watters

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    Is this still connected to the system described in the first post? Do you have a fan curve for the fans? At what static pressure can they produce 1000 CFM?
     
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