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CFM Calculations For New Tapping

05/14/2017 3:15 AM

We have a blower of capacity 5000 cfm. It's suction line is of size 16 inch. The line have 10 no. Of tappings of size 8 inch. We have to provide 3 more tappings of size 6 inch. What will be the cfm we will get at that tapping???

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#1

Re: cfm calculations for new tapping

05/14/2017 5:35 AM

Nobody can answer that without a lot more detail.

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#2

Re: cfm calculations for new tapping

05/14/2017 9:12 AM

Think about all the ways these 'tappings' could be configured and how these different configurations could affect the air flow. And since you've not said anything at all about this, there is no way your question, as stated, can be answered.

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#20
In reply to #2

Re: cfm calculations for new tapping

05/16/2017 9:12 AM

They will knock three times on the door, then comes the end. If you are not Dr. Who, it is not really a big problem to solve.

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#3

Re: cfm calculations for new tapping

05/14/2017 10:17 AM

Will the tappings be 1 meter or 1 km long? Will all of the vents or some of the vents be open? Are there any louvers or other flow restrictors? Insect nest? Filters?

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#4
In reply to #3

Re: cfm calculations for new tapping

05/14/2017 11:39 AM

16inch line is 300 feet long. 8 inch tappings are placed at a distance of 20 ft from each other and of length 12 ft each. Tappings are provided with damper at it's end.

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#6
In reply to #4

Re: cfm calculations for new tapping

05/14/2017 9:30 PM

My questions were intended to make you think instead of answer. Unfortunately for you, you choose the latter.

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#21
In reply to #4

Re: cfm calculations for new tapping

05/16/2017 12:05 PM

OK, but:

- how far from the blower is the closest 8" tapping to be ?...

- where are each of the 6'' tappings to be located?...

- length of 6" tappings ? ...

- cross-sectional dimensions, materials, intrusions, etc. ? ...

- [ { 300' - ( 20' /tapping x 10 ea. = 200' ) } = 100' ] => are these remaining feet to remain un-tapped, or what? ...

(seems like at least one of your previous homework assignments should have addressed this sort of thing. Yes/no/other ?...)

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#5

Re: CFM Calculations For New Tapping

05/14/2017 4:14 PM

We have to provide

Just putting it out there, if you need the tappings why not put them in and measure the resulting cfm in this particular real world application? You can always take one or more tappings back out if the cfm is too low or increase the blower capacity.

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#7
In reply to #5

Re: CFM Calculations For New Tapping

05/14/2017 10:03 PM

I suppose to have calculate it theoretically first.

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#13
In reply to #7

Re: CFM Calculations For New Tapping

05/15/2017 3:01 AM

Why, if providing them is obligatory as previously stated?

What happens if the real world, once assembled, does not agree with the calculation?

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#8

Re: CFM Calculations For New Tapping

05/14/2017 10:30 PM

About 150 cfm give or take....

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#9
In reply to #8

Re: CFM Calculations For New Tapping

05/14/2017 10:47 PM

Thanks for the answer. I need a calculations or steps to calculate it.

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#10
In reply to #9

Re: CFM Calculations For New Tapping

05/14/2017 11:25 PM

Is this for a homework assignment or will your boss not accept an answer from an online forum.

<sigh> Have you tried an internet search for the math to perform the calculations? If you have been asked by someone in your company to do this and are going to take credit for the answer you really should put some work in yourself. We don't do homework here sorry.

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#11
In reply to #9

Re: CFM Calculations For New Tapping

05/15/2017 12:40 AM

Well in a theoretically straight duct ignoring static loss you can determine the cfm per square inch....take the total square inches of outlet and divide it into 5k, then multiply it by the square inches of each outlet...of course this would just be theoretical in nature...Your velocity in in a 16" duct would approach 60 feet per second at 5000 cfm, basically a jet engine...the static loss would be significant and the duct design would be critical...

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#19
In reply to #9

Re: CFM Calculations For New Tapping

05/16/2017 3:14 AM

No-one here is stopping any of you at that facility doing that.

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#12

Re: CFM Calculations For New Tapping

05/15/2017 2:57 AM

10 X 8" = 80"; 5000/80" = 62.5; 62.5 X 98" = 6125 CFM; I do not know it is correct or not.

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#17
In reply to #12

Re: CFM Calculations For New Tapping

05/15/2017 12:00 PM

The method to solve this can be approximated by considering the flow as incompressible, but each section of duct is calculated with the velocity and the friction loss, including the loss at each branch.

You have a velocity head loss at each opening, consider as entrance loss, plus branch flow tee loss, and tee run friction loss, all velocity dependent.

Each duct section has its own velocity, so the losses are different at every place in the duct.

And just to make it more difficult, the compressor you are using may move a different amount of air depending on the pressure at the suction. So then you match two equations, the actual flow the compressor develops given a suction pressure, and the suction pressure you get when moving a certain amount of air through the duct. At one place, both values are the same, that is the solution.

So the values you need are air flow friction losses for round duct, and fittings, and velocity head (Vsquared/2g) calculation where the pipe changes size.

The flow through any opening along the trunk line will be different, unless the duct tapers between ports to keep the velocity constant. If your trunk line was very large compared to your taps, you could ignore those differences, but that is not your case.

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#18
In reply to #12

Re: CFM Calculations For New Tapping

05/15/2017 12:05 PM

The formula for the area of a circle is pi * r(radius) squared....so an 8" duct would have 3.142 * 16 = A

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#14

Re: CFM Calculations For New Tapping

05/15/2017 3:06 AM

One may presume from the way the question is worded that there are several others that might be involved in this particular project. Surely, then, this is a topic for discussion and arrival at a decision amongst the <...We...> rather than a bunch of complete strangers at a global Engineering forum?

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#15

Re: CFM Calculations For New Tapping

05/15/2017 4:54 AM

<...What will be the cfm...at that tapping?...> Going past it or going down it (rhetorical question - NNTR)? It depends upon whatever is downstream of the tapping that can present resistance to flow. If an installed isolation damper is closed, for example, then the cfm will be zero and the upstream flow will redistribute itself around the other tappings.

How could the forum possibly answer this question (rhetorical question - NNTR)? After all, the current installation cannot be seen from here.

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#16

Re: CFM Calculations For New Tapping

05/15/2017 7:22 AM

It's Its

Grammar is the difference between knowing your sh*t, and knowing you're sh*t.

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#23
In reply to #16

Re: CFM Calculations For New Tapping

05/16/2017 1:57 PM

Let's eat, grandma.

Let's eat grandma.

Proper punctuation can save a life.

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#22

Re: CFM Calculations For New Tapping

05/16/2017 12:20 PM

In spite of how hard you try, you just cannot, will not, should not suck the world through a straw. It does not matter how hard you blow. Try playing a harmonica for a week, and see if you don't at least suck once.

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