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Flow Rate Calculations

Flow Rate Calculations

Flow Rate Calculations

Hi guys,

I am new engineer how is working on replacing Fume Hood inside a lab with a new one.

I would like to know how to do the required flow rate so that I will select the blower accordingly. We are shooting for 100 f/min velocity through the face of the hood. I also I would like to know how to find the total static pressure.

The duct diameter is 10 inches.
Please advise me
Thank you very much

RE: Flow Rate Calculations

These calculations are valid for a canopy type fume hood

Flow through hood

Q2 = 1.4 * P * D * V   {cfm}


P = perimeter of tank or table below the hood {ft}
V = capture velocity (100 - 150 fpm)
D = free space above tank or table  {ft}

When the hood is above a hot area, an additional flow Q1 of air or vapors is


To remove the vapours it is necessary that Q2 > Q1

For practical values   Q2 = 1.2 * Q1

Calculation of Q1

Valid for D <= 3 ft

1) Hood above hot area without vapor generation

 Q1= 5.4 * S * \3/--( h * dt1.25)  {cfm}

S = Area hot plate { ft2}
t = temperature difference hot plate and ambient air {oF}
h = heigth hot plate above the floor {ft}

2) Hood above tank with hot liquid (vapor generation)

 Q1= 290 * S * \3/--( h * G)  {cfm}

S = Area top side of tank { ft2}
G = Vapor formation {lb/ft2/min}
h = heigth tank with hot liquid {ft}


Steven van Els

RE: Flow Rate Calculations

hi there
i am constructing a natural drain  here on my farm
the top of the drain is 12 ft wide
    width of bottom is 2.7ft
    gradient{fall}     420
    depth of drain is  5 ft
can someon eplease calculate the flows  in GPM  when the water depth is  2ft,3ft,4ft

regards and awaiting a reply

John Lochhead
email is jlochhead@fencepost.com

RE: Flow Rate Calculations


You should start your own thread.  In it you should also state the length of the channel you are constructing as well as whether it will be concrete, grass, gravel, dirt, etc.

I'm assuming that the depth you are concerned with is the depth at the very beginning of the channel (drain).  If you supply the additional data I've described, the flow at the end of the drain can be calculated.

I'm also assuming that the dimensions (width, depth)of the drain are uniform from beginning to end.


RE: Flow Rate Calculations



for some information.

RE: Flow Rate Calculations

Greetins MeEn,

I would like to suggest to you that you provide plenty of isolation valves and locations for low point drains when you do have to isolate sections of piping. An old Maintenance Plumber told me that the first time you use a valve the valve has paid for itself.

Trying to plan for the future is almost impossible...... but something you might be able to do is this: Contact the people who will be actually using and maintaining the system... ask them where they would like valves / drains / filters / room for access or maintenance.....

You are very fortunate to have this opportunity to design this system and it sounds like you are working hard to make it a good one !

Good luck,

RE: Flow Rate Calculations

what are the steps to design piping for refinery

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