Airflow volume pressure

HowardM · 21 Jan 2017

Not being a mathematical person, i am seeking guidance about airflow rates.
I know this is a well trodden path, but i have to try my own ideas.
The problem is, fans are usually free air rated, and i need the well known figure of 1000cfm at 10" vacuum.
That is my question really then, can it be calculated, what free air fan rating would be equivalent.
Thanks
Howard

sunnybob · 21 Jan 2017

Its not that simple.
The air flow alters depending on how long the ducting is. Friction slows the air which in turn increases the vacuum reading

The diameter, length, and fittings involved all alter the flow. 10" vacuum could be achieved with a reduced diameter pipe of a foot length, or the opposite would be a foot diameter and a 100 foot length.

Youre better off telling us exactly what you want to do, and then most likely a couple of us have already tried it.

Fitzroy · 21 Jan 2017

10" of vacuum is not very much, 1atm is 9.8m of water or 385" of water so a 10" vacuum is (385-10)/385 or 97.4% of normal atmospheric pressure, so 1000 (s)cfm ( or standard cubic feet per minute) is only 1000/.974 = 1026 (a)cfm (or actual cubic feet per minute) at the specified reduced pressure. How much air 10" of vacuum will move is then dependent of the size of the fan and the ducting, as Howard alludes to. For example most gas systems run with a gas velocity or 15-25 m/s, to transport 1026 acfm at a velocity of 20m/s you would require
1026/35.29/60 = 0.48m3/s
0.48 / 20 = 0.24m2 (x-sectional area of ducting)
square root of (0.24 * 4 / 3.142) * 100 = 17.4cm diameter of ducting (c. 6")

using http://www.engineeringtoolbox.com/press ... d_852.html you can calculate the specific pressure drop that will be experienced. Based on 1000cfm air, 3.28' (1m) length, 6" id duct, and 14.3psi (97.4% of 1atm 14.7psi) you get a pressure drop of .00631psi.

.00631 psi = .00631 / 14.7 * 385 = .17" of water.

So your system could have up to 10/.17 = 58m of equivalent 6" ducting before you would exceed the 10" of available suction and then flow would reduce (note fittings, bends, gates etc all have an equivalent lenght, ie a 90degree bend is equivalent to 5-8 pipe diameters)

For interest the same flow in a 4" pipe has a specific presure drop of .047 psi or 1.2"/meter, so you would see flow reductions after only 8m of ducting. Pressure drop at equivalent volumetric rate is a function of d^5! You see the need for large duct diameter in a HVLP system!

Regards

Fitz (woodworker by night, chemical and process engineer by day)

HowardM · 21 Jan 2017

Thanks for the formulae and the simplified explanation Fitzroy, its a big help.

I can move forward on the fan selection now.
Howard

siggy_7 · 24 Jan 2017

I'm a little intrigued as to how you know both your required airflow and the driving pressure required to deliver it. Anyway, your question is relatively straightforward to answer, but the blower needs to be specified based on both the unrestricted flow rate and the stall pressure. Fortunately, the stall pressure is quite easy to predict even if it's not specified - it's just related to rotational speed and diameter.

I presume that by 10" you mean 10" water pressure? I'm also going to take a small leap and guess that as you're posting this on a Woodworking forum what you're interested in is centrifugal fans like used in LPHV dust extractors. To achieve your pressure, you need a fan diameter of around 42cm assuming you are using a direct drive 3,000RPM motor (which all dust extractors I've ever seen in the UK are). A bigger fan would give you a higher stall pressure. Whilst you're very unlikely to get a specific performance curve that shows pressure vs flow rate for your blower in question, this page gives you a starter for 10: http://www.engineeringtoolbox.com/fan-types-d_142.html You can see that the maximum pressure of radial fans is typically reached at a flow rate of about 50% of the unrestricted maximum value. So, a blower with a diameter of at least 42cm and an unrestricted flow rate of at least 2,000cfm should meet or exceed your requirements.