# 101 in air flow dynamics please?

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1. Apr 21, 2015

### Nabil

Hi everyone,

I've got a newbie question which is probably more complex than I imagine so please bare with me. As a small DIY project I've trying to build a small exhaust filter system (A) for another device that produces undesirable fumes (B). I'm trying to work out if an axial fan (generates greater air flow) or a centrifugal fan (more air pressure) would be better.

The setup

• The exhaust (A) is connected to an inlet on the device (B) with an opening of an area of 4cm2. the outlet for A has an area of 20cm2
• Device B besides the small 4cm2 outlet has an opening on the top with an area of appox 200 cm2. It is otherwise a closed environment. It has a volume of approx 60,000 cm3 or 0.06 m3
• Environment has normal room temp of 21c and normal atmos. pressure.
The fans

Ideally I would like to be able simulate the effect of different extraction rates will have on the air flow in Device B. And considering picking up a solid works 30 day trail package to run some simulations. But for now to I've been trying to figure out what kind of fan would work best for my setup. My two considerations are:
• An axial fan rated flow rate of 18 CFM (0.514 m3/min) and max + air pressure of 0.12 inH2O (3.04 mmH20).
• A centrifugal fan rated flow rate of 4.5 CFM (0.12 m3/min) and max + air pressure of 0.39 inH2O (7.85 mmH20).
I'm guessing what I need to figure out is the resistance that will be caused by the 4cm2 inlet. I understand enough that the volume of space decreases, the air flow increases and pressure drops. But not sure what fluid dynamics equation I need to use to do my calculation. Bernoulli's equation seem like what I might need but not sure how to use it.

Any suggestions / advice would be most appreciated!

- Nabil

2. Apr 21, 2015

### SteamKing

Staff Emeritus
Any particular reason the connection between spaces A and B has an area of only 4 cm2? There's going to be quite a bit of restriction thru this connection.
Can this connection be enlarged?

3. Apr 21, 2015

### Nabil

Hi SteamKing,

It's to do with the design of the device. I might be able to double it to perhaps 8 cm2, but I'm interested doing the calculations. If even 8 cm2 is too restrictive than I might need to start again with the problem. I know the centifugal fan should be fine with it as the outlet hole on the fan is it self about 4 cm SUP]2[/SUP]. But the airflow is quiet low. Based on other things I have researched, I think I want to be emptying out the volume of space in Device B at least about 3 time every minute for which I need a flow rate of 0.18 m3/min ideally. So I guess what I'm really trying to figure out is, hypothetically will the axial fan cut it...

4. Apr 21, 2015

### MarkJW

Try this calculator for pressure drops across an orifice:

http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm#calc

This is specifically for an orifice plate, but is probably ok for an initial estimate as long as the chamber after the 4 cm^2 outlet is significantly larger than 4 cm^2.

Treating the inlet and outlet as circular, back calculate the radii (A=Pi r^2, r =SQRT[A/Pi])
Inlet = 200m cm^2 --> 7.98 cm
Outlet = 4 cm^2 --> 1.13 cm

Note that you can mix units on this calculator.

Case 1: 0.12 inH2O (axial fan)
Case2: 0.39 inH2O (centrifugal fan)

Q[CFM], case 1 = 1.01 CFM
Q[CFM] case 2 = 1.82 CFM

You would need to look at fan curves to see if either of those fans will operate under these conditions. The centrifugal might work. I suspect you need a larger orifice. I would scope out this before trying CFD to see if you can find a design space that works.

Next step after determining your pressure drops and fan compatibility is to calculate air changes per minute and compare this with the expected fume generation rate.

Good luck!

5. Apr 21, 2015

### jack action

First, you need to define the volumetric flow you need. Once this is done, you can start estimating the pressure drops across every components. This will give you different pressures reading like this:

Where:
P3 > Patm > P1 > P2;
P3 - Patm => pressure drop across filter;
Patm - P1 => pressure drop across entrance;
P1 - P2 => pressure drop across duct;
P3 - P2 => pressure drop across fan (what you need to know).