Calculating CFM

[time]

I have a PACE air extraction unit that is rated at 167 CFM. I am limited to a maximum duct length of 16 feet per port (there are four ports per unit). I need a duct length of at least 30 feet, and still maintain adequate air extraction for solder stations. Could I vary the duct sizes and still have adequate air extraction?

Thanks,

Time

 

[ChasBean1]

Yes. But to keep the duct sizing from having to be an exact science, provide a volume damper at each of the four branches. This way, your main could be a standard size (e.g., 8 inches) and each branch can also be a standard size (e.g., 4 inches).

 

[abeltio]

What the vendor is saying is that with a certain size of duct (e.g. equal to the size port) you can only have 16ft of ducting per port.
If you increase the duct size you are limiting the pressure drop so the net effect is to have the same pressure drop as with the duct size the vendor had in mind.
If I understand your question correctly...in other terms your problem is: what is duct size that with double the length (30/16 ~ 2) will give the same pressure drop as a given duct size, maintaining the flow constant.

So... considering circular ducts
H = constant

1. H = f1 *( L/D1)* (V1)^2
2. H = f2 *(2L/D2)* (V2)^2 ; same dp with double length of duct

Q1 = Q2; flow constant (depends on renovations/hr required)
V1*A1 = V2*A2
3. V2 = V1*(A1/A2) = V1 *(D1/D2)^2

Solving for D2:

D2 = [(2*f2/f1)^(1/5)]*D1

f1 = friction factor for D1 @ V1(depends on reynolds number)
f2 = friction factor for D2 @ V2

The above equation requires some iteration... you assume a D2 diameter, figure out f2... put it in the equation until the equation is satisfied.

If D2 starts to get too big...you may have to consider a bigger extractor.

Of course after you figure out your duct size with this method you have to check again the whole system and confirm the new duct size.
Ideally you should have the extractor a bit oversized so you can install the dampers mentioned by chasbean1 to balance the whole system otherwise the extraction at the most distant station will be significantly different from the flow at one nearest to the extracion unit.
HTH
Saludos
a.