Calculating air flow through a tube

[mess]

TL;DR Summary

I have a fan blowing air into a tube, I would like to learn how to calculate the air flow so that I can figure out efficiency/losses between fans (axial, centrifugal) and tube diameter/length.

This is for a campervan project I am working on, I am trying to take outside air that is cold (Canadian winter) and mix it into the return air in my diesel heater. I need to hit a specific minimal CFM to maintain proper Co2 levels inside and I would like to be able to calculate this.

The axial fan I am using is 30 CFM, the tubeis 1.5" in inner diameter and the length is 4 feet. I don't have any pressure measurement's but I assume that would be needed? I can probably get an air pressure sensor and stick it in the tube just after the fan if its what is needed.

 

[bob012345]

There is a web based fluid flow simulator you can use called SimScale. They have hundreds of examples and one may be close to what you want which you can copy and modify and run all for free up to 3000 core-hours of processor time.

https://www.simscale.com/

 

[anorlunda]

This is for a campervan project I am working on, I am trying to take outside air that is cold (Canadian winter) and mix it into the return air in my diesel heater. I need to hit a specific minimal CFM to maintain proper Co2 levels inside and I would like to be able to calculate this.

To calculate it, you need to the inside pressure. Blowing cold air in could increase pressure, but hot gases going up the chimney could decrease pressure (unless you leave windows open.)

 

[bob012345]

Alternatively, here is a flow calculator that might be useful;

https://www.tlv.com/global/US/calculator/air-flow-rate-through-piping.html

 

[russ_watters]

The axial fan I am using is 30 CFM, the tubeis 1.5" in inner diameter and the length is 4 feet. I don't have any pressure measurement's but I assume that would be needed? I can probably get an air pressure sensor and stick it in the tube just after the fan if its what is needed.

You need to obtain a performance curve for the fan ("fan curve") to obtain the pressure available to push through the ductwork at different flow rates. Here's a static pressure calculator:
https://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html
All of the elements of the system matter, though, so you'll need pressure drops for every component.

My gut tells me this could be a problem, because a 30 CFM axial fan sounds like a computer case fan, and they generate close to zero static pressure.

On the other hand, how much airflow are you expecting to need (is this system the outside air only part?)? How many people and how many CFM per person are you calculating? I wouldn't expect a van to be tight enough for CO2 to be an issue (though water vapor/condensation maybe).

 

[jrmichler]

Does your axial fan look like this?

Because if it does, it probably has a fan curve something like this:

You can find the pressure loss in your tube by searching duct loss calculator. Here's one that looks good: https://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html. I plugged in 15 CFM, 1.5" diameter, and 4 feet long, and it said 0.1" w.c. pressure loss. That number sounds right to me.

If you have a size reduction at the beginning, add in the velocity pressure for the entrance loss. The velocity pressure for the same conditions is 0.09" w.c., and the total fan pressure needed is the sum of the velocity pressure and the duct friction loss. The total fan static pressure needed to push 15 CFM through your 1.5" duct is thus 0.1 + 0.09 = 0.19" w.c. If you want to measure these pressures, there is no need for a pressure gauge. Use a water manometer. A water manometer is a piece of clear plastic tubing, bent into a U-shape, and with some water in it. The pressure is the difference in water height between the two sides of the U, and is measured with a ruler.

Velocity pressure can be calculated from the Bernoulli equation, but an easier method used in HVAC work is Velocity pressure = (Velocity in FPM / 4005)^2. The velocity pressure is 1.0" w.c. at 4005 FPM (feet per minute), and is proportional to the velocity squared.

 

[mess]

You need to obtain a performance curve for the fan ("fan curve") to obtain the pressure available to push through the ductwork at different flow rates. Here's a static pressure calculator:
https://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html
All of the elements of the system matter, though, so you'll need pressure drops for every component.

My gut tells me this could be a problem, because a 30 CFM axial fan sounds like a computer case fan, and they generate close to zero static pressure.

On the other hand, how much airflow are you expecting to need (is this system the outside air only part?)? How many people and how many CFM per person are you calculating? I wouldn't expect a van to be tight enough for CO2 to be an issue (though water vapor/condensation maybe).

I have several fans ranging between 20-150cfm, some with the computer fan design but higher power, and some centrifuge fans from vacuums that do create a good deal of pressure. but I also need it to be quiet that's why i need to find the best fit.

The calculations from another post say i need about 14cfm under normal circumstances for 1 person, probably about 9cfm when I am sleeping.

Here is my original post showing CO2 becomes an issue within just an hour or less:
https://www.physicsforums.com/threa...entration-in-a-campervan.996832/#post-6426144

 

[cmb]

'Return air in my diesel heater'. Not sure what this means.

Are you trying to pipe exhaust gases into your camper? Where is this CO2 coming from?

 

[mess]

'Return air in my diesel heater'. Not sure what this means.

Are you trying to pipe exhaust gases into your camper? Where is this CO2 coming from?

It means that the air which the diesel heater intakes for heating (not combustion) is being mixed with the air I am pulling from the outside through a vent :)

And nope, exhaust gases are all outside the van. CO2 is coming from my breathing.

 

[mess]

Does your axial fan look like this?
View attachment 273757
Because if it does, it probably has a fan curve something like this:
View attachment 273759
You can find the pressure loss in your tube by searching duct loss calculator. Here's one that looks good: https://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html. I plugged in 15 CFM, 1.5" diameter, and 4 feet long, and it said 0.1" w.c. pressure loss. That number sounds right to me.

If you have a size reduction at the beginning, add in the velocity pressure for the entrance loss. The velocity pressure for the same conditions is 0.09" w.c., and the total fan pressure needed is the sum of the velocity pressure and the duct friction loss. The total fan static pressure needed to push 15 CFM through your 1.5" duct is thus 0.1 + 0.09 = 0.19" w.c. If you want to measure these pressures, there is no need for a pressure gauge. Use a water manometer. A water manometer is a piece of clear plastic tubing, bent into a U-shape, and with some water in it. The pressure is the difference in water height between the two sides of the U, and is measured with a ruler.

Velocity pressure can be calculated from the Bernoulli equation, but an easier method used in HVAC work is Velocity pressure = (Velocity in FPM / 4005)^2. The velocity pressure is 1.0" w.c. at 4005 FPM (feet per minute), and is proportional to the velocity squared.

Thats very helpful thank you! so seems i will have to go with a centrifugal fan to get that pressure up. I will be placing a barometric pressure sensor in the tube after the van so that i can get an idea of the flow rate

 

[mess]

If you have a size reduction at the beginning, add in the velocity pressure for the entrance loss. The velocity pressure for the same conditions is 0.09" w.c., and the total fan pressure needed is the sum of the velocity pressure and the duct friction loss. The total fan static pressure needed to push 15 CFM through your 1.5" duct is thus 0.1 + 0.09 = 0.19" w.c.

I understand the Friction Head Loss in Air Ducts - Online Calculator provided .0979 with those parameters, and so i see how you got 0.1, but i don't understand where you got the 0.9 from. I don't understand the size reduction at the begging part you mentioned.

I also tried looking up velocity pressure as you mentioned but didn't find much, is that the same as dynamic pressure?

 

[jrmichler]

Velocity and dynamic pressure are two names for the same thing. The Wikipedia entry for dynamic pressure has a good explanation: https://en.wikipedia.org/wiki/Dynamic_pressure. The Wikipedia article has a nice diagram in it. When you transition from a large duct to a small duct, the air speeds up. It takes pressure to make the air speed up. That pressure is in addition to the friction loss in the duct.

Here is a blower that would deliver 15 CFM through your 1.5" duct:

It was found in a quick search in Digikey. It is rated for 12 volts, draws 4 watts of power, and costs $18.00 plus shipping. Since the blower outlet is about the same size as your duct, the velocity pressure loss would be negligible, and you would need only a simple transition piece to connect it to the duct. There's more information in the datasheet: https://www.delta-fan.com/Download/Spec/BFB0712HH-A.pdf. It's not the only blower available, it's the first one I found in a quick search. Digikey is not the only supplier to check, Newark and Mouser may also have blowers.