sizing ductwork for exhaust fan 1

[brownrc]

Hello,

It was suggested by a coluege that I visit this forum for some help. I need advise regarding the sizing for some exhaust ductwork in my fab shop.

I need to vent a work area, and need to use two 16 inch exhaust fans that I have in the shop already. They are mounted side by side. It will require 25 feet of ducting to get to the work area.

In the HVAC field, is there a calculation that someone can explain to me to properly size this duct to minimize the loss in airflow from having too small of a duct size?

The fans are grainger #3xk40
can be viewed here:

http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId=1611631423

I look forward to any help you folks can lend,

thank you,
ryan

 

[imok2]

This is one example:
Flow Rate 2200 CFM
Velocity 1400 FPM
Pressure 14.7 PSI
Temperature 100 F
Duct length 25 FT
Duct Roughness 0.0003 FT

These are the results
The duct diameter is: 16.97 inches
The air density is: 0.0709 pounds per cubic feet
Reynolds Number = 185772.47
Friction Factor fa = 0.01719927
Friction Factor f = 0.01735941
loss PSI = 0.00127703
loss inches of H2O = 0.03538384
loss inches of H2O per 100 ft = 0.14153536
For more options go here:

http://www.connel.com/cgi-bin/airduct.pl

 

[walkes]

If you are trying to capture anything, like welding smoke you will likely have a problem. If you need any type of capture velocity you may have a problem with pressure drop at the duct inlet. You also have to take into account the effect of backdraft dampers or motorized dampers in the system. By the time you add up all of your pressure drop you may be in an unstable region of fan operation (check fan curves).

 

[chrisirish1973]

Dear brownrc;

I'm an HVAC prefessional engineer. I do not recommend attaching ductwork to the fans you listed above. These fans are designed to operate without any ductwork and do not have enough power to pull air through a ductwork system. I afraid if do attach ductwork to these fans, you have very little airflow and burn out the fans in a short amount of time.

Call a fan manufacturer distributer and explain your situation and he can probably recommend a new fan with some ductwork sizes. The system you request is not difficult to design so most people in the HVAC business can help you.

Chris

 

[KiwiMace]

ChrisIrish1973 is spot on - these are propeller fans - note the flow rate spec drop significantly at only .25" pressure.

The type of fans you'd typically need for ducted installations are axial, inline centrifugal or mixed flow. If this is important to you, get this system designed for you.

With the provision of a properly sized intake grille located on the other side of the space, you can use these fans to generate a decent amount of cross-flow ventilation. Roughly you want about 12-14sqft of grille per fan.

Prop fans often have nasty stall region so you want to make sure that they work to a low static pressure or they'll cycle between two points on the curve - dammed annoying if you work under one, and not very effective in a ventilation sense.

 

[brownrc]

Thank you all for your explainations. I was pleased to see so many people helping me. Unlike yourselves, I am obviously not an HVAC professional and I specialize in another field.

I understand what you say about the fan curve not being proper for this install. Yes I do require capture velocity.

Is there any way that I could mount a suitably sized "booster fan" at/near the duct entrance to make this acceptable?

See proposed picture at:

http://www.ryanbrownracing.com/pics/fan_ductwork.GIF

Again, thanks for the help... I like to be aware of the concepts to this project before I call a professional in the area,

ryan

 

[KiwiMace]

This fan isn't going to work ducted, Ryan. You need a different fan. You'll likely also need separate systems for localised exhaust and general exhaust and ventilation.

 

[chrisirish1973]

I don't recommend installing two fans in series. It is best remove the existing fan and install a inline centrifugal fan with some ductwork.

I'm assuming you are only using this to remove heat. If you are using this in a garage setting where a high level of combustable fumes are be exhausted, you might want to have an explosion proof fan motor. This is a spark resistant motor that won't ignite fumes in the exhaust airsyteam.

You might want to consider hiring an consultant engineer. They are sometimes listed in the yellowpages. This might be an overkill but I not sure what are you are doing.

 

[brownrc]

OK, I will abandon the extended duct idea. I appreciate your patience, since the vast majority of your forum does not deal with people who are of the "try it and see how it works" mentality. I know when those folks wander into my "world", it can be frustrating... but once and awhile, they actually teach us something!

The reason I thought it might work is because right now, the two fans are at the top of a 70" wide x 28" deep x 168" tall duct that pulls air out of an engine dyno room.

The room is fed outside air by two blowers and the whole package works extremely well. Although that practice may be frowned upon from an engineers standpoint, in practice, for the last 8 years the room moves a ton of air thru it, even with just the fans on, all with their original motors.

But then again, maybe a 28" x 70" passage is not considered a "duct", so therefore there would be no real restriction to it, as I have never used a manometer to see what the pressures are in the system... it just plain worked.

If this was to get your "blessings", I would have tied into the existing fans, as that would have been much more convenient... but I shall heed your advice and install a fan much closer to the new work area (a chassis dyno) and look at installing a similar type of exhaust fan, but without the ducting.

ryan