How to Calculate Static Pressure in Exhaust Duct

If your ducts are sealed and connected, your static pressure will vary according to how many dryers are running...I don't see that you need an exhaust fan with the duct going straight up the hot air will rise...Lint buildup may be a concern on such a long run, and I would address the maintenance issue here...

http://www.captiveaire.com/MANUALS/AIRSYSTEMDESIGN/DESIGNAIRSYSTEMS.HTM#Fan_Static_Pressure

From Wikipedia:

where

is pressure head (Length, typically in units of m);

is fluid pressure (force per unit area, often as Pa units);

and is the specific weight (force per unit volume, typically N/m³ units) is the density of the fluid (mass per unit volume, typically kg/m³)

is acceleration due to gravity (rate of change of velocity, given in m/s²)

My gut feel is to plan for the lowest output (1 dryer running) and have effective backflow preventers at each dryer.

It seems to me this might create a problem with the exhaust fan drawing air through the dryer at an excess volume, cooling the dryer tub and increasing drying times.....

I don't know that we have concluded that an exhaust fan is really necessery. I guess we need to consider how much water is in the air.

Well I would vent the duct at the bottom....I would run the 4 exhaust ducts separately for the horizontal 20' run, then elbow up into the vertical run with the larger 16" duct left open at the bottom, this would allow natural convection...and place a drain pan underneath the duct to catch any condensate that might form....but that might not work here....

SolarEagle, thank you for the feedback!

We thought about that being a problem as well if keeping the fan always at a static speed. There were suggestions to use a DCV type of system where speed of the fan will be controlled based on the pressure increasing or decreasing in the duct. This would potentially elliminate the issue with underdrying the load.

How to Calculate Static Pressure - calculate the pressure loss in the duct, using your preferred method. I use Fanning-Moody (it has various names) where you work out the Reynolds No. etc and get a friction factor f.

Then Δp = 4*f*L/D*0.5*ρ*V².

Subtract that from the starting pressure, subtract the dynamic pressure, and you have static pressure. I estimate the pressure loss in the duct about 0.45 inch w.g. and dynamic pressure 0.14 inch w.g.

But more details needed. Does the inlet to the duct just come from the room, or is each dryer piped to it? You say each dryer produces 400 CFM. Does that mean each dryer has a fan? If so you may not need an additional exhaust fan (as others have said). If not and each dryer is piped to the duct, you need to know the pressure loss of the dryers and include that, and the duct static pressure is below atmospheric.

Each dryer pocket has a blower fan producing 400 CFM. Given that the total duct length is 95 feet i afraid that exhaust will not be able to escape the duct without an additional exhaust fan. Here is the quick sketch of planned installation.

http://flickr.com/gp/borisai/bD1w1F

Thanks!

Boris.

You need to know what Δp the fans are designed for, to judge whether they can cope with more. Is your arrangement normal, or does the dryer manufacturer expect the exhaust to go via a short pipe through the wall to outside? If the latter you can play safe by fitting an additional exhaust fan, rated for the duct pressure drop.

You probably will not need a fan but if you feel you do install an inline fan on the horizontal (you can get them in plastic) with a drain tap.

As Lyn mentioned if the fan is too big you will remove efficiency by pulling air through the dryer and even with back draft dampers on the other dryers you will probably pull them open as well. To much pull through the dryer vent could cause a negative pressure in the space and if they are gas dryers, this could affect their combustion air causing sooty burns again reducing efficiency.

If it were me, I would install the vent system with no fan and size your riser for the dryers MFT suggested cfm at the standard 0.1 friction loss per 100 ft this would be a 14" duct run. If you oversize your riser (larger than 14") you could get some "birdnesting" type of flows. Birdnesting is when too much cold air is in the riser causing a large static head on the dryers not allowing even a forced air dryer to vent properly then over heat. Or it could be a real bird nest on top of the vent causing the blockage (for that call Del the cat he will get those birds gone for you).

If you use a secondary fan for the dryer vent system, again, if it were me, I would base the run of the fan on stack temperature. If your stack/vent temperature begins to get too hot then you know that additional force/venting is required to remove the heat. This should also allow you to maintain some sort of efficiency as well.