PDA

View Full Version : Help with Duct Sizing

mechie09
06-15-2007, 02:38 PM
I'm adding a piece of ductwork to an industrial oven. On the supply side, there is a supply fan, a filter, an elbow and about 10 ft of straight duct.

Current there is 1700 SCFM of air going through the straight circular duct. But that is not enough to meet the requirements, I need 3901 SCFM going through there (it's a 12 inch circular duct).

So I am going to need to increase the size of the duct to meet the volume flow rate. But I do not know how much to increase the duct size by.

By the way, I don't really understand the theory behind how static pressure works for this sytem so if somebody can explain that as well, that wold be great.

Help is much appreciated

Mstrav
06-15-2007, 05:47 PM
you have a photo? what hp (horsepower) is the motor?

there are lots and lots of variables, is this for food?

mechie09
06-16-2007, 07:26 AM
Sorry, I have no photo.
The fan is about 30 HP.
It is not for food. It's for baking metal.

I'm looking for a method of how to solve this kind of problem instead of the solution.

joken
06-16-2007, 12:08 PM
2.75 inches of static = about 4000cfm thru your 12" round. Your fan may do it but the velocity may cause issues. Bigger duct = same volume with less velocity. Check the specs on the fan. If you have no fan data I would check the motor amperage and see if you can increase the fan speed with different sheaves. Might be cheaper to pay a balancer for an hour to solve your problem. Static pressure in a duct is a pressure measurement in inches of water colum. Ken in Oregon

emcoasthvacr
06-16-2007, 09:16 PM
Google "fan laws" "Duct sizing" and you should get the fan laws published by fan manufacturers and info for Duct sizing.

Fans are tested according to ASHRAE and AMCA standards.

Since you know the flow rate, HP or RPM, and current diameter, you can calculate the required flow rate needed. If you use HP, make sure you account for the efficiency of the fan, which varies with manufacturers -- using RPM is more straight forward.

For example, duct systems use the equation (change in pressure 2 / change in pressure 1) = (volumetric flowrate 1 / volumetric flowrate 2) squared

Then use a Ductulator to size the duct, or ASHRAE Fundamentals for Duct sizing.

I would reference the manufacturer of your fan though.

kdprice111
06-16-2007, 11:12 PM
If you are getting 1700 cfm out of 12" pipe your static pressure is around .5 inches. In order to get 3900 cfm you will need to increase pipe size to 18". At this size you will have your FPM around 2600 FPM / 1700 CFM you were getting 2100 FPM, so you sould have know problem with volume. Make sure the motor can get to that FPM, so you are going to have to amp motor and make sure you are under nameplate FLA. Even if it is close you may need to change sheave to change motor speed while staying under FLA, because I doubt your motor is rated as a countious duty motor, so it could be easy to harm motor windings.

mechie09
06-18-2007, 08:04 AM
Thanks for the help so far. Two questions:

1. For those people who say 1700 scfm = so and so static pressure. How do equate the two? Is there a formula that gives you the static pressure based on cfm?

2. For my analysis, I was going to find the static pressure of the fan, which I have based on manufacture specs. Then, find the static pressure loss due to friction of the duct I size and make sure they're equal. Is that correct?