4000 CFM fan with 3 in static - Please check

[Rumananoor]

Selected 4000 CFM fan with 3 in static for supply air normal application.

Please check if my calculation is correct I have a 20”X14” duct with 113ft running and three 90 degree elbows and two 45 degree elbows.

Thanks

 

[mech5]

It seems a little high to me. According to your numbers, your velocity is around 2200 fpm. At this velocity for your size of duct, you have around .36 in WC loss per 100 ft of duct. Using 1.5*113 ft to account for the elbows, I get around .6 in WC.
Now, this is just for the duct mentioned.
Have you take into account:
-fliter losses?
-AHU/fan losses?
-dampers?
-registers?
All this needs to be taken into account when specifying the ESP for a fan or AHU fan.
I hope this helps.

 

[ElSid1]

Don't forget to specify if this is SCFM or ACFM! Major headache for me when reviewing others work. Their is a difference!

 

[Rumananoor]

Thanks Mech5,

Will you please explain the way you calculated? It will be really helpful for me.

Any easy calculation methods? Thanks in advance

elSid1 -Its SCFM.Thanks

 

[ElSid1]

Rumananoor,
It is best if YOU post your work. Easier to correct and see your assumptions. Mech5 gave you the basics items that most forget.

 

[mech5]

I used a Ductilator to arrive at the numbers for the pressure drop per 100 feet and the velocity. You can use a manual one or an electronic ductilator. For the duct length, I used the length you gave us and multiplied it by 1.5 (to account for any elbow, turn, etc...). Now keep in mind this is a rule of thumb for quickly calculating the ESP. If this is a critical application, you must go into more detail and account for EVERYTHING in the system. Now, you stated that this was for normal supply air. This ESP seems kinda high. You typically don't see this amount of ESP for normal supply air. For normal supply air, typically it is in the range of 0.25 to 1.00 or so. You have to be careful with the static pressure because if you specify a fan that has 3 inches of static pressure and the contractor installs a typical gauge duct for supply air, then you will have a problem. The duct will balloon and pop all the seams along with the insulation creating a problem for everyone.

 

[Rumananoor]

Hi mech5,

Thank you for your kind email and clarification of your calculation. It is great to know that you are very detailed in your approach in calculating the static pressure.

Based on my calculation the fan is for a kitchen fan so the static loss at the hood is 1 in. WC and the losses at the bends are greater since they are without veins. I did not calculate the loss at the fan and I did add 10% as a safety. I recalculated the losses again based on SMACNA and achieved a static pressure of 2.8"?

Regards,

 

[ChasBean1]

Replace "calculations" with "estimations." Real conditions, relative roughness of duct, etc. are standardized in the "calculations." 2,100 cfm is too high by experience. Air noise/velocity is excessive. You need a larger duct.

 

[sherifheity]

i want to add to all the above , you have to check what is the air outlet?.....i saw jet nozzles having 200 Pa Pressure drop!

i suggest if you feature all the duct accessories you have from the AHU discharge up to the air outlet.

regards,

Sherif Abulfetuh