estimating static pressure 1

[NCCaryME]

I need some advice on estimating operating conditions for a fan. I have a simple exhaust system moving air through approximately 180' of 66" round duct, through a fan, and then discharging the air into a stack. I know the desired air flow, and the inlet and outlet velocity of the fan. How can I use this information to estimate the staic pressure at the fan during operation?

 

[walkes]

If you can refer to ASHRAE Fundamentals Chapter 35 Duct Design. It will guide you through the pressure drop through various fittings.

Otherwise you can guess using a ductulator to get a ball park number.

 

[tombmech]

Yes, your round duct will have a straight-line static pressure drop per length, whereas fittings and transitions will typically have pressure losses an order of magnitude higher. Those fitting losses are empirical data and are tabulated.

SMACNA is another source besides the ones Walkes mentioned.

 

[NCCaryME]

Thanks for the sources. I think maybe my question is easier than this. Assuming I know (or can estimate) the pressure losses upstream and downstream of the fan, will that tell me the static pressure at which the fan needs to operate?

 

[walkes]

yes

 

[PhilBW]

The required static pressure for the fan is the total pressure drop through the ductwork before and after the fan. Your exhaust system will start at approximately atmospheric pressure at the inlet to your exhaust duct. The static pressure will steadily decrease because of the pressure drop as the air goes through the duct. The fan will increase the static pressure to something above atmospheric. There will again be a static pressure drop in the ductwork after the fan, and should reach atmospheric pressure again at the discharge. The amount the fan increases the static pressure equals all the pressure drop through the ductwork. That is how you start at atmospheric pressure and end up there as well.

One word of caution. If additional fans are discharging into the stack you mention, the stack could be slightly pressurized. It is not unusual to see 3-4" of static pressure in a stack. If that is the case, that must also be added to the static pressure requirements of the fan.

 

[cme]

Those tables in smacna and ahsrae are absolutely useless because the charts do not reflect system effects that are inherent in typical layouts.

Statics are really a stab in the dark however experience gives you insight to the true operating characteristics.

Its amazing on how many poor layouts are being designed even by "experienced" designers.

I go on the heavy side because I like to sleep well at night. Many equipment has VFD's and sheaves are inexpensive if you need to turn it down somewhat. It's going in the other direction that causes you pain.

 

[tombmech]

With respect, CME, you may be trying to make a legitimate point. However, calling ASHRAE and SMACNA fitting loss data "absolutely useless" is irresponsible - to say the least.

I strongly object to characterizing those organizations and their data that way.