Static Pressure versus Volume Flow Rate

[Jimbo10]

A new centrifugal type exhaust fan is installed to a duct system with any dampers fully open. With the fan (DOL) operating, the static pressure measured in the duct on a manometer is 300 pascals. Plotted on the applicable fan curve, this equates to a volume flow rate of 1800 litres/sec. A volume flow rate of 1100 litres/sec is required for correct operation. Could the static pressure measured in the duct be adjusted with a damper in the duct to reflect the required volume flow rate or is there no way that measured static pressure can be equated to volume flow rate?

 

[MintJulep]

The pressure values on the fan curve are deltas - the pressure across the fan.

You cannot equate a single-point static pressure measurement to flow.

 

[Jimbo10]

So if the fan is a roof mounted exhaust fan (no duct thereafter), wouldn't the discharge side static pressure basically be at atmosphere? If so, would the fan curve be applicable?

 

[lilliput1]

If you need to be precise, see if you can rent a calibrated balancing cone to measure the airflow at each intake. Make sure it comes with instructions. The device consist of an airflow measuring station with a field assembled hood that is placed over the air terminal so all air flowing in or out goes through the calibrated airfow measuring station. Using the fan curve and measured pressure drop to estimate flow will not be accurate unless there is adequate straight run of duct to not cause fan system effect deficiency.

 

[Jimbo10]

Unfortunately, it is near impossible to gain access to the exhaust intake (there is only 1). Would a pitot transverse test through multiple areas of a straight section of duct be accurate. If so, could you enlighten me as to which is the most cost effective item with which to do this?

I have a manometer with +ve and -ve inputs. Could a pitot probe be connected to the +ve & a static probe be connected to the -ve to provide me with the (nett) velocity pressure?

 

[MintJulep]

If one side of the fan is open directly to atmosphere (no ductwork), then a yes, a single static pressure reading is in effect a delta, since the reference pressure of the manometer is also atmosphere.

If that is what you are doing, then go ahead and just close down your dampers (assuming they are on the inlet side of the fan) until you hit the fan's static pressure that corresponds to you desired flow.

If you have a proper pitot tube, it will have two connections. One for the total pressure port at the tip, and one for static pressure ports located radially. Connected as you note, the preasure reading will be the velocity pressure.

 

[Jimbo10]

Thank you very much for your assistance in this matter. It is greatly appreciated.

I have not yet acquired a pitot tube. Could you recommend one that could easily be connected to my Comark manometer.

 

[MintJulep]

http://www.dwyer-inst.com/htdocs/airvelocity/Series160Price.cfm

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