Static pressure to measure airflow? 3

[chims]

Has anyone ever used static pressure as a method of determining airflow?

Example: Seal the outside air damper up tight, so the return will equal the supply airflow. Measure the static pressure in the return air duct at the inlet of the AHU. Then use the fan laws Q1/Q2=square root of (P1/P2). If you need a specific volume of outside air then you can determine what your return air needs to be in order to get the require outside air. Then you control the static in the return duct to the new static pressure determined by the fan laws. It seems pretty simple to me and cheaper than airflow measuring stations, but I'm wondering if there other caveats to be aware of.

 

[ChasBean1]

Hence the statement, "attach a VAV system to it and all goes out the window." According to Chims, this is a simple and hypothetical system. It does have one inlet, one fan, one outlet. I understand the system effects, but I think the point was whether we can use pressure change during throttling as a means for estimating flow change (temporarily without regards to the attached system). I think we can.

 

[quark]

My point is exactly what MintJulep stated in his Jan21st post. The fan system is very dynamic. If we calculate the pressure with respect to a certain flow rate and then close the damper partially and allow more fresh air, two things happen. Pressure in the return duct gets reduced due to reduced flow and increases because of the obstruction created by controlling damper(plus when we open the fresh air damper, the thing becomes even worse). So, I think it is difficult to correctly assess the flowrate in the two duct works.

If no system is attached to the fan (or if the system is not recirculatory type) then it is quite possible and it is by the same method we check flow rate of a pump when no metering system is available.

But this is really interesting discussion and if possible I would like to have a copy of the ASHRAE article either from the original poster or any of you (Ufortunately my ASHRAE Membership started from June, 2003).

Regards,

 

[chims]

Quark,

Let me know what your email address is and I can send you a copy of the the ASHRAE article that discusses this issue.

Chims

 

[quark]

builblock1@yahoo.com

Thanks in advance,