air flow and air pressure measurements

[mechanicaldup]

Where can I guidelines on the correct way of doing air flow and air pressure measurements?

What do you recomend to monitor the operation point (pressure or flow) of a fan.

<With pump installation you normally use a ordinary pressure gauge.>
What sensor would you recommend?

 

[MintJulep]

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

 

[dpoppeli]

Good basic description but.. geez, who still uses inclined manometers to measure pressure? Seems like Dwyer would put a plug in there for there Magnehelic gages.

 

[tombmech]

Actually, an inclined manometer is still one of the most precise ways to measure flow. The principles involving Total Pressure, Static Pressure, and Velocity Pressure are self-evident by the physical representation of the manometer. So, it is an unparalleled teaching tool.

I agree that its value in the field is limited. Although, its use in precision applications is not uncommon, and spying an inclined manometer fastened to the side of a duct or air movement device is not a singular event.

The problem in measuring flow directly exists with every instrument, including the manometer: the velocity field. Friction and turbulence are a reality in every instance. At best, the velocity field may be laminar with some small fall off at the edges of a duct. Regardless, methods to measure this field are well documented: a duct traversal is performed and a sampling grid over the cross-sectional duct area is created. This is fine for field measurements with a handheld instrument, but what about for permanent control?

Often, elaborate annubars or circular flow rings are used to sample the flow field with statistically-leveling multiple sample points. However, turbulence still leads to some unreliability and inaccuracy. To limit this error, straight distances of undisturbed flow are required before and after the sampling grid location. This puts a limit on location flexibility, and the devices become structurally inconvenient. All of this adds cost. Further, ducts may have particle contaminants. Even dust can affect readings. Some mfrs developed positive pressure annular annubars for industrial processes, but I have even seen these type clog up.

The most reliable method of control is pressure - specifically, the measurement of static pressure. Pressure sensors are reliable, precise, and cheap. Once flows are established through Test and Balancing (measurement), static pressure becomes a reliable indicator of flow - depending on the rating of the fan. However, even if flow vs. velocity varies, the static pressure remains a reliable control of fan performance. Some applications combine documented ratings with a static pressure sensor and current measurements on the fan power. That nails it pretty good for a tight, closed control loop.

 

[dpoppeli]

Pressure-only measurement is not going to tell you the same thing as velocity/flow measurement. I think the original poster needs to elaborate on the application however.

 

[tombmech]

With respect, that's what I stated if you read carefully.

I agree he needs to elaborate.

 

[quark]

Flow control is more apt and predominant in pharma industry. Pressure control, after established balancing, can be ok if the system is operated at constant pressure. The typical pressure drop across filters vary in the below given limits approximately.

10 micron - 20 to 80 Pa (clean to clogged)
5 micron - 20 to 80 Pa
3 micron - 50 to 150 Pa
HEPA (EU12) - 250 to 750 Pa

For systems with HEPA, when controlled on pressure, either we have to run the system at redundant flowrates or system is to be balanced time and again. The current practice is to use flow control with variable speed systems.

The type of control depends upon application and no single control can guarantee all the processes.

 

[tombmech]

This is just to add to your comments, it is not a disagreement at all:

As you say, this "... depends upon application and no single control can guarantee all the processes."

The design of the fan, its rating, and the overall system pressure dictate whether the unloaded and loaded pressure drops of a filter are enough to significantly affect flow. Even in the case of HEPA's, the system pressure may mitigate the effects of the increase between unloaded and loaded filters.

In biological containment scenarios, for instance, variable speed on the exhaust fans are not even recommended, yet the system can still be designed to accomodate the drop without affecting flow. The figures of pressure drop across HEPA's may be much higher, too.

Interestingly, the control of static pressure can ensure the success of controlling flow through the use of pressure-independent flow control devices. This is because "pressure-independent" still has its limits. If you control the pressure within those limits, then you have the system controlled.