Pressure Swings

[SAK9]

Guys,

I am looking into pressure swings in a manufacturing space at a particular time of the day.I believe this is due to the flow variation in the exhaust fan due to wind. The fan is installed on the roof and equipped with a side discharge.Does anyone have any good idea as to how to isolate the fan discharge from the effects of wind?

 

[imok2]

If you know the time of day and the most prevelent wind direction at those times then you could install a wind deflector (as short as possible if it's an axil flow fan)also you can experiment with the deflector at various angles
and take room readings over time to find the most efficient position

 

[SAK9]

Thanks Imok for the reply.What should be the geometry of the deflector?Where should I install it,at the discharge duct outlet?

 

[lilliput1]

If it is caused by the wind, remember wind can come from any direction. Also the windward side would see an increase in pressure while the leeward side would see a decrease in pressure. If pressure control is really critical, consider zoned active space differential pressure control systems. Determina an interior base reference pressure & compare w/ the zone pressure using differential pressure transmitters. Control pressure w/ VSD OA makup air units heated & cooled if the space is air conditioned.

If pressurization not that critical, consider pressurizing the entire building & having gravity backdraft damper operated roof vents to relieve excess pressure. The dampaers bust be reqularly maintained clean.

I have seen roof exhaust vent duct that are elbows with flat metal sail on top. The elbow rotate about a vertical axis & the elbow sail rotates it to always discharge away from the wing. This would minimize wind effect on the fan performance but would not counter the wind pressure effects on the building windward versus leeward side.

 

[ChasBean1]

Hi SAK9, don't take this the wrong way, but based on very limited data, I would initially tend to disagree with your assessment. Wind is usually whimsical and not timed. I'm leaning toward a timed setback, although (you know us here) there are very few givens in your problem! Vr, CB

 

[SAK9]

Good to see you more active in the forum again,CB.certainly miss your punchlines!

We have kind of ruled out reasons such as control instabilities,frequent door openings due to shift changes etc.If you can think of any other reasons, do let me know.

The swing happens in the late afternoons when the wind speed around that time is about 7m/s~8m/s and it does not happen everyday.The fan is discharging into the wind.On the face of it the wind seems to create the swings.We are trying to reorient the fan discharge.Will post the results soon

 

[ChasBean1]

At that windspeed I get about a 0.3 in w.c. stagnation pressure at the outlet (if it's faced right into the wind) acting against the discharge... pretty significant. Out of curiosity, are there constant air volume boxes in the exhaust system?

 

[SAK9]

No CB,It is a manaully hard balanced system,pretty basic I would say!

 

[lilliput1]

Can you configure the discharge to a vertical stack with open type rain cap (larger diameter, higher than stack discharge)?

 

[imok2]

58-inch Wind Diverter Kits!
Scorching summer winds choke fan output. Now, block blistering winds with new 58-inch Wind Diverter Kits! Protect your large tunnel ventilation fans and stop ventilation rates from crashing to dangerous levels!
SAK9, This may be of some help

http://www.osborne-ind.com/lvstcksub/ventilation/58_in_wdk.htm

 

[friartuck]

You must have a very sensitive system.

If you install a 'tee' discharge on the outlet, the air will flow initially straight ahead and then split left and right. If the wind blows from one side or the other,(into one end of the tee- it will have no effect on the fan discharge.

Personally, I would install a VJ vertical jet discharge fan rather than one with a single sided discharge.I would also use a BC fan which is less susceptible to external forces.


You will also get problems if the inlet and outlet louvres are on different sides. Where a wind prevails, the louvres should be on the same side of the building to cancel each others affect out.




Friar Tuck of Sherwood