Hi Ask A Wise Engineer! Hi Extraction Fan Causing Building Roof Implosion?

Hi Extraction Fan Causing Building Roof Implosion?

Hi Wise Engineer!

Hi THUNDER STRUCK! 

Hi Q.) For quick extraction of unbreathable gases generated during possible fire after the fire was extinguished, in big commercial buildings there are used large fans on the roofs plus the normally closed automatic air intake shutters in basement.

These fans are powerfull - with 1000 and more Pa of static pressure. 

Does someone heard about a case when such a fan, turned on by a fault - by that the shutter staid closed, the building being rather hermetic with the doors closed, and its roof trusses being already undersized for a load placed on the roof - would cause the implosion (collapse) of the overloaded roof ? 

So to say, the SP of 1000 or 500 or even 50 Pa - that is, the additional load of 100 or 50 or even 5 kg/m2 - turning out to be "the last straw that broke the camel's neck" ?

Is it plausible ?

Hi Bound; Answer Alternatives:

Hi Ans.1); 

I installed one of these systems in the atrium of the International Conference Centre in Birmingham, UK. 

The purpose is the remove smoke so that people can evacuate the building, and then the vents/fans are shut off to starve the fire of air. 

The fans are interlocked so they will not start until the inlet vents (in this building windows at first floor level) are open. 

It is fitted with two independent systems each having their own pre-stored air supply and fed the window actuators through copper pipes so a fire or power failure/shut off will not disrupt the operation. 

Shutting down the fans once the building is evacuated can only be manually selected by the fire crews.

Hi Ans.2); 

One Pa equals 1.450377×10⁻⁴ psi, therefore 1,000 Pa equals 1.450377 x 10-¹ or 0.1450377 psi. 

Much more pressure than this can be achieved by blowing through a straw in a soda glass or beer glass if you choose. 

The fans you are referring to are high volume low pressure units. They have large clearance around the fan blade and high pitched blades. 

Their rotational speed is comparatively slow to other fans. The fans shown in #3 are of this type. They are used in barns, hen houses and such in order to cool the animals with a high volume of air but low pressures to prevent animal health problems. 

They are also used in large commercial flower nurseries to keep flowers and such cooler during hot summer months. 

Closing all the vents and doors of a cheaper "hoop" style green house with a polyethylene sheet used as the walls and ceiling while the fan is exhausting air won't even cause your ears to "pop". If the plants are watered prior to using the fan, evaporation cooling makes them even more effective.

The large fans on the top of buildings are high volume low pressure fans. This prevents the damage you were referring to but moves great amounts of air. 

The Positive Pressure Fans used by firefighters to exhaust smoke or flames from a building are also of this type. 

The fans used to blow up hot air balloons prior to applying the propane heat also are this type. In that case anything other than a low pressure high volume fan would damage the nylon balloon.

Hi Ans.3); 

Yes, this is plausible. I haven't personally encountered any instances, but I know of blast freezers that have squished from cooling, and controlled atmosphere rooms that had plywood pulled off from interior walls when certain valves failed.

Hi Note Final);

- "Life is like riding a bicycle. To keep your balance you must keep moving."