To Suck or Blow

[stu3k]

Hi Folks
Building this in shed to store/dry wood (scotlands pretty wet all the time)and am putting ply on front and fitting 5 inch a solar powered extractor fan on front of middle section to keep air flowing.
Now question is should it be blowing Air in or sucking it out
Whats the experts point of view ?​

 

[Dalmatian90]

Blow.

Positive pressure gets things moving a lot more energetically even if the volume of air moved is the same. All those extra air currents, eddies, etc will get the moist air moving towards the exits faster, instead of lingering about lazily in the corners, between stacks, etc.

 

[AKKAMAAN]

I used to have a house with air heating through a forced air ventilation system. The system was set up to "suck" air out of the wet rooms, like laundry room, bathrooms, and kitchen. Tempered, fresh air was supplied, through a heat pump, into bedrooms and the living room. The system was balanced that way so slightly more air was sucked out than pushed in. Negative relative air pressure inside the house. That made the house extremely dry inside, actually had some antique wooden furniture ruined, because of the dry air. Joints etc literally fell apart. That wood, in the furniture, had never been that dry period!! Same thing with my pine wood ceiling in the entire house.....

I had no different room temperature than in a "normal" house, and the air movement it self dried the wood.

So my point is that sucking air out of the wood pile is better than pushing air into the wood pile. I believe, when sucking the air out of the wood pile, the new fresh air will distribute more evenly into the wood pile. Also there is a risk that moist will be pushed into the wood pile.
Air temperature is not as critical as air movement........
Negative relative air pressure in the pile will help a lot aswell...

Thats my 2¢.....

 

[treedogg]

:agree2: or you'll be suckin' the wet.

 

[stu3k]

Thanks folks Sucking it is!!

 

[avalancher]

I did an experiment just the other day like this.
I have a 24X60 workshop.I have one fan on one end,blowing air out permanently mounted in the wall.I also have a roll around type large fan that I also use when it gets real hot in here.
I stuck the roll around fan in one door at one end blowing in and covered what was not covered by fan with a piece of plywood.Temp in the shop eventually dropped five degrees.With a piece of paper dangling, I noted how fast the air was moving back out the other door on the other end.

I then popped the fan out of the door and started the fan going on the other end blowing out.this fan is smaller by 4 inches but the fan is driven at the same speed.Temp dropped and additional ten degrees within 15 minutes.Wind speed in the open doorway was considerably faster that with the fan that was pumping air into the building.There is no doubt in my mind that a fan is more efficient at drawing than pushing air.

 

[jags]

Blow into an empty plastic pop bottle and you will create a small amount of pressure. Suck the air from an empty plastic pop bottle and you will crush the sidewalls. Air hates vacuums and always tries to reach equilibrium.

Its also the reason that the old power brake units on cars were run from vacuum and not pressure.

 

[Dalmatian90]

Blow into an empty plastic pop bottle and you will create a small amount of pressure. Suck the air from an empty plastic pop bottle and you will crush the sidewalls. Air hates vacuums and always tries to reach equilibrium.

Then why doesn't the bottle explode when you blow into it? Isn't the air just as intent on reach equilibrium with the air around the bottle?

A person can't build up enough pressure to cause the bottle to fail from pressure. Some acid and aluminum foil, things end up different.

A human can blow only about 2psi at standard temperature and pressure. However they can generate about -4psi vacuum, and the physiology is a bit different (the muscles in the mouth, like cheek and tounge, are used when sucking a vacuum / straw / you have a dirty mind...when blowing it's the diaphram that generates the pressure, and you still have a dirty mind)

 

[AKKAMAAN]

When you "suck" air out of a bottle, your mouth and lungs create vacuum, the lower pressure inside makes the outside pressure to "push" on the bottle so it shrinks....same with a hydraulic pump....it is the atmospheric pressure that push the oil into the pump....no atmospheric pressure means no "suction", and pump cavitates....

Thats also the reason why a cabin on a high elevation aircraft need to be pressurized for the travelers, or they will need pressurized oxygen supplied by masks....

 

[turnkey4099]

Then why doesn't the bottle explode when you blow into it? Isn't the air just as intent on reach equilibrium with the air around the bottle?

A person can't build up enough pressure to cause the bottle to fail from pressure. Some acid and aluminum foil, things end up different.

A human can blow only about 2psi at standard temperature and pressure. However they can generate about -4psi vacuum, and the physiology is a bit different (the muscles in the mouth, like cheek and tounge, are used when sucking a vacuum / straw / you have a dirty mind...when blowing it's the diaphram that generates the pressure, and you still have a dirty mind)

It takes a whale of a lot less force to collapse the bottle than it does to blow it up. You can crush a bottle with just a thumb and finger but you can't tear it apart without some strenous pulling.

Mythbusters have done some tests as to the pressure a plastic bottle will hold. It is surprisingly high.



Harry K

 

[sdt7618]

Hi Folks
Building this in shed to store/dry wood (scotlands pretty wet all the time)and am putting ply on front and fitting 5 inch a solar powered extractor fan on front of middle section to keep air flowing.
Now question is should it be blowing Air in or sucking it out
Whats the experts point of view ?​

Welcomes from a fellow Scot,

Where about are you in our little land.

Just me 2p worth, but, even how wet we get on the west, coast, my wood stays outside all summer, then under the cover of a shed come Sept, gaps at top and bottom to let air flow and a large roof overhang to stop horizontal rain( we get lots!!)

Suck..

 

[stu3k]

Thanks i stay in sunny Inverness ! Dont have lot of room to store wood and not got round to building proper woodshed, just got small woodburner in living room and noticed the benefits of proper dry wood last winter.

 

[sdt7618]

certainly helps to have seasoned wood, made that mistake a few years back.

 

[jags]

However they can generate about -4psi vacuum, and the physiology is a bit different (the muscles in the mouth, like cheek and tounge, are used when sucking a vacuum / straw / you have a dirty mind...when blowing it's the diaphram that generates the pressure, and you still have a dirty mind)

Not trying to be picky - but vacuum is measured in inch pounds, not psi. And you are correct, I do have a dirty mind.

OK - to be fair I was simplifying, the design of the bottle, and blaa, blaa, blaa,yada, yada, yada, I was trying to create a simple example of pressure v.s. suction.

It still holds true that most people can suck better than blow. And it is more effective to create a vacuum in a room than to try and pressurize (trying to bring this back to the op)

 

[AKKAMAAN]

vacuum is measured in inch pounds, not psi.

Vacuum is a state of pressure lower than the atmospheric pressure. no pressure at all is complete vacuum and zero (0) psi. atmospheric pressure is about 15psi. in daily speaking we set atmospheric pressure to zero, and all we measure is related to that. pressure measures in psi.
you need to back up your inch pounds for vacuum.....

 

[jags]

Vacuum is a state of pressure lower than the atmospheric pressure. no pressure at all is complete vacuum and zero (0) psi. atmospheric pressure is about 15psi. in daily speaking we set atmospheric pressure to zero, and all we measure is related to that. pressure measures in psi.
you need to back up your inch pounds for vacuum.....

Yep, PSI can't go below zero, after that it is known as vacuum.