Vertical exhaust air ducts
Vertical exhaust air ducts
(OP)
Why is it than in some handbooks for air ventilation, the height pressure (density x height) is not considered for fan pressure calculations?
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Vertical exhaust air ducts
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Vertical exhaust air ductsVertical exhaust air ducts(OP)
Why is it than in some handbooks for air ventilation, the height pressure (density x height) is not considered for fan pressure calculations?
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RE: Vertical exhaust air ducts
RE: Vertical exhaust air ducts
RE: Vertical exhaust air ducts
RE: Vertical exhaust air ducts
If you have a 10m vertical duct with 300m3/hr the height pressure is about 12 mm H2O, this can be important in relation with the friction pressure loss, depending of the type of duct. Is this wrong? If you only have a straight duct, then according to my calculations it could be important.
RE: Vertical exhaust air ducts
RE: Vertical exhaust air ducts
Same way in hydronic systems, in closed loops we don't take height into account. but in open loops.
RE: Vertical exhaust air ducts
v1=0, v2=v (v=velocity inside the duct, suposing air comes out at the same velocity it has inside the duct)
p1=p2 + SP(z2-z1) SP: Specific Weight of air
Therefore substituting in the Bernoulli formula you will have that the work needed by the fan is only the friction losses plus velocity pressure.
Is this correct?
RE: Vertical exhaust air ducts
Here is an example:
THERMAL GRAVITY EFFECT CALCULATION
D Pse : Thermal Gravity Pressure Differential, in.wc
ra: Ambient (Room) air density, lb/ft3. 0.0745 lb/ft3 for 75 oF
r: Density of the air in the duct, lb/ft3. 0.08 lb/ft3 for 40 oF
Dh: Elevation difference between the air intake and discharge, 4.0 ft.
DPse = 0.192 x (ra - r) x Dh
DPse = 0.192 x (0.0745 - 0.08) x (4.0) = -0.004 in.wc (pressure change due to elevation/density diff.)
RE: Vertical exhaust air ducts
How much did you sweat last time you had to carry air up the stairs??????? adn how much potential energy could you extract from your airdam power plant (like hydrodam, except filled with air)? If you needed energy to blow air up the duct, then you should be able to run a generator on a windmill in the duct when the air falls back, right?
RE: Vertical exhaust air ducts
p,st = 0,192 x (ro,a - ro) x (z2 - z1)
ro,a - ambient air density
ro - supply air density.
You have to care about direction: if you supply cold air upwards (fan is at the bottom) ro > ro,a, stack effect is downwards and will act as additional pressure loss. If you supply warm air in the same direction, stack effect will "help" fan. The opposite applies to downward design flow.
To assess whether is negligible or not, calculate, compare with total pressure drop calculated for system and you will see.
When in doubt, it is much more responsible to check it (as you are seemingly attempting) than to find resort in general "rule of thumb" assumptions.
Rules of thumb are good only if you are certain that you can apply them.
RE: Vertical exhaust air ducts
I think HerrKarl example of obtaining potential energy from a duct full of air makes clear that the above reasoning of the Bernoulli ecuation for a non compressible fluid is correct: the height pressure diffence is cero. The same thing happens if you pump wáter through a pipe inside water, that is you have water at the outlet of the pipe. It would be different if you exhaust the air, lets say, through a vertical duct, but into another room that is pressurized, then the difference in height pressure (or potential energy) would not cancel.
RE: Vertical exhaust air ducts
RE: Vertical exhaust air ducts
Actually there are solar power plants that work with a tall chimney and hot air rising up, so this can be a significant amount. but you likely don't have a 1000ft tall duct and have hot air at some hundred °F.
RE: Vertical exhaust air ducts
RE: Vertical exhaust air ducts
RE: Vertical exhaust air ducts