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Saturday, September 4, 2010

How a Roof Turbine Works (or doesn't)

What an oxymoron:wind turbine. That is exactly what it isn't:wind driven.

Here's the dynamic. Cut a hole in your roof and the hot air rises(if you have intake)but rain gets in. So you put a cap over the hole which slows the escaping air but keeps the fierce Dallas storm out.

You want more air to escape so you put on a turbines whose cap size has been effectively reduced to the small plate all the fins have been spot welded to. I know because one of my winter jobs, when roofing was slow, was to build turbines for Mr. Vernon Johnson at Coolcrafters in Arlington Texas on South Peycos Drive.

The escaping hot attic air makes the turbine spin even on still days. Of course "even" but especially on still days. That is because a wind spun turbine won't let the air out as fast, if at all.

You don't believe me so I'm going to prove it to like Aristotle proved to me the world was flat.

Imagine that we live on a large planet like Jupiter and the gravity is so dense the air is as thick as water. If the flowing water above your roof is spinning the turbine then the hot water in your underwater attic cannot get out as fast because of the pressure of the flow of water over the roof.

This makes sense and the fluid dynamics are the same. The hot air's escape is impeded by the wind spinning the fins rather than rising air spinning the fins.

Other attic ventilation systems like vent ridge and low profile vents have a better "down wind" side because the turbine makes air flow around it, it being elevated and spinning, thereby making airplanes fly.

Did I skip something? Oh yeah, that Italian dude who predicted that F-16's would lift off the ground when they got moving fast because of the vacuum caused by the shape of the wing, was proven right. The Wright Brothers knew it and started a roofing company in Irving over a hundred years ago. Their roofs flew and flew.
Here's what Wikipedia says about his theory:

In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.[1][2] Bernoulli's principle is named after the Dutch-Swiss mathematician Daniel Bernoulli who published his principle in his book Hydrodynamica in 1738.[3]

Bernoulli's principle can be applied to various types of fluid flow, resulting in what is loosely denoted as Bernoulli's equation. In fact, there are different forms of the Bernoulli equation for different types of flow. The simple form of Bernoulli's principle is valid for incompressible flows (e.g. most liquid flows) and also for compressible flows (e.g. gases) moving at low Mach numbers. More advanced forms may in some cases be applied to compressible flows at higher Mach numbers (see the derivations of the Bernoulli equation).

Bernoulli's principle can be derived from the principle of conservation of energy. This states that in a steady flow the sum of all forms of mechanical energy in a fluid along a streamline is the same at all points on that streamline. This requires that the sum of kinetic energy and potential energy remain constant. If the fluid is flowing out of a reservoir the sum of all forms of energy is the same on all streamlines because in a reservoir the energy per unit mass (the sum of pressure and gravitational potential ρ g h) is the same everywhere.[4]

Fluid particles are subject only to pressure and their own weight. If a fluid is flowing horizontally and along a section of a streamline, where the speed increases it can only be because the fluid on that section has moved from a region of higher pressure to a region of lower pressure; and if its speed decreases, it can only be because it has moved from a region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest.

See, I told you so. Don't confuse him with his evil cousin Venturi, who squeezed air into small pipes and made it hurry.

That's why, when those storms blow in from Lewisville and north Carrollton into Irving and Dallas, turbines start to fly.

So if you want turbines, go get a real job. They are about the least effective form of roof ventilation you can buy for your attic. The only thing worse is a roof exhaust vent without matching intake vents.

That behind the times roofer who wants to add a turbine on the lower ridge on the back of the house is your attics worst enemy and he is going to cost you a lot of money.

Attic ventilation is my mantra and passion. If it isn't your roofing company's passion then he shouldn't be your roofer.


Anonymous said...

My house has three turbines for attic exhaust and they seem to work great. Attic temperature on hot days is reasonable, and in the mornings the temp in attic is same as exterior. Stays very dry in all seasons (live in Wisconsin). Many homes around here now have ridge vents, but I stuck with my turbines when I reroofed because they were working just fine. I can see why people may not like them aesthetically, but I kind of like the hypnotic spin!

Anonymous said...

WOW Jon- I have to say that you seem to know what you're talking about. Although it could be a case of "If you can't dazzle 'em with brilliance then baffle 'em with B.S." Anyway,I have always suspected that roof turbines were a gimmick. Hot air rises, provide an opening up for exhaust and below for intake and let the physics do the work. Joe in IN

Chaz LaBrock said...

I have been staring at a bunch if them on the building next door and sat with my coffee trying to logically work out how the hell they actually work.. and that is what lead me here.. I can see hot air doing some spinning and even maybe causing it to accelerate to a point..if you look at the blade shape it would fling hot air out..I have noticed on more blustering days they get up to a higher spead so I'm thinking this helps "fling"that hot air out faster..could the cool wind air not cause the hot air to expand thus helping the whole process heat and pressure have a tendancy to be in collusion if I have my rudimentary physics right..??

JGaston said...

Jon, say I set up a freewheeling (not connected to a motor) fan outside in the wind. The fan will start turning, but that does not mean that this rotating fan, which is powered by the wind from behind, is also now blowing the wind in front. Actually the wind velocity will be lower after passing through the fan because some wind kinetic energy is used to turn the fan. Doesn't this apply to an unpowered roof turbine also?
1.) With a hot attic and no wind, the expanded attic air is at higher pressure than outside air. Put a hole in the roof and the hot high pressure air will move outside to equilize pressure. Put a roof turbine on that hole, it starts spinning, but it is blocking the flow of air to the outside, not speeding it up. If it did, then it's as if you had some sort of a perpetual motion machine.
2.) On a windy day, the same logic applies. The wind passing over an open, unobstructed hole in the roof would result in lower pressure outside the roof hole. Air inside the attic would move to lower pressure through the hole. With a turbine, the wind spins the turbine fan but that does not result in lower pressure. The turbine is actually blocking air from escaping, wasting the wind's kinetic energy to turn the turbine. Therefore, with no other concerns such as rain, mice, rats, bats, insects, etc, a simple unobstructed hole would be best. Conclusion, roof turbines put on a good show, but that's all. In other words, since we do have to be concerned with rain, etc, then a turbine that no longer rotates is just as effective as when it rotates.

The Roof Forum said...

The turbine still acts as a static vent in my mind. There is a Bernoulli created downwind side. The purpose of the turbine , in my mind, is to reduce the resistance caused by a stationary bonnet or cap. Without exhaust the pressure in the attic increases as it heats up. The exhaust provides for pressure relief but when coupled with intake vents a venturi is created removing heat and humidity. The turbine manufacturers claim the spinning head creates more pull but I cannot fathom that in my head. Spinning is a function of wind, not the other way around. The manufactures
even produce numbers unlike net free area (NFA) for calculation the amount of ventilation needed and CFM for active ventilation.

Anonymous said...

Swirl your finger around a drain and watch how much faster the water exits a basin compared to naturally under it's own weight.

Sophia said...

How interesting, If the flowing water above your roof is spinning the hot watering your underwater attic cannot get out as fast because of the pressure of the flow of water over the roof. See Details: