What powers draft in a chimney?

[johnstra]

Nicely stated, TE.

 

[ddahlgren]

I did not mean to be rude about not wanting to crank out the math is mainly because it has been a long while since I have had to do it and so many variables it turns into a long drawn out deal that I don't have time to do. I am working 10 hrs a day 6 days a week and doing house work pay bills etc. in my so called spare time.

The simplification only because I did not want to assume everyone in the conversation has taken physics 101. Thinking in absolute pressure is not obvious to everyone like you or me and possibly everyone but have no way to know that. I have never met anyone here in person and chat during the burning season to learn listen and offer what I can.

In the end not trying to be rude or insulting and apologize if taken that way by anyone.

 

[Ashful]

Did you look at the link I provided? You do not *have* to add energy to the system to get flow. There are always dynamics available to drive flow through a chimney. Adding energy biases the system so you get the desired flow. Maybe I misunderstood your original question?

I read most of it. The significant thing I took away from it is the concept of Neutral Pressure Plane, which the author argues will naturally tend to be higher in the house due to the location of primary leaks (second floor / attic), and that this will want to make the chimney negatively draft. He explains that you're fighting that tendency with the fact that the house is (hopefully) warmer than the outside, thus forcing positive (upward) draft. Now back to the OP, in which the indoors is as cold as (or colder than) outdoors, and it seems the author is arguing that the chimney will go into negative draft.

No house is "cold", there is always some heating effect even if just from the sun, so there will almost always be some stack effect.

We're talking about Highbeam's workshop, and in the case of my workshop, it is very often colder inside than outside... especially in March thru June.

 

[johnstra]

even if your shop (or a tent or whatever) is colder than outdoor ambient, it's at least partially enclosed and therefore subject to the stack effect. There will be relatively warmer air higher in the structure, so there will still be a NPP and the air in the structure will try to achieve equilibrium with the air external to it.

 

[oldspark]

Everything I found on the subject is exactly what johnstra stated in his last post, humidity can play a part also.

 

[georgepds]

There's more to it. It's not the delta P or delta T between one end and the other. Think about the straw in your water glass much higher P at the bottom than at the top but no flow. Even if your glass is 100 feet deep.

That's because the gravity forces ( density * accel due to gravity * height) balance the pressure differential in the straw. If you immersed the straw (to get rid of the problem where the water has to bubble up into the air) , and heated up the water inside it (say with an electric coil) to lower the density you'd start a flow moving up it... buoyancy at work