Operation of the flues through the communicating vessels
Have you ever wondered how fireplaces worked?
Well the operation of the flue pipe of a chimney is based on the principle of communicating vessels, or an application of a principle of gravity that provides that says that a fluid contained in two (or more) vessels, in communication with each other, tends to assume and keep the same level, provided it has the same density, in each pot.
This also applies to air because it too is a fluid that has a mass and its weight depends on its density; that is, the quantity of matter per unit of volume.
The density of a fluid is also given by its temperature: the higher the temperature, the lower its density.
An example of the operation of the flue pipe can be that by having two vertical flue pipes of identical height, placed in communication with each other and at the base a horizontal section and equipped in the center with a shutter that separates them, at room temperature the two columns of air contained in the fireplaces will have the same density (and therefore the same weight).
If instead we give one of the two columns a source of heat and the temperature of this increases, this will decrease in density and part of this gas will be expelled from the flue pipe.
At this point, by opening the shutter and putting them in communication with each other, we will notice that the second column, containing the cold air, will occupy part of the place of the first column.
The contact of the cold air with the hot air will also heat this, making it also lighter and pushed upwards by the chimney.
So the union of these two flues plus continuous heat determines the continuous operation of the system.
The behavior of flue pipe
In this scheme, one must take into account is that the pressure difference is minimal:
A chimney is in good “draft” when the difference between the external pressure and the internal pressure is between 10 and 20 Pascal.
The fireplaces, in fact, seem to suffer from weather conditions because, depending on the weather changes, there may also be changes in atmospheric pressure.
To make the “draft” of a flue pipe easier, it will be necessary to raise it so as to raise the external air column. To do this you can use a chimney with many curves to increase its length.
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