Natural draft
It is the draft produced by density difference between the two fluids. Consider the hot gas column shown in the figure. The hot gas column is surrounded by atmospheric cold air. A monometer mounted at point 1 and 2 reads the draft  and  . The relation between draft and density difference can be obtained as follows:
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(4) |
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(5) |
By equation 1 and 4 and we get.
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(6) |
When the chimney (stack) is opened to atmosphere, d2 = 0
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(7) |
Due to flow of gases, the draft is reduced by frictional losses, so that the static draft is the maximum draft which the stack of a given height can produce under limiting condition.
By using mechanical energy balance equation (equation 7 of lecture 21) and using equation 7 (express in equation 7 in terms of and  ) we get the mechanical energy balance equation expressed in terms and draft:
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(8) |
Reference
R. Schuhmann: Metallurgical Engineering, Volume 1 Engineering Principles
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