Simple Chimney
In a simple chimney, no work is extracted, W˙ = 0, the chimney serves to drive an air flow. Equation (13.24), with W˙ mass flow and chimney height,
The draught of the chimney depends on the square root of the height H. The mass flow also has a non-linear dependence on the ratio between the temperatures outside (T0) and at the foot of the chimney (T1) with a maximum for T1/T0 = 2.
In the past, the chimney effect was used for instance to drive the combustion air for a coal power plant. As the above analysis shows, a well working natural draught chimney requires a relatively high temperature (T1 = 2T0), which implies discharge of large amounts of warm gas, and correspondingly high entropy generation, and work loss. In modern power plants, and other applications, discharge is effected by fans, which allow low exhaust temper- atures. High chimneys are still build today, not to increase draught, but to expel the exhaust into higher layers of the atmosphere for better dispersion of the exhaust.