Flue-gas Stack - Flue-gas Flow-rate Induced By The Draft

Flue-gas Flow-rate Induced By The Draft

As a "first guess" approximation, the following equation can be used to estimate the flue-gas flow-rate induced by the draft of a flue-gas stack. The equation assumes that the molar mass of the flue gas and the outside air are equal and that the frictional resistance and heat losses are negligible:.

Also, this equation is only valid when the resistance to the draft flow is caused by a single orifice characterized by the discharge coefficient C. In many, if not most situations, the resistance is primarily imposed by the flue stack itself. In these cases, the resistance is proportional to the stack height H. This causes a cancellation of the H in the above equation predicting Q to be invariant with respect to the flue height.

Designing chimneys and stacks to provide the correct amount of natural draft involves a great many factors such as:

• The height and diameter of the stack.
• The desired amount of excess combustion air needed to assure complete combustion.
• The temperature of the flue gases leaving the combustion zone.
• The composition of the combustion flue gas, which determines the flue-gas density.
• The frictional resistance to the flow of the flue gases through the chimney or stack, which will vary with the materials used to construct the chimney or stack.
• The heat loss from the flue gases as they flow through the chimney or stack.
• The local atmospheric pressure of the ambient air, which is determined by the local elevation above sea level.

The calculation of many of the above design factors requires trial-and-error reiterative methods.

Government agencies in most countries have specific codes which govern how such design calculations must be performed. Many non-governmental organizations also have codes governing the design of chimneys and stacks (notably, the ASME codes).