The Long and Short of Stacks

Many furnace stacks are not only too tall but also too large. This may be because the steel shell of the stack often needs a protective refractory lining, which may be difficult to install in a small-diameter stack. Stack dimensions should be determined by calculation for each individual case. 

A thumb guide for determining stack cross-sectional area (inside the lining) is to make it equal to about 60% of the sum of the areas of all exhaust ports or flues, provided that they were properly sized. This reduction to 60% is reasonable because the gases cool down on their way through the stack and because one large duct creates less frictional resistance than many small ducts of the same total cross-sectional area. 

The method of calculation of stack size varies with local conditions, but one must first picture the pressure pattern through the combustion system and the furnace, as suggested in figure 7.4. 

From figure 7.4 it is possible to write an equation of pressure balance, similar to balancing one s checkbook or applying the law of conservation of energy (1st Law of Thermodynamics) in a heat balance. 

The following is a listing of where to find numbers to fill equation (7.6)  

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