Dampers could be placed either in the stack or on the inlet of a plenum chamber around the burners. A stack damper is the oldest and most common arrangement, particularly for furnaces with horizontal tubes. One of the main advantages of a stack damper is its lower cost and simpler linkage than a damper on a plenum chamber. Another advantage of a stack damper is that closing it to reduce the supply of total air also reduces the draft level throughout the heater and thus decreases the air infiltration. Also, assuming that the damper fits loosely, less air will be admitted to a furnace fire with a stack damper, than with one on the plenum chamber.
On the other hand, a stack damper also has some disadvantages. For example, closing the damper too much may put the top of the heater under pressure and force the gases through the casing, peepholes, and header compartments. The escaping gases may scorch paint around the openings, warp doors and peephole covers, etc. This may also result in large leaks through the roof, which may overheat the roof beams sufficiently to cause the roof to sag, leading ultimately to major repairs.
Where more than one furnace is connected to a single stack, it is best to locate dampers in the breeching from each furnace. A single damper in the stack is highly undesirable for at least three reasons:
• In the event of a tube rupture, closing a flue damper in the breeching will confine the fire to a single furnace. A stack damper would leave the breechings interconnected and allow fire and smoke from one furnace to spread to the other furnaces, complicating the fire fighting.
• When strong winds are blowing, each furnace of a group may have a different optimum damper setting, depending upon its relative position in the wind’s path.
• Individual fireboxes need individual draft control. Each has its own duty, firing note, leakage of air, etc., and therefore requires its own independent draft control.