A stack is designed to maintain negative pressure throughout the heater under operating conditions. For a discussion of draft, see Section 270.
The draft produced by a column of hot flue gas depends on the density difference between the hot gas and the ambient air. The draft effect of the stack must overcome the pressure drops listed below and still maintain a negative pressure at all points in the heater. Friction loss through the firebox is assumed to be negligible.
• Intake losses (plenum, burner, etc.)
• Air preheater loss (when applicable)
• Frictional loss across the convection tube bank
• Entrance loss into the stack
• Damper loss
• Friction loss in the stack
• Exit loss from the stack
Every heater manufacturer has his own method of calculating these pressure drops. Since the manufacturer is responsible for the performance of the heater and stack, the Company has not established rigorous rules for calculating pressure drops.
However, a simple, approximate guideline is given next, to help you check the vendor’s design.
When setting new stack height, also check to be sure the height meets the minimum requirement for personnel protection. See Section 461 for this criteria.