Excessive draft also can cause the burner to overfire. This caused wet steam and tripping of the low-water cutoff. It caused violent surges and waves inside the boiler. We had a vertical fire-tube steam boiler where the flame was drawn into the rear tubes of the boiler. Flame impingement means the burner flame is touching the metal surfaces of the boiler that were not designed to have flame directly on them. It is similar to what happens when you keep bending a paper clip. Embrittlement is the loss of ductility of the metal. This could develop higher than desired levels of carbon monoxide.įlame impingement also could cause embrittlement of the boiler metal, lowering the life of the boiler. In addition to increased operating costs, the high draft can cause flame impingement on the boiler. Excessive draft inside a boiler can cause other strange behaviors. If the draft inside the chimney is greater than the setpoint, the damper will open and allow air from the boiler room inside the chimney, rather than stealing heat from the boiler.ĭraft controls are typically used when the stack or chimney height is greater than 30 ft. It is set for the desired draft conditions using weights and adjustment screws. The barometric damper is installed in the flue between the boiler and the chimney. High draft will pull the flue gases too quickly through a boiler, not allowing the heat to be transferred into the boiler. How do you provide a buffer to handle these erratic swings?Ī barometric damper is a great solution. The draft conditions inside the chimney could cause swings of 10, 20 or even 100 times that amount. The boiler requires a stable environment and the chimney is like the “wild child.” Most older boilers were designed to have a draft at the outlet of the boiler to be about -0.05 in. When a tall chimney is attached to a boiler, the draft readings will vary greatly. On another building, the engineer factored the building draft into his HVAC design and made a system using the chimney effect to provide free ventilation to the building without fans.
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