Plenums for Noise Control
Intense noise accompanies the high rates of heat release inside fired heaters. Combustion roar escapes from the firebox mainly through the burner air registers. Inherent in today’s burners, combustion roar can be effectively suppressed by acoustically absorbent air intake plenums.
A typical plenum is shown in Figure 500-6. As much of the plenum interior as possible should be lined with acoustic material. The material should have high absorption coefficients in the frequencies of combustion roar, at least 0.80 at 250 Hz. The density of the acoustic material should be 6 pounds per cubic foot and its thickness should be at least 4 inches.
Plenums should be fabricated of relatively heavy steel plate about 1/8-inch thick, undercoated with a soft-setting, sound deadening material and constructed so as to completely enclose the burner registers. The acoustic lining should be positively retained (e.g., with expanded metal screen).
If the heater is oil fired, provision should be made for inspection, drainage and clean out of the inevitable oil leaks. The acoustic lining should be omitted where drips collect.
All of the combustion air, both primary and secondary, should be taken from the plenum chamber. There must be no line of sight from the burner or the plenum interior to the outside.
Lined baffles should provide at least two reflections of escaping sound on absorbent surfaces. Hand holes, penetrations for external air register control, oil gun, etc., should be well sealed.
Today it is common practice to provide plenums on all new fired heaters. Retrofitting plenums to existing heaters has been done in some cases but it is very difficult. For an expanded discussion, see “Furnace Roar Control,” Chemical Engineering Progress, Vol. 71, No. 8, August 1975.