Visualizing Furnace Pressure

Visualizing furnace pressure requires measuring it by an inclined manometer with one leg connected to a tap through the wall to the furnace interior and the other manometer tap simply receiving pressure from the atmosphere just outside the furnace. To control the effects of furnace pressure, one must determine the elevation within the furnace of the zero pressure level (i.e., zero AP inside to outside the furnace) and understand how it affects interior furnace gas flows. (See pp. 58-69 of reference 52.) 

The hottest gas within a furnace (or any enclosed chamber) rises to the top, creating a higher pressure at the furnace s higher elevations and a lower pressure at the furnace s lower elevations. (This is stack effect within the furnace.) The zero gauge-pressure plane or neutral pressure plane is the locus of points where the pressure inside the furnace is the same as the atmospheric pressure outside the furnace at the same elevation. The neutral or zero plane is the boundary between + and — pressures within the furnace. If there are leaks through the furnace walls, furnace gases will leak outward from the space above the neutral plane and air will leak inward to the space below the neutral plane. (See fig. 6.13.) 

In most industrial heat-processing furnaces, it is desirable to have the entire furnace chamber at a positive pressure with an automatic furnace control system having a setpoint of 0.02 in. wc (0.5 mm) at the elevation of the lowest part of the load(s) or better yet, at an elevation just below the lowest leak. To keep out tramp air inleakage, raise the furnace pressure enough to drive the neutral pressure plane below the furnace bottom, in a liquid bath furnace, below the liquid surface level. 

Furnace pressure or draft is normally controlled by a damper in the stack, thus choking off the outfiow of gases and pressurizing the furnace. (See sec. 6.6.3.) If negative furnace pressure is needed, use a speed control on an induced draft fan, a pressure (volume) control on an eductor jet, or a barometric damper. (See sec. 6.7.1 on Turndown Devices.) 

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Checks to the air and flue gas system include visually inspecting the furnace and periodically monitoring all fans, levels of draft, furnace pressure, excess air demands, and combustion efficiency. 

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