Various types of oil are cracked in an “oxygenstarved, natural gas fired” furnace and quenched with water to prevent it from burning properly. The soot created is Carbon Black. A large quantity of Tail Gas is produced as a by product of the cracking process. Tail Gas, a very dirty and wet product, is used as fuel to fire Dutch Ovens that dry Carbon Black Beads.Pigment Carbon Black
  Pressure must be maintained slightly negative inside the Carbon Black Furnace relative to atmospheric pressure to insure the Carbon Black dust stays within the furnace.
The Tail Gas (fuel) and Air flow into the Dutch Oven must be measured and controlled to maintain the proper fuel/air ratio to maximize combustion efficiency so that:
  The static pressure must be maintained slightly negative (typically -0.5" H2O or less) in the furnace. The drafts in the furnace and reference area make the DP signal very noisy and not suitable for control. When DPs are controlled in the -0.05" H2O range, the noise problem is compounded.
The Tail Gas flow measurement is very difficult because it is dirty, hot and wet. Other pitot averaging technologies have been tried, but failed because the sensing ports constantly clogged resulting in grossly inaccurate and unreliable measurements. Thermal technologies cannot be used because the wet dirty gas affects their accuracy and speed of response. The output of a thermal probe in a wet, dirty gas becomes erratic or can drop to zero.Carbon Black N330
  The available straight duct between the fan and boiler is usually very limited, making an accurate and repeatable Combustion Air Flow Measurement very difficult to obtain.
  Similarly, the straight run is also limited at the fan inlet, resulting in turbulent, distorted velocity profiles.