Many coal-fired power plants measure various pressures associated with their coal pulverizing mills. The red dots in the figure at right shows the typical locations for these measurements for a Raymond Bowl type mill:
- The pulverizing mill section
- The ductwork from the classifier carrying coal fines to the exhauster, and
- The exhauster -- the fan and ductwork carrying pulverized coal and fines to the boiler burners.
A certain "sweet spot" of pressure indicates smooth and proper operation of the mill, as well as the exahuster ductwork. The impulse lines that transfer the pressure signal to a rack of transmitters may be long -- 30 to 50 feet in some cases. The sensed pressure is often in the range of 30 to 40 inches of water column. If the pressure drops, the mill could be plugged or the fuel supply cut short. Higher than normal pressure may indicate plugging of downstream coal ducts leading to the furnace. Improper pressures often lead to flame instability, productivity losses, and pollution.