We have experience in design, fabrication and calibration of many different airflow measurement devices. We frequently find that plants are unaware of poor airflow management and control. A good first step to good air management is metering and properly distributing the air. We have found that measurement of primary air is critical to boiler optimization, and control of the primary air should be within ±3%. Because many other optimization steps assume accurate measurement of primary air, air metering and control should be one of the first steps in the optimization of a pulverized coal-fired boiler.

 

Depending on the location in the ductwork, venturis or flow nozzles are preferred. In most cases ductwork is rectangular, but when possible a circular venturi is preferred for smoothest signal and maximum pressure recovery.

In most cases, the primary airflow is assumed and not accurately measured. It is necessary to accurately measure the primary airflow and coal flow for control and maintain an accurate air/fuel ratio. From testing at some plants, it has been found that the air/fuel ratio is twice as much as required, which adversely affects fuel fineness and distribution. Pulverizer optimization is the first major and necessary step in achieving optimum combustion and accurate measurement of the primary airflow to the pulverizer.

 

Accurate measurement of the secondary airflow is equally important as the primary airflow measurement in achieving optimum combustion. Accurate secondary airflow measurement is needed to supply equal amounts of airflow to each burner compartment. Unequal distribution of the secondary airflow will cause high furnace exit gas temperatures and secondary combustion, which will cause overheating problems in the back-pass as well as poor unit performance.

 

Averaging pitot tubes are economical, functional and can be accurate--depending on the flow conditions. With averaging pitot tubes, we have found laminar flow to be the key to repeatable and accurate tube measurements. This can be accomplished by careful flow straightening and increasing the velocity in the averaging pitot tube measurement plane. The setup should be accurately calibrated by a multi-point hand traverse and checked periodically to ensure proper quantification and control of primary and secondary air distribution.