THERMAL POWER PLANT - AUTO FURNACE DRAFT CONTROL
The furnace draft control is kept in ‘auto’ with a set point of –5.0 to –8.0 mmwc. The ‘vane-scoop combination controls’ of the running ID Fan/Fans are also placed in ‘auto’. Furnace draft controller measures Furnace draft & compares with set point.
Output signal is generated & fed to ID Fan ‘Vane scoop control’. Inlet guide vane control is quicker compared to speed control using the variable fluid coupling. However, throttling the inlet guide vane leads to reduction in operating efficiency of the fan.
Flow control by controlling the speed (like fluid coupling scoop control) is more energy efficient than
inlet vane control though the later is quicker in response. The purpose of the modification of ID Fan operation control logic was to ensure efficient and stable furnace draft control with combined response of vane & scoop.
Previous status
As per the original design philosophy, the vanes would modulate first from 35% (lower limit) to 55% (upper limit) as per demand signal. Once it reached limits, the vane remained fixed & demand signal went to operate Scoop. Scoop would take a % of position and desired furnace draft was achieved.
Area of concern : The main drawback of the previous control logic was that during high load (full load) the vane opening was limited to 55% whereas the scoop was nearly at 100% position. It was an energy inefficient control resulting in very high ID
Fan power consumption.
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