Ashrae Journal - December 2008 - (Page 41) Drum Pressure PT Oxygen Analyzer Combustion Airflow Drum Pressure Air Actuator Jackshaft Actuator Firing Rate Oil Gas Flow Gas Oil Flow Oil Gas Figure 2: Fully metered combustion control ἀow transmitters are used to monitor fuel and air ἀo w. The controllers modulate fuel control valves and air dampers(s) to maintain the correct fuel-air ratios despite changing conditions. Oxygen Analyzer Figure 1: Single point positioning (jackshaft) combustion control system fuel and air control devices are mechanically linked. Adjustable fuel valves or linkage assemblies allow fuel ἀow and airἀow to be characterized at all Ḁring rates. Air Actuator ficiency guarantees, ASME PTC 4.1 efficiency calculations, and the discussion above assume: • The boiler is fired by hand; • Firing rate is fixed at 100% and run to steady state; and • No changes in ambient conditions occur. Few boilers operate under these conditions in the field. The objective of good burner combustion controls is to operate the burner in real-life conditions as close to these optimum conditions as possible—despite changing boiler loads, varying ambient temperatures, and other environmental factors. Fully metered combustion control systems monitor fuel and air flow and will adjust fuel valves and air dampers to keep burner flow rates constant despite changing conditions. Oxygen trim can be added to these systems to help ensure that the stack oxygen setpoint established during burner commissioning is maintained over time. In the past, fully metered combustion control systems were limited to large watertube boilers because of their high capital cost. Newer combustion control systems offer fully metered combustion control with oxygen compensation available off the shelf at substantially lower prices (Figure 2). Parallel-positioning combustion control systems offer oxygen trim without the expense of fuel and air meters. These systems safely compensate for changing ambient conditions and will maintain peak low excess air operation over time. They are extremely economical December 2008 Fuel Actuator Firing Rate Oil Gas Figure 3: Parallel positioning with oxygen trim fuel and air are electronically characterized in the controller. Air is biased to maintain stack oxygen on the setpoint curve established during burner commissioning. and now are often applied to boilers as small as 150 hp (1470 kW) (Figure 3). www.info.hotims.com/16020-42 ASHRAE Journal 41 http://www.wrightsoft.com http://www.wrightsoft.com http://www.info.hotims.com/16020-42
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