BOILER CONTROL SYSTEMS
Boilers have to provide steam or hot water whenever heat is needed. A conventional BMCS (boiler management control system) is often set to provide a continuous hot water or steam supply at anytime the OA (outside air) temperature drops to 60°F for more than 30 min and an AHU (air handling unit) is calling for heat. The BCMS should include a software on/ off/auto function. Boiler manufacturers’ recommendations provide specific guidelines in this area of operation.
Unless a low-limit for water temperature is used, hot water boiler burners are not controlled to provide water temperatures based on outdoor temperatures, because the reset schedules require water temperatures to be supplied below the dewpoint temperature of the flue gas. Some boilers require incoming water temperatures to be above 140°F before going to high-fire. In this case, if a building is using a hot water system and the boiler is locked into low-fire because the incoming water is too cold, the system may never recover.
The following are three ways to control the output of a commercial boiler:
- On/off (cycling) controlis most common for small boilers up to 1,000,000 Btu/h capacity. The oil or gas burner cycles on and off to maintain steam pressure or water temperature. Cycling control causes losses in efficiency because of the cooling (which is necessary for safety) of the fireside surfaces by the natural draft from the stack during the off, pre-purge and post-purge cycles.
- High-fire/low-fire controlburners provide fewer off-cycle losses since the burner shuts off only when loads are below the low-fire rate of fuel input.
- Modulating controlis used on most large boilers because it adjusts the output to match the load whenever the load is greater than the low-fire limit, which is usually not less than 15% of the full load capacity. Steam pressure or hot water temperature is measured to determine the volume of gas or oil admitted to the burner. Boiler firing and safety controls are boiler-manufacturer furnished and code approved. A BMCS usually enables a boiler to fire, provides a set-point, controls pumps and blending valves, and monitors operation and alarms.