Page 143 - Boiler plant and distribution system optimization manual

128 Boiler Plant and Distribution System Optimization Manual

6. OVER FIRE DRAFT CONTROL 7. CROSS LIMITING CONTROLS

Cost Cost
Low Moderate

Potential Savings Potential Savings
Moderate (2% to 10%) Moderate

Description Description
One of the first and simplest devices used to Fuel delivery responds almost immediately
control excess air was with over fire draft control. to control system positioning whereas it takes lon-
Because of the change in draft on cold and hot ger for the air to respond, so on upward swings
days and the lack of precise draft control expe- you can get a black smoking condition until the
rienced with barometric dampers, over fire draft air catches up with the fuel.
control systems are used to maintain a constant This smoking on load changes is a waste of
negative pressure in the fire box. This establishes fuel and the soot deposit it leaves behind fouls the
predictable conditions for air fuel ratio control. heat exchange surfaces causing higher stack tem-
Also, when the boiler shuts down, the over fire peratures. This is one of the reasons that pneu-
draft control system damper closes, preventing matic and electronic controls are used. They can
heat from escaping up the stack (Figure 10.26). be designed for this cross limiting feature which
prevents smoking. If a boiler is smoking on load
Advantages/Disadvantages swings, then it is changing load too fast, and cross
The over fire draft control is a simple, low limiting is needed.
cost ($1,500) device and much cheaper than oxy- Cross limiting systems simply do not allow
gen trim. the fuel valve positioning signal to be greater
than the actual measured air flow requirement,
thus eliminating smoking on load changes.

Figure 10.26—Over Fire Draft Control system