The Case for Draft Controls: Then and Now
The inset article appeared in Fuel Oil & Oil Heat magazine in 1964. Since then a lot has changed in the boiler industry, but properly designed and functioning draft controls are still important for many combustion applications.
In 1964, many more boilers were of brick-set construction, required to be run at negative draft or balanced draft pressure. Because the furnaces were not air tight, the furnace walls were kept cool by a constant stream of cool air drawn in by the slightly negative pressure of the furnace. Allowing these furnaces to “go positive” for even a short amount of time could result in damage to the boiler casing or injury to boiler operators. Boilers made in this era typically had tall stacks to induce a negative pressure (or draft) in the boiler, or induced draft fans. To control the negative pressure generated by a tall stack or an induced draft fan, stack outlet dampers were installed and controlled to maintain a setpoint typically about 0.1” negative pressure measured at the back of the furnace. Then as now, proper draft control was also important for flame stability and maintaining the correct fuel air ratio in the boiler.
There are still many balanced draft boilers in operation that require draft controls for the same reasons they did in 1964. However, even air-tight forced draft boilers built today often need draft controls to help stabilize burners using flue gas recirculation for NOx control. Flue gas recirculation is often induced by the combustion air fan. If the stack draft is too negative, the forced draft fan will not be able to induce enough flue gas to meet the required NOx emissions. If the stack draft is not repeatable, the fan will induce varying amounts of flue gas recirculation that will make the fuel air ratio control unstable. Burners utilizing high flue gas recirculation rates, and ultra low NOx burners have narrow limits of flammability and require precise fuel air ratio control. The combustion controller can’t precisely control the air flow through the burner if the boiler draft is constantly changing.
Boilers that operate with excessively negative pressure will draft too much air through the furnace, resulting in poor burner turndown and poor efficiency because excess air cannot be controlled--especially at lower firing rates. When these older boilers are retrofitted with low NOx burners using flue gas recirculation, the high draft condition needs to be controlled because the re-circulated flue gas is diluted by fresh air (called tramp air) that leaks through the boiler casing. The diluted flue gas is less effective at reducing NOx. To meet typical NOx guarantees, effective draft controls need to be installed, and the boiler casings often need to be repaired to reduce air leakage.
The burner manufacturer is the best authority on the need for draft controls. Generally, burner manufacturers will require draft controls if the stack is over 50 feet high or if more than one furnace shares a common stack. “Stub” stacks (those less than 50 feet high) usually don’t produce enough draft to cause repeatability problems. Multiple boilers using a common stack will require draft controls because draft conditions change depending on how many burners are on line, and the rate at which they are firing. The final decision will also involve the sensitivity of the burner design, the NOx and other emission requirements, the temperature extremes expected at the plant location, and the turndown requirements of the boiler.
The JC-22D Draft Monitor and Controller is a microprocessor based draft controller, indicating instrument, and alarm monitor.
Modern draft controls can be stand-alone, incorporated into the flame safeguard controls, or incorporated into a plant-wide distributed control system. Regardless of the control hardware used, modern draft controllers should contain the following functionality:
- A true draft range transmitter with internal filter to reject low frequency boiler rumble.
- PID control of the process variable to minimize response time and overshoot.
- Interface with flame safeguard controls during purge, light off, and release to modulate states.
- Feed forward from firing rate controls to improve response time during rapid load changes.
- A high draft contact with time delay to trip the burner in the event the furnace draft becomes too positive and can’t be controlled.
BurnerMate Universal controller includes flame safeguard, combustion control, and integrated draft control. Features include advanced flame safeguard with first out annunciation, parallel positioning combustion control with oxygen trim, draft, and feedwater control.
Modern draft controllers are easy to set up, have large easy to read displays, and are pre-programmed to include all the essential draft control functions. Although the requirements for draft controls have changed over time, their installation in certain combustion applications has remained a constant over time.
- Understanding Fuel Savings in the Boiler Room
- 09 November, 2011
- Because of their relatively low capital cost and high impact on boiler efficiency, burner and combustion control retrofits offer boiler owners the quickest paybacks based on energy savings.
- The Case for Draft Controls: Then and Now
- 04 January, 2011
- Modern draft controllers are easy to set up, have large easy to read displays, and are pre-programmed to include all the essential draft control functions. Although the requirements for draft controls have changed over time, their installation in certain combustion applications has remained a constant over time.
