By David Eoff
Danbury, CT – In 1964, Preferred Instruments published an article in the Fuel Oil & Oil Heat magazine. During that time, draft controls were used primarily to control excess draft from tall chimneys and lower excess air to conserve fuel. (Heating oil was 25¢ a gallon in 1964!) Additional benefits included more reliable burner performance, reduced burner emissions, and increased safety by tripping a boiler off line if the draft turned positive.
Check out the first page of that article below.
Today, draft controls are still common on all types of boilers, but for very different reasons. Namely, boiler construction. Since then, 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 airtight 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 recirculated 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.
Boiler construction today is almost entirely different, but draft controls are still required in many applications for mostly different reasons. Boiler combustion chambers now are entirely steel encased and air tight. The burners always include forced draft fans sized to pressurize the burner windbox, the furnace, and sometimes even part of the stack. The boiler and breeching are designed to withstand this positive pressure without the need for cooling air.
Today draft controls are required to accurately maintain draft conditions in the furnace and compensate for changes in outside conditions including:
- Changes in ambient air temperature
- Changes in stack temperature as the boiler warms up or changes firing rates
- Changes in wind velocity blowing across the stack
- Changes in draft conditions caused by multiple boilers connected to a common breeching.
Precise draft control is required now because we expect the burner’s fuel-air ratio controller to hold excess air levels typically below 15% at high fire to conserve fuel. The fuel-air controller can’t effectively maintain low excess air levels when draft conditions are changing.
Just as importantly, modern low NOx burners are more sensitive than their 1964 counterparts. Ultra low NOx burners are extremely sensitive to draft conditions (and ambient temperature, stack oxygen, phase of the moon, operator’s attitude, etc.) Too much draft can cause the burner to run lean, become unstable, and flame out. Too little draft can cause the burner to burn back into the burner internals and damage equipment. Most burner manufacturers require draft controls be installed with their burners if any of the following conditions are present:
- Boiler stack is taller than 100 ft. (sometimes 50 ft. is the limit)
- An induced draft fan is running in the stack
- Two or more boilers share a common stack
If your applications meets one of the above conditions and you don’t install draft controls, the burner manufacturer will offer little help if you experience burner stability problems during start-up. A typical response will be, “Install draft controls and call us if the problem persists.”
To meet the increasingly demanding requirements of modern low NOx burners, draft controls have become much more sophisticated.
–Draft range transmitters have replaced the diaphragms in direct-sensing draft controllers. This is an important advance because the sensing line and diaphragms in direct-sensing draft controllers were prone to fill with condensation and quit working. Transmitters have this same weakness, but are easier to install higher than the breeching tap to ensure they stay dry. Draft transmitters usually include filters to help reject boiler pulsation and transmit just the boiler draft. Once the draft signal is digitized, it is easier to manipulate in a digital controller.
–PID controllers have replaced proportional only controllers. Although the derivative component of the PID control is rarely used, the integral component helps the controller respond faster to quick load changes. Floating point, and gap PID controls utilize a lower proportional gain when the draft is close to setpoint to help eliminate controller oscillation during normal operating conditions.
–Firing rate is often used as a feed-forward input to the draft controller. During commissioning, the technician determines the best draft damper position at 20%, 40%, 60%, 80%, and 100% firing rates. During quick load changes the draft controller monitors the burner firing rate and quickly moves the stack damper to these predetermined positions. As the firing rate begins to level out, the PID controller takes over again to trim the damper position to hold the draft setpoint for that load.
–Modern draft controllers have an adjustable start position—a separate damper position or draft setpoint used only for burner lightoff. If the technician is fighting an unstable pilot, he can position the draft damper or draft setpoint where he needs it to ensure a stable pilot flame and reliable main burner lightoff.
–A digital draft controller can generate a high or low draft pressure alarm, a low draft pressure shutdown contact to the burner management system, and can communicate digitally to a plant-wide control system.
Although positive pressure boiler designs have reduced the need for draft controls, the sensitivity of low NOx burners has actually increased the use of draft controls in recent years. As burner performance standards for low excess air, low NOx operation have increased, the performance requirements for draft controllers have increased proportionately. As the inset article at the beginning of this post shows, Preferred Instruments was one of the earliest providers of boiler draft controls. Today, Preferred Instruments continues to manufacture the most advanced draft control products available to handle any draft control application.
The JC-22D stand-alone draft controller interfaces with virtually any burner management system and combustion controller to safely monitor and control furnace draft in virtually any application.”
The BurnerMate Universal boiler controller incorporates burner management, combustion control, feedwater control, and draft control for complete boiler control in one easy-to-use package. The BMU incorporates all the draft control functions of the JC-22D. Because it is integrated with the other boiler control functions, the only field device required is a draft transmitter and draft damper actuator.