Specifying Burners and Combustion for Process Heating
Careful selection of burners and combustion controls ensures safer operations.
The spectrum of applications found in the process industries means that burner designs must vary widely. Smaller burners are utilized in paint booths, ovens and furnaces while larger burners are specified for incinerators, thermal oil heaters and oxidizers.
When deciding on a process heating burner, it is important to take into consideration the following criteria:
- Maximum input.
- Required turndown.
- Local codes.
- Environmental concerns.
- Operating application.
Maximum Input. Maximum input is determined by the original equipment manufacturer (OEM) and should not be exceeded by the supplied burner. Remember that specific heat is the amount of heat required to raise the unit weight of an item through a 1° temperature rise. For example, 1 BTU (a measurement of heat energy) is the amount of heat energy required to raise one pound of water by 1°F.
Required Turndown. Turndown refers to the burner’s ability to fully modulate throughout the firing range. For example, a 10:1 turndown burner, firing at a high fire of 1 million BTU/hr, is also capable of firing at 100,000 BTU/hr at low fire. In other words, it can fire from 100 percent of its maximum firing rate down to 10 percent of its maximum firing rate. Many process heating burners are capable of turndowns up to 40:1.
Local and National Codes. Any national, state and local codes or regulations pertaining to your geographical area must be considered. Typical codes include those defined by Underwriters Laboratory (UL), the National Fire Protection Association (NFPA), Factory Mutual (FM) and, in the case of Controls and Safety Devices for Automatically Fired Boilers (CSD-1), the American Society of Mechanical Engineers. Outside of the United States, safety standards defined by the Canadian Standards Association (CSA) or the CE marking, used for products sold within the European Economic Area (EEA), may apply.
Environmental Concerns. Burner components can be adjusted to cover any factors outside of a typical NEMA 1 indoor site. Examples include C1D2 for hazardous applications, NEMA 4 for outdoors or NEMA 12 for indoor dusty environments.
Applications. It is important to decide which type of burner is the best for the actual heating process. Some applications require a specialized burner. For instance, an immersion burner can fire into a small tube (1.5 to 6” in diameter) that is submerged in a liquid. This type of burner is designed to provide heat for an acid bath, parts washer or metal finishing.
Retrofitting an existing process heater with a new burner could lead to fuel or electricity savings. Adding a variable-frequency drive (VFD) can help prolong the motor life and reduce electrical consumption. The higher turndown allows the burner to run more efficiently. This is accomplished by avoiding a costly purge of the furnace with cold air to ensure that any unspent fuels are safely removed from the firing chamber.
The stoichiometric combustion point is illustrated. When stoichiometric combustion occurs, all fuel is burned completely, producing only carbon dioxide and water with no other byproducts.
Combustion is a chemical process where fuel — whether in solid, liquid or gaseous state — has a reaction with air to produce heat or energy.
It is important to understand the basics of combustion before finalizing your burner choice. The “Three T’s of combustion” offer a good starting point. Without time, temperature or turbulence, proper combustion cannot be achieved.
- Time. The firing chamber must be large to allow for enough time to complete the combustion process.
- Temperature. The temperature must be high enough to properly ignite the fuel.
- Turbulence. The mixing of fuel and air should allow for each particle of fuel to come in contact with the necessary oxygen.
I would be remiss if I did not also cover stoichiometric combustion. In addition to being a great word in a game of Scrabble, “stoichiometric” defines perfect or complete combustion. In other words, complete combustion means the fuel-to-air ratios for hydrocarbon fuels are matched perfectly, and all fuel is burned completely, producing only carbon dioxide and water with no other byproducts.
Unfortunately, operating at stoichiometric combustion is only theoretical: it is not practical in the real world to commission a burner at these fuel-to-air ratios. The major risk of such a design is that changes in air density would not allow for complete combustion of fuel.
This is the reason that most burners are commissioned for a minimum of 3 percent oxygen. Obviously, it is much safer to have excess (or extra) air than fuel during the combustion process. This excess air also can be brought directly into the furnace or oven and used for temperature uniformity to bring the temperature down for the particular application.
The Importance of Safety
The flame safeguard is an industry term used to define the safety controller device and amplifier. The flame safeguard is used to ensure safe and reliable operation of the burner.
Once there is a call for heat, the flame safeguard ensures multiple safety steps are accomplished in sequence before main burner light off occurs. First, the safety interlocks or switches are verified. Next, the flame safeguard will start the blower motor and fan for pre-purge to remove any unspent fuel from the combustion area. The fan will operate for a minimum of 30 seconds and up to the amount of time for the required full air changes in the firing chamber. As an additional safety measure, a pressure switch will verify this process. Then, the pilot light off occurs, which is verified by the flame safeguard. That is followed up with the main gas valves opening and light off.
The flame safeguard will continue monitoring the flame until the call for heat is over. In the event of a flame failure, the flame safeguard will promptly shut off the fuel supply. Depending on the application or codes, a post-purge process might also be utilized.
It is important to note that a majority of “incidents” happen due to people bypassing or jumping terminals related to the safety steps outlined. Nothing is worth the risk of human life created by skipping the safety steps.
Depending on the process or application, there are various ways of flame detection. While a thermocouple has the capability to sense heat, its response time typically is too slow to be safely used. Common flame detection technologies include:
- Flame rectification or flame rod.
Preventive maintenance for flame-detection technologies depends on the method used.
Flame Rectification or Flame Rod. Check for cracked ceramics and sufficient ground. Move the flame rod and ignition rod to opposing sides of the burner. Check the OEM’s recommended gap dimension to ensure rod to ground.
Infrared. Infrared is best used to capture the last two-thirds of the flame. Be careful when using infrared scanners with refractory because the glow from the heated refractory can simulate a flame. Infrared flame detection is not recommended for use with oil.
Ultraviolet (UV). Ultraviolet flame detection includes units that provide a UV self-check for continuous operation. Keep the UV lens clean, and be aware of line of sight to capture the first one-third of the flame for the best detection. A UVSC scanner uses a shutter to cycle every few seconds and to prevent burn in of the flame.
Photocells. Though not as prevalent as the previously mentioned detectors, photocells work on same principle as a flame rod. The cathode is coated and the anode is not, which allows for electrode detection.
It is much safer to have excess air than fuel during the combustion process. This excess air can be brought directly into the furnace or oven and used for temperature uniformity to bring the temperature down for the particular application.
When specifying a burner for process heating, there are several options to consider for improving combustion, maximizing efficiency and reducing fuel consumption.
Parallel-Positioning Control System. Over time, a traditional single jackshaft or linkage-type burner can lose its effectiveness by developing hysteresis. (The moving parts can wear and no longer remain in the same position as when the burner was tuned.) When the fuel-to-air ratio is no longer in tune, the burner can run in a rich or lean condition — or even both. Also, a linkage-type burner is more likely to suffer from human error by a non-authorized self-appointed burner technician improperly tweaking and adjusting a burner in the field.
Each of these potential issues can be alleviated by incorporating a parallel-positioning control system. This system uses two separate motorized actuators or servos for both the fuel and air valves. An additional actuator can be added if the burner has a flue-gas return (FGR) for a low NOX application. Using a parallel-positioning control system with older linkage-type burners also can improve the burner’s turndown. (Older linkage-type burners without a parallel positioning control system typically are limited to turning down to 35 percent of the maximum firing rate.)
A typical parallel positioning system will have 900 positions of movement through 90 degrees of rotation. This level of precision can provide the best possible conditions for getting the proper match of fuel and air for the required load.
Variable-Frequency Drives to Control Fan Speed. In conjunction with a parallel-positioning system, adding a VFD to control fan speed helps decrease electrical consumption and allows the VFD to serve as a motor starter. A VFD also can lead to higher turndown. Because the fuel and air are adjusted in parallel, the fan speed can be adjusted at a tighter tolerance to provide more or less air at the same damper position. VFDs are typically beneficial for motors 5 hp and up.
Adding O2 Trim. Another option to consider for improving efficiency is using O2 trim, which works by adding an O2 sensor or analyzer in the stack. The sensor adjusts the fuel-to-air ratio based on the ever-changing temperature and air density. This is almost like having a burner tech onsite to constantly tune the burner. It allows for tighter tolerances and approximately a 3 percent improvement in efficiency, which can be significant on larger burners. It is important to consider stack temperatures, however, when sourcing an O2 probe. Some applications could have temperatures higher than are recommended.
While there are many options to consider when sourcing a burner, consider consulting with your local burner representative or OEM to ensure the safe and efficient operation of your heating system.