The heat treater often has choices for burners and combustion systems when purchasing new equipment or rebuilding older furnaces and ovens. While natural gas prices are highly competitive today, a key consideration is to match the heating and temperature uniformity needs of the applications with the best and most cost-effective systems. Let’s learn more.
Air for combustion can be supplied to the burner in several ways. Primary air is a term used to describe air supplied and mixed with fuel prior to ignition. This is usually controlled through orifices and valves where all combustion air is mixed with the fuel and is ready to ignite as soon as it reaches the burner nozzle. The term “pre-mix” is used for burners and combustion systems where the air and gas are mixed prior to the burner nozzle. When they are mixed at or within the burner nozzle, it is called a “nozzle-mix” system.
There are various pre-mix systems, but all must proportion the fuel and air separately to create an air/gas mixture that will effectively burn at the burner nozzle. The air and gas volume flow must be controlled and mixed prior to being piped to the burner nozzle. A mixing device blends them to a thoroughly combustible mixture.
Air and gas are mixed at the burner nozzle in nozzle-mix burners. Some nozzle-mix burners keep the air and gas separated until the point of ignition, whereas others mix some of the primary air prior to the point of ignition. Sealed-nozzle mixed burner systems depend entirely on primary air. By contrast, secondary air is supplied to the flame after it is ignited and is brought in at the burner. An example of both types is in atmospheric burners, which use about 70% primary air and 30% secondary air.
Direct-Fired, Indirect-Fired Combustion Systems
Combustion systems can be divided into two general categories: direct-fired and indirect-fired systems. In direct-fired applications, the products of combustion are exposed to the work, whereas indirect systems fire into radiant tubes or find the work protected from the flame by such items as retorts and muffles. In most direct-fired combustion systems (e.g., box furnaces), secondary air is pulled into the furnace through leaky doors, other openings and negative furnace pressure.
Recuperative systems also provide many benefits. There are two types of air preheaters: recuperators (Fig. 1) and regenerators. Recuperators are gas-to-gas heat exchangers placed on the furnace stack. Internal tubes or plates transfer heat from the outgoing exhaust gas to the incoming combustion air while keeping the two streams from mixing. Recuperators are available in a wide variety of styles, flow capacities and temperature ranges. Regenerators include two or more separate heat-storage sections. Flue gases and combustion air take turns flowing through each regenerator, alternately heating the storage medium and then withdrawing heat from it. Cost justification is based on a payback analysis (Equation 1).
(1) Typical payback period = (Cost of combustion air preheating system obtained from the supplier or contractor) ÷ (Reduction in fuel usage, Million BTU/hour × number of operating hours per year × cost of fuel per Million BTU)
Sizing a Combustion System
Heat input must balance with the heat demand to avoid over-temperature or under-temperature conditions, which could negatively affect the heat-treatment process. Using burners with heat output too great will cause them to cycle more. It is much preferred to have burner output more closely match the heat required in order to reduce cycling and smooth out temperature swings in the furnace, which are created during cycling. Temperature control within a cycle is a very important process-control parameter.
One of the benefits of utilizing indirect heating via radiant tubes in furnaces is the possibility of rapid temperature changes during any portion of the cycle (e.g., dropping from carburizing to hardening temperature prior to quenching). This feature can be accomplished by turning off the fuel to the combustion system and allowing cool combustion air to flow through the tubes. The furnace must be equipped with this feature to ensure burners properly relight after the cooling segment.
Controls and Safety
Anyone who operates or maintains burners or combustion systems must understand the basic function of the various components that work together to make their system work effectively and safely. Each component has an important function. Some are for operation and control, while others are strictly safety devices that automatically shut the system down to prevent damage and personal injury.
When looking at a combustion system for the first time, people are often a bit overwhelmed with the amount of valves, controls and other components. This initial fear is normal because no one should operate a combustion system without first understanding all of the components involved, their function and their safe operation. Having a healthy respect also provides good motivation to learn what each device is doing and how they are controlled for safe start-up, operation and shutdown.
System Monitoring for Safe Operation
Combustion systems have layers of safety checks and devices designed to protect operators and equipment from harm. In the event that a safety device is not working properly, other devices in the system are designed to back them up. An example of this is the fact that two main gas shutoff valves are used in series. If there is a leak or failure of one, the other is there to shut off the gas. Another example involves normally closed valves (i.e. valves held open when power is applied). These valves will automatically shut in the event of an electric power outage.
The first line of defense is the flame monitor, which must be used in all combustion systems. The absence of flame at the burner will trigger the system to shut off the main gas valves.
In pre-mix combustion systems, a second line of defense is a device called a flame arrester. If the combustion were to burn back into the pre-mix supply line, the flame will stop at the flame arrester. A flame arrester is defined in NFPA 86 as “a device that prevents the transmission of a flame through a flammable gas/air mixture by quenching the flame on the surface of an array of small passages through which the flame must pass.”
A flame arrester installed in a pre-mixed system that includes a way to both extinguish the flame and shut off the fuel supply is called an automatic fire check. An automatic fire check utilizes a spring-loaded shutoff valve that is held in place by two bimetallic rods during normal operation. If a flashback occurs, the heat from the flame will cause the bimetallic rods to bend and release the spring-loaded shutoff valve.Fire checks should be inspected a minimum of twice a year to ensure they are functioning properly.
The combustion safety system continually monitors and controls the system. There are different requirements depending on the stage of operation. The different stages are categorized as follows:
• Pre-ignition – A timed pre-ignition purge cycle
• Trial-for-ignition period – A fixed period of time (15 second or less) for pilot and main burners to ignite
• Operation – Safety sequencing has logic systems. Activation of any safety interlock will result in a safety shutdown. Safety interlock devices can be hardwired in series, use relays or be connected to a PLC system. PLC system requirements are spelled out in detail for safety standards and must include at least one manual emergency switch.
• Restart – Occurs when flame is not detected. In some instances, the standard allows for an automatic trial to re-ignite the burners.
• Failure – Occurs when one of the interlocks fails to operate within its design parameters
• Shutdown – Occurs when the system ceases to function either due to failure or by an action of the operator. There is often a post-purge cycle initiated at this time.
A number of safety checks must be made prior to ignition. Monitoring devices send electronic signals back to the combustion control system to ensure that certain conditions are all met prior to ignition. The monitors provide continual feedback signals during combustion to ensure that all of the conditions are met to allow the system to continue operation. The control system will use this information to determine whether to allow the process to continue, try to relight the system or shut it down. Shutdown begins with the closing of the gas valves, followed by a purge time in which the air must continue to run in order to ensure that any uncombusted fuel has been evacuated or vented.
Understanding the basics of combustion will provide tangible benefits to the heat treater, including faster heat-up times and load recovery (due to higher flame temperatures and greater heat transfer); greater efficiency (more available heat); reduced pollution (minimum exhaust volumes, reduced fuel use); and cost savings (more cost competitive vs. alternative energy sources). IH
1. Herring, Daniel H., “Quest for Fire – Combustion Basics,” Industrial Heating, October 2009.
2. Mr. Thomas Bannos, TS Thermal LLC, technical and editorial contributions, private correspondence.
3. Mr. Ralph Poor, Surface Combustion (www.surfacecombustion.com), private correspondence.