In my last column ("Flame Temperature: What Is It?"), I delved into flame temperatures and the factors affecting them. To round out your understanding of the subject, I'll look at what happens to the flame and its temperature after combustion begins. To make sure we're all starting with a common understanding of the process, I'll review a few key points from the last column.
The adiabatic flame temperature of natural gas, burned with the chemically correct, or stoichiometric, amount of air is about 3,600°F (1,980°C). Adiabatic flame temperature is defined as the temperature the combustion gases would reach if they didn't give up any of their heat to their surroundings. It's an unachievable ideal, because they do, indeed, lose heat to nearby oven walls, workloads and anything else in their line of sight. In addition, at high temperatures, some of the CO2 and water vapor in the products of combustion undergo a sort of reverse combustion, reverting to CO, hydrogen and oxygen and stealing back part of the heat they had previously released. This process is called dissociation.
When all is said and done, the maximum flame temperature attainable in most industrial ovens and furnaces is about 3,000 to 3,300°F (1,650 to 1,815°C), depending on how quickly the process can remove heat from the flame and transfer it to the workload.
Now, I'll look at what becomes of the temperature and heat contained in the combustion gases as they progress through an oven or furnace. I'll start with a simple case -- a burner firing directly into the chamber containing the workload. This is typical for many industrial furnaces, especially those operating at high temperatures.
Imagine someone has developed a temperature-measuring device that not only floats in air but is small enough to ride that airstream through piping, valves and burner passages. It can withstand temperatures higher than 3,000°F and contains a radio transmitter that sends you the temperature data it collects. You pop it into the burner's combustion air supply and watch the temperature curve develop as the transmitter beeps its way through the oven. In a few minutes, you'll have a graph that looks something like figure 1.
The temperature starts out close to ambient. As the air carrying the recorder mixes with the gas and ignites, the readings rise sharply to somewhere around 3,000°F. Following that peak, there's a roller-coaster drop -- nearly all the fuel has been consumed, and the hot combustion gases have begun giving up their heat to the oven and load. At first, the heat transfer rate is rapid because of the high temperature differential between the gases and the oven. As this differential decreases, the heat transfer rate decreases too, and with it, the rate of temperature decline. Eventually, the temperature curve flattens out as it approaches the furnace setpoint temperature. This is an indication that most of the useful heat transfer has taken place, and it's time to discard these combustion gases. Although there's still some transferable heat in the gases, moving it to the furnace and load will take a long time. It's time to make room for a fresh batch of hotter gases. Depending on the oven or furnace, this point usually comes when the gases are still 50 to 150°F (28 to 83°C) higher than the furnace's operating temperature.
In most ovens and dryers, there's another wrinkle -- the hot combustion gases don't transfer their heat directly to the load. The burner flame is confined to a chamber remote from the oven work chamber, and the combustion products are mixed with a stream of makeup air or recirculated oven gases before they're exposed to the load. If you used your floating temperature recorder in these situations, the curve would look something like figure 2. The combustion gases hit peak temperatures like before, but as they blend with the recirculated gases or makeup air, they quickly drop to a temperature that might be no more than 50°F higher than the load. As they're pushed or pulled over and through the load, they transfer heat to it, losing temperature in the process. When their work is finished, they may have cooled to only a few degrees higher than the oven and load. In some ovens, part of them will be recirculated back to the combustion chamber for a fresh charge of heat. In single-pass ovens, they will be exhausted out the stack.
One thing is obvious from this comparison: Firing directly into the work chamber promotes rapid heat transfer because of the high combustion gas-to-load temperature differential. Why, then, don't most ovens do it this way? Why not dispense with the cost and complexity of a recirculating fan and simply fire into the work chamber? There are three reasons -- to protect a relatively delicate workload from those extreme 3,000oF-plus temperatures; to ensure good temperature uniformity throughout the oven; and to promote efficiency. Exhausting gases at 10°F (5°C) or so above oven temperature wastes a lot less fuel than doing it at 100°F (55°C).