Come on in -- there are a couple of seats left in the back. All right, just to make sure you're in the right room, this is Burner History 101.
Most modern industrial processes are heated by gas burners. We take for granted that they will generate the heat we want, operate on demand and not misbehave. For the most part, they don't disappoint us, but it's been a long road to today's standards of reliability and efficiency. Let's take a look back at how the modern industrial gas burner came to be. It will give us some insights into why burners are made the way they are and what it takes to keep them running properly.
In those early days, there were few devices that would be recognized as burners. Furnaces and ovens were equipped simply with sets of air and gas openings (figure 1), and the two streams mixed and burned inside the furnace proper. Despite the simplicity of this approach and the obvious opportunities for things to go wrong, it worked well, especially on high temperature furnaces. In fact, this concept has survived into the present with some regenerative glass-melting furnaces and, until a few years ago, open-hearth steelmaking furnaces.
This "throw 'em together and let 'em burn" approach wasn't suited to all applications, though. Flames often were too large and slow mixing to fit many combustion chambers, and on the lower BTU gases, they were prone to going out unless the furnace was really hot or the air -- and gas, sometimes -- were preheated.
The search for better mixing, flame intensity and stability occupied many researchers, including Robert Bunsen, whose namesake burner is familiar to anyone who ever sat in a chemistry lab. Bunsen designed a mixing tube that used a jet of gas to entrain air and create a partial premix, which then passed through a burner nozzle, where it was ignited. In present-day terminology, this is an atmospheric burner (figure 2a), so-called because the pressure of the air-gas mixture is very close to atmospheric pressure. The gas entering the orifice at the base of the mixing tube is at a pressure of only a few inches water column, so it lacks the oomph to entrain 100 percent of the air it needs for combustion. The premix contains only about 40 percent of the required air; the remainder is entrained into the flame from the area around the nozzle. Consequently, atmospheric burners have to be operated in an environment where there is plenty of free air. That's why there are generous air openings near the atmospheric burners on your kitchen range and water heater. Bunsen's burner, and similar designs developed by others, were the ancestors of today's premix industrial burners.
You may be asking yourself if raising the gas pressure wouldn't increase the amount of air that could be entrained, allowing the burner to be operated in a sealed environment with little or no free air. The answer is yes -- a gas pressure of a few psig will do the trick -- but most of the city gas distribution systems of the day couldn't operate at pressures that high. Where high pressure supplies were available, though, burners could be operated with 100 percent premix. The mixing or venturi tubes that feed these burners look very similar to those used with atmospheric burners. Known as injectors or inspirators (figure 2b), they squirt a jet of gas into the throat of the mixing tube, creating a suction strong enough to entrain all the combustion air that's needed. This creates a hotter, more intense, faster-burning flame better suited to industrial processes. As high pressure natural gas pipelines began to spread across the countryside, more and more industrial users took advantage of the higher performance of these burners.
People in areas of low pressure gas weren't out of luck, though. By the early 1920s, proportional mixers (figure 2c) also were coming into use. These operate on the venturi principle, just like injectors, with one important difference -- the combustion air entrains the gas instead of the other way around. Air under pressure, usually from a blower or fan, flows through the throat of the mixer, creating a suction that draws in the gas. The gas can be at zero gauge (atmospheric) pressure because it's being pulled, rather than pushed, into the mixing tube. In fact, proportional mixers are paired with gas regulators known as zero governors or atmospheric regulators. These regulators maintain the gas supply at zero gauge pressure to ensure the gas-air ratio doesn't fluctuate. Proportional-mixer/zero-governor systems allow a burner to operate with 100 percent premix and with greater flexibility than an atmospheric system, but this comes at a higher cost -- the fan or blower needed to drive the system.
The bell's about to ring, so we'll call it a day. In Burner History 102 [see link at bottom of page], I'll look deeper into the premix burners themselves. Oh, and don't forget the reading assignment!