The Internet has made reciting and debunking urban myths a popular pastime these days. In fact, the demand for new myths seems so strong that I’m starting to see 20- and 30-year old tales being recycled, long after I thought they had been put to rest.
Every industry has its equivalent of those urban myths, and ours is no exception. And like the stories that circulate over the Internet, some of them seem to have nine lives, resurfacing time and again to lead another generation astray. Some of these myths are better described as misconceptions or misunderstandings; regardless, they can give people mistaken notions about certain aspects of process heating.
Here are a few of the more durable myths and misunderstandings, along with the truth.
Flame Temperatures for Natural Gas, Liquid Propane and Similar Fuels are around 3,500oF when Burning with Ambient Combustion Air. Indeed, they are, if you’re talking about the adiabatic flame temperature, defined as the temperature the flame would reach if it didn’t lose any heat to its surroundings. However, all flames do (that’s the point, isn’t it?), so real-life temperatures are much lower. Remember, flame temperature is actually the temperature of a mass of gases produced by combustion. If all the energy released by combustion went into those gases, they would heat up to the adiabatic temperature. However, even before the combustion process is complete, those hot gases are throwing off some of their energy to their surroundings, so they’ll never reach the adiabatic temperature.
How hot will they get in real-life situations? It depends on a lot of things, like how fast combustion takes place (fast mixing and burning promote higher temperatures), the temperature of the flame’s environment (colder surroundings will suck the heat out of a flame faster) and the fuel-air ratio (stoichiometric, or on-ratio, mixtures produce the highest temperatures). In all my years in the business, I don’t recall seeing flame temperatures higher than 3,200 to 3,250oF (1,760 to 1,788oC), and that was on furnaces with immensely thick insulation to retard heat losses and extremely precise fuel-air ratio control.
Short-Wavelength Infrared Radiation Penetrates Deeper into Products. I have to be careful here -- this might be true for product materials that are optically transparent or semi-transparent, but it certainly isn’t the case for opaque materials such as sheet metal, castings or wood. Infrared radiation is electromagnetic waves -- it is converted to heat only when it strikes a surface. Once that happens, heat migration below the surface is dictated by the thermal conductivity of the material being heated, not the wavelength of the radiation that brought it there. Short wavelength radiation may well heat things faster, because it is created by higher temperature heat sources. Higher temperature differentials between the source and the product will increase the rate of transfer to the product surface, creating higher surface temperatures and speeding up conduction into the piece, but it’s not because radiation penetrated deeper. It didn’t penetrate at all.
Excess Air Is the Amount of Air Added to a Fuel-Air Mixture beyond What’s Needed to Completely Burn the Fuel. True and false -- it depends on what this means to you. To some, it means that you mix the high fire amounts of fuel and air, and then add more air to get the excess air ratio. Most combustion ratio control systems don’t work that way -- they are designed to provide a certain amount of air sufficient for whatever firing rate you choose and then limit the fuel flow to a less-than-stoichiometric amount, producing a lean ratio. The result is excess air, but probably a more correct term would be “fuel deficiency.”
Combustion Efficiency and Oven (or Furnace) Efficiency are Interchangeable Terms. First, there’s no generally accepted definition of combustion efficiency, although some people equate it to available heat. Available heat is defined: it’s the portion, or percentage, of the total heat input that remains in the oven or furnace after the exhaust gases and the heat they contain have been thrown away. The available heat is divided up to replenish heat losses through the walls of the appliance, radiation losses out openings, heat conducted out by conveying equipment and cooling water or air flows, heat stored in the furnace or oven structure and, finally, heat contained in the work load.
Oven or furnace efficiency is defined simply as the amount of heat energy in the work load divided by the total heat input to the process, so numerically, it is always lower than the percent available heat. This brings up another point. Many modern flue gas analyzers contain a thermocouple in the stack probe to measure the exhaust composition and temperature simultaneously. Knowing these two variables, you can calculate the percent available heat, and the digital processors in many analyzers contain a program that does just that. However, they report available heat as percent efficiency, which it is not. To confuse matters further, analyzers manufactured in Europe are likely to calculate the available heat on the basis of the net heating value of the fuel, which for natural gas or liquid propane is 90 to 93 percent of the fuel’s gross heating value. The resulting percentage will be 7 to 11 percent higher than the available heat calculated on the basis of the gross heating value, which is the custom in the United States.