An Introduction to the Combustion Process
What, exactly, is energy and heat transfer?
There are plenty of articles on combustion testing, equipment adjustments, and efficiency, but few on basic combustion knowledge or understanding. To some people, promoting setting gas pressure, clocking meters (natural gas only), and checking delta T provides for the best furnace safety and performance. It seems that using combustion analyzers to set up equipment; measure oxygen (O2), CO, and flue temperature; and then evaluate equipment performance and safety should be the 21st century method to test and adjust combustion devices.
Whatever our comfort zone may be, if we do not have some understanding of the combustion process and heat transfer, neither method has much meaning or value.
This article is based on more than 30 years of field testing, measuring, and auditing equipment performance. Although some of the information requires interpretation, it’s still based on proven field performance.
WHAT IS ENERGY?
Combustion is the burning of some kind of fuel — whether it’s natural gas, propane, oil, coal, or wood — to produce usable heat or energy. This energy, in turn, transfers, at first, to a device like a heat exchanger. The exception would be when the flames and gases of combustion make direct contact with whatever needs to be heated. This article will examine how to maximize the usable energy from a fuel source and how to transfer that source though a heat exchanger to air or water.
For simplicity, let’s cover two main types of energy: potential energy (fuel) and kinetic energy (thermal energy).
Potential energy is the most energy stored that must convert to usable energy (kinetic energy). Converted kinetic energy, through heat transfer, becomes thermal energy.
For example: A cubic foot of natural gas may contain a potential energy of 1,000 Btu per cubic foot, a gallon of propane may contain a potential energy of 90,000 Btu per gallon, and a gallon of oil may contain 140,000 Btu of potential energy.
Until converted through combustion, none of the energy these fuels store is available for any useful heating process.
So, potential energy needs to convert to thermal energy through combustion. Then, the thermal energy contained in the flame and combustion gases can transfer to a cooler object by heat. In heating, the cooler object is the heat exchanger, but, it’s not the only object that will absorb heat. Even so, the heat exchanger now contains a percentage of thermal energy that is available and then transfers this energy to air or water. If this seems complicated, well, it is — and it isn’t.
A proper combustion process burns 99.99 percent of the fuel, which should also be the amount of thermal energy or Btu released from the potential energy of the fuel for transfer to the heat exchanger. We assume this and use it to calculate the operating efficiency of equipment. But, is this what it appears to be? Is it possible this thermal energy could be transferred or absorbed by something other than the heat exchanger? Besides the heat exchanger, where could this thermal energy go?
Natural gas can have a potential energy of 1,000 Btu per cubic foot, as stated earlier. To convert to 100 percent thermal energy and only transfer this energy to the heat exchanger, natural gas must burn at perfect combustion (stoichiometric) with no excess air (0 percent O2) to produce a flame temperature around 3,600°F.
Suppose perfect combustion was attainable. How much cooler would the flame temperature and combustion gases have to be before exiting the heat exchanger for 100 percent energy transfer? The answer: about 70°, or the temperature of the combustion air used. But, is the heat exchanger the only thing that can absorb this heat? At 0 percent O2, the answer is yes. But, that’s not the normal O2 content of the combustion gases.
A good O2 level in residential furnaces is about 6 percent, which produces a flame temperature around 2,800°. This means 800° of thermal energy was already exchanged or lost before any energy was transferred to the heat exchanger. Where does it go?
When discussing oxygen, we’re talking about air. But, air contains about 79 percent nitrogen (N) and only 21 percent oxygen. Nitrogen, which is often ignored, is a thief that absorbs heat or dilutes it. At 6 percent O2, there would also be around 24 percent nitrogen, which steals the 800° for itself.
A CUP OF JOE
A simple way to explain this: Consider a cup of coffee with cream and water added.
Coffee = Fuel
Cup = Heat Exchanger
Cream = O2
Water = N
A coffee cup contains 100 percent coffee (50 drops) at 180° and is almost full to the brim. Coffee is the thermal energy and the cup is the heat exchanger. At 180°, the coffee contains 100 percent of its thermal energy, which would be able to bring the whole cup temperature up to about 180°.
That might burn our hands. We have a little room in the cup, so we add some cream that is 40° (let’s say six drops) to cool it down. With those six drops of cream, 24 drops of water tag along. That makes the coffee 127.5° and most of the cup approximately 127.5°. The same amount of thermal energy is in the cup, but 30 percent of it was absorbed by the water and can no longer be exchanged to the cup.
Let’s change our coffee scenario a bit. We’re in a restaurant and our coffee is too hot. So, we tell our waitress. She’s having a bad day, so she dumps 12 drops of cream into our coffee, which increases the drops of water to 48. Now, our coffee is only 103.6° and lukewarm. But it’s the hottest the cup (heat exchanger) can get. Therefore, 103.6° is the hottest the cup (heat exchanger) can get.
At this point, we assume the cup is full, so it would be about the same temperature from top to bottom. But, what if the cup was only 60 percent full when the 12 drops of cream and 48 drops of water were added? That means 40 percent of the cup is barely heated at all.
Assuming air or water is flowing past the cup (heat exchanger) versus us holding it with our hand, the cup is under-performing.
TEMPERATURE AS AN INDICATOR
Temperature is an indicator of how many Btu (thermal energy) a substance contains. At 6 percent oxygen, the flame temperature is 2,800°. This is 800° less than 3,600°, meaning less available Btu to transfer to the heat exchanger. I estimate about 90 percent of total thermal energy produced is available to transfer to the heat exchanger.
What if the oxygen content is 12 percent? That would put the flame temperature at 2,000°, which is even less thermal energy to transfer. Now, my estimate is approximately 80 percent. So, how can a furnace be 80 percent-plus or 90 percent-plus efficient if the available thermal energy of combustion is only 80 percent?
This exercise shows only a certain amount of available thermal energy is transferred by the process defined as heat. There are two types of heat created in the combustion process: sensible heat and latent heat. All fuels contain carbon (C) and hydrogen (H). Most fuels, including natural gas, are initially given two Btu values: higher heating value (HHV), and lower heating value (LHV), based on the carbon and hydrogen content.
Natural gas is 90 percent carbon and 10 percent hydrogen. In the combustion process, carbon combines with oxygen and produces carbon dioxide (CO2) or sensible heat. So, a cubic foot of natural gas rated at 1,000 Btu potential energy contains 900 Btu of carbon and 100 Btu of hydrogen. During combustion, hydrogen combines with O2 and produces H2O or latent heat.
But, because the hydrogen is now combined with O2, the percentage of latent heat is 18 percent at perfect combustion (stoichiometric) or 0 percent O2. At 6 percent O2, the latent heat volume is closer to 14 percent and puts the sensible heat volume at 86 percent. If you have a noncondensing furnace, what is the maximum sensible heat available at normal combustion? Answer: Less than 90 percent with 6 percent O2.
Here are two questions to ask following the first part of our discussion: First, when discussing Btu input of a heating appliance, what is more important, the cubic feet of natural gas, the gallons of propane or oil we are supplying, or the actual amount of thermal energy produced and available for transfer by the fuel through the combustion process?
Second, does combustion analysis provide us the best means to maximize the amount of thermal energy produced? No, it doesn’t consider enough data, even though combustion analysis is the industry norm.
WHAT IS HEAT TRANSFER?
Heat transfer is the exchange of thermal energy from a hotter substance to a cooler substance. This is done by radiation, convention, or conduction, with radiant heat being the most efficient. When reading technical articles on heat transfer, three things are always listed as contributors: temperature, mass, and specific heat capacity (heat exchanger). Recently, I realized there are two other important factors: line of sight and residence time.
Temperature, by definition, is the measure of approximate thermal energy (Btu) of the molecules of a substance. The hotter the substance (flame temperature), the more thermal energy is available for exchange.
When we discussed energy, we mentioned three different flame temperatures: 3,600° (perfect combustion), 2,800° (at 6 percent O2), and 2,000° (at 12 percent O2). Inside the heat exchanger, where the flame exists, radiant heat is the main method of energy transfer.
Radiant heat increases or decreases to the fourth power of its absolute zero (Rankine), according to the Stefan-Boltzmann Law. What does this mean? Okay, I guess I have to include just a little scientific jargon. How does this affect convective or conductive heat?
If you increase the temperature of a flame or surface from 2,000° to 2,100°, there would be about a 6.7 percent increase in convective or conductive heat to or from the heat exchanger. However, the most important place is at the flame within the heat exchanger, where most of the heat transfer is radiant.
You would find similar discussions on this from manufacturers’ technical data on radiant heaters. Now, looking at the same increase of 100°, there is an increase in the radiant heat transfer of 22 percent, according to Stefan-Boltzmann. What do you think happens when the flame temperature is increased from 2,000° to 2,800°? For radiant energy to be most effective, hotter is better.
Therefore, temperature is the first requirement for maximum heat transfer. A second requirement is that the flame has to be big enough or have mass.
For example, a candle burns at approximately 2,500°. If you put a candle in a furnace heat exchanger, it doesn’t heat very well. The candle may be hot enough, but it’s not big enough. The flame and the gases of combustion must be of a certain mass to effectively heat the volume of the heat exchanger surface. Therefore, flame size and combustion gas quantity produced inside the heat exchanger should be whatever size or mass is necessary to produce the rated output of the equipment.
This does not mean to keep increasing gas pressure or orifice size or over-fire the equipment. It means adjusting equipment with a combustion analyzer to verify safe operation and create the necessary temperature (Btu) and mass to deliver the Btu output listed on the manufacturer’s rating plate.
Usually, the third requirement listed for heat transfer is heat capacity, which is determined by the design of the heat exchanger and its ability to absorb and dissipate thermal energy.
A deterrent that would affect the heat capacity of a furnace heat exchanger is soot. On a boiler or water heater, it would be scale. These deterrents greatly reduce the ability of the heat exchanger to absorb heat. This reduces heat capacity.
Another deterrent is when a heat exchanger cannot transfer the heat it absorbs. On forced-air equipment, this issue involves proper airflow. On a boiler, this involves pump flow. These would fall under the category of residency time on the delivery side of the heat exchanger. Delta T and flue temperature are measurements that should tell you if you face any of these issues. Unfortunately, without knowing how to interpret the real factors of heat transfer, many look at delta T and flue temperature as a fuel indicator and make wrong decisions.
A fourth factor impacting heat transfer that is not always considered is line of sight or proximity. A continuation of the Stefan-Boltzmann Law to explain this would be radiant heat intensity varies as the square of the distance from the heat source. A simpler way to say this might be: The closer the flame is to the heat exchanger, the more heat gets radiated. Although the flames in Figure 1 are the same size and temperature, their distance from the heat exchanger will greatly affect their abilities to transfer heat. If the flame temperature on the right were also cooler, the difference in heat transfer would even be greater.
The final factor, again, now on the inside of the heat exchanger, is residence time. No matter how big or how hot the flame or gases, the amount of time making contact with a median is quite important. A bonfire can be big and hot, but if you ride your bicycle past it, you hardly feel the heat versus standing in front of it. You can swipe your hand right through a candle flame and feel nothing.
Temperature, mass, heat capacity, and line of sight can all be sufficient, but too little contact time or residency nullifies their effects. On the fire side, or inside a heat exchanger, contact time is determined by draft or flow pressure. On the outside of the heat exchanger, contact time is determined by airflow or pump flow.
Only a combustion analyzer and proper combustion diagnostics can determine if all the above are correct, safe, and efficient and conformed to the equipment’s listed rating. Determining true performance of mechanical equipment and adjusting them properly requires measuring their output.
Those familiar with proper equipment testing already have a decent understanding of these principles. Choosing to ignore these principles leads to poor, unsafe, and inefficient operation of HVAC systems.
Publication date: 12/21/2015