Heating Theory: Chapter 2
Combustion
In this topic, we will discuss combustion and its components. We will also investigate the dangers associated with combustion and how to handle them safely. Skip to quiz!
Combustion Components
Combustion is simply the process of burning. Recall that a chemical reaction occurs when two chemicals mix. An example is the reaction between iron and oxygen creating rust on your bike! Combustion is a chemical reaction between oxygen in the air and fuel.
There are three main components we need for combustion:
Fuel,
Air or oxygen, and
Heat.
We can remember these using the combustion triangle.
Fuel is the substance we combust. Wood, oil, and natural gas are examples of fuels. Oxygen is the element that reacts with the fuel. Heat is required to start the combustion reaction between fuel and oxygen. Fuel cannot just begin burning on its own!
During combustion, the fuel heats up to high temperatures and reacts with oxygen in the air. This is called burning and produces heat and light in the form of a flame. Lighting a matchstick is an example of combustion. The friction of the match striking against the box causes the match to heat and react with oxygen, creating a flame.
Another example of combustion is burning firewood. The firewood is the fuel. The surrounding air has oxygen. You use a lighter or matchstick to start the fire, and this is the heat that sets off the combustion reaction!
All heating systems rely on fuel. They pull air from outside to react with this fuel. The initial heat required to start the combustion reaction is provided when we turn the system on. Combustion then occurs, providing us with heat to warm our air or wat
Complete Combustion
Recall that combustion occurs when fuel is heated and reacts with oxygen in the air, creating
a flame that gives off heat and light. Complete combustion occurs when all of the fuel burns. When this happens, we are left with ash and smoke. This smoke contains water vapor and carbon dioxide.
An example of complete combustion would be burning all of the firewood at a campfire. When the fire is out, we are left with nothing but ashes and smoke. At the end of complete combustion, all of the fuel is used up!
Incomplete Combustion
Incomplete combustion is when the fuel does not all burn because there is not enough oxygen for it to react with. Like complete combustion, water vapor is produced. However, instead of carbon dioxide, carbon and carbon monoxide are produced. The carbon settles as a black layer of powder, known as soot.
The carbon monoxide produced by incomplete combustion is a dangerous gas. It is colorless and odorless, so it is hard to detect. If you breathe in a small amount, you might feel dizzy, but you can lose consciousness or even die if you breathe in a large amount. One place where incomplete combustion occurs is in car engines.
Carbon Monoxide Safety Procedure:
Never use a gas stove or oven to heat your home.
Never burn charcoal in a fireplace inside your home.
Have fuel-burning appliances inspected and serviced annually by a licensed contractor.
Install carbon monoxide alarms and replace them every five years.
Furnaces
A furnace is a heating unit that uses combustion to heat entire homes or buildings. Furnaces provide warmth by sending hot air, steam, or water to different spaces that we want to be heated. Furnaces are the most popular way to heat homes in the United States!
Furnaces combust fuel inside of a heat exchanger. A heat exchanger is a component that makes the transfer of heat from the burning fuel to the air possible.
Think of a heat exchanger as a shell. We combust fuel inside tubes in this shell, and it heats up. This hot air leaves the shell through these tubes. Outside air passes over this shell and is heated up by hot air from the shell. This heated air moves through our homes to keep us warm!
Furnaces that heat water instead of air are called boilers. Instead of exchanging heat with the air, the heat exchangers in boilers heat water. You can use this hot water in your home for showers, cooking, and other day-to-day tasks!
Combustion, or burning, has three components: fuel, oxygen, and heat. Complete combustion occurs when all of the fuel burns. Incomplete combustion occurs when there is not enough oxygen for the fuel to burn completely. It produces toxic carbon monoxide, so we must take safety precautions.
Types of Fuel
We will also investigate the dangers associated with combustion and how to handle them safely. Skip to quiz!
Types of Fuel Gases
We use different types of gases as fuel for combustion. Some common gases we use are propane, butane, and natural gas.
Propane is a gas that is harmless to humans and has no color or smell. It is also clean-burning, meaning it produces very little smoke and other pollutants. If you own a grill, odds are it uses a propane tank as fuel to start the combustion reaction that cooks your food!
Recall that the pressure on a substance is directly related to its condensation temperature. Condensation temperature is the point at which a gas becomes liquid. So, if we increase the pressure on a gas enough, it will become liquid.
We use this property by storing propane in tanks and increasing the pressure until it becomes liquid. Propane takes up 270 times less space as a liquid than as a gas, so we can store 270 gallons of propane gas in a 1 gallon tank. This means we can save money by storing propane as a liquid!
Propane is a non-toxic gas that burns cleanly, meaning it is good for people and the planet! Propane is also relatively cheap. Because propane is cheap and burns cleanly, we can even use it to fuel the stoves in our homes.
Butane is a gas that catches fire instantly. Like propane, it has no color or smell. Some things we fuel with butane are cigarette lighters and portable stoves.
Butane is hard to burn as a liquid, so we can’t use it at temperatures below its condensation point of 31.3℉. Butane is also very dangerous, as inhaling it can cause you to lose consciousness or even stop breathing.
Natural gas, like butane, catches fire instantly. And, like both propane and butane, it has no color or smell. Natural gas produces no smoke and is not toxic. We can use natural gas for electricity, heating, transportation, or simply lighting a fire!
Natural gas is easier to get from oil rigs than propane. It also requires fewer steps to process it and make sure it is ready for use.
Unfortunately, natural gas is a greenhouse gas. This means that when released in the air, it contributes to global warming. Global warming is a huge threat to the survival of Earth’s species, including humans.
This Venn Diagram shows the properties, benefits, and drawbacks of propane, butane, and natural gas. The shared qualities of the gases are shown in the overlap between their circles in the diagram.
Fuel Efficiency
Fuel efficiency measures the amount of heat or energy that is produced by a fixed amount of fuel. For example, a car that can drive 30 miles per gallon of fuel burned is more fuel-efficient than one that can drive 20 miles per gallon of fuel. Because they require less fuel, fuel-efficient vehicles help us save money and the environment!
Fuel efficiency is measured by Annual Fuel Utilization Efficiency (AFUE). AFUE is the ratio of annual heat created to annual fuel consumed. An AFUE of 90% means that 90% of the energy in the fuel becomes heat, and the other 10% is wasted.
Recall that a furnace is a heating unit that uses combustion to heat entire homes or buildings. Furnaces provide warmth by sending hot air, steam, or water to different spaces that we want to be heated.
The efficiency of a furnace is determined by the AFUE. Recall an AFUE of 90% means 90% of the energy in the fuel becomes heat, and the other 10% is wasted. The minimum furnace AFUE allowed in the U.S. is 78%.
The types of furnaces available on the market vary in their AFUE. Standard Low-Efficiency Gas Furnaces have AFUE’s of 60-65%. Mid-Efficiency Furnaces have AFUE’s of 80-85%. High-Efficiency Furnaces are used today and have AFUE’s above 90%.
Furnaces combust fuel inside of a heat exchanger. Recall that a heat exchanger is a component that allows the transfer of heat from the burning fuel to the air.
High-efficiency furnaces use a second heat exchanger to capture the heat from the smoke produced by the first heat exchanger. This lowers the temperature of the water vapor in the smoke, causing it to condense into liquid water. This process allows us to use the wasted heat from the first heat exchanger to create more heat!
Safety
Inspecting heat exchangers is important to maintain safety. A cracked or damaged heat exchanger can allow carbon dioxide from the burning fuels to enter the air in your home. This can cause suffocation. If there is incomplete combustion in the furnace, carbon monoxide can enter your home and cause death.
To avoid explosions, you should:
Store heat sources and fuels separate from each other.
Avoid accidental fires.
Install smoke alarms.
Have the fire-fighting equipment necessary to put a fire out.
Some common fuels used in combustion are propane, butane, and natural gas. Furnaces combust fuel inside of a heat exchanger. We can add a second heat exchanger to improve efficiency. We must take safety precautions to avoid explosions.
Question #1: Which of the following is NOT one of the main three components of combustion?
Heat
Water
Fuel
Oxygen
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Answer: Water
The three main components of combustion are fuel, oxygen, and heat.
Question #2: Combustion is the chemical reaction between oxygen and _______, producing heat and light in the form of a flame.
Hydrogen
Water
A fuel
None of the above
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Answer: A fuel
Combustion is a chemical reaction between oxygen and fuel and produces heat and light in the form of a flame.
Question #3: Complete combustion occurs when _____ the fuel burns.
Some of
Most of
All of
None of
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Answer: All of
Complete combustion occurs when all of the fuel burns.
Question #4: What does incomplete combustion produce that complete combustion does not?
Water Vapor
Carbon Monoxide
Carbon Dioxide
None of the above
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Answer: Carbon Monoxide
Complete combustion produces ash and smoke, which contains water vapor and carbon dioxide.
Incomplete combustion also produces water vapor, but instead of carbon dioxide, it produces carbon and carbon monoxide.
Question #5: Which of the following is a good carbon monoxide safety procedure?
Use gas stoves and ovens to heat your home.
Install carbon monoxide alarms and replace them every five years.
Burn charcoal in a fireplace in your home.
Inspect fuel-burning appliances on your own.
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Answer: Install carbon monoxide alarms and replace them every five years.
The only proper safety procedure out of these options is installing carbon monoxide alarms and replacing them every 5 years.
It might seem like a good idea to inspect your own fuel-burning appliances, but you should actually leave this to a licensed contractor.
Question #6: Furnaces combust fuel in a _______________.
Heat exchanger
Boiler
Tank
Heat pump
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Answer: Heat exchanger
Furnaces combust fuel inside a heat exchanger, which makes the transfer of heat from the burning fuel to the air possible.
Question #7: Which of the following is NOT a benefit of propane?
It burns cleanly
It is not toxic to humans
It is easier to store as a gas than as a liquid
It is relatively cheap
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Answer: It is easier to store as a gas than as a liquid
Propane is a non-toxic gas that burns cleanly, and it is also relatively cheap.
However, it is easier to store as a liquid than as a gas.
Question #8: Butane catches fire instantly.
True
False
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Answer: True
Butane is a gas that catches fire instantly in room temperature air.
Question #9: Natural gas is a _____________ that contributes to global warming.
Molecule
Fuel
Greenhouse gas
None of the above
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Answer: Greenhouse gas
Natural gas is a greenhouse gas, meaning that when it is released in the atmosphere, it traps heat and warms up the planet.
Question #10: Which of these gases burns cleanly, is harmless to humans, and is easy to store and burn as a liquid?
Butane
Natural Gas
Propane
None
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Answer: Propane
Factory wiring is wiring that the manufacturer has done. The technician does not need to worry about wiring factory wiring. Generally, a solid line represents factory wiring.
Question #11: How can we improve the efficiency of a furnace?
It is impossible to improve the efficiency of a furnace
By adding more fuel
By adding more heat
By adding a second heat exchanger
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Answer: By adding a second heat exchanger
High-efficiency furnaces use a second heat exchanger to capture the heat from the smoke produced by the first heat exchanger.
Question #12: Which of the following is NOT a good explosion safety tactic?
Keep heat sources and fuels close together
Avoid accidental fires
Install smoke alarms
Have the fire-fighting equipment necessary to put a fire out
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Answer: Keep heat sources and fuels close together
High-efficiency furnaces use a second heat exchanger to capture the heat from the smoke produced by the first heat exchanger.