Getting Heat To Do Work – Fundamentals of Engineering Module #8
Heat is SO lazy. It never wants to work, always calls in sick. Getting heat from an object to do work for us is ideal though, it’s one of the main reasons boilers are such workhorses and continue to do work for us in the 21st century.
I guess fancy pants people can analyze the energy conversion from heat to work using our now always referred to post on the Laws of Gases (Boyle, Charles.)
The energy transformation from heat to work is of major interest in the building engineering world. Kind of anywhere that uses steam really. To see how this conversion happens, let’s, once again for the billionth time, consider the pressure, temperature, and volume relationships that hold we know hold true for gases. We have already covered this so…
Robert Boyle, or Bobby as I liked to call him – good ol’ Bobby Boyle, was among the first to study the compressibility of gases. In the middle of the 17th century, he called it the “springiness” of air. Idiot. Somehow Booby figured out that when the temperature of an enclosed gas was kept constant and the pressure doubled, the volume was cut to half of the former value. As the pressure being applied was decreased, the volume increased. We know this.
Boyle concluded that for a constant temperature, the product of the volume and pressure of an enclosed gas remains constant. This conclusion became Boyle’s law. If you’re ever talking to him though, he calls it Bobby’s law of springy gas fun.
You can demonstrate Bobby’s law of springy gas fun by confining some amount of a gas in a cylinder that has a tightly fitted piston. Kind of like one of those syringes you play with from your Doc. Finger over the hole, push the plunger down, and you’re trying to compress the air inside. For this case though, you can apply force to the piston to compress the gas in the cylinder to a specific volume. If you double the force applied to the piston, the gas compresses to half its original volume.
Changes in the pressure of a gas also impact its density. As the pressure increases, its volume decreases. But, no change occurs in the weight of the gas. This means that the weight per unit volume (density) increases. So now we know that the density of a gas varies directly as the pressure if the temperature is held constant.
Then later in the same century, Jacques Charles proved that all gases expand the same amount when heated 1 degree if pressure is held constant.
We have gone over these a couple times now in the last week or two but here are their laws one more time:
Boyle’s law — when the temperature is held constant, an increase in the pressure on a gas causes a proportional decrease in volume. A decrease in the pressure causes a proportional increase in volume.
At atmospheric pressure (sea level), a balloon has a given volume, any old volume, with respect to temperature and pressure. As the balloon falls say 1 mile below sea level, the volume of the balloon decreases due to the increase in atmospheric pressure. As the balloon climbs to 1 mile above sea level, the balloon expands as the atmospheric pressure pressing all around it decreases.
This is the same principle behind sinking in a submarine and using the escape hatch. If we sank in like 500 ft of water, there was probably very little chance of me trying to survive a swim to the ocean surface. Of course that’s what the procedure was and what they wanted you to do. But with a very little actual shot at survival, I probably wouldn’t have gone for it.
The rough procedure was as follows, see just below for details:
- Sink to the bottom of the ocean and land on the floor.
- Poop your pants.
- Exhaust all other options.
- Stand at the bottom of a LET hatch.
- Put on a PLASTIC hood, called a Steinke hood, pronounced Stanky.
- Have someone puncture your ear drums – not kidding.
- Climb ladder into LET.
- Close bottom hatch.
- Open upper valve allowing 35F ocean water from a depth of 500 ft to fill up the LET that you and a few other guys are sitting in.
- Open upper hatch and swim to the surface.
- Yell ho, ho, ho the whole way up so that your lungs don’t explode as you ascend.
Somehow we got lucky, sinking in a section of the ocean that doesn’t go on forever. Some submarines have like 2-3 LET’s or logisitic escape trunks. They are basically little round pods that bolt in place at the 3 access points to a sub. They can be removed to get big stuff in/out of the submarine but out at sea they provide a barrier between the ocean and inside. Like I said they are little hamster balls.
And that whole thing was to show how the law above was like your lungs expanding on the way up. This is why we have to say ho, ho, ho – to allow for the volume expansion inside our lungs as the pressure outside of them decreases. And like we know how long or short to say ho, ho, ho from whatever depth.
Charles’s law — when the pressure is held constant, an increase in the temperature of a gas causes a proportional increase in volume. A decrease in the temperature causes a proportional decrease in volume.
Charles’s law can also be stated like this — when the volume is held constant, an increase in the temperature of a gas causes a proportional increase in pressure. A decrease in the temperature causes a proportional decrease in pressure.
Tanks A and B are of the same size and have an equal volume of gas inside. Tank A has a pressure of 10 psi when heated to 40°F. Tank B has a pressure of 12 psi when heated to 100°F. Unlike balloons or lungs, steel tanks do not expand to accommodate changes in temperature and pressure. This concept demonstrates that changes in temperature are inversely proportional to changes in gas pressure when the volume is held constant.
Let’s say we got a hot little boiler all fired up, a little bit o’ steam has been formed. With the main steam stop valves still closed, the volume of the steam remains constant while the pressure and the temperature are both increasing. This is probably the easiest example to use in order to grasp this whole pressure/temperature/volume relationship stuff. When operating pressure is reached and the steam stop valves are opened, the high pressure of the steam causes the steam to flow to the steam header/manifold/whatever. The pressure of the steam thus provides the potential for doing work. The actual conversion of thermal energy to work is done elsewhere.
Notes about Steam
Steam is water to which enough heat has been added to convert it from a liquid to a gas.
When heat is added to water in an open container, steam forms. It quickly mixes with air and cools back to water that is dispersed in the air, making the air more humid. If you add heat to water in a closed container, the steam will build up pressure. If you were to add precisely enough heat to convert ALL of the water to steam at the temperature of boiling water, you’d get saturated steam.
Saturated steam is steam saturated with all the heat it can hold at the boiling temperature of water.
Superheated steam is steam hotter than the boiling temperature of water.
The boiling temperature of water becomes higher as the pressure over the water becomes higher.
Wet steam is steam at the boiling temperature that still contains some water particles.
Desuperheated steam is steam that has been cooled by being passed through a pipe extending through something like a steam drum. More on steams drums & boilers some other time.
The advantage of desuperheated steam is that it is certain to be dry, yet not so hot as to require special alloy steels for the construction of the piping.
Notes about Combustion
Combustion refers to the rapid chemical union of oxygen with fuel. Perfect combustion of fuel would result in carbon dioxide, nitrogen, water vapor, and sulfur dioxide. The oxygen required to burn the fuel is obtained from the air.
Air is a mixture that contains oxygen, nitrogen, and some other malarkey gases. Only oxygen is used in combustion. Nitrogen is an inert gas, it has zero effect on combustion, so let’s forget about it.
The chemical combinations that result during combustion frees up heat energy. Actually, what happens is a rearrangement of the atoms of the chemical elements into new combinations of molecules. Or even another way, when the temperature of the fuel is increased to the ignition point, a chemical reaction occurs as long as oxygen is hanging around.
The fuel begins to separate and unite with specific amounts of oxygen to form an entirely new substance. Heat energy is given off in this process. A good fuel burns rapidly and produces a large amount of heat.
Perfect combustion is the objective and ideal. However, this has been impossible to achieve in either a boiler or the cylinders of an internal combustion engine. Theoretically, it is simple. It consists of bringing each particle of the fuel, heated to its ignition temp, into contact with the correct amount of oxygen.
The following factors are involved:
- Sufficient oxygen must be supplied.
- The oxygen and fuel particles must be thoroughly mixed.
- Temperatures must be high enough to maintain combustion.
- Enough time must be allowed to permit completion of the process.
Complete combustion can be achieved. This is accomplished by more oxygen being supplied than would be required if perfect combustion were possible. The result is that some of the excess oxygen appears in the combustion gases. This isn’t necessarily a good thing – to have excess oxygen in the combustion gases.
We should now understand how we get heat to do work. We can use it to convert water into steam and then get that steam to do some work or we can use heat for combustion which also can accomplish work like in a piston on a car engine. It’s important that we keep trying to relate how those laws connect to the theories here. By controlling pressure like in a vessel, we can modify the boiling temperature. We can superheat steam, desuperheat it, have wet steam, etc.
Boilers make the steam, pipes deliver it, and then it is used to do work. If you’ve got any questions or comments, make them known. If you do but don’t want to take the time to leave a comment or suggestion, zip it.