Last time we discovered that the boiling point of water varies. It’s dependent upon the amount of pressure exerted on its surface, which varies due to a variety of reasons, including where it is in relation to sea level. Before we see what happens under higher than atmospheric pressures, such as exist in an electric utility power plant boiler, let’s cover some basics.
In the power plant, water is heated in a boiler specifically to produce steam, unlike our tea kettle where the primary purpose is to produce hot water. The steam produced is used to spin turbine generators, which in turn generate electricity, as I explained in a previous blog on steam turbines.
Unlike a tea kettle, which is open to the atmosphere on your kitchen stove, the boiler in a power plant is an enclosed, reinforced steel vessel. See illustration below.
The reinforced steel boiler vessel is designed to withstand great internal pressure as temperatures rise within. In addition to providing a safety feature, the enclosed space provides a sheltered environment for collecting steam so it can later be put to use spinning power generating turbines down the line. In other words, surface water inside the boiler is closed off from the surrounding atmosphere, allowing its internal pressure to build for our specific purposes.
As heat energy is added to water within the boiler, the water boils and steam bubbles break out from its surface, filling the empty space above the surface with pressurized steam. This steam will try to expand here, but it can’t, because it’s being constrained by the reinforced steel vessel within which it is enclosed. Instead, steam pressure builds up on the surface of the water inside the boiler until it is high enough to be released through an attached pipe which is connected to a nearby turbine.
We’ll talk more about this pent-up energy and how it is put to use within the power plant in next week’s blog.
Archive for July, 2013
If you’ve ever baked from a pre-packaged cake or cookie mix, you’ve probably noticed the warning that baking times will vary. That’s because the elevation of the area in which you’re doing the baking makes a difference in the baking time required. Living in New Orleans? Then you’re at or below sea level. In Colorado? Then you’re above sea level. Your cake will be in the oven more or less time at the prescribed temp, depending on your location.
Last time we learned how the heat energy absorbed by water determines whether it exists in one of the three states of matter, gas, liquid, or solid. We also learned that at the atmospheric pressure present at sea level, which is about 14.7 pounds per square inch (PSI), the boiling point of water is 212°F. At sea level there are 14.7 pounds of air pressure bearing down on every square inch of water surface. Again, I said sea level for a reason.
The boiling point of water, just like cake batter baking times, is dependent upon the amount of pressure that’s being exerted on its surface from the surrounding atmosphere. When heat energy is absorbed, it causes the water or cake batter molecules to move around. In fact, the temperature measured is a reflection of this molecular movement. As more heat energy is absorbed, the molecules move more and more rapidly, causing temperature to increase.
When the water temperature in our tea kettle reaches its boiling point of 212°F at sea level, the steam molecules in the bubbles that form have enough energy to overcome the atmospheric pressure on the surface of the water. They become airborne and escape in the form of steam.
If we’re up in the Rockies at say an altitude of 7000 feet above sea level, the atmospheric pressure is only about 10.8 PSI. There’s just less air up there. That means there’s less air pressure resting upon the surface of the water, so it’s far easier for steam molecules to form into bubbles and leave the surface. As a result the boiling point is much lower in the Rockies than it is at sea level, 196°F versus 212°F.
So what if the water was boiling in an environment that had even higher pressures exerted upon it than just atmospheric? We’ll see how to put this pent-up energy to good use next week, when we begin our discussion on how steam is used within electric utility power plants.
If you took high school chemistry, you learned that water is created when two gases, hydrogen and oxygen are combined. You may have even been lucky enough to have a teacher who was able to perform this magical transformation live during class.
Depending primarily on the amount of heat energy absorbed, water exists in one of the three states of matter, gas, liquid, or solid. Its states also depend on surrounding atmospheric pressure, but more about that later. For our discussion, the water will reside at the atmospheric pressure present at sea level, which is around 14.7 pounds per square inch.
Last time we learned that the heat energy absorbed by water before it begins to boil inside our example tea kettle is known as sensible heat within the field of thermodynamics. The more sensible heat that’s applied, the more the water temperature rises, but only up to a point.
The boiling point of water is 212°F. In fact this is the maximum temperature it will achieve, no matter how much heat energy is applied to it. That’s because once this temperature is reached water begins to change its state of matter so that it becomes steam. At this point the energy absorbed by the water is said to become the latent heat of vaporization, that is, the energy absorbed by the water becomes latent, or masked to the naked eye, because it is working behind the scenes to transform the water into steam.
As the water in a tea kettle is transformed into steam, it expands and escapes through the spout, producing that familiar shrill whistle. But what if we prevented the steam from dispersing into the environment and continued to add heat energy? Ironically enough, under these conditions temperature would continue to rise, upwards of 1500°F, if the stove’s burner were powerful enough. This process is known as superheating. Now hold your hats on, because even more ironically, the heat added to this superheated steam is also said to be sensible heat.
Confused? Let’s take a look at the graph below to clear things up.
Sensible heat is heat energy that’s added to water, H2O, in its liquid state. It’s also the term used to describe the heat energy added to steam that’s held within a captive environment, such as takes place during superheating. On the other hand, the latent heat of vaporization, that is the heat energy that’s applied to water once it’s reached boiling point, does not lead to a further rise in temperature, as least as measured by a thermometer.
Next time we’ll see how surrounding air pressure affects water’s transition from liquid to steam.
| In our house the whistle of a tea kettle is heard throughout the day, no matter the temp outside. So what produces that familiar high pitched sound?
When a tea kettle filled with room temperature water, say about 70°F, is heating on the stove top, the heat energy from the burner flame will transfer to the water in the kettle and its temperature will steadily rise. This heat energy that is absorbed by the water before it begins to boil is known as sensible heat in thermodynamics. To read more about thermodynamics, click on this hyperlink to one of my previous blog articles on the topic.
So, why is it called sensible heat? It’s so named because it seems to make sense. The term was first used in the early 19th Century by some of the first engineers who were working on the development of boilers and steam engines to power factories and railways. Simply stated, it’s sensible to assume that the more heat you add to the water in the kettle, the more its temperature will rise.
So how high will the temperature rise? Is there a point when it will cease to rise? Good questions. We’ll answer them next week, along with a discussion on another form of heat energy known as the latent heat of vaporization.
| Last week we began our discussion on perception and how without visual cues individuals exposed to the same verbal information will oftentimes arrive at different conclusions as to what they just heard. We also touched on the fact that peoples’ attention spans are extremely limited, so much so that experts studying human behavior conclude that even when the subject matter is extremely interesting to the listener, their ability to absorb information is limited to mere minutes.
No one would argue the fact that we live in the Visual Age. Most images are flashed at us for only a few seconds. Our appetites for visual arousal seem never to be satiated, and venues that lack a visual component are suffering shrinking audiences. Orchestras come to mind. Even pop stars and professional sporting events today employ huge screens to “up” their visual content in the hopes of keeping their audiences engaged.
This phenomenon was addressed way back in 1982, when the internet and its cornucopia of images was just a baby. Dr. Donald E. Vinson, a leader in courtroom behavior and trial strategy, wrote in the March, 1982 edition of Trial Magazine, “Attention span and the arousal of attention are fundamental to all perception,” a sentiment which Benjamin Franklin echoed in his statement, “If you would persuade, you must appeal to interest rather than intellect.”
Dr. Vinson also states that, “Perceptions are constantly subject to emotional reinforcement, and those which are positively reinforced will tend to be better remembered.” And what’s supplying these necessary emotional reinforcements? Stimulation to the senses. And what type of stimulation is most effective? In Dr. Vinson’s opinion, “A moving stimulus is also more effective than a static one.”
So will shortened modern-day attention spans result in speed trials that are equivalent to short movies? Anything is possible, but our legal system as it stands today isn’t able to produce this just yet.
So how is an attorney trying to impress their message upon jurors from diverse backgrounds and life experience going to succeed in getting their message across? And what if that message is complicated and requires certain technical aptitudes? Will verbal descriptions be enough? Considering the fact that the average adult American reads at or below the 5th grade reading level, it is unlikely.
Research findings discovered more than 30 years ago still hold true today. Messages are most effectively received when employing visual cues, and the most effective visual cue is one that moves.
There’s a good reason why my blogs seldom lack a visual component…