The Rankine Cycle is one of many thermodynamic cycles that can be used to extract useful work from two heat reservoirs at two temperatures, labeled Th (temperature high) and Tl (temperature low.) The Rankine cycle consists of four steps.
In step one, a liquid is pumped from Tl into a vessel at Th. This pumping occurs at constant volume (liquids are incompressible) but the change in pressure requires that work be added to the liquid.
In step two, the liquid in the vessel then boils, a change of state, where Th is constant, but energy is transferred to the working fluid. Boiling is a change of state from liquid into gas with a corresponding increase in volume, but the temperature remains the same. The heat taken from the Th reservoir to boil the working fluid and expand the resultant gas is called enthalpy and consists of two parts. The first part is the heat required to change the molecules from liquid to gas (boiling) and the second part is the heat required to expand the resultant gas.
In step three, the the expanded gas is then admitted to a cylinder where the heat (in the form of enthalpy) added to the gas at Th now does useful mechanical work by pushing on the cylinder surface. As the cylinder moves, the pressure and the temperature of the gas decrease as the volume increases.
Step four occurs when the gas contained in the cylinder is put in contact with Tl and condensation occurrs. during condensation, the remaining enthalpy in the gas is given up, first as the gas reduces in volume and temperature, and finally when the gas changes back to a liquid. This final change greatly reduces the volume of the fluid, allowing the cycle to begin again at step one.
A power plant uses this Rankine cycle to generate electricity. To maximize efficiency, Th is made as high as possible and Tl is made as low as possible. In addition, a power plant uses a superheater just as most modern steam locomotives di
Alas! I have been unable to convince the professors who teach classes on thermodynamics that we should take a field trip to the nearest railroad museum…sigh…
Serious question, what would be required for a railroad museum to invite a class of science or engineering students on site to play with the steam locomotives? Insurance would be a bitch to obtain but let’s pretend for a moment that that problem was solved, what kinds of lesson plans would be required and who could teach them.
What do forum members who are members of museums have to say about this?
I have been unable to convince the professors who teach classes on thermodynamics that we should go on a field trip to the nearest railroad museum and play with the steam locomotives…sigh…
Serious question, why not open the roundhouse to science and engineering students? Let’s assume for the moment that there would be no problem with insurance, What kinds of lesson plans could be used? Who should teach the classes? How much could be accomplished in one day? What sort of equipment should be used? Are there any museums shops that have the ability to take indicator diagrams? Should the museum charge the school or offer the program as a means of recruiting a new generation of members?
Maritime museums take students out for marine science cruises all the time.
You could ask the railroad museum you have in mind if it would be possible to bring a group of students to tour a steam locomotive and perhaps see it operated. I think asking them to let a group of college students “play with” their locomotive is a little unrealistic. However, looking at it and talking to someone who actually does run it may well be possible.
If your thermodynamics teachers are unwilling to sponsor you perhaps another faculty member would be. You don’t have to teach thermodynamics to understand a steam locomotive and another professor may share your interest. Of course you would most likely have to do this on your own time.
Looking back on my own education, thermodynamics was the hardest course I ever took. Many years later I learned that steam locomotives were designed based on trial and error rather than thermodynamic principles. I think seeing the locomotive itself would have been helpful. I hope you pursue your idea.
One of the problems teaching science today is that there are so many on line resources. Too many people seem to think that simulations and interactive web sites are suitable substitutes for actual hands on learning.
I see little or no support for a field trip where the students just look at the locomotive, even if accompanied by an expert who is there to answer questions. I can already hear the university administrators asking “can’t you do that on-line?”
Perhaps the best way around the insurance problem would be for the museum and a university to jointly offer a for-credit class. Anyone at the University of Scranton listening? UConn and the Coast Guard Academy are not that far from Essex Connecticut, Penn State and East Broad Top?
As a matter of fact, I am a professor in a college of engineering, and one term I taught a course on energy policy, in which I included a treatment of thermodynamics. And I took the students on a field trip – to the local Rankine-cycle electric power plant.
My group was paired up with some other visitors to the plant, which included a man from Estonia, who shared with us that with the (then) recent breakup of the Soviet Union, Estonia was cut off from cheap oil under the old Soviet system, but Estonia had oil shale. What they were doing was mining the oil shale and using it is a boiler fuel in a cogeneration scheme of electric power and district heating. He also told me the heating value of the oil shale, burned as if it were a low-BTU coal, and I did a quick conversion of Kcal/Kg into BTU’s per pound – it was a pretty “lean fuel” indeed.
The encounter with the man from Estonia was more valuable than anything to be learned looking into the open fire hole of one of the boilers or what combination of tu
I’m a mechanical engineer and fully agree with Paul’s comment. Such a trip would add little or nothing to a students education in engineering. It might, however, serve some useful purpose for American History students.
Steam locomotives are NOT Rankine cycle engines in any form unless they are condensing. A Rankine cycle engine is by definition a closed loop system where the condenser produces a vacuum on the exhaust side. This vacuum is essential to the effiientcy of the Rankine cycle engine. An engine discharging against a back pressure such as a locomotive is not a Rankine cycle engine.
The ideal thermodynamic cycles are always approximations to practical heat engines. You could also qualify the steam locomotive as using an “open” Rankine cycle, where the condensing takes place in the open air after the vapor exits the stack, that water works its way through rain back into rivers, streams, and ground water, and such water is pumped back into the tender.
Yes, there is substantial thermodynamic irreversability in that arrangement, both in the back pressure between cylinder exhaust and the stack and in the atmospheric condensing temperature being at 212 deg-F, considerably warmer than the usual outdoor temperature in which rain forms and falls. But even in one of those fancy condensing-steam powerplants, you have pressure drops and temperature drops not found in the ideal Rankine cycle.
If one is going to split philosophical hairs, the “Rankine-ness” of the steam locomotive is the use of a fluid-to-vapor phase change, where the passage of fluid into the evaporator (boiler) requires only an injector or a small pump and not an energy-robbing compressor as in a gas cycle such as a gas turbine. A lot of power is required to operate such a compressor, and it is energy-robbing because of the unavoidable losses when transfering that much power.
If one is further going to split philosophical hairs, a super-critical steam power plant is not longer Rankine cycle either, but its non-Rankineness is deeper than that of a powerplant without a condenser, on account of migrating
Interesting though there with the man from Estonia. Little story that happen to me. I live is Harrisburg PA and when my daughter was 7 years old, on Saturdays I take her to the Strousburg Railroad to look at the trains. My wife was a nurse working 12 hour shifts on the weekend, so it was one way Dad and daughter could leave the house quiet for her to sleep. Everytime we go to Strousburg, I take the time to explain the various working parts of whatever they were running that weekend, "this is the tender, see the fireman, his job is, this is the water injector and this is how it works, the cross-compound air pump was a hoot as it was a steam engine in mintature that moved while we stood there. By the time I got to the pilot (yes Emily its called a pilot not a cow-catcher) and the vairous hoses (this is break air line, this is signal) I had a large crowd of people following along! Once the Conductor tagged along and said “mister, your good!”
The point being its the personal stories that give the machines interest and connection. To draw on my hobby car experience, you can show a Trans Am, Trans Sport, Can Am, GTO restored to better than factory new, but its the stories the owner tell (my dad ordered it new in 1960 and I’m the third generations driving her after 50 years) that draws the folks in and leaves an lasting impression.
The advantage of pumping a liquid into a boiler as opposed to compressing a gas, is that most liquids can be treated as incompressible fluids at the pressures used in practical Rankine cycles. This is even true for a super-critical steam plant as long as the temperature of the water being pumped is sufficiently below the temperature of the critical point (i.e. temperatures low enough that water still resembles an incompressible fluid).
Steam locomotives look like Rankine cycle engines to me as well.
The problem with Brayton cycle engines is attaining high isentropic efficiency for the compressor, though some simple cycle combustion turbines have higher thermal efficiencies than the best steam plants.
Erik
P.S. It’s been 34 years since I had an engineering thermodynamics course or a thermal hydraulics course, so my terminology may be a bit rusty.
Steam locomotives are NOT a “subset” of Rankine cycle engines because they are NOT Rankine engines. Rankine cycle engines are by definition closed loop systems where the condenser produces a vacuum at the exhaust. This vacuum contributes to the power of the engine by permitting power to be produced at less than atmospheric pressure. You can no more remove this part of the cycle than you can remove the exhaust stroke from you auto engine.
I have always understood the ideal Rankine cycle to be described as follows:
1: Isentropic compression of a liquid
2: Constant pressure heat addition
3: Isentropic expansion
4: Constant pressure heat rejection
Because the heat addition and rejection processes involve phase change of a simple substance, these processes are also at constant temperature.
As far as I know, there is no requirement that the condensing process create a vacuum. The vacuum produced in steam power plants is because water is the working fluid, which at normal ambient temperatures happens to condense at pressures below atmospheric pressure. Although this is advantageous because it results in a greater pressure ratio and higher efficiency, a vacuum is by no means a requirement of the Rankine cycle.
As Paul pointed out in his earlier post, the water used in a steam engine does go through a cycle because the rejected steam is condensed in the atmosphere and returned back to the tender as liquid water.
Glad you are enjoying this thread, but what is a minuate?
By the way, were a person to find this thread too technical, a lively discussion of Thomas the Tank Engine is taking place another thread.
I rather enjoy Thomas the Tank Engine – morally uplifting stories, emphasizing such principles as getting along with others, not forcing oneself to be the center of attention – mixed with some interesting train watching.
But if you will excuse me, I will go back to discussing technical minutiae . . .