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 did. A superheater is an elaboration of step two. After the liquid is boiled into a gas and has undergone its initial expansion, the gas is sent through an array of tubes at an even higher temperature than Th. This adds more enthalpy to the gas and causes it to expand more, but still at the same constant pressure as in the initial expansion at Th.
A power plant also uses a condenser, cooled by either air or water, to allow Tl to be much lower than 212 F. The best condensers use a wet well and operate with a final pressure of about 2" Hg, less than one tenth of an atmosphere. This greatly lowers Tl and increases the efficiency of power conversion.
The best water-based Rankine Cycle power plants achieve efficiencies of about 40%. That is, they are able to convert about 40% of the heat energy in the fuel into useful work.
The advantage of the Rankine cycle is that after the working fluid is condensed into a liquid, it is incompressible. This makes the work required to force the working fluid into a vessel at Th a very small portion of the work that can be extracted as the boiled vapor expands to do useful work.
Steam locomotives operate as an open loop Rankine Cycle engine except for the few that were condensing engines. The atmosphere is Tl and the boiler temperature is Th. (Note that Th is always much lower than the peak temperature of the burning fuel, a source of loss in a steam locomotive. To minimize this loss some locomotives incorporated smokebox reheaters along with compounding. The smokebox reheater exposed the exhaust steam from the high pressure cylinder to the hot gases in the smokebox, thus recovering some heat that would otherwise go up the stack. This also further dried and expanded the intermediate steam before sending it on to the low pressure cylinders.) Steam locomotives also often incorporate superheaters, which helped extract more heat from the furnace, thus adding enthalpy to the steam even though the pressure did not increase.
A steam power plant normally uses a turbine to expand the steam and thereby extract work from it. The best turbines consist of a large number of stages, each one expanding the steam incrementally. This slow, even expansion is necessary to extract the maximum work possible. A turbine may also use a reheater to recover heat from flue gases. This is analagous to the smokebox reheater mentioned above.
A simple steam locomotive, on the other hand, expands the steam in one great burst, losing considerable efficiency in the process. A compound locomotive expanded the steam in two or more steps, thus allowing more work to be obtained from the same amount of steam.
The Mallet design, with two moderately sized cylinders on the fixed frame and two huge cylinders on the articulated frame, was only one of many compound locomotive configurations. It was invented for a specific purpose, to haul sizeable loads up grades and around curves that would frustrate a locomotive with a conventional rigid frame. To ask Mallets to do anything else is often counterproductive. So they were slow. Who would want to be going any faster than twenty miles per hour around ten degree curves or on grades of 2 to 4%?
Other compound designs were certainly capable of high speeds. The Atlantic design that was owned by Santa Fe (knicknamed "Bull Mooses" for their odd appearance) that used them to pull the special for Death Valley Scotty were compounds and they made as much as 90 to 100 miles per hour. Chapelon's locomotives were also high speed, high horsepower designs. It was a matter of designing for the task at hand. The arrangement of the cylinders varied widely and the number from a total of two to four or more.
Why, then, did compounds eventually lose the edge they supposedly held? Because they could be complicated mechanically, and designing and operating one to obtain the best efficiency was beyond all but a few people who understood the underlying thermodynamic principles. The vast majority of engineers and firemen wanted simple machines that were reliable and easy to operate and repair.
Why were condensers not used? Because they are inherently large and heavy. Where does a designer put such a piece of equipment? How does the designer arrange to reject the waste heat to the atmosphere, where it ultimately must go? At low pressures, the discharge steam from the engine requires large piping to conduct the steam to the condenser. Where are these pipes located? Not an easy problem to solve.
Putting on an efficient exhaust nozzle arrangement such as a Kylchap or Lempour was quite effective in lowering the final back-pressure at the cylinder exhaust, allowing the steam locomotive to approach the efficiency of a Rankine cycle engine operating between atmospheric pressure and temperature and the boiler pressure and superheater temperature. Using advanced and properly designed compounding coupled with good insulation of the boiler, steam piping, cylinders and other hot parts of the engine and paying attention to eliminating steam leakage at all joints paid off. Steam locomotives could have done, and still can do much better in terms of efficiency.
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 female dog 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?
Kevin Olsen
Montclair State University
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.
Any ideas?
Kevin
A & H,
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?
Atlantic and Hibernia 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. Any ideas? Kevin
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 turbine sizes they had in the turbine gallery. Think of it, there as a foreign "somewhere else" where they have to burn rocks to stay warm and keep the lights on! We are such a rich country that we have coal (shipped by rail by the Wisconsin and Southern, but that has all changed as that plant is going off coal and over to natural gas).
Steam engines are kewl and I go view them every chance I get, but I don't know how scheduling a field trip to a railroad museum or even to see a locomotive under steam advances the teaching mission in a thermo course.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
Paul Milenkovic Steam engines are kewl and I go view them every chance I get, but I don't know how scheduling a field trip to a railroad museum or even to see a locomotive under steam advances the teaching mission in a thermo course.
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.
Mark
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.
tdmidget 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 over into the realm of a Brayton cycle with higher pumping power required and a fuzzy phase transition of the working fluid.
And yes, you give up considerable efficiency for not using a condenser. But even if you have a locomotive with a condenser, an air-cooled condenser has much less vacuum as for practical considerations it has to reject heat at much higher temperature, and condensing steam locomotives did not offer much if any improvement in thermal efficiency on account of this.
But all practical heat engines are only approximations to the ideal cycle, and any open cycle (is a gas turbine really Brayton cycle if it exhausts to the atmosphere?) can be regarded as a closed cycle if you track through how water or air gets recycled in the environment.
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.
Thanks for sharing.
Paul Milenkovic 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 over into the realm of a Brayton cycle with higher pumping power required and a fuzzy phase transition of the working fluid.
Paul,
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.
tdmidget 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.
Anthony V.
Nothing quite as interesting as engineers arguing over the minuate of engineering principals.
Never too old to have a happy childhood!
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 . . .
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