Rankine Cycle Introduction (LearnChemE.com, 4min) The Carnot cycle becomes impractical for common large scale application, primarily because H2O is the most convenient working fluid for such a process. When working with H2O, an isentropic turbine could easily take you from a superheated region to a low quality steam condition, essentially forming large rain drops. To understand how this might be undesirable, imagine yourself riding through a heavy rain storm at 60 mph with your head outside the window. Now imagine doing it 24/7/365 for 10 years; that's how long a high-precision, maximally efficient turbine should operate to recover its price of investment. Next you might ask why not use a different working fluid that does not condense, like air or CO2. The main problem is that the heat transfer coefficients of gases like these are about 40 times smaller that those for boiling and condensing H2O. That means that the heat exchangers would need to be roughly 40 times larger. As it is now, the cooling tower of a nuclear power plant is the main thing that you see on the horizon when approaching from far away. If that heat exchanger was 40 times larger... that would be large. And then we would need a similar one for the nuclear core. Power cycles based on heating gases do exist, but they are for relatively small power generators.
With this background, it may be helpful to review the relation between the Carnot and Rankine cycles. (LearnChemE.com, 6min) The Carnot cycle is an idealized conceptual process in the sense that it provides the maximum possible fractional conversion of heat into work (aka. thermal efficiency, ηθ).
Comprehension Questions:
1. Why is the Carnot cycle impractical when it comes to running steam through a turbine? How does the Rankine cycle solve this problem?
2. Why is the Carnot cycle impractical when it comes to running steam through a pump? How does the Rankine cycle solve this problem?
3. It is obvious which temperatures are the "high" and "low" temperatures in the Carnot cycle, but not so much in the Rankine cycle. The "boiler" in a Rankine cycle actually consists of "simple boiling" where the saturated liquid is converted to saturated vapor, and superheating where the saturated vapor is raised to the temperature entering the turbine. When comparing the thermal efficiency of a Rankine cycle to the Carnot efficiency, should we substitute the temperature during "simple" boiling, or the temperature entering the turbine into the formula for the Carnot efficiency? Explain.
Chapter 5 - Thermodynamics of Processes
Chapter 5 - Thermodynamics of Processes
05.2 - The Rankine cycle
05.2 - The Rankine cycle
Rankine vs. Carnot: LearnChemE
Using XSteam Excel (4:46) (ms
Using XSteam Excel (4:46) (msu.edu)
This utility is helpful once you have learned how to interpolate reliably. It saves the tedium.
Using XSteam Matlab (4:20) (m
Using XSteam Matlab (4:20) (msu.edu)
This utility is helpful once you have learned how to interpolate reliably. It saves the tedium.
Rankine Example
Thermal Efficiency with a 1-Stage Rankine Cycle. (uakron.edu, 12min) Steam from a boiler enters a turbine at 350C and 1.2MPa and exits at 0.01MPa and saturated vapor; compute the thermal efficiency (ηθ) of the Rankine cycle based on this turbine. (Note that this is something quite different from the turbine's "expander" efficiency, ηE.) This kind of calculation is one of the elementary skills that should come out of any thermodynamics course. Try to pause the video often and work out the answer on your own whenever you think you can. You will learn much more about the kinds of mistakes you might make if you take your best shot, then use the video to check yourself. Then practice some more by picking out other boiler and condenser conditions and turbine efficiencies. FYI: the conditions of this problem should look familiar because they are the same as the turbine efficiency example in Chapter 4. That should make it easy for you to take your best shot.
Comprehension Questions:
1. The entropy balance is cited in this video, but never comes into play. Why not?
2. Steam from a boiler enters a turbine at 400C and 2.5 MPa and exits a 100% efficient turbine at 0.025MPa; compute the Rankine efficiency. Comment on the practicality of this process. (Hint: review Chapter 4 if you need help with turbine efficiency.)
Rankine Example Using Steam.xls
Rankine Example Using Steam.xls (uakron.edu, 15min) High pressure steam (254C,4.2MPa, Saturated vapor) is being considered for application in a Rankine cycle dropping the pressure to 0.1MPa; compute the Rankine efficiency. This demonstration applies the Steam.xls spreadsheet to get as many properties as possible.
Comprehension Questions:
1. Why does the proposed process turn out to be impractical?
2. What would you need to change in the process to make it work? Assume the high and low temperature limits are the same. Be quantitative.
3. What would be the thermal efficiency of your modified process?
05.3 - Rankine modifications
05.3 - Rankine modifications
Improving Thermal Efficiency
Improving Thermal Efficiency with a 2-Stage Rankine Modification (uakron.edu, 15min) Power is to be generated between temperatures of 500 C and 65 C (0.025MPa) with the steam quality not to drop below 100%. This coursecast compares a simple Rankine cycle and one modification in which the single turbine expansion is replaced with two turbine stages. The thermal efficiency increases from 27% to 39% as a result of this modification. Such an increase could equate to millions of dollars per year at a decent sized electric power plant. These considerations motivate careful analysis of thermal efficiency under multiple permutations of modifications. Ultimately, the Carnot efficiency cannot be surpassed, however. Also, the optimal configuration for a particular facility (like cogeneration at a chemical plant) will depend on other demands like the need for medium pressure steam dedicated to other purposes.
Comprehension Questions:
1. Do you think that turning this process into a 3-stage Rankine cycle would increase the thermal efficiency another 12%, from 39% to 51%? Explain.
2. Suppose the turbines were 85% efficient. How would you approach this problem in that case?
Process Reporting
Organizing your calculations in a spreadsheet (uakron, 18min) Scratching numbers on a crude process flow diagram (PFD) may be convenient for a rough calculation, but it is not the kind of document that would want to submit for a project report or even for homework. Organizing your calculations can make them easier to convey and defend, and you may even catch a typo or two relative to your rough calculation. Did you ever wonder, "How can I show how I did my calculations so I can get some partial credit? Just hand in the spreadsheet? But this lame prof wants a hard copy...?" Then you definitely need to watch this video. If you are indeed writing a project report, you might like to review how to construct your PFDs using a process simulator.
Comprehension Questions:
1. Create a spreadsheet version of the Table associated with Example 5.3 in the textbook. Add a column to indicate the order in which the calculations were performed. Add another column to indicate the variables that specified each state in the table. Use the space under this table to indicate how the work, heat, and efficiency were computed.
05.4 - Refrigeration
05.4 - Refrigeration
OVC Intro
Refrigeration Cycle Introduction (LearnChemE.com, 3min) explains each step in an ordinary vapor compression (OVC) refrigeration cycle and the energy balance for the step. You might also enjoy the more classical introduction (USAF, 11min) representing your tax dollars at work. The musical introduction is quite impressive and several common misconceptions are addressed near the end of the video.
Comprehension Questions: Assume zero subcooling and superheating in the condenser and evaporator.
1. An OVC operates with 43 C in the condenser and -33 C in the evaporator. Why is the condenser temperature higher than than the evaporator temperature? Shouldn't it be the other way around? Explain.
2. An OVC operates with 43 C in the condenser and -33 C in the evaporator. The operating fluid is R134a. Estimate the pressures in the condenser and evaporator using the table in Appendix E-12.
3. An OVC operates with 43 C in the condenser and -33 C in the evaporator. The operating fluid is R134a. Estimate the pressures in the condenser and evaporator using the chart in Appendix E-12.
4. An OVC operates with 43 C in the condenser and -33 C in the evaporator. The operating fluid is R134a. Estimate the pressures in the condenser and evaporator using Eqn 2.47.
5. An OVC operates with 43 C in the condenser and -33 C in the evaporator. Assume the compressor of the OVC cycle is adiabatic and reversible. What two variables (P,V,T,U,H,S) determine the state at the outlet of the compressor?
Reading A PH Chart
How To Read the Pressure-Enthalpy (PH) Diagram for Propane (uakron.edu, 9min) A chemical process may need refrigeration to operate the condenser of a distillation column at cryogenic conditions. The process in this video operates between -100F and 80F in its refrigeration coils.
Comprehension Questions: Assume zero subcooling and superheating. (ie. The "approach temperature" is zero.)
1. Download the table of saturation properties for propane from the student supplements section of chethermo.net. Estimate the pressures and enthalpies exiting the condenser and evaporator. How do these compare to the values reported in the video?
2. Suppose the condenser outlet operated at 100 F. Estimate the condenser pressure (MPa).
3. Suppose the condenser outlet operated at 100 F. Estimate the condenser outlet enthalpy(J/g)
4. Suppose the condenser outlet operated at 100 F. Estimate the condenser inlet temperature(F)
5. Suppose the condenser outlet operated at 100 F. Estimate the condenser inlet enthalpy (J/g)
COP of OVC
COP of OVC Cycle (uakron.edu, 7min) operating at -100F in the evaporator with a compressor that takes the saturated vapor from the evaporator to 10 bars and 180F. This presentation continues the example of compressor efficiency, using similar outlet conditions to those developed previously. You may wish to refer back to that example if you haven't watched it lately. Also, you may want to refresh your chart reading skills before you watch this first video because it is based on chart reading for the compressor work calculation. As an alternative to chart reading, (uakron, 10min) you might be interested in using the ideal gas equation of state to estimate the compressor work and the propane tables to look up the saturation properties. The significance of PHi/PLo is somewhat more obvious when using the ideal gas equation. This last presentation is slightly different from the compressor efficiency problem in Section 4.10 because the compressor inlet saturated vapor (exiting the evaporator) is determined from the propane table instead of the SCVP eq. Comparing the chart calculation to the calculation based on tables and the ideal gas assumption shows that the estimates are not so different. You should have the skills to perform the calculation either way.
Comprehension Questions:
1. Suppose you wanted to run a refrigeration process between the same condenser and evaporator temperatures using HFC134a as the working fluid. How would that affect the COP? How would the pressures in the process change? Would there be some practical concerns about the pressure ranges involved with either propane or HFC134a?
2. Suppose you replaced the throttle with a turbine. How would that affect the shape of the cycle on a P-H diagram?
3. What was the numerical value of the compression ratio, PHi/PLo, applied in the presentation using the ideal gas assumption?
05.5 Liquefaction
05.5 Liquefaction
Joule-Thomson Coefficient
Joule-Thomson Expansion (LearnChemE.com, 7min) describes the Joule-Thomson coefficient - (dT/dP)H. For non-ideal fluids (including liquids), the temperature usually drops as the pressure drops. From a molecular perspective, it requires energy to rip molecules apart when they are in their attractive wells, and this energy must be taken from the thermal energy of the molecules themselves if the system is adiabatic. This video refers to the PREOS.xls spreadsheet to be used more in Unit II, but you can get the idea of how the Joule-Thomson expansion provides a basis for any liquefaction of any chemical, including the liquefaction that occurs in refrigeration and the one that occurs in a process designed to simply recover liquid product (e.g. liquefied natural gas (LNG), aka. methane).
Comphrehension Questions:
1. Referring to the table for R134a in Appendix E-12, compute the fraction liquid at 252K after throttling from a saturated liquid at 300K.
2. Referring to the table for R134a in Appendix E-12, compute the fraction liquid at 252K after expanding a saturated liquid at 300K through a reversible turbine.
05.6 Engines
05.6 Engines
Turbojet Engine
Acceleration of an Airplane by a Turbojet Engine (LearnChemE.com, 10min) calculates the initial acceleration of an airplane powered by a turbojet engine. Demonstrates application of the energy and entropy balance for individual components of a composite system and for the system as a whole.