Current page :. Follow us on Twitter Question, remark? Contact us at contact myengineeringtools. The compression of gases in compressors is often idealized as isentropicwhich means that there are no friction, no leakage of gas in between high and low pressure side, and that the compressor is perfectly insulated.
In reality this is of course not achieved which means that the discharge temperature is higher than for an isentropic compression at same compression ratio. As a consequence, at the same discharge temperature, the compression power transmitted to the fluid will be lower, which means that an actual compressor will require more power than the same isentropic compressor to reach the same pressure.
The polytropic efficiency is given by the manufacturer of the compressor. If you have an existing compressor or consider a new one, you can find such references on manufacturers datasheets.
If this is not available, you can approximate the polytropic efficiency of centrifugal compressors as the polytropic efficiency is linked to the flow at inlet of the compressor. The polytropic efficiency calculation formula is the following  :. The gas isentropic coefficient is the specific heat ratio of the gas, which means that the heat capacity at constant pressure is divided by the heat capacity at constant volume. You can access the specific heat ratio of different gas on My Engineering Tools.
The specific heat ratio of the gas is used in the calculation of the power of an ideal compressor running in isentropic or adiabatic compression. However this ideal ratio is changing in a true centrifugal compressor for which the outlet temperature differs from the adiabatic one. The isentropic coefficient k must then be replaced by the polytropic coefficient n. The following formula can be used to calculate the polytropic compression power for a centrifugal compressor :.
It is finally required to divide the compression power calculated at STEP 4 by the polytropic efficiency to calculate what will be the actual power consumed, considering the losses happening in the compressor during the compression process.
For references of industrial compressors and discussions with experts, the reader can refer to the following company no link with MyEngineeringTools. Reciprocating compressors are usually cooled, which means that the discharge temperature is close to the isentropic discharge temperature. For these compressors, the isentropic power calculation is therefore relevant and can be used to calculate the reciprocating compressor power.
The Engineer wishes to calculate the actual compression power to foresee for a centrifugal compressor.
The following formula can be used to estimate a centrifugal compressor polytropic efficiency when used under those conditions :.
At low pressure, Z is assumed to be equaled to 1 here. You can access a free Excel calculation nest zone control to make the compressor power calculation explained above : Compressor Power Calculator.Log In. Thank you for helping keep Eng-Tips Forums free from inappropriate posts. The Eng-Tips staff will check this out and take appropriate action.
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Join Us Close. Are you an Engineering professional? Join Eng-Tips Forums! Join Us! By joining you are opting in to receive e-mail. Promoting, selling, recruiting, coursework and thesis posting is forbidden. Students Click Here. Related Projects. Can any one explain in simple terms as understood by a mechanical Engineer about Polytropic Head in a Centrifugal Compressor?
Also for a definition: Head, polytropic. The energy in foot pounds required to compress polytropically and to transfer one pound of a given gas from one pressure level to another. I'm not sure if it would help or not but perhaps a discussion of a polytropic process would help. The extra energy goes into the gas as an increase in temperature. For values of n less than k, there is heat being removed during the compression process.
Note that an isentropic process is not the most efficient in terms of power required.You must log in to edit PetroWiki. Help with editing. Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. More information.
This page provides an overview of the primary categories of natural gas compressor services and a description of the different classifications and types of compressors available to the industry. Adiabatic and polytropic compression theory are discussed with supporting definition of terminology. Major components and construction features of centrifugal and reciprocating compressors are emphasized.
Installation, safety, and maintenance considerations also are discussed in their erspective pages. Compressors used in the oil and gas industry are divided into six groups according to their intended service. These are:. Flash gas compressors typically handle low flow rates and produce high compression ratios.
Gas lift compressors are frequently used in oil handling facilities where compression of formation gases and gas lift gas is required. Gas lift compressor duty is frequently of low to medium throughput with high compression ratios. Many gas lift compressors are installed on offshore facilities. The reinjection of natural gas is employed to increase or to maintain oil production.
Reinjection compressors can be required to deliver gas at discharge pressures in excess of 10, psi. Reinjection compressors also are used for underground storage of natural gas. Compressors, applied to these services, have large compression ratios, high power requirements, and low volume flow rates. Gas transmission through pipelines results in pressure drop because of friction losses. Booster compressors are used to restore the pressure drop from these losses.A polytropic process is a thermodynamic process that obeys the relation:.
The polytropic process equation can describe multiple expansion and compression processes which include heat transfer. In addition, when the ideal gas law applies:. For an ideal gas in a closed system undergoing a slow process with negligible changes in kinetic and potential energy the process is polytropic, such that.
For certain values of the polytropic index, the process will be synonymous with other common processes. Some examples of the effects of varying index values are given in the following table.
A solution to the Lane—Emden equation using a polytropic fluid is known as a polytrope. From Wikipedia, the free encyclopedia. For the term used in biogeographysee Pantropical. For polytropism in viruses, see Amphotropism. The classical Carnot heat engine. Classical Statistical Chemical Quantum thermodynamics. Zeroth First Second Third.
System properties. Note: Conjugate variables in italics. Work Heat. Material properties. Carnot's theorem Clausius theorem Fundamental relation Ideal gas law.
Free energy Free entropy. History Culture. History General Entropy Gas laws. Entropy and time Entropy and life Brownian ratchet Maxwell's demon Heat death paradox Loschmidt's paradox Synergetics.
Caloric theory Theory of heat. Heat ". Thermodynamics Heat engines.
Nucleation Self-assembly Self-organization Order and disorder. Polytropes: Applications in Astrophysics and Related Fields.
Categories : Thermodynamic processes. Hidden categories: All articles with unsourced statements Articles with unsourced statements from June Namespaces Article Talk.Jump to content. Low Flow in Pipes - posted in Ankur's blog.Isentropic efficiency with example
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Posted 24 September - PM. Community Forum Software by IP. Sign In Create Account. Featured Articles Check out the latest featured articles. File Library Check out the latest downloads available in the File Library. New Article Product Viscosity vs. Featured File Vertical Tank Selection. This topic has been archived. This means that you cannot reply to this topic. Thank you Attiq Ahmad.
What is Polytropic Process – Definition
A Polytropic Compression Process is typical of a dynamic-type compressor — such as a centrifugal compressor. A reciprocating compressor typically follows an Adiabatic Compression Process very closely.
If you plot the compression curves for Isothermal, Adiabatic, and Polytropic compressions on a Pressure versus Volume graph, you will find that the Polytropic curve falls between the other two, but closer to the Adiabatic curve. Polytropic Head The Polytropic Head is an expression used for dynamic compressors to denote the foot-pounds of work required per pound of gas. I strongly recommend that you seek out more information on these subjects through the Google Search engine.
You will find a wealth of information on the Polytropic Process and other related processes in Thermodynamics.
If you want to secure good responses that you can track then you should only post one thread on the subject and follow it through to its completion. I think it posted due to my fault. Anyway its always nice to hear some valueable advice from your side.
Attiq Ahmad. I will be happy if u can provide me the calculations or steps for the same. If you're still here, can you explain how a compressor can be Isothermal?A polytropic process is any thermodynamic process that can be expressed by the following equation:. The polytropic process can describe gas expansion and compression which include heat transfer. Thermodynamic Processes. We hope, this article, Polytropic Processhelps you. If so, give us a like in the sidebar.
Main purpose of this website is to help the public to learn some interesting and important information about thermal engineering. Main Menu. Definition and characteristics of Polytropic Process. Thermal Engineering. Polytropic process — main characteristics Polytropic processes with various polytropic indexes.
Nuclear and Reactor Physics: J. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed. Lamarsh, A. Baratta, Introduction to Nuclear Engineering, 3d ed.
Glasstone, Sesonske. Nuclear and Particle Physics. Physics of Nuclear Kinetics. Addison-Wesley Pub. January Advanced Reactor Physics: K. Ott, W.A compressor is device to raise the pressure of a compressible fluid. Work is required as an input to the compressor in this process.
The first law of thermodynamics, which is just the application of energy balance, when applied in the context of steady flow compression gives us the following relationship which was derived in Chapter 02 as For the sake of brevity alone, we shall write total enthalpies as if they were just static enthalpies.
Practically, the kinetic energy effect might be insignificant, but if we find that the accuracy has fallen below our expectation, we can use total enthalpies instead of static enthalpies without altering the structure of these equations. For a perfect gas the enthalpy difference can be arrived in a more simple manner and we can write as. Please note that compressors using interstage cooling or diaphragm cooling should never use this approximation as the error would be unacceptable.
While the first law helps us to establish the power consumption of an actual compressor, we have not been guided about the power that would be consumed by an ideal compressor. So what is an ideal compressor or a compression process? Theoritically there are three cases of process compression which can be used for benchmarking the performance of an actual compression process. These are:. These processes have been illustrated on the P-v diagram above.
We have already demonstrated in Chapter 05, that out all processes that can occur between two states, the reversible process is the most efficient.
Polytropic Head Of Centrifugal Compressor
So while benchmarking, the compression processes, reversibility will also be a condition for computing the ideal head. The equation for work done in a reversible steady flow process was derived in Chapter 05 as equation 19 and is reproduced below:.
For the case of compressor where we want to treat work input as positive quantity and after neglecting the kinetic energy and potential energy terms, the ideal work of compression can be expressed as:. In an isothermal process, the heat developed in the compression process is allowed to escape by arranging sufficient cooling and the temperature remains constant. Practically, a finite temperature gradient is required, for a heat transfer to happen. Therefore, implementation of isothermal compression would require, infinite amount of heat transfer area and infinite amount of time for this thermal interaction.
While, it may be impractical to build a compressor that truly behaves like an isothermal machine, we can try to build machines that are more and more closer to this, by increasing the amount of intercooling during the compression process. On evaluating the above integral the ideal work in for the compression process assuming isothermal compression as reference is.
Isothermal compression is best visualised on a T-s diagram. The area under the curve shows the heat rejected by the compressor.