Ludtke, "Process Centrifugal Compressors". I have to say that K. Ludtke is only speaking for dynamic compressors. For positive displacement compressors his statement is nearly perfectly incorrect. Zdas, thanks for this precision. I take the occasion to ask you a question, just to better my understanding of positive displacement PD machines.
But we know that when the gas behavior departs from ideal gas, isentropic exponent "split" into an "isentropic volume exponent kv" and an "isentropic temperature exponent kt". Why don't we pick these two exponents instead of the adiabatic exponent for the purpose of performance calculation?
In other words, this would be equivalent of assuming PD compression process as subset case of a polytropic path, that is to say polytropic efficiency set equal to one, subsequently we would use kv for calculation of head and kt for temperature e.
Would this be more accurate or am I completely off the mark, in which case please excuse my ignorance. I don't know that you are completely off the mark, but you would be limiting some calculations unnecessarily.
If you can assume an adiabatic process and I've done some very detailed work that convinced me that the departures from adiabatic performance are outside the uncertainty of the rest of the calculations, so I'm comfortable with PD machines being adiabatic , then you have access to some effective simplifying assumptions that make life in the PD world easier than in the dynamic compression world.
For example, I can calculate a performance envelope for a PD machine with just knowledge of the frame MAWP, available HP, interstage cooler capacity for a multi-stage machine, and maximum acceptable temperature. It ends up a lot simpler than we see for dynamic machines. This is new to me, that the polytropic exponent n used for compressor poly head and power calcs is not the same as the value of n to be used for actual discharge temp calcs for centrifugals.
Does this Ludtke book give us an idea of how different these two values of n are for some typical gas and operating pressure range? I've done so many of these calcs for projects in the past, and I dont recall any significant discrepancies with compressor vendor predictions, so I suspect these two values must be nearly the same?
Gents, Please excuse my ignorance, would you please give a simplified answer for my thread? Don't know about pressure ranges, guess accuracy is purpose driven. They would also depart from each other, thus resulting in inaccuracies from discrepancies to serious deviations in compression process calculations. Polytropic exponent n is influenced by compression polytropic eff also, so how does this GERG program compute these values without polytropic eff input?
I'll do a google search on this in any case - would be good to find thermodynamic differential expressions that show the difference between these. And yes, if you would like to understand better about polytropic exponents in particular, K. Ludtke is an excellent reference too. In short, the point raised here is that isentropic volume exponent kv, isentropic temperature exponent kt and heat ratio depart one another when real gas applications come into play.
An example was given to illustrate the concept. I used GERG which is practical for deriving selected thermodynamic properties, but it could have been another equation of state. The polytropic exponents volume and temperature are derived based on kv, kt and polytropic efficiency, that is to say via simple mathematical formulas.
Hope this clarifies. This also probably explains why the GPSA procedure for compressor calcs remains unchanged all these years as it requires thermodynamically derived isentropic exponent values. I don't think I succeeded to pass the point, sorry my communication failure Other equation of state will demonstrate the same behavior, the departure effect will be captured mathematically in a different manner and to a lower accuracy with the GERG setting the benchmark.
But this is accessory to the topic raised. Does the cyclic integral of heat have to be zero i. Do include a brief explanation. In large compressors, In almost every project, issues arise because tasks fall behind, schedules have to changed, resources are impacted, and scope may change. This dynamics of projects cause challenges for the project manager and stakeholders because ways of compressing The volume at Create an Account and Get the Solution.
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