rQuestionEngineering
Mechanical
What is a good analogy for a layman to understand Polytropic Work ?
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What is a good analogy for a layman to understand Polytropic Work ? 1
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georgeverghese
Chemical
Polytropic work (or power) is almost the same as actual brake shaft power to the compressor
Shaft power = Poly power + minor losses.
These minor losses are due to bearing friction, seal losses, and a nominal 1% of poly power may be allocated to it at the design stage, until compressor vendor advises.
Poly power = Poly head x mass flow / Poly eff; all in SI units,
where poly head = kN.m/kg, mass flow = kg/sec
Shaft power = Poly power + minor losses.
These minor losses are due to bearing friction, seal losses, and a nominal 1% of poly power may be allocated to it at the design stage, until compressor vendor advises.
Poly power = Poly head x mass flow / Poly eff; all in SI units,
where poly head = kN.m/kg, mass flow = kg/sec
Polytropic work is associated to so called small step efficiency.
In short, isentropic efficiency measures how far you deviate from isentropic compression process (adiabatic and reversible).
For real gases and as the compression ratio increases, there is another effect that needs to be taken into account and this effect is associated with the shape of compression path on a T-s diagram.
So, if on a T-s diagram you divide a certain compression path from A to B into very small steps and then measure the isentropic work on each step and compare the sum of these versus the isentropic work going A to B in one step, you would see that you will not find a same value.
In fact, polytropic efficiency measures departure from isentropic work plus incorporates the effect of shape of compression path on T-s diagram as the compression ratio increases. When compression ratio is very small polytropic efficiency virtually equals isentropic.
Thus it is useful parameter to compare compressor performance at different pressure ratios.
In short, isentropic efficiency measures how far you deviate from isentropic compression process (adiabatic and reversible).
For real gases and as the compression ratio increases, there is another effect that needs to be taken into account and this effect is associated with the shape of compression path on a T-s diagram.
So, if on a T-s diagram you divide a certain compression path from A to B into very small steps and then measure the isentropic work on each step and compare the sum of these versus the isentropic work going A to B in one step, you would see that you will not find a same value.
In fact, polytropic efficiency measures departure from isentropic work plus incorporates the effect of shape of compression path on T-s diagram as the compression ratio increases. When compression ratio is very small polytropic efficiency virtually equals isentropic.
Thus it is useful parameter to compare compressor performance at different pressure ratios.
<p style="font-size:14px font-family:'Arial nova'">Polytropic Work is the amount of work being done following Polytropic compression process.
To understand polytropic compression, you have to understand two other processes :
(1) Isobaric process, wherein pressure remain constant and volume changes with temperature.
(2) Isothermal process, wherein temperature remains constant and pressure changes with changes in volume.
Polytropic process is a process in between the above two processes.
</p>
To understand polytropic compression, you have to understand two other processes :
(1) Isobaric process, wherein pressure remain constant and volume changes with temperature.
(2) Isothermal process, wherein temperature remains constant and pressure changes with changes in volume.
Polytropic process is a process in between the above two processes.
</p>
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