Basics Compressor Calculation

Basic Ba sics s Comp ompre resso ssorr Calcu lcula lation tion

Sven-Olaf Zillmann Project Manager Sales Head of Team Pipeline Process Industry

MAN Diesel & Turbo SE

Sven Olaf Zillmann

Basics Calculation Compressor

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Basics Compressor Calculation For the calculation of a compressor two steps are necessary: 1. Thermodynamic calculation 2. Mechanical calculation


Sven Olaf Zillmann


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Basics Compressor Calculation Thermodynamic calculation

The compressor is a volumetric machine due to the geometry of the impeller


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Basics Compressor Calculation Thermodynamic calculation Step 1: get the usual process data The needed data are: Volume (mass) flow, inlet & outlet pressure, inlet temperature, molecular weight (gas composition)


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Step 2: Calculate the polytrophic head for the design point

h = R * z1 * T1 * n / (n – 1) * [(p2/p1)(n-1)/n - 1 ]

h = polytrophic head

[ kJ / kg ]

p1 = suction pressure at compressor flange

[bar a]

p2 = discharge pressure at compressor flange [bar a] T1 = suction temperature at compressor flange [K] z1 = compressibility at suction condition

typical value z1 = 0,92

R = gas constant = 8.315 / MW [kJ/kg K] MW = molecular weight n = polytropic exponent


typical value n = 1.4

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Step 3: calculate the gas power

P= m* h/

P = gas power

η

[kW]

m = mass flow delivered to the process [kg/s] h = polytropic head [kJ/kg] η

= average polytropic efficiency (eta)

typical value η = 0,84

The massflow can be calculated from the volume flow pressure unit is bara, temperature unit is Kelvin, diameter unit is meter

Radial and axial bearing losses: typically 50 - 80 kW


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Step 4: calculate the number of impeller

Nimp = h / (μy * u22)

Nimp = Number of impeller h = Head needed at design point [ J/kg ]

μy

= head coefficient,

typical value 0.48 to 0.54

u2 = peripheral speed

[ m/s ]

typical value = 250 to 280 m/s


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Basics Compressor Calculation Mechanical calculation

Two mechanical calculation has to be done:

1. Lateral analysis To find out the lateral eigenfrequencies of the compressor. The excitation comes from the gas dynamic and the mass distribution on the shaft.

2. Torsional analysis To find out the torsional eigenfrequencies of the whole train. The excitation comes from the driver e.g. motor or gas turbine. A motor with a frequency converter is more complicated than a gas turbine.


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Basics Compressor Calculation Mechanical calcul ation

The lateral analysis must be done to ensure the stability of the compressor. The analysis shows if the compressor runs in or near the natural eigenfrequencies (lateral critical speed).


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Basics Compressor Calculation Mechanical calcul ation

The torsional analysis must be done to ensure the stability of the whole train. This includes the compressor, gear (if necessary) and the driver. Especially if a VFD motor is the driver of the train. A VFD creates an excitation by running in part load. This is typically for an VFD.


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Disclaimer

All data provided in this document is non-binding. This data serves informational purposes only and is especially not guaranteed in any way. Depending on the subsequent specific individual projects, the relevant data may be subject to changes and will be assessed and determined individually for each project. This will depend on the particular characteristics of each individual project, especially specific site and operational conditions.


Sven Olaf Zillmann


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Do you have any more questions?

Sven-Olaf Zillmann

Benjamin Omlor

[[email protected]]

[[email protected]]


Sven Olaf Zillmann


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18.11.2013

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Basics Compressor Calculation

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