Engineering task and calculation objective
The QKUK module estimates the coupling power of a compressor from the delivered nominal (standard) volume flow and the specific power consumption. This makes it possible to determine the drive power requirement of a compressed air or process gas compression in an early project phase, before a detailed thermodynamic compressor design or a manufacturer's quotation is available.
The starting point is the specific power consumption, i.e. the power to be expended per unit of standard volume flow for the required pressure increase. This base value is adjusted to the actual operating conditions by corrections: the suction temperature and the intercooling return temperature enter as a temperature correction, and the number of compressor stages via a correction factor for the n-stage compressor. The result is the power required at the coupling as the sizing quantity for motor, gearbox and energy demand estimation.
Such estimating calculations are the usual basis in plant engineering for budgeting the drive power, energy costs and cooling water demand of a compressor station; the binding design remains the task of detail engineering with the compressor manufacturer.
Calculation workflow
- Define the delivered flow: The standard volume flow of the gas to be compressed is entered as the base quantity; the specific power consumption is referred to it.
- Apply the specific power consumption: For the required pressure increase, the specific power consumption is applied; it describes the drive power of the compressor required per unit of standard volume flow under reference conditions.
- Correct for temperature effects: If the suction temperature and the intercooling return temperature deviate from the reference conditions, the power requirement is adjusted via the temperature correction – higher suction or intercooling temperatures increase the compression work.
- Account for the number of stages: The number of compressor stages enters via the correction factor f of the n-stage compressor: more stages with intercooling bring the compression closer to the isothermal limit and reduce the specific power consumption.
- Calculate the coupling power: From the standard volume flow, the corrected specific power consumption and the correction factor, the coupling power is obtained as the basis for selecting the drive motor and estimating the energy demand.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Nominal vlome flow | VN | m³/h |
| Specific power consumption | L | kWh/m³ |
| Number of compressor stages | n | – |
| Suction temperature | T1 | °C |
| Cooling temperature | TR | °C |
Calculated results
| Quantity | Symbol | Unit |
|---|---|---|
| Temperature correction | dT | °C |
| Correction factor(n-staged compressor) f | Kompressor) | – |
| Clutch performance | Pku | kW |
Frequently asked questions
What does the coupling power include, and what does it not include?
The coupling power is the mechanical power to be delivered at the compressor shaft or coupling, including the internal and mechanical losses of the compressor. Not included are the losses of the motor, the frequency converter and, where applicable, the gearbox – the electrical power consumption is therefore higher by their efficiencies.
Why does multi-stage compression with intercooling reduce the power requirement?
The compression work grows with the gas temperature at the stage inlet. If the gas is cooled back to the intercooling return temperature between stages, the process approaches isothermal compression as the theoretical minimum. The gain is larger the higher the overall pressure ratio; this is why the number of stages enters the calculation via the correction factor f.
How accurate is such an estimating calculation?
It typically achieves an accuracy in the range of a few percent up to about ten percent, depending on how well the specific power consumption matches the actual machine type (screw, reciprocating, turbo compressor) and the operating point. A margin is customary for motor selection; the binding power figure comes from the manufacturer's design per ISO 1217 or the applicable acceptance codes.
What must be observed regarding the standard volume flow?
The reference state: standard volume flow (e.g. 0 °C, 1.01325 bar), normalised volume flow and suction volume flow differ considerably. The specific power consumption must be referred to the same reference state as the volume flow used, otherwise the result is systematically wrong.