efficiency describing the deviation of the efficiency of the analyzed compressor compressing air according to a certain polytrope to the ideal compression process according to isentropy. This indicator takes into account the discharge pressure and can therefore be used to compare compressors or compressor stations of similar size, even if they operate at different operating pressures. Isentropic efficiency compares the actual energy consumption of a compressor or compressor station with the theoretical energy consumption of an idealized compressor. Since this idealized consumption cannot be realistically realized, the actual number does not matter much, but is extremely useful when comparing several compressors or compressor stations with each other. This indicator has a different nature than the specific power of the compressor, which cannot be compared in systems operating at different pressures, it is a measure that can be applied to compressors built for different working pressures and operating at different actual pressures. To calculate the isentropic efficiency, we can use this formula. All entries must be expressed in basic ISO units:
As “E/V” is actually SSP (specific power) we can also write:
- SSP in this case must be in Joules/cubic meter
- p1 … input absolute pressure in Pascal (usually atmospheric pressure)
- p2 … absolute discharge pressure in Pascal (atmospheric + gauge pressure)
- V … volume of compressed air at inlet conditions (at input pressure and temperature (FAD)) in cubic meters
- E … total energy used in Joules
- γ … ratio of specific heats (assumed 1.4 for air)
How to read Isentropic efficiency:
- < 50% – poor inefficient system
- 50-70% – average system efficiency
- 70-80% – good system efficiency
- 80-100% – very good system efficiency
- 100% – better then theoretically best system – system measurement or calculation error
- 100% – measurement or calculation error
Usage of isentropic efficiency is considered more suitable for the following reasons:
- Simplified comparison between different operating pressures / pressure ratios;
- Isentropic efficiency is widely accepted in other (i.e. non-industrial air) technical fields like energy technology;
- Isentropic efficiency is less sensitive regarding deviation of measurement conditions (operating point) and gas properties when comparing to specific power requirement;
- Compressors without internal or inter-stage cooling are physically not able to compress isothermally;
- Due to additional losses, no compressor with internal cooling is able to reach isentropic compression.
Simply stated, isentropic efficiency is a ratio that indicates how the real energy consumption of an air compressor compares to that for an idealized compression process. The value is expressed as a percentage, and a compressor with a higher number is more efficient at converting electrical energy into compressed air potential energy than a compressor with a lower number.
There is no ideal compression, as there are inevitable losses in the real world. For a bare compressor, the power consumed at its shaft (actual work) includes internal losses such as frictional (e.g., created by lubricant film between rotating and static components), inertial (e.g., created by speed changes of reciprocating masses on piston compressors), flow resistance (e.g., pressure losses at intake and outlet ports, including valve losses on reciprocating units) and mass–flow losses (e.g., gas density changes resulting from oil injection heating of rotary screw threads).
For a packaged compressor, the bare compressor losses are combined with driver efficiency (e.g., electric motor efficiency), flow resistance (e.g., pressure losses introduced by oil separation vessels, filters, heat exchangers, pipe works, etc.) and mass-flow losses (e.g., gas density changes resulting from heating effects introduced by protective enclosures).
Isentropic efficiency, which can be calculated and measured independently of compression technology (positive displacement, dynamic, etc.), is not measured directly; rather, it is derived from power consumption, pressure ratio, and delivered capacity. Within the range of pressures typically found in industrial compressed air systems, this measure as it pertains to a specific machine does not change with changes in pressure. In order for an isentropic efficiency number to be meaningful, it is essential that both the ideal and actual work values be calculated or measured at the same operating pressure.
Fixed speed compressors have one isentropic efficiency number. Variable speed compressors have their isentropic efficiency determined as a weighted average of efficiencies at various, standardized load levels. This reflects the performance of compressors running between 40% and 100% of their capacity. If the demand is in a smaller range than that, it may be prudent to check the specific power curve on the data sheets and compare compressors operating within that range at the anticipated pressure rating.
- Isentropic Efficiency compares the actual performance of a device to the performance under idealized circumstances for the same inlet and exit states
- In the simplest form, we are measuring how efficiently the compressor is converting electrical energy into compressed air potential energy
- It is measured as a percent and the higher the %, the more efficient the compressor.
- The better the compressor design is at reducing the real world losses, the higher the IE will be and the lower its specific power will be.
- Isentropic Efficiency makes it far easier to compare compressors with different full-load operating pressures
IE = Energy (kW) for the Ideal Compression Process /Actual Energy (kW) Consumed
- The key is that IE incorporates discharge pressure into the calculation such that the IE rating of compressors with varying discharge pressures are comparable
- The IE metric makes the comparison apples-to-apples on full-load operating pressure, which is critical
The better the compressor design is at reducing the real-world losses, the higher the Isentropic Efficiency will be and the lower its Specific Power will be.
Ideally, use of both numbers, isentropic efficiency and specific power, will allow individuals to make the best choice for their specific application and situation. When evaluating compressor systems, the use of isentropic efficiency provides the customer with additional valuable information to evaluate the suitability of products for the customer’s specific application. By itself, efficiency does not provide a means to make a selection because it needs to be quoted with capacity and rated pressure. For two compressors that are rated at similar capacities and rated pressures though, using efficiency values does provide a direct means of comparison.
