Two-stage screw compressor: around 10% less energy than a standard screw
A two-stage screw compressor compresses air in two steps with intercooling in between, close to the theoretical isothermal cycle. Around 10% less energy than a standard screw at constant load; with a permanent magnet drive, on a variable load, the two gains combine.
Why compress in two stages
Gas compression is exothermic: the more you compress, the higher the temperature climbs. With no cooling at all, an ideal adiabatic compression from 1 to 8 bar absolute would take air from 20 °C to around 260 °C. In a lubricated screw, the injected oil absorbs most of that heat as compression proceeds, so the air leaves the airend well below that value. The more the air heats up during compression, the more work compression requires: this is what cooling between two stages reduces.
A two-stage machine compresses in two steps: stage 1 raises the air to 3-4 bar, the air is cooled between the two stages (by an intercooler, or, on a lubricated screw, by oil injected into the channel that links the two stages), then stage 2 finishes at 8 bar. The second stage works on cooled, and therefore denser, air, which saves energy. This comes closer to theoretical isothermal compression (the most efficient, but impossible in practice).
The actual figures
For lubricant-injected screws at 100 psig (6.9 bar), the US Department of Energy's Sourcebook gives 16 to 19 kW per 100 cfm for a standard screw and 15 to 17 kW for a two-stage screw. Low bound against low bound, high bound against high bound, the gap is 6 to 11%: around 10%. With a variable speed drive, the two gains combine, each on the consumption the other has already reduced: on a variable load, against a load/unload screw, the total runs from about 18 to 27%; against a screw under inlet modulation, where the drive removes close to a third, it approaches 40%.
| Standard screw, fixed speed | Baseline 100% |
|---|---|
| Standard screw, permanent magnet VSD | −9 to −19% against a load/unload screw at 60% demand (DOE Sourcebook) |
| Two-stage, fixed speed | ≈ −10% (6 to 11%, constant load) |
| Two-stage, PM VSD | Both gains combine: about 18 to 27% against a load/unload screw |
Where two-stage gains the most
- 24/7 industry: petrochemicals, steelmaking, cement, paper mills
- Continuous plastics processing: extrusion, high-speed PET blow moulding
- Sites with a stable 70-100% load most of the time
- High-pressure applications, up to 13 bar: in theory, the saving from cooling between stages grows with the pressure ratio
- Energy-intensive sites: electricity already accounts for 76% of a compressor's cost over ten years on two shifts, five days a week (US Department of Energy, 2000), and its weight grows with running hours
Initial extra cost and annual gain
A two-stage compressor costs more to buy than a standard screw of equivalent power: the intercooler adds weight and footprint, and the mechanics are more complex. The exact amount is set out in the quotation, as it depends on the configuration.
What you can calculate without a quotation is the saving. On a 75 kW machine running 8,000 hours a year at full load, i.e. around 600,000 kilowatt-hours absorbed, a specific power difference of 6 to 11% represents roughly 36,000 to 66,000 kilowatt-hours saved every year. Multiply by what you pay per kWh: the order of magnitude of the annual gain is immediately clear. The investment itself is stated in the quotation.
The extra stage saves energy in every loaded hour, not only on continuous duty; the gain grows with the hours. It is the benchmark technology on energy-intensive sites.
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