How Two-Stage Compression Works in a Screw Air Compressor

Spec sheets for industrial screw compressors often say "single-stage" or "two-stage". The difference matters when you size a compressor for long running hours or a higher discharge pressure, yet it is rarely explained in plain terms. This article sets out the principle in a textbook way: what happens inside a two-stage screw compressor, why the air is cooled between stages, and which questions help you decide whether two stages suit your plant.

Why compress air in stages

Compressing air raises its temperature. Hot air takes up more volume than cool air at the same pressure, so more mechanical work is needed to push it to the final pressure.

In theory, the least work is needed when heat is removed continuously during compression. This ideal is called isothermal compression, and reaching it would take an infinite number of intercoolers. Real machines approximate it instead: the pressure rise is split into two or more stages, and the air is cooled between them.

The U.S. Department of Energy's compressed air sourcebook describes the same idea in practical terms. Multi-stage designs include inter-stage cooling to reduce air temperature, and multiple stages are used either for better efficiency at a given pressure or to reach higher pressures.

A quick look at a single-stage screw compressor

A rotary screw compressor has two intermeshing rotors. Air enters the spaces between the rotor lobes, those spaces shrink along the length of the rotors, and the trapped air is compressed until it reaches the discharge port. In a single-stage machine, one pair of rotors carries out the whole pressure rise in a single pass.

In a lubricant-injected design, the injected lubricant does three jobs. It lubricates the rotors and bearings, carries away most of the heat of compression, and seals the clearances between the rotors and the housing.

How a two-stage screw compressor works

A two-stage machine divides the compression into two steps, with cooling in between:

1. Intake and filtration. Ambient air passes through the air filter and enters the first stage.
2. First-stage compression. The first rotor set compresses the air to an intermediate pressure, often called the inter-stage pressure.
3. Inter-stage cooling. The partly compressed air is cooled before it reaches the second stage. A heat exchanger called an intercooler normally does this. In lubricant-injected multi-stage designs, the DOE sourcebook notes that lubricant may be removed and air-cooled between the stages, or the air and lubricant mixture may pass through a curtain of lubricant as it enters the next stage.
4. Second-stage compression. The cooled air, now at a smaller volume, enters the second rotor set and is compressed to the final discharge pressure.
5. Separation and aftercooling. The air leaves the second stage and passes to the separator and aftercooler before it enters the plant's air network.

How the inter-stage pressure is chosen

The split between the two stages is a design decision. For an ideal two-stage process, the work is lowest when the inter-stage pressure is close to the geometric mean of the suction and discharge pressures. Real designs apply corrections for pressure drops in the intercooler and for differences in stage efficiency.

A simple illustration: if air enters at 1 bar absolute and leaves at 9 bar absolute, the geometric mean is 3 bar absolute. Each stage then handles a pressure ratio of about 3, instead of one stage handling 9. The figures are an arithmetic example only and do not describe any particular product.

Two stages in oil-injected and oil-free machines

Two-stage designs appear in both families of screw compressor.

• Oil-injected (lubricant-injected) machines. According to the DOE sourcebook, two-stage versions can reduce specific power, and some can reach higher discharge pressures than single-stage units.
• Oil-free (lubricant-free) dry-type machines. With no injected fluid to take away the heat of compression, most designs use two stages with an intercooler between them and an aftercooler after the second. The sourcebook gives single-stage dry-type units a pressure range up to 50 psig and two-stage units up to 150 psig.

The motor side: permanent magnet motors

Staging determines how the air is compressed. The motor determines how efficiently electrical energy becomes shaft power. The two choices are independent.

In a permanent magnet (PM) motor, the rotor carries magnets instead of conductive windings, and the magnets do not need to be energised. The motor is synchronous, so no slip is needed for it to run. A 2013 U.S. Department of Energy report on high-efficiency electric motors notes that PM motors can be more efficient than induction motors, especially at part load. A motor efficiency class describes the motor alone. It says nothing about the efficiency of the complete compressor package, so ask for system-level test data when you compare machines.

HUADA and two-stage compression

HUADA is the flagship industrial air power brand of Fujian Huade Group. It was founded in Quanzhou, Fujian, China in 1985, operates from a 50,000 m² plant and exports to 20+ countries. The operating company is Quanzhou Huade Mechanical & Electrical Equipment Co., Ltd. HUADA's two-stage machines combine two-stage compression with an IE5 permanent magnet motor. See the HUADA two-stage PM-VSD screw compressors page for the range.

When two stages make sense

Two-stage compression is not automatically the right answer. Consider it when several of these apply:

• The compressor runs for many hours each year, so energy is a large share of the lifetime cost.
• The required discharge pressure is toward the upper end of the typical industrial range.
• The load is fairly steady, or is paired with suitable capacity control.

Two stages also bring a second rotor set, an inter-stage cooling path and extra components to inspect and maintain. The efficiency benefit has to be weighed against purchase price and servicing for your own duty cycle. The DOE sourcebook recommends analysing the plant's load profile, meaning how air demand varies over time, before selecting equipment.

Questions to answer before you specify

• What discharge pressure does the process really need, after allowing for pressure drops in dryers, filters and piping?
• How many hours a year will the compressor run, and how much does demand vary?
• What are the ambient temperature, cooling method (air or water) and electricity tariff at your site?
• Do you need oil-injected or oil-free air? Our article Oil-Free vs Oil-Injected Screw Compressors: How to Choose compares the two.

Send your answers to HUADA's engineers and they can match a two-stage configuration to your duty profile. To get started, request a quote through the inquiry form on our Contact page.