Why do dry oil-free screw compressors usually have two stages?

The DOE sourcebook (2016) says that, with no injected fluid to remove the heat of compression, most dry-type designs use two stages with an intercooler between them and an aftercooler after the second stage.

Can a water-injected screw compressor supply medical air?

In England, HTM 02-01 (2026) says water should not be used as a sealant in medical air compressors because of microbial contamination risks and potential problems with water treatment.

Single-stage vs two-stage oil-free screw compressors

Q: Single-stage vs two-stage oil-free screw compressors

The US Department of Energy compressed air sourcebook (third edition, 2016) says most dry-type oil-free rotary screw compressors use two stages with an intercooler, because no injected fluid removes the heat of compression; single-stage dry units operate up to 50 psig and two-stage units can generally reach 150 psig.

The sourcebook describes two types of lubricant-free (oil-free) rotary screw compressor: the dry type and the water-injected type. In both, no lubricant enters the compression chamber; lubricant is used only for the bearings and gears, which are isolated from it. In the dry type the rotors do not touch, and lubricated timing gears outside the compression chamber keep their positions; the lack of a sealing fluid also requires higher rotation speeds than in lubricant-injected screws. Dry-type units range from 25 to 4,000 hp, or 90 to 20,000 cfm.

Side by side

	Dry type, single-stage	Dry type, two-stage	Water-injected type
Discharge pressure (DOE sourcebook)	Up to 50 psig	Generally up to 150 psig	100 to 150 psig may be reached with only one stage
Heat of compression	No injected fluid to remove it	Intercooler between the stages and aftercooler after the second stage	Water injected into the compression chamber removes it and seals internal clearances
Coolers and separators	Dry type: an air cooler after each stage and a lubricant cooler	Dry type: an air cooler after each stage and a lubricant cooler	A moisture separation device removes the injected water and condensed moisture

Efficiency

The Compressed Air Challenge fact sheet on component-level opportunities (May 2026, Rev. 01) states that two-stage rotary-screw compressors are more efficient than single-stage units, that many multi-stage compressors save energy by cooling the air between stages, and that many industrial compressors only have a single stage because equipment manufacturing costs are lower. The same fact sheet says lubricant-free rotary screw and reciprocating compressors are generally less efficient than lubricant-injected machines.

For hospital medical air

NHS England's HTM 02-01 (2026) lists rotary screw, reciprocating and scroll compressors as the most common types, and says compressors may be of any type, provided they are suitable for continuous running on load and for high-frequency start/stop operation or are controlled by variable speed drives (Part A, 7.31 and 7.32).
It says oil-free compressors should be considered as replacements for oil-filled compressors, because they reduce filtration requirements, minimise the risk of oil contamination and improve system performance (7.34).
It notes that rotary compressors are sealed and cooled by oil or water, and that water should not be used as a sealant because of microbial contamination risks and potential problems with water treatment (7.36 and 7.37).

For how oil-free and oil-injected compressors compare on a PSA oxygen plant, see oil-free vs oil-injected compressors for PSA; for medical air compressor types, see scroll vs screw vs piston.