# Variable speed vs fixed speed compressors

A fixed speed compressor meets lower demand by starting and stopping or by loading and unloading, while a variable speed drive (VSD) compressor reduces its speed when demand falls; the ULTRASCREW datasheet states up to 35% energy savings for its variable speed version.

Demand on a hospital air plant varies. HTM 02-01 (2026), the NHS England guidance, notes that surgical air is used at high flows of up to 350 L/min for minutes at a time, while air for patient ventilation is used at lower flows, typically under 80 L/min, but continuously. The US Department of Energy describes how compressors follow such demand: start/stop and load/unload controls respond to lower demand by turning the compressor off or unloading it so that it does not deliver air for a period, while variable speed controls reduce the speed of the compressor in low demand periods. DOE also notes that compressors running at part load are generally less efficient than at full load.

## Side by side

|  | Fixed speed | Variable speed (VSD) |
|---|---|---|
| How it follows demand | Starts and stops, or loads and unloads | Reduces compressor speed in low demand periods |
| Duty it must handle (HTM 02-01) | Continuous running on load and high frequency start/stop operation | Speed control by the drive; sized individually for the system |
| Sizing rule in HTM 02-01 | Duplex to octuplet plants with each compressor sized as a share of design flow | Where the VSD gives primary control, no more than 80% of the required flow |
| Ultra Controlo examples | ULTRACECO SCR fixed speed models | ULTRASCREW VSD: up to 35% energy savings, constant output pressure, 5 to 14 bar; ULTRACECO SCR variable speed models |

## What the NHS England guidance asks for

HTM 02-01 says all plant designs should consider variable speed drive compressors and that compressor selection should maximise energy efficiency. It recommends a single dual-purpose plant for medical and surgical air, incorporating VSDs, as the most sustainable and efficient solution, because it can modulate flow to suit demand. It also sets a test: the supplier states plant efficiency as free air delivered to the pipeline per kWh, the plant must reach at least 5 m³/kWh at 100% and at 10% of design flow, and power at zero flow must be less than 1% of the power at 100% design flow. For dental air systems above 500 L/min, HTM 02-01 asks for a VSD compressor system.

## VSD in a multi-compressor plant

With at least two compressors in a medical air plant under HTM 02-01, the controls that coordinate them also affect efficiency. DOE notes that most multiple-compressor controls run the appropriate number of compressors at full load and let one compressor trim at part load to match demand, and that uncoordinated controls lead to compressors working against each other, short-cycling or blowing off, all signs of inefficient operation. DOE also advises building a pressure and demand profile before investing in additional controls.

In the Ultra Controlo range, the ULTRASCREW oil-injected screw compressor has a variable speed version that its datasheet credits with up to 35% energy savings, operation at constant output pressure and a 5 to 14 bar range. The ULTRACECO SCR two-stage rotary compressor, with oil-free compression, is listed in fixed speed and variable speed models at 60 Hz, with VSD packages under 75 kW using DCBL motors. Pressure set point also costs energy: the ULTRASEC datasheet estimates that each extra bar on the compressor takes about 7% of the installed motor power.

### Should every compressor in a medical air plant be variable speed?

HTM 02-01 asks designers to consider VSD compressors, sizes a VSD unit that provides primary control at no more than 80% of the required flow, and says VSD compressors should be sized individually for the system.

### How much can a VSD compressor save?

It depends on the demand profile, which DOE advises measuring first; the ULTRASCREW datasheet states up to 35% energy savings for its variable speed version.

## Sources

- [US DOE, Compressed Air Tip Sheet #7: Compressed Air System Control Strategies (August 2004)](https://www.energy.gov/sites/prod/files/2014/05/f16/compressed_air7.pdf)
- [NHS England, HTM 02-01 Medical gas pipeline systems, Part A (2026), paras 7.2, 7.6 to 7.10, 7.32, 7.33, 7.43, 15.196, Table 16](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-a-design-installation-validation-verification-aug-2026.pdf)
- [ULTRASCREW datasheet, p. 2 and 3](https://www.ultracontrolo.com/products/ultrascrew)
- [ULTRACECO SCR datasheet, p. 3 and 4](https://www.ultracontrolo.com/products/ultraceco-scr)
- [ULTRASEC datasheet, p. 2](https://www.ultracontrolo.com/products/ultrasec)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/compare/vsd-vs-fixed-speed-compressors/