A compressed air plant is all the machines that produce a site's air: compressors, receivers, dryers, filters and the system that controls them. As soon as consumption exceeds what one compressor can cover alone, or as soon as an air stoppage stops production, the question arises: one large machine or several smaller ones? This guide gives the method for choosing and organising the plant.
One large machine or several compressors?
A single machine costs less to buy and takes up less space. But if it stops, for a breakdown or simply for servicing, the plant has no air. And if consumption varies a lot, it spends part of the time far from its ideal operating point.
Several compressors cost a little more to install, but bring:
- Continuity: when one machine is stopped, the others carry on.
- Better efficiency: only the number of machines needed is started.
- Flexibility: the plant can grow with the factory by adding a machine.
In practice, as soon as air is vital to production, a plant of two machines or more is almost always the right choice.
Combining variable speed and fixed speed
The most common configuration in industry combines:
- A variable speed compressor, which follows changes in consumption. It covers the part that moves during the day.
- One or more compressors that supply the base load, the consumption that is always there.
The variable speed compressor must be sized to cover the gap between two steps, otherwise control "gaps" appear: the variable machine is either flat out or at minimum, and the base machines start and stop too often. AIRSTAHL S and K Series are variable speed. To understand the different control modes, see load/unload and variable speed control.
Controlling the plant: the master controller
Without common control, each compressor reacts to pressure on its own: pressure bands overlap, machines hand over badly and network pressure rises more than necessary. A master controller (also called a sequencer or central controller):
- Starts and stops the machines according to real demand, with a single pressure band for the whole plant.
- Gives priority to the variable speed compressor to follow the variations.
- Rotates the machines to balance hours and wear.
- Makes it possible to lower the set pressure, because the plant responds better. Every bar less shows up on the electricity bill (see compressor energy cost).
Planning for standby: the N+1 rule
The most widely used rule is N+1: the plant has one more machine than needed to cover the peak. If one machine stops, the others still cover consumption.
Two variants exist when a complete standby machine seems too expensive:
- A standby that covers only the vital flow, the air without which the line stops.
- A rental solution prepared in advance, with the connection already planned on the header.
The choice between these options is detailed in our article does a factory need a standby compressor?.
A worked example
A plant measures its consumption over a week: about 2,000 l/min base load, peaks at 5,000 l/min, on two shifts.
- Option 1, a single machine sized for the peak: no standby, and a machine that spends much of its time at low load.
- Option 2, a plant of three machines of about 2,500 l/min each, one of them variable speed: two machines cover the peak, the third is the standby (N+1). At night and in off-peak hours, a single machine runs.
Among our online models, a machine like the S 220 PM (3,200 l/min) gives the order of magnitude of a machine of this size. Beyond that, our industrial compressors from 30 to 132 kW are sized on quotation. The figures in this example are indicative: only a measurement of your consumption lets you choose.
Receivers and air treatment in a plant
- The receiver absorbs peaks and gives the controller time to start a machine. See sizing an air receiver.
- Dryers and filters are sized for the total flow of the plant. With several dryers, one can be isolated for servicing without stopping the air.
- Isolation valves on each item make it possible to work on one machine without shutting down the plant.
Common mistakes
- Adding a compressor because "there isn't enough air" without looking for leaks: part of the air produced often escapes from the network. See our leak detection.
- Overlapping pressure bands between machines without common control.
- A variable speed compressor that is too small to fill the gap between two machines.
- Forgetting ventilation when adding a machine in the same room: see compressor room.
Want to review your plant or build a new one? Our technicians measure your consumption during a compressed air audit, then we propose the right configuration.
For your trade: Compressor for industry and production, Compressor for recycling and waste sorting, Compressor for printing and packaging.