Because raising system pressure penalises every user and every leak, to serve one machine.

System pressure sets the operating cost of the entire installation. Increasing it means the compressors work harder continuously; it means every leak in the distribution system flows more air, and leakage is proportional to pressure; and it means every tool, cylinder and blow-off consumes more air than it needs, because most pneumatic devices use more air at higher pressure without doing more work.

A widely used rule of thumb holds that a modest increase in system pressure produces a noticeable percentage increase in energy consumption across the whole site.

A booster confines the requirement to the one point that has it. The rest of the plant continues at its normal pressure, and only the small flow the high-pressure application needs is raised further.

It is also usually the cheaper capital option, since the booster is small compared with the compressors that would otherwise be uprated.

Considerably less, because most of the work has already been done.

The energy required to compress air depends on the pressure ratio - the ratio of outlet to inlet pressure - not on the absolute pressure difference. Taking air from atmospheric to a high pressure is a large ratio. Taking air that is already at plant pressure to that same high pressure is a much smaller ratio.

So a booster fed with pre-compressed air does far less work per unit delivered than a standalone high-pressure compressor drawing from atmosphere.

The whole-system picture is what matters, though: the main compressor still had to produce that air in the first place. The comparison that favours the booster is against a dedicated high-pressure compressor for the same duty, or against raising site pressure.

Where the high-pressure demand is very large, a dedicated machine may be better; boosters excel when one user needs high pressure and the rest do not.

A booster powered by the compressed air itself rather than by an electric motor - with a large drive piston working a smaller compression piston.

Plant air acts on a large-area drive piston, and that force is applied to a small-area compression piston on the same rod. Because the same force acts over a smaller area, the output pressure is higher than the drive pressure by the ratio of the areas. The unit cycles automatically, and it stalls when the outlet reaches its ratio, holding pressure without consuming anything further.

The advantages are that no electrical supply is needed at all, that it is inherently suitable for hazardous areas, that it stops automatically when the demand is satisfied, and that it can be installed at the point of use.

The cost is efficiency - it consumes a substantial volume of drive air per unit delivered.

So air-driven boosters suit intermittent, modest-volume, point-of-use duty; electric boosters suit continuous or larger requirements.

Single high-pressure users on an otherwise normal plant air system.

Typical cases are PET bottle blowing, which needs high-pressure air for the blow stage while the rest of the machine runs on plant air; laser cutting assist gas; leak and pressure testing rigs; clamping and press applications requiring more force than plant pressure gives; nitrogen and gas boosting; and cylinder filling.

Tool and equipment testing, hydraulic accumulator charging, and pilot-plant or laboratory duties are others.

What they share is that the high-pressure demand is a small part of total site consumption but at a pressure the site does not otherwise need.

Where several such users exist, it is worth checking whether one larger booster with local distribution serves them all - though a high-pressure distribution system carries its own cost and safety requirements, so point-of-use boosters are often simpler.

Clean and dry, and generally better than the plant average - because compressing further concentrates whatever is already in the air.

A booster takes air that has already been compressed and compresses it again. Any moisture, oil or particulate present is carried through and concentrated by the further reduction in volume, so contamination that was tolerable at plant pressure can be problematic at booster outlet.

Moisture is the particular issue: air that is at its dewpoint at plant pressure will condense as it is compressed further, producing liquid water in the high-pressure system.

So boosters normally require filtration and drying on the inlet, specified for the outlet pressure rather than the inlet. On breathing air and cylinder filling duty the requirements are considerably stricter and are set by the applicable standard.

Confirm the required inlet specification with the manufacturer and treat it as part of the installation.

Usually yes - a high-pressure receiver on the outlet lets the booster run efficiently and supplies short peaks.

A booster delivers a relatively modest flow. Applications like bottle blowing or pressure testing draw air in short, sharp demands that exceed that flow considerably for a brief moment.

A receiver on the high-pressure side stores the output between demands and supplies each peak, so the booster only needs to be sized for the average consumption rather than the peak. That can reduce the booster size substantially and therefore the cost.

It also allows the booster to run in longer, more efficient cycles rather than starting and stopping constantly, which matters for wear on a reciprocating machine.

The vessel must of course be rated and certified for the higher pressure, with appropriate relief protection - and it is a pressure vessel subject to whatever periodic inspection regime applies locally, which should be factored into the installation from the start.

Reciprocating-machine maintenance - valves, rings, seals and lubrication - with attention to the higher pressures involved.

Most boosters are reciprocating, so the wearing parts are the familiar ones: valves that open and close every stroke, piston rings and seals, and the lubrication that supports them. Higher pressures mean higher loads and temperatures, so these items work harder than in a standard compressor and intervals are correspondingly shorter.

Seals on high-pressure boosters are frequently the limiting item and are a routine replacement.

On air-driven boosters the drive-side seals and the cycling valve wear as well, and a booster that has become slow or that cycles without building pressure is usually reporting seal wear.

Relief valve testing matters more here than on a plant air system, because the consequences of an overpressure are greater. Include it in the schedule, and treat the high-pressure side as a system requiring its own isolation and venting procedure.