Compressed air is the most expensive utility in a factory

The efficiency of compression, and where the energy actually goes.

Compressed air is the most expensive utility a factory buys because generating it turns the large majority of the electrical energy fed into the compressor into heat before a single useful puff of air reaches a tool or a cylinder, so every job done with compressed air is paid for at close to nine times the electricity that job would have used if an electric motor had done it directly.

Where the electricity goes

Squeezing air into a smaller volume takes real work, and almost none of that work stays stored usefully in the compressed air. Most of it becomes heat the moment the gas is compressed, heat that cooling fans or water jackets then have to remove before the compressor can keep running without damaging itself. Only a modest remaining share of the electrical input ends up stored as usable pressure in the air leaving the compressor. That share shrinks further as the air travels through pipework, fittings and the leaks described earlier in this set, and what arrives at the point of use is a small fraction of what was paid for at the electricity meter.

The heat cannot simply be kept and handed on with the air. Hot air leaving the compressor is cooled on purpose, first in an aftercooler and then in the receiver tank and pipework, until it matches the temperature of the building, partly to protect the equipment and partly to drop out the water it carries. A tank of hot air sealed and left to cool loses pressure as it does so, which shows that the heat being thrown away was carrying part of the push the compressor worked to create, and none of that energy returns once the air is at room temperature.

The warm barrel of a bicycle pump

Pumping up a bicycle tyre by hand soon makes the pump barrel warm to the touch, and that warmth shows exactly where much of the effort has gone: into heating the air being compressed, with only the remainder reaching the tyre's final pressure. Anyone who has pumped a tyre hard on a warm day and felt that heat has experienced, in miniature, the energy loss a factory compressor suffers on a vastly larger scale.

Short, fast strokes heat the barrel quicker than pumping the same total volume slowly with long, easy ones, because rapid compression leaves the heat no time to escape through the pump's walls before the next stroke piles more on top. The same reasoning explains why a compressor working flat out in continuous duty loses proportionally more of its input to heat than one allowed to idle and cool between cycles.

Losses that multiply along the pipe

Compression heat is the largest loss, commonly accounting for most of the electrical energy a compressor consumes, since bringing air up to a useful working pressure requires that much thermodynamic work however good the compressor's mechanical design. Large compressors are built in stages with cooling between each one for this reason. They compress the air partway, remove the heat before it can build further, and only then compress it the rest of the way, clawing back some of what a single brute-force stage would throw away as a hotter discharge.

What survives that first loss then travels through pipework with its own resistance, through fittings and filters adding further pressure drop, and past every leak along the route, each bleeding away a further share. The losses multiply. A system with four separate losses that each keep only three-quarters of what reaches them delivers less than a third of its starting energy, even though no single loss looks alarming on its own.

One part in ten

A typical industrial compressed air system delivers something on the order of one part in ten of its electrical input as useful work at the point of use. A well-matched electric motor turns roughly nine-tenths of its input into shaft work, so the same mechanical job done with air can take close to nine times the electricity. A gap that size makes switching a continuous, heavy-duty application from air to electric drive one of the most effective energy savings many factories can make.

The same arithmetic changes how a leak should be read. Every cubic metre of air escaping through a whistling fitting carried roughly ten times its useful energy in electricity to produce, so a leak wastes far more than the air itself appears to be worth.

Getting some of the heat back

The heat a compressor sheds is ordinary, usable heat, and a system plumbed to capture it recovers a real share of what would otherwise be blown outside through a cooling fan. Warm air can be ducted from the compressor room into a workshop that needs heating anyway, or routed through a water jacket to preheat water used elsewhere in the building.

Recovering the heat leaves the efficiency of turning electricity into compressed air exactly where it was, since the pressure delivered at the point of use is no higher. It does improve the honest accounting of the whole installation, because energy that heats a building which needed heating anyway has ended up doing a useful job after all.

Choosing air for the right jobs

Earlier articles in this set showed the advantages compressed air holds over electric drive for the right job. Its running cost has to be counted honestly against those advantages, and the fact that it feels like "just air" makes it easy to assume it is cheap. A factory that treats every whistling leak, every oversized pressure setting, and every job that could run on an electric motor as small, forgivable inefficiencies is paying a multiple of what the same work would cost through a more direct source of power, one modest-looking line on the electricity bill at a time.

Compressed air earns its keep on simple, robust, centrally distributed motion, and it loses that justification fast when it is used simply because a pipe happened to be running past the point where power was needed. A job that runs for a few seconds many times a day tends to favour air, because a compressor already running for other equipment can supply that short burst at very little extra cost. A job running continuously for hours gains nothing from that shared infrastructure and pays the full cost of the conversion for every minute it operates, which is where the electric motor's ninefold advantage shows most plainly on the bill.