Compressed air: where the energy goes

Compressed air looks free to the people using it: open a valve and there it is. For whoever pays the electricity bill, it is a different story. The US Department of Energy describes it as “one of the most expensive utilities in a plant”. Most of the electrical energy going into a compressor ends up as heat, and of the air produced, some is lost through leaks or feeds uses that could be done another way.

The underlying problem is organisational. Maintenance makes sure there is air, production consumes it, and the energy manager sees the total bill. Nobody is responsible for how much air is produced or what it is used for.

Measure first

Without baseline figures you cannot show what you have gained. In the compressor room you need two things: the electrical power drawn (a meter or power analyser on the compressors’ supply) and the flow of air delivered to the network (a flow meter on the main header). If you have no flow meter, start with power and with the loaded and unloaded hours from each compressor’s controller.

Let the logging run for at least one full week, weekend included. From it you get three things: specific energy consumption (kWh per cubic metre of air), the demand profile by shift, and the consumption when nothing is being produced.

The weekend test is the cheapest diagnostic you have. With the lines stopped but the network still pressurised, everything the compressors still produce goes into leaks and into consumers that never switch off. Compare the flow (or power) in that interval with the average during production. The ratio gives you a first estimate of the share of losses. The DOE leaks tip sheet says leaks often waste 20-30% of a compressor’s output and gives 5-10% of total flow as a reasonable target for a plant.

Leaks

The best way to find them, according to the same DOE sheet, is an ultrasonic detector, which picks up the high-frequency hiss that the ear misses against the noise of the hall. Soapy water works too, but it is slow.

Steps that work:

• Do the walk-round during breaks or at the start of a shift, when background noise is lower.
• Tag every leak on the spot with a number and enter it in a list with the location, estimated size and date.
• Repair the large leaks first. Flow through an orifice grows with the square of its diameter, so a few large holes weigh more than dozens of small ones.
• Check after the repair and close the tag only then.
• Repeat the walk-round periodically. A one-off campaign is undone within a few months.

The usual places are quick couplings, hoses, fittings, filter-regulator-lubricator units, condensate drains, valves and end-of-line equipment.

One detail often forgotten: once the leaks are fixed, the compressors have to actually produce less. If one runs unloaded for longer instead of stopping, the saving stays on paper. Adjust the controls after the campaign.

Pressure

Network pressure is usually set by the most demanding consumer, then raised “to be safe” with every complaint. Each extra bar costs twice: the compressor uses more energy, and leaks and unregulated consumers draw more air, because their flow rises with pressure. Carbon Trust says that reducing pressure by 10% can save 5% of energy.

How to bring it down without stopping production:

• Find out what pressure each major consumer actually needs, from its datasheet and not from habit.
• Measure the pressure drop between the compressor room and the points of use. A large drop means clogged filters, undersized pipes or hoses that are too long.
• If a single machine needs a pressure well above the rest, treat it locally (a booster or a buffer receiver next to it) rather than raising the whole network for it.
• Lower it in small steps, a few days apart, and watch for stoppages or scrap.

Inappropriate uses

The DOE tip sheet on inappropriate uses names them directly: cleaning, drying and process cooling with compressed air, cooling people, air-driven vacuum generators, vortex coolers without a thermostat, and idle or abandoned equipment left under pressure. The alternatives are ordinary: low-pressure blowers, fans, brushes, efficient nozzles, dedicated vacuum pumps.

The open hose used to blow off swarf or dust is the most visible case. A nozzle fitted to the gun reduces both flow and noise. For machines that stand idle between cycles, a solenoid valve that cuts the air at the inlet when the machine is stopped also shuts off the leaks inside it.

Walk the hall with this list in hand and note every use. The conversation with line supervisors is easier when you can show how much air a particular station draws at night.

Compressor control

With several compressors, each on its own pressure switch, you often end up with two or three running partly loaded or unloaded at the same time. An unloaded compressor uses energy without delivering air. A central controller that starts and stops machines according to demand, with a single one taking up the variation, solves much of this.

A variable speed compressor makes sense where demand varies a lot during the day or between shifts. As a configuration rule, it takes the variation while the fixed-speed machines run at full load on the base demand. If the measured profile shows almost constant demand, variable speed adds little. Hence the order: first measure and reduce demand, then choose the equipment. Otherwise you buy a compressor sized for the leaks.

Well-sized air receivers give the control system time and reduce frequent starts.

Heat and condensate

Almost all the electrical energy drawn by the compressor leaves as heat, through the cooling oil and the cooling air. On air-cooled compressors, the warm air can be ducted into the hall or the warehouse in winter, with a damper that sends it outside in summer. On oil- or water-cooled ones, a heat exchanger can preheat domestic hot water or process water. Ask the compressor supplier which recovery kit exists for your model.

Condensate drains are a silent source of losses. Timed ones open at fixed intervals whether or not there is condensate, and release air every time. Ones stuck open blow continuously. Drains with a level sensor (no air loss) remove the problem. Check them on every leak walk-round.

Who is responsible and what to do monthly

Name one person responsible for compressed air as a system, from compressor to point of use. They do not have to do everything themselves, but they hold the figures and the action list.

A monthly routine that fits in one working day:

• Read the specific consumption (kWh per cubic metre) and compare it with last month.
• Repeat the no-load measurement on a weekend or during a planned shutdown.
• Do the ultrasonic walk-round on one area of the hall, by rotation, and update the tag list.
• Check the drains, the pressure drop across filters and the pressure set at the compressors.
• Report in one line at the production meeting how many leaks were found and how many repaired.

If you want a first step for next week, schedule the power logging over a weekend and look at what the compressors use while the plant is standing.

If you have data from your plant or a question, join the discussion in the WhatsApp group “Energy & Utilities”.