Putting the squeeze on compressed air costs

With energy prices on the rise and huge savings there for the taking, Mark Allinson of ABB argues that the time has come to take control of compressed air.

Wasting compressed air is like watching money go up in smoke.

People too often treat compressed air as if it is as free as the air around us, when in fact it's the most expensive utility available.

This is because compressors turn around 90% of the electricity they use into waste heat, not compressed air.

This effectively makes compressed air around 10 times more expensive than electricity, in terms of both the company's bottom line and the cost to the environment.

There has never been more pressure to save energy, with businesses facing a "double whammy" of rapidly rising fuel costs and environmental energy taxes.

The good news is that compressed air provides the perfect target for an efficiency drive, with savings of 30% in energy costs available on most UK sites through simple good housekeeping.

According to the UK Government's Energy Efficiency Best Practice Programme, overall industrial energy savings of 10% could be realised by optimising compressed air systems.

With this in mind, it's staggering that a survey of typical industrial sites found that compressed air leakage accounted for 39% of demand on average.

The first step in tackling this sort of waste is to monitor it.

To do that requires the right sort of flowmeter and thermal mass flowmeters are ideal for this type of application.

First, thermal mass flowmeters work by measuring the amount of heat that a gas carries away from a heating element as it flows past.

A reference probe checks the ambient temperature of the surrounding gas, while a measurement probe senses the heat transfer from the heating element.

The amount of energy required to keep the measurement system in equilibrium depends directly on the mass of the passing gas.

This is a direct measurement of the mass flow so it is more straightforward, and hence easier (and often cheaper) to implement than techniques that derive the mass indirectly.

For example, a volumetric flowmeter would also need to know the temperature and pressure of a gas in order to compute its mass flow, which means buying, installing and maintaining extra instrumentation.

In addition, thermal mass flowmeters take measurements using two small probes on the end of an insert.

This causes only a minor obstruction in the surrounding flow, so that correctly sized thermal mass flowmeters offer an extremely small pressure drop, typically between 1 and 2mbar.

Vortex meters for example, can produce a pressure drop of anywhere between low tens to low hundreds of millibars for a similar measurement system, and the pressure drop across an equivalent orifice plate can be even higher.

In compressed air applications this can have a knock-on effect on process efficiency, as tool power drops by some 2.5% for each 100mbar drop in compressed air pressure.

Thirdly, accurate leak detection needs a meter that can perform accurately at very low flows.

Thermal mass flowmeters have a turndown ratio of 150:1, compared to between 30 or 40:1 for vortex meters and just 4 or 5:1 for orifice plates.

In addition, thermal mass meters do not have the low flow cutoff floor found on vortex shedding devices.

Of course, there are traditional ways of detecting leaks without using any flowmeters, such as listening for hissing sounds or coating joints with soap and checking for bubbles.

It's helpful when looking for leaks to realise that there are some components of a compressed air system that are especially vulnerable, such as pneumatic cylinders, flanges, filters, tools, presses and drop hammers.

But even if you know where to start looking, site surveys are laborious, time consuming and must be repeated regularly.

All that adds up to higher personnel costs.

This sort of survey will also only detect leaks that are big and accessible enough for a human operator to spot.

As a rule of thumb, this approach leaves 10% of leaks unrepaired at any given time, which is worrying if you consider that a single 5mm hole in an air line costs around GBP 1400 per year.

Thermal mass meters can detect tiny leaks in comparison.

They also operate continuously, so they can spot a problem as it develops, rather than waiting for the next survey.

And once in place, they can help with other aspects of good housekeeping, such as monitoring the amount of compressed air used by each consumer.

This enables energy managers to allocate compressed air costs to different areas of production separately, which encourages individual process or machine operators to minimise their consumption.

Not only are the potential rewards of metering compressed air increasing, but the price of meters is also becoming more affordable, thanks to innovations such as ABB's ECO 2 model.

This is a member of the company's Sensyflow family that is specifically aimed at the compressed air market.

Because it's tailored to a single specific gas mixture - air - and a limited range of line sizes, it costs only around half the price of ABB's more comprehensive models.

The extra specificity also improves the response time of the ECO 2 from around 0.5s for standard industrial meters to just 25ms.

People's attitude towards compressed air has traditionally been rather blase.

But the possibility of realising substantial savings at a time when energy costs look set to rise for the foreseeable future surely means it's time to bring compressed air usage under control.

The following low- or no-cost measures should help minimise the cost of compressed air systems.

Set up a leak reporting and repair programme.

Switch off compressors during unproductive hours - an idling compressor can draw between 40 and 60% of its full load power.

Make sure everyone is aware of the true cost of compressed air.

Identify any misuse of the compressed air system - for example, using air blasts for cleaning down benches or for cooling when less wasteful solutions are available.

Check the pressure required at each usage point to see whether part of the system can be served by a 3bar low-pressure main, rather than a 7bar version.

After all, the higher the pressure, the more air will escape through a given size hole.

And investigate whether waste heat from the compressors can be used elsewhere, for example, for space heating.

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