COmPReSSeD AIR L
ike many other services and utilities used in factories and process plants, compressed air is widely regarded as a
free resource. Just like electricity or water, it is there to be used at the turn of a tap, the flick of a switch or the press of a trigger. Yet, far from being free, it is the most expensive
utility there is. With compressors turning about 90 per cent of the power they use into waste heat, compressed air is ten times more expensive than electricity and everyone who uses it needs to be aware of just how much it can cost. Clearly, waste of this expensive resource needs
to be minimised. Fortunately, there are some easy wins in saving money on compressed air costs. The first priority is to set up a leak reporting
and repair programme. This will give you an idea of where the troublesome connectors and lines are sited and allow you to formulate a repair strategy to ensure they are kept fully working. Some of the traditional ways of detecting leaks
include listening for hissing sounds or coating joints with soap and checking for bubbles. When looking for leaks, bear in mind 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. Compressors should also be switched
off during non-working or unproductive hours. This can be a big saver as an idling compressor can draw between 40 and 60 per cent of its full load power. Do you find that people use air blasts for
cleaning down benches or for cooling? This is a very wasteful practice and these and other misuses of the compressed air system should be stopped and other methods suggested that can achieve the same results. Pressures at each usage point should be
checked to see if they are really required, and whether for example, a seven bar supply can be replaced by a three-bar, low-pressure main - the higher the pressure, the more air will escape through a given size hole. There is also the possibility of using much of the waste heat produced by the compressors
TURNING UP THe HeAT ON COmPReSSeD AIR meASURemeNT
While many people tend to view compressed air as being as free as the air around them, the energy needed to generate and distribute it and the environmental impact it can have make it anything but. Here, David Bowers MInstMC, product manager Pressure and Process Flow for ABB’s Measurement & Analytics business, explains how using thermal mass flowmeters to accurately measure compressed air consumption can be a vital starting point for helping to change attitudes and encourage a more sustainable approach.
elsewhere, for example, for space heating. With businesses facing the dual challenges of
rising fuel costs and environmental energy taxes, there has never been a better time to save on utility costs. The Carbon Trust estimates that UK industry uses over 10TWh of electricity to produce compressed air, making it the direct root cause of
over five million tonnes of CO2 emissions a year (source: The Carbon Trust, ‘Compressed air – opportunities for business’).
A MeAsure oF Control They say you cannot control what you cannot measure, so an accurate measurement technique is needed if you really want to get a handle on what your compressed air system is costing.
This requires the use of a suitable flow meter. The most commonly used flow meters for
gases measure the operating volume flow. This requires additional measurements of pressure and temperature to calculate the mass flow rate, which means buying, installing and maintaining extra instrumentation. These add cost and increase the complexity of the measurements - in addition they decrease the measuring system accuracy. A better choice is thermal mass flow meters
which provide mass flow rate in kg/h directly without any additional measurements or calculations. Using the normal density of the gas, the normal volume flow rate can be calculated, e.g. in Nm³/h. They 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
26 APRIL 2021 | FACTORY&HANDLINGSOLUTIONS
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