COMPRESSED air is often taken for granted in most manufacturing facilities. Yet the cost of this source of energy is easily overlooked or underestimated. In fact, the value of compressed air as an energy source is several times higher than that of electricity.
So, even small leaks add cost over time. In addition to finding and plugging the leaks, it is important to have a regular maintenance schedule, as well as ensure ongoing awareness. It’s the old saying, ‘If you can measure it, you can improve it’.
Large air leaks are rarely difficult to identify, but once the obvious leaks have been eliminated, finding the smaller ones is not always as simple. The sounds generated by smaller air leaks are often not detectable by the human ear, especially in a noisy factory environment.
So a more effective and practical approach is to use a hand-held ultrasonic leak detector with good-quality earphones. Such devices are available with extension kits to reach around corners or fitted with parabolic antennas to detect leaks further away.
In terms of keeping track of and trending the flow of air, there are several methods and instruments available. However, one very important aspect is to ensure that the readings provide meaningful information.
The volume of air is sometimes referred to as actual volume. Since air expands and contracts with changes in pressure and temperature, measuring just the volumetric air flow provides only part of the picture.
The term standard or normal volume is a more useful parameter as it provides a reference measurement. For example, 1 normal cubic metre (Nm3) of air is generally defined as 1 actual m3 of air at a pressure of 1013.25 mBar (a) and at a temperature of 0°C. So, measuring and logging air consumption in Nm3 provides an easy way to, as they say, compare apples with apples.
One interesting aspect of the term normal volume which is often overlooked is that, despite the terminology, it is in fact not a unit of volume but a unit of mass.
Consider the example of a balloon which holds 1m3. If the pressure inside the balloon is 1025 mBar (a) and the temperature is 0°C, then you have 1 Nm3.
Squeezing the balloon will increase pressure and make it smaller, but the content (mass) inside the balloon will <[lb]>remain the same.
As normal volume is a unit of mass, consequently measuring the mass flow of air provides a practical method of determining the flow in Nm3 without the need for pressure or temperature compensation, or the associated cost of installation and upkeep of such gauges.
There are several types of mass flow meters on the market. One example is the insertion mass flow meter which can be fitted and removed from the pipe work under pressure using a simple ball-valve assembly.
This simplifies regular testing as the same sensor can easily be moved between different testing points, providing a stream of valuable datalogging with timestamps and flow rates directly in normal volume. Kits with a GSM modem are available to upload the data directly to a website.
One starting point when deciding to evaluate air losses is to first measure the normal air flow on a day or at a time when the compressor is running, but with no factory production.
The results can be surprising. Regular measurements, as the leaks are gradually identified and plugged, will provide a quantifiable measurement of cost savings to the bottom line and to the environment.
[*Lars Akerlund is managing director of Pricam Automation.]
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