Four birds with one stone
January 1st 2006 Experts estimate that a great many of the compressors used in compressed air systems could really be dispensed with because they only run to make up for the unnecessary loss of air and pressure. Moreover, effective compressed air controlling would enable considerable cost cutting. But one of the keys to success was missing: only now has it become possible to bundle all the data required for a comprehensive strategy to improve economic performance. Flow rate measurement and control in compressed air systems is key says Beko Technologies
To achieve the overriding objective - minimising the costs of compressed air generation - it is important to take a close look at four specific areas: - determination and reduction of the leakage quantity,
- allocation of consumption and costs in the case of several points of use within the plant,
- increase of the capacity utilisation factors of individual compressors and, where appropriate,
- verification and breakdown of consumption quantities in the context of energy contracting.
If these four areas have to be analysed separately, using flow rate and consumption measurement methods that are not interlinked, the costs of these procedures will easily exceed any possible savings. And that is the reason why most companies have so far not made a concerted effort to optimise the economic performance of their compressed air system - although a considerable part of the energy bill is spent on simply nothing: air that disappears into the atmosphere.
It is of course not easy to devise a data collection system that integrates all the different parameters required for the respective analyses.
Cost unit analysis: revealing responsibilities
Very few compressed air systems consist of only one compressor supplying a single point of use. Usually, compressed air systems have a central compressor station and a widely branched pipe network that feeds compressed air to, for instance, the company's assembly line, paint-spraying shop, and measuring room. Any system for the measurement of flow rates and compressed air consumption must therefore be able to systematically allocate the amounts consumed to the different sections of the total plant and to document this accordingly.
In the first place the strategic collection of measuring data permits the cost determination and allocation of consumption quantities within the company.
But there is an added benefit: the resulting transparency promotes more careful handling of the expensive "raw material" compressed air at the monitored points of use.
Capacity utilisation analysis: activating hidden resources
The capacity utilisation of individual compressors can be simply determined by comparing the on-load hours with the idle running hours. Quite often the capacity utilisation will be below 80%, i.e., the compressors consume electricity during noload operation without producing any compressed air. Frequent changes between on-load and no-load operation will produce alternating stress, thus reducing the lifetime of the bearings and the compressor stages.
It would, however, be possible to step up the capacity utilisation by matching the compressor performance to the timed steps of the production sequence. An "out-ofwork" compressor could then function as a standby machine, for instance, to take over in the event of failure of another compressor or to meet increased demand for compressed air.
To this end, it must be realised that the optimisation potential can only be tapped if the consumption curves are known on the basis of suitable flow rate measurement methods.
Energy contracting: creating trust
For many companies, energy contracting is an ideal solution to avoid the initial high investment costs for their own compressed air systems (and possible system expansion) and the regular maintenance costs. For the compressed air "supplier", the contracting principle can be an equally attractive business proposition, provided it is properly calculated. Ideally, this should be a balanced win-win situation for both sides.
However, to produce a sound cost/benefit analysis and ensure mutual satisfaction there is one precondition: the consumption quantities must be determined correctly and documented in detail, besides having to be fully verifiable.
And this can only be accomplished by a flexible and technically mature system for the measurement of flow rates and compressed air consumption.
Leak localisation: closing the gaps
In systems with normal maintenance some 20 to 30% of the generated compressed air is lost through leaks - and in systems that are less well maintained, the loss may even be as much as 50%. This means: up to half the compressor energy used is entirely wasted because of existing leaks - and so are the corresponding costs.
In view of these clearly significant figures, one has to ask why hardly any companies make a determined effort to close the "air gaps" in the system. The answer is simple: nobody really knows where the leaks are.
And likewise, it is not known how many leakage spots there are and what the concrete loss amounts to. Not exactly good conditions for winning the fight against expensive leaks.
Consequently, there has been a strong need in this sector for an inclusive system that can tackle flow rate measurement in conjunction with leak localisation.
Four different tasks - a common solution
Leak elimination, cost unit analysis, capacity utilisation analysis, and monitoring of energy contracting: these previously separate fields can now be handled simultaneously and comprehensively.
Flowmeter - a new measuring instrument from BEKO Technologies, the compressed air specialist in the German city of Neuss on the river Rhine - has been designed to "kill four birds with one stone". It offers a flexible, four-pronged solution for all the above measurement and analysis tasks together with full and interlinked evaluation and documentation of the data collected.
The device includes a measuring section adapted to the pipework diameter, a measuring valve, a rod-shaped measuring probe and a flow rate calculator with digital display. Optional evaluation of up to 32 measuring points is possible by data transfer to an external computer for which appropriate software is available.
The measuring section is installed in the pipework of the compressed air system. At the centre of the measuring section pipe sits the measuring valve that receives the probe. The probe is fitted with two sensors at its tip. After inserting the probe into the measuring section, the sensors will occupy successive positions in the airflow path.
Sensor 1 is heated to a specific temperature, whereas sensor 2 is used solely for temperature measurement.
Sensor 1 is heated constantly to above ambient temperature, while a second sensor measures the ambient temperature.
Compressed air flowing across the sensors would normally lead to cooling of the heated sensor. To avoid this, the sensor is supplied with additional heat. The electricity required for heating is measured and serves as a basis for determining the mass flow rate. Taking the set reference values of temperature and pressure into account, the flow rate calculator then converts the mass flow rate into the corresponding volumetric flow rate, which is shown on the display.
Advantage of this new method: The heated sensor is not heated to a constant temperature, but instead kept constantly above ambient temperature. Therefore, the sensor can measure both minimal and extremely high flow rates very precisely and without loss of accuracy.
Compared with the usual thermal sensors, the Flowmeter sensors have a significantly smaller mass, making them also suitable for the measurement of leaks down to the smallest amounts.
Calibration at nine different points
An important advantage of the Beko system, compared with similar products, is the calibration of the sensor, measuring valve, measuring section and flow rate calculator at a total of nine points. Normally, calibration is restricted to the highest point of the measuring range. This has a significant effect on the accuracy in the lower and middle range.
Flowmeter, by contrast, registers deviations of as little as two percent across the entire measuring range from 1 to 100 % of the volumetric flow rate. This excellent precision of Beko's Flowmeter is verifiable using flow rate nozzles tested by the German Federal Institute of Physics and Metrology (PTB).
Easy cleaning of sensor during operation under pressure
In the case of very polluted compressed air it is advisable to clean the sensor occasionally. After loosening a clamping sleeve, the sensor is moved from the pipe to the outlet of the measuring valve. The valve is then closed, and the sensor can be unscrewed, cleaned and reinstalled in reverse order.
Flexibility and scope
Flowmeter is available as a stationary or portable device to provide maximum flexibility of application. The portable version incorporates a flow rate calculator and a sensor probe that can be inserted, via a ball valve, into one or more measuring sections - even when the compressed air system is under pressure. As with the stationary unit, the measuring data from the portable flow rate calculator can also be transferred to an external computer.
While the flow rate calculators of the stationary and portable Flowmeter devices already supply valuable first information, the PC software makes it possible to obtain very detailed analyses of the flow rate characteristics and consumption of the compressed air system.
For these software-based analyses, the data from up to 32 measuring instruments can be centralised for separate or joint graphical and tabular evaluation. It is easy to understand that this results in a multiplicity of measured data and relevant figures for controlling, but to expand on this would lie outside the framework of the present article.
Determining and localising leaks
Flowmeter shows the extent of leakage in the compressed air system by registering any existing flow at times when there is no demand for compressed air, e.g., during the night when the plant is not operating. If the compressor starts up during such rest periods, this indicates that compressed air has escaped through leaks and that the compressor is now compensating for the loss.
By monitoring over time the frequency of compressor start-up, Flowmeter also reveals the extra consumption of a machine as a result of wear.
If the measuring section of Flowmeter is appropriately positioned, or if there are several measuring sections within the compressed air system, the leakage areas can be defined more closely and weighted according to quantities lost.
The problem field is thus analysed and "encircled". What is missing now is the exact localisation of the leak. The second phase of the Beko solution - the portable Leak Detector - is designed specifically for this purpose. This separate instrument localises the usually very small leaks with millimetre precision. The human ear cannot hear leaks with cross sections below 1 mm2, but these leakage points still produce ultrasound because the air molecules create friction as they escape at high velocity and come up against the edges of the material. And registering ultrasound frequencies is the speciality of Leak Detector.
Even up to a distance of 20 metres, Leak Detector pinpoints the trouble spot with unfailing accuracy. Compressed air pipes running along the ceiling of high production halls, for instance, can therefore be examined from the ground.
The same applies to piping installed underground, hidden behind wall panels or other objects, or found in places generally difficult to access.
Due to the long working range of Leak Detector, which weighs only 400 grams and has a length 17 cm, it is also not necessary to switch off certain production machines for safety reasons while the 'detective' is exploring the pipework.
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