by Rolf Gloor, Gloor Engineering, CH-7434 Sufers and Christian Bachmann pcb Pressebüro, CH-8501 Frauenfeld for EEMODS'02 in Treviso (I).
In Switzerland, 150,000 compressed air units use approximately 750 GWh electricity, which constitutes 1.5% of the annual national electricity consumption. In industrial companies, compressed air units require up to 25% of the operational electricity consumption. Pneumatic processes have however a bad overall efficiency, which enables economical possibilities to save energy between 5% and 50%. A study, conducted in Switzerland and funded by the Swiss federal Office of Energy, prooved that in Switzerland theoretically 300 GWh and practically about 100 GWh (0.2% of the national electricity consumption) electricity per year could be saved by optimizing compressed air units.
Generally it can be found that larger compressed air units indicate a superproportional electricity consumption, and this offers also larger and economical possibilities to save energy. The 10,000 larger compressed air units (starting from 15 kW of compressor capacity) cause approximately 80% of the electricity consumption. With annual costs of electricity for compressed air of over 5,000 Swiss Francs, energy saving measures are interesting for the owners of these systems. A compressed air specialist can undertake locally appropriate optimizations. In the remaining small systems, savings efforts are less economical. The user could execute standardised measures with the help of a simple check list here. Various possibilities in optimizing compressed air systems were tried out in two companies, a kitchen manufacture and a weaving mill.
A modern manufacture of kitchen furniture with 20 workers uses approximately 140,000 kWh of electricity per year, whereby the compressed air unit requires 10% (14'000 kWh) of the consumption. The air consumers consist of 5 automatic machines (40%), various hand equipment (40%) and leakage (20%). The compressed air supply takes place to 90% by a 5.5 kW screw compressor and to 10% by a 5.5 kW piston compressor. Without the already available time switch, which switches the compressors off outside of the working time, the consumption would be increased by over 25,000 kWh per year. By optimization of the screw compressor (operating mode start-stop, reduction of the pressure level, reduction of the slowing-down time, maintenance) the current consumption has been reduced to 8,400 kWh/a, which corresponds to a saving of 40%.
A cotton weaving mill with 121 weaving looms uses 180,000 kWh of electricity per year with a new 30 kW screw compressor. A detailed load analysis revealed the amazing result of a constant load from Sunday evening to Saturday noon of 28 kW. On workfree Sundays, the compressed air unit was normally switched off. As a trial, the compressor was operated on one Sunday, requiring a capacity of 22 kW. In the weaving room, the sound of escaping air could be heard, which came from leaks in brokenshaken fittings from standing machines. In the usual operation procedure these leakages could never be noticed because of the loud engine noises. The repair of the air fittings reduces the power consumption of the compressed air unit by 70% with a payback time of only a few months.
In 2000, on behalf of the Swiss Federal Office of Energy, a study on the possibilities of energy saving in compressed air installations in Switzerland has been undertaken. As a result, energy-saving potentials of 5 to 50 % have been calculated, depending on the type of industries and applications.
The energy efficiency of pneumatic processes is low. But little is known about the amount of energy that could be saved by optimizing these processes. The goals of this study were to estimate the total energy consumption of compressed air installations in Switzerland, to calculate the optimizing potential, to identify energy-saving measures and to analyze the market situation.
The total energy consumption of compressed air installations in Switzerland was estimated in two different ways: by market survey in suppliers of compressed air units and by the percentage of energy consumption for the generation of compressed air in some important industries. Both methods lead to corresponding estimates of 150,000 compressed air units installed in Switzerland, consuming 750 GWh of power annually, which is 1.5% of national power consumption and up to 25% of industrial power consumption.
Saving potentials vary with different types of applications between 5% and 50%. The nationwide saving potential is, in theory, 300 GWh annually. As an optimistic, but reasonable assumption, 100 GWh could be saved annually by optimizing compressed air installations.
Table 1: Statistics of compressed air installations in Switzerland
The main goal of the study described above was to identify measures to be taken on various levels, from optimizing the installations up to influencing the market mechanisms.
The largest 10,000 units use approximately 80% of the power consumed by all of the 150,000 compressed air installations in Switzerland. Annual energy costs of these larger units exceed 5000 Swiss Francs.
Energy-saving measures must begin with consumers, because the whole system depends on their need of compressed air. As a second step, the distribution is to be optimized, mainly by replacing leaky elements. The saving potential of compressors, however, is limited to 10% in most cases.
Main players in the compressed air market are the users in a considerable number of industries, the suppliers of compressors and of fittings, the engineers and fitters, the manufacturers and traders of air-driven tools and of machines using compressed air. In most cases, a plumber installs the pressure lines during the construction of a building. In simple cases, the engineers planning compressed air systems often use standard configurations. In more complex installations, they use to take the advice of compressor suppliers.
There is, in brief, a number of possibilities to influence the market players: Centers of competence, documentation, check lists, labels, setting examples, contracting, subventions of energy-efficient products.
The feasibility of energy-saving measures has been shown in two industrial companies: a kitchen manufacture and a weaving mill.
This example shows a substantial optimizing potential. However, this potential is too small to justify the consultation of external expertise. But the appropriate measures are so simple that they can be taken easily by installation owners, best with some assistance of maintenance men.
Schneebeli AG is a kitchen manufacturer with 22 employees in Ottenbach, Switzerland. Main pressure consumers are a CNC working center, an edge gluer and a plate saw with pneumatic cylinders, an aircleaned band grinding machine, various working machines and presses, and 12 working benches equipped with airdriven hand equipment and cleaning guns. The elements of the pressure supply are a pressure line network, a refrigeration dryer, an ultra filter, and 4 pressure containers. The pressure generators are mainly a screw compressor of 5.5 kW, and a piston compressor running only in marginal hours.
As a first step, the following values have been measured daily and added up weekly:
Further assessment led to the conclusion that there is only limited saving potential in pressure consumers and in the pressure line network. Therefore, we focused our efforts on the compressor. Its running time of 50 hours a week was indicating a considerable saving potential.
Figure 1: Power consumption of the enterprise, of the screw compressor and of the refrigeration dryer from September to December 2001.
Changing the operating mode of the compressor from continuous to stop and go lowered the power consumption by 35%.
Optimizing the time schedules of piston compressor, screw compressor and refrigeration dryer by limiting the availability of pressure to the main working hours lowered the power consumption by 8%.
Reducing the after-running time and pressure limits of the screw compressor lowered the power consumption by 17%.
By all these measures, the power consumption of the compressed air unit has been reduced by a total of 30% without any investment in material.
By the optimizing measures described above, annual cost savings are about 1000 Swiss Francs. This amount is considerable, but by far too small to justify an external consulting. It is necessary to give installation owners a simple guide, so that they know how to optimize the compressed air units themselves. The best moment to do this is a planned maintenance, in collaboration with the maintenence man. An article in trade journals will communicate this message.
This example shows a very high energy optimizing and cost saving potential.
Jenny Fabrics is a manufacture of cotton tissue in Ziegelbrücke, Switzerland, operating 120 gripper looms in a 6 days, 24 hours schedule, resulting in a production time of 6500 hours annually. Each gripper loom needs only small amounts of compressed air for thread woof and for cleaning.
Vibrations cause huge leakage losses in the compressed air supplies of a large number of looms. The losses exceed the consumption of the machinery by a factor of three. The leaks have been detected in an acoustic search with pressure supply turned on and looms turned off. This test cannot be done during working hours because of the loud noise.
Figure 2: Power consumption log of the compressor. Running the compressor on one Sunday showed high power input caused by leaks.
By repairing the leaky fittings, 150,000 kWh of electric power and costs of over 10,000 Swiss Francs have been saved annually. An investment of 3000 Swiss Francs in material and in 30 hours of manpower was needed. The payback time is only a few months.
This example shows that permanent vibrations in textile machinery may lead to hidden leakages of pressure supply, causing high losses of energy and cost. An article in trade journals of the textile industry will communicate this message.
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© GLOOR ENGINEERING, CH-7434 SUFERS, 10.09.2002 update 19.01.2003