PCN Europe: Why is the compressed air consumption so high in many large filter systems?
Brandis: It is because the controllers of the systems are often obsolete. They release the compressed air blast for the filter cleaning in defined time intervals and with constant air pressure - regardless of the actual degree of filter contamination. The intervals of the air blasts are often too short. This needs more compressed air than is actually necessary.
PCN Europe: What does that mean for the plant operators?
Brandis: They have relatively high operating costs because compressed air is quite expensive. But this is not the only disadvantage.
PCN Europe: What other problems may occur from time controlled cleaning?
Brandis: The high number of compressed air blasts reduces the service life of the filter hoses. The cleaning pressure is important too. You must know that each compressed air blast partially expands the filter fabric. Many compressed air blasts mean that the filter medium must be renewed more often because of the high wear. This also leads to higher costs for the plant operators. And then there is precoating.
PCN Europe: What exactly is that?
Brandis: A precoating is an auxiliary layer, e.g. of lime or potash, which is applied to the filter fabric to influence the filter effect. Even finest gas particles deposit on this layer, that would normally pass an unprecoated filter. If the filter is cleaned too often or too strong, the precoating can detach from the fabric more easily and the filter performance is significantly reduced. Then the precoating has to be renewed, which is expensive.
PCN Europe: Your company has now developed a solution that regulates the cleaning pressure depending on the filter contamination. Is that going to reduce the compressed air consumption of large filter systems?
Brandis: Yes, significantly. With the new pre-pressure control of our master solenoid valve controller HE 5750, the valves only release as much compressed air as is actually required for filter cleaning. The cleaning pressure can either be set differential pressure controlled or volume flow controlled. The requested nominal cleaning pressures for the differential pressure or for the volume flow are determined in a table. Let´s say for example: “At a differential pressure of 50%, I would like to have a cleaning pressure of 3 bar.” If the cleaning pressure in the filter system falls below 3 bar after a cleaning process, the HE 5750 opens the central shut-off valve. Compressed air is then refilled into the system, until the desired cleaning pressure is reached again.
PCN Europe: How exactly does pressure regulation via a central shut-off valve work?
Brandis: If the cleaning pressure exceeds the previously set value, the HE 5750 activates the shut-off valve, which is then closed. Once the shut-off valve is closed, compressed air can no longer flow into the pressure tanks that feed the cleaning valves. The pressure tanks are therefore filled up to the respective calculated cleaning pressure. The pressure is released indirectly via the valves only, i.e. during the cleaning process.
PCN Europe: Are there any findings on how much the operating costs of large filter systems can be reduced by using pre-pressure control?
Brandis: Yes, there are indeed: we have calculated that for a medium-sized filter plant with a compressed air consumption of approx. 130.000 m³ and compressed air costs of approx. 16.000 euros per year, 3.500 euros can be saved per year using the pre-pressure control. Considering that the filters have to be replaced less frequently because of the lower cleaning pressure, that will make another 5000 euros of savings per replacement period (see info box). Well-known customers have been convinced by this efficiency and reliability: our HE 5750 is in use e.g. in Dubai, in one of the largest energy from waste plants worldwide. Also the Basin Electric Dry Fork carbon power plant in Gillette/Wyoming (USA) uses our controller for many years. It is the world´s largest plant with a circulating fluidised bed dry desulphurisation.
Saving potential
Werner Brandis, Managing Director of AXXERON HESCH electronics GmbH, has calculated the saving of compressed air by pre-pressure control. “Let´s take for example a medium-sized filter system with 96 valves, a typical compressed air consumption of 0.05 m3 per valve opening and 2.628.000 valve openings per year. That will make a compressed air consumption of 131.400 m3 per year during a 24/7 permanent operation. With a medium compressed air price of 12 cents per cubic metre, that will make 15.768 euros of annual costs.
Assuming that the medium pressure in the tanks is reduced by 1 bar using pre-pressure control, that will make a decrease of air consumption and thus of compressed air costs of 22 percent. For our example that will be an annual saving of 3500 euros. Since the filter elements must be replaced less frequently thanks to the lower cleaning pressure, there are another 4.800 euros of savings per replacement period.”
