Determination of the optimum cycle time of bag filters using the filter expert system "ProExpertise"

Concepts for Future Energy Efficient Filter Plants
Within the scope of the discussion about energy costs of industrial installations, the optimisation of filter plants has an increasing significance. Their energy demand results mainly from the energy consumption of the motors which drive the main ventilating fan as well as the compressed air consumption for the cleaning of the filter media. The minimisation of energy consumption of the entire installation including the filter, is today one of the most important factors when selecting equipment. In addition, the air/gas volumes to be dedusted with filtering separators have considerably increased within the industry during recent years, which leads to rising filter dimensions and increasingly longer filter bags. Here the installed cleaning technology and the operating mode (on-line or off-line mode) play a key role. A major help to operators of filter plants, for the optimisation of the operating parameters is a suitable method of measuring these factors.

Determination of the optimum cycle time of bag filters using the filter expert system „ProExpertise“
The pressure losses and hence the ventilating fan power demand can be precalculated with this expert system, on the basis of the basic equation of the cake-building filtration. Amongst other factors, the optimum operating point of a bag filter is based on the geometrical parameters of the filter. This also includes for example the filter surface, the bag length, the design of the raw gas chambers and clean gas chambers, and the control dampers. Other criteria are the process parameters such as the volume flow, the raw gas concentration, the temperature, etc.

By trials in a bench-scale unit (by directive VDI 3926) as well as in a half-technical filter plant, the required filter media and filter cake resistance for the data evaluation were determined empirically for different dust kinds and filter media. The dusts were:

• Pural Sb (Al2O3) dust: critical dust for filtration that doesn’t agglomerate and penetrates the filter media with a high tendency
• Kiln/raw mill dust: taken out of a product flow in a cement plant, raw mill dedusting (without separator)
• Cement, 5900 Blaine

The residual pressure loss (the resistance of the filter media directly after the jet-pulse cleaning) was determined in the test plant in an experimental bench-scale unit (10 filter bags of 4-m length). The residual pressure loss is also a function of the required regeneration energy, i.e. a function of the cleaning pressure (from 0.1 to 0.5 MPa).Additionally , the results were validated by operating data of existing plants.
All filter media were submitted to a defined ageing process to guarantee stationary test terms. The following filter media were field-tested:

• Standard polyester (PES) needle felt as a reference
• ePTFE membrane laminated on a glass fabric
• ProTex® PES: a special microfiber needle felt quality (with reduced differential pressure)
• ProTex® m-Ar: a special meta-Aramid (Nomex) microfiber needle felt quality (with reduced differential pressure)
• ProTex® PI: a special Polyimid (P84) microfiber needle felt quality (with reduced differential pressure)

Figure 1a and 1b show the input / output form of the filter expert system as well as the questionnaire for the analysis of the basic data. Process parameters and the geometrical data of the different modular Intensiv-Filter systems can be given in the data bank directly. Also, the evaluation and calculation of bag filters independent of manufacturer is possible, if certain geometrical data are known. The results of the calculations with "ProExpertise" are shown in tabular form and graphically.
Figure 2 shows exemplarily the operating costs for a kiln/raw mill dust removal. The consumption of compressed air is the prevailing factor, with very short cycle times and leads to rising operating costs. The operating costs further increase after crossing a minimum value (optimum operating point). This can be explained by the raised filter cake resistance and the raised energy consumption of the main ventilating fan. Low values of the pressure loss tend to show in the lower operating costs, as a result of the lower costs for compressed air compared with the costs for the motor power of the ventilating fan. This results in an optimum operating point, with relatively short cycle times.

With high raw gas dust content the optimum operating point further moves to shorter cycle times, because the power demand is determined by the high resistance of the quickly increasing filter cake. The image completely changes if little dust must be separated. The operating point of the filter plant with maximum energy efficiency moves with very low raw gas dust loadings (e.g., ≤ 20 g/m³) to longer cycle times of the cleaning.

Another task of the filter expert system exists in the comparison of different filter dimensions. Here filters with varying length bags and with different numbers of bags can be analysed, with regard to energy demand and operating expenses. Additionally the filter expert system includes the possible variations concerning the type of filter medium used. Therefore, for example,the energy savings can be calculated by using filter bags of the ProTex® generation in comparison to standard-filter bags. At the same time the compressed air necessary for the jet-pulse cleaning is calculated, for the respective configuration. With "ProExpertise" we can pre-calculate from an energy point of view the optimum operating point that is cycle time and cleaning pressure. The plant operator can also decide whether a planned increase of the plant capacity, i.e. a rise of the air-to-cloth ratio with the existing filter plant can be carried out, or whether upgrade measures become necessary. The basic data of "ProExpertise" is extended continuously by other dust and product data, filter media data and operating data. The next stage of expansion will also include the costs for depreciation, demand for service and spare parts, like filter bags and cleaning components.

To sum up, it can be said that "ProExpertise" is a tool for the engineer, which predicts operating expenses for existing and new installations with high accuracy by the input of the essential geometry and operating parameters of the bag filter. The operating costs for the dust removal and the product separation can be reduced considerably by setting of optimum operating parameters and if necessary by upgrade measures on energy-efficient filter media.