CEE Oct-Dec 2012

" 780 760 740 720 700 620 S80 5000 10000 15000 20000 25000 10000 These results show that the most efficient solutions in this example are a centrifugal blower equipped with inlet guide vanes and variable vane diffuser vanes, the "Turbo eve blower' that runs at constant speed, or the 'High Speed Drive blower' that runs at a variable speed and is also equipped with variable van diffusers vanes. Comparing the other solutions with the optimum solution shows the following average power increase (mechanical aerators are eliminated, as even the best additional consumption exceeds the optimal solution by 60 per cent: see Figure 12). Tobl Pressure WnerO.pth Flow In NMl/h It is also seen that the choice of a Figure10: System pressure graph used for the 6.5 meter water column example. 'Cheaper' centrifugal blower, which runs at constant speed and only regulates the flow by means of diffuser vanes; the 'Turbo VVD blower' is a poor solution in terms of energy efficiency, as this type of blower cannot be optimized for operation at a lower operating pressure than its design pressure. • • The 3 plant loadings of 86 per cent, 67 per cent and 58 per cent are weighted equally. In order to accommodate the total air requirement (30,000 Nm 3 /h) the calculation shows three blower units in respect of the large centrifugal and positive displacement blowers, while six blower units have been considered for the smaller High Speed Drive units and for the Screw Compressors. Choice of blower sequence philosophy One can choose to start and stop the blower units in accordance with the Cascade or Parallel principles. In the case of Cascade operation only one unit is regulated, while the others being online are kept either at a minimum (50%) output or at a maximum (100%) output. In the case of Parallel operation all the on-line units are kept as far as possible at the same load. This means that with a plant load of 25,800 Nm 3 /h the configuration for Cascade operation will be 2*100% +1*58% while for Parallel operation the configuration will be 3*86%. Blower types used Rotation: Motor speed held constant or variable by using a VFD. Motor and VFD losses are calculated on the basis of information from ABB Motors. Calculation results In this section the main results of the calculations are shown in graphic form. The average efficiency is stated as the kg of 0 2 the blower station can deliver per consumed kWh, so that the higher the value, the more efficient the overall blower system (see Figure 11 ). At the same time, the calculations show that in this example there do not appear to be any particular advantage or disadvantage in respect of parallel or cascade blower sequence control (here .... .. ... & ..... l j i ...... 2 l • ~a~ 1,..... c ! ... II ..... ..... tlltbiM ~ ..,_YYD SO.. -~ ~ ~ ~ ....., ~ Drtw_.r ~ - Figure II: 0\'era/1 average aeration eftlciency -- - Ftgure 12: Additional consumption compared wtth the oprtmal ;oluuon ., ... 19