CEE Oct-Dec 2012

Fir;ure 7· 1-ltgh ~peed drwe blow<•r untt with diffuser •·ane; ftt;lll< II: r, ntfllllt;.II illua "' "tth 1 uuhl<' in!< t 1 ,1m·' measured as the mass flow, from a minimum of 45 per cent up to 100%. In the case of a positive displacement blower, by its nature the amount of air can only be regulated efficiently by controlling the speed, while for the dynamic blower type the amount of air is regulated either by using adjustable vane systems located in front (pre– rotation vanes or inlet guide vanes, see Figure 8) or up stream of the impeller (diffuser vanes, see Figure 9). The most advanced centrifugal blower types use a combination of inlet guide vanes and diffuser vanes, called Combined Van Control (CVC). Alternatively, dynamic blowers can also be regulated solely by adjusting the tip speed, High Speed Drive Blower or by a combination of vane system and speed regulation. If a variable van system is not used, the turn down capability will often be limited to the range 65 per cent to 100 per cent, which will require a higher number of units to be installed as to obtain an overlap between units at low plant load conditions. Figure 7 shows a High Speed Drive Blower type of equipment with variable diffuser vanes for flow regulation . It would be too much to go through all the technical advantages and disadvantages of the different types of blower technologies, and their regulation capabilities, in this article, so comparisons are drawn between the energy efficiency of the descriqed configurations by means of hypothetical examples as follows. Design data for the calculation example are: • • • • Plant data used in the example calculation Design plant load 300,000- 400,000 PE Maximum air amount 3*10.000 Nm 3 /h =8,940 kg 0 2/h Alpha factor is set at 0.60 for bottom aeration and 0.85 for surface aeration 3 water depths have been used for the membranes: 5.0, 6.5 and 8.0 m water column. • In all calculations "normal" plate diffusers are used, with a density of 15 per cent. There is no emphasis on optimization of this part of the process. • The constant pressure loss in the regulation valves, etc. is 28 millibars and the maximum dynamic pressure loss is set to 77 millibars (see the system graph in figure 10). • The climate data is equivalent to Central European conditions, i.e. a maximum ambient temperature of 40°C and 60 per cent humidity, i.e. the maximum amount of actual air is 35 .961m 3 /h. • The energy consumption is calculated as 24- hour averages for spring, summer, autumn and winter climatic conditions. 18