CEE Oct-Dec 2012

900,"" 250."" 200."" 150,"" 100,"" SO."" 0."" 3 Relatht SOTt 3 metre= 100% s 6 7 I F~~ltfl' J: ReiJitve cOtnfJre~..or l>orA> ,1~ ,1 lunclton ol ltquid depth The other conflicting factor relates to the fact that the compression work performed by the air blowers increases as the delivery pressure increases. The bar chart below shows the theoretical relative rise in the compression work performed by the air blowers at various depths (see Figure 3). Choice of the best aeration technology There are three important needs that must be met when choosing air blowing equipment for the air supply: • Achieving the lowest possible energy consumption at most common plant loading situations • The ability to efficiently be able to regulate the amount of air supplied so that it matches the plant load all the time • Good reliability, i.e. high availability factor and low maintenance costs. The air blower technologies in use can be divided into 2 main groups: 1. Positive displacement blowers: blowers (Roots-type - rotary piston) or rotary screw compressors. 2. Dynamic blowers; centrifugal blowers called turbo blowers In the positive displacement blower, air is forced into compression chambers (a and b) as the rotor turns (see Figure 4). The air is then compressed when the chamber is opened to the pressure side (c) or as the pressure chamber volume reduces when the rotor turns (screw compressor), shown in Figure 5. In the dynamic machine, air is drawn into an impeller, which rotates at high speed and therefore transfers energy to the air as dynamic (kinetic) energy after which , the air velocity is reduced again in the blower outlet and converted to static pressure (see Figure 6.) In order to ensure a flexible air supply that could accommodate plant load fluctuations, the air supply station typically has three duty units and one stand-by blower that can take over in case of a fault or servicing on the other units. In order to ensure an overlap between the units, it is necessary for each unit to be able to regulate the amount of air output, Figure 4: Posiu\ e dispiJcement blower Figure 5: Screw compressor F(i!,urc• f>: Cenlrilu~al h/ower with compn'~sor I\ heel 17