Various arrangements of perforated pipes have been tried in different types of fermentation vessel with or without impellers. In small stirred fermenters the perforated pipes were arranged below the impeller in the form of crosses or rings (ring sparger), approximately three-quarters of the impeller diameter. In most designs the air holes were drilled on the under surfaces of the tubes making up the ring or cross. Walker and Holdsworth (1958) commented that in production vessels, sparger holes should be at least 6 mm (1 /4 inch) diameter because of the tendency of smaller holes to block and to minimize the pressure drop.
(volume of air"1 volume of medium"1 minute-1) with a power input of 1 W kg-1, Nienow et al. (1988) found that the power often falls to below 50% of its unaer-ated value when using a single Rushton disc turbine which is one-third the diameter of the vessel and a ring sparger smaller than the diameter of the agitator. If the ring sparger were placed close to the disc turbine and its diameter was 1.2 times that of the disc turbine, a number of benefits could be obtained (Nienow et al., 1988). A 50% higher aeration rate could be obtained before flooding occurred, the power drawn was 75% of the unaerated value, and a higher KLa could be obtained at the same agitator speed and aeration rate. These advantages were lost at viscosities of about 100 m Pas.
Orifice spargers without agitation have been used to a limited extent in yeast manufacture (Thaysen, 1945), effluent treatment (Abson and Todhunter, 1967) and later in the production of single-cell protein in the air-lift fermenter which are discussed in a later section of this chapter (Taylor and Senior, 1978; Smith, 1980).
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