Magnetic Drives

The problems of providing a satisfactory seal when the impeller shaft passes through the top or bottom plate of the fermenter may be solved by the use of a magnetic drive in which the impeller shaft does not

Stirrer Gland

ponents; 1, flexible coupling; 2, stirrer shaft; 3, bearing housing; 4, ball journal fit on mating parts; 5, two slots for gland leaks, only one shown; 6, 'O'-ring seal; 7, seal body; 8, stationary counter-face sealed to body with square-section gasket; 9, exit port for condensate, fitted with unequal stud coupling; 10, rotating counter-face; 11, bellows; 12, shaft muff; 13, as 11; 14, as 10; 15, entry port for condensate, as 9; 16, as 8; 17, as 6; 18, shaft bush support; 19, leak holes; 20, Ferobestos bush; 21, ground shaft).

ponents; 1, flexible coupling; 2, stirrer shaft; 3, bearing housing; 4, ball journal fit on mating parts; 5, two slots for gland leaks, only one shown; 6, 'O'-ring seal; 7, seal body; 8, stationary counter-face sealed to body with square-section gasket; 9, exit port for condensate, fitted with unequal stud coupling; 10, rotating counter-face; 11, bellows; 12, shaft muff; 13, as 11; 14, as 10; 15, entry port for condensate, as 9; 16, as 8; 17, as 6; 18, shaft bush support; 19, leak holes; 20, Ferobestos bush; 21, ground shaft).

pierce the vessel (Cameron and Godfrey, 1969). A magnetic drive (Fig. 7.16) consists of two magnets: one driving and one driven. The driving magnet is held in bearings in a housing on the outside of the head plate and connected to a drive shaft. The internal driven magnet is placed on one end of the impeller shaft and held in bearings in a suitable housing on the inner surface of the headplate. When multiple ceramic magnets have been used, it has been possible to transmit power across a gap of 16 mm. Using this drive, water can be stirred in baffled vessels of up to 300-dm3 capacity at speeds of 300 to 2000 rpm. It would be necessary to establish if adequate power could be transmitted between magnets to stir viscous mould broths or when wanting high oxygen transfer rates in bacterial cultures. Walker et al. (1987) have described the development of a magnetic drive suitable for microbial fermentations up to 1500 dm3 which could be used when

Rotating and stationary seal interface

Sterile fluid/lubricant chamber

Rotating and stationary seal interface

Stationary seal

Rotating and stationary seal interface

Sterile fluid/lubricant chamber

Rotating and stationary seal interface

Stationary seal

Double Mechanical Seal

Upper seal assembly

Lower seal assembly

Upper seal assembly

Lower seal assembly

Fio. 7.15. Double mechanical seal (New Brunswick Scientific, Hatfield, England).

higher containment levels are specified. The stirring mechanism is ideal for animal cell culture to minimize the chances of potential contamination. This application is discussed later in this chapter.

on the baffles and the fermenter walls. Extra cooling coils may be attached to baffles to improve the cooling capacity of a fermenter without unduly affecting the geometry.

Baffles

Four baffles are normally incorporated into agitated vessels of all sizes to prevent a vortex and to improve aeration efficiency. In vessels over 3-dm3 diameter six or eight baffles may be used (Scragg, 1991). Baffles are metal strips roughly one-tenth of the vessel diameter and attached radially to the wall (see Figs 7.1 and 7.2 and Tables 7.2 and 7.3). The agitation effect is only slightly increased with wider baffles, but drops sharply with narrower baffles (Winkler, 1990). Walker and Holdsworth (1958) recommended that baffles should be installed so that a gap existed between them and the vessel wall, so that there was a scouring action around and behind the baffles thus minimizing microbial growth

The aeration system (sparger)

A sparger may be defined as a device for introducing air into the liquid in a fermenter. Three basic types of sparger have been used and may be described as the porous sparger, the orifice sparger (a perforated pipe) and the nozzle sparger (an open or partially closed pipe). A combined sparger-agitator may be used in laboratory fermenters (Fig. 7.17) and is discussed briefly in a later section.

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