Jackets and Coils

Jackets are used for the circulation of steam and cooling water during the heating and cooling cycles of sterilization of the fermentor (191). Since microbial reactions are exothermic, the heat produced during fermentations leads to a rise in the temperature of the broth, necessitating the need to maintain the temperature at the optimal value. The heat transfer is dependent on the area of contact surface of the cooling or heating fluid with the fermen-tor and the temperature gradient of the medium. Normally, steam is used as the heating fluid; and water, chilled water, or chilled brine are used as cooling fluids. The contact surface area of the jacket with the fermentor should be maximal and the pressure drop of the circulating fluids in the jacket should be minimal for better process performance. The layout of the external jackets could be a double jacket, a full pipe, or a half pipe (limpet coil), depending on the required area for heat transfer, the heating or cooling medium, and the circulation velocity (192).

The choice of jacket material usually is not critical, and it usually is best to leave the vessel fabricator some degree of flexibility to allow for his usual shop practices. Some guidelines are:

1. Stainless steel jacket cover made of SS304 to extend vessel life

2. Jacket is appropriately designed to increase vessel strength

3. For vessels less than 5 m3 providing jacketing at the bottom, dished end may not contribute significantly to heat transfer

4. Jacket cover to the height equivalent to the operating volume of the fermentor

5. Jacket inlet and outlet lines should have end connections (flanged or sanitary coupling) between the external pipes and the jacket

6. Jacket design should favor minimum accumulation of solids to avoid ionic concentrations, which cause corrosion

If internal cooling coils are used, they can not only remove the excess heat from fermentation, but also act as baffles for better mixing. A minimum spacing of 150 mm between coils ensures that they withstand the thermal and mechanical stresses generated during fermentations. Joints between pipes should be butt welded, covered with a welded concentric tube, and lead tested (193).

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