The use of water jackets or pipe coils within a fermenter as a means of temperature control has been described in Chapter 7. In many small systems there is a heating element, 300 to 400 W capacity being adequate for a 10-dm3 fermenter, and a cooling water supply; these are on or off depending on the need for heating or cooling. The heating element should be as small as possible to reduce the size of the 'heat sink' and resulting overshoot when heating is no longer required. In some cases it may be better to run the cooling water continuously at a steady rate and to have the heating element only connected to the control unit. This can be an expensive mode of operation if the water flows directly to waste. For small-scale use, Harvard Apparatus Ltd (Fircroft Way, Edenbridge, Kent, U.K.) make a unit, the Thermocirculator, which will pump recirculating thermostatically heated water through fermenters of up to 10 dm3 capacity and give temperature control of ±0.1°.
In large fermenters, where heating during the fermentation is not normally required, a regulatory valve at the cooling-water inlet may be sufficient to control the temperature. There may be provision for circulation of refrigerated brine if excessive cooling is required. Steam inlets to the coil and jacket must be present if a fermenter is being used for batch sterilization of media.
Low agitation speeds are often essential in animal cell culture vessels to minimize shear damage. In these vessels, heating fingers can create local 'hot-spots' which may cause damage to cells very close to them. Heating jackets which have a lower heat output proportional to the surface area (water or silicone rubber covered electrical heating elements — Chapter 7) are used to overcome this problem.
Flow measurement and control
Flow measurement and control of both gases and liquids is important in process management.
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