Vertical single-shaft mixers can be equipped with indirect temperature control for heating and cooling tasks. The options range from a simple vessel jacket to full temperature control of the mixing shaft and mixing tools. Which solution is suitable does not depend solely on the desired product temperature or the required heating or cooling capacity. Product condition, thermal conductivity, fill level, viscosity, possible phase changes, mixing duration, hygienic requirements, and possible vacuum or pressure operation are equally decisive.

In enclosed mixers, heat transfer is generally indirect. The temperature-control medium remains separate from the product and releases heat to the mixed material via the vessel wall or temperature-controlled internals, or absorbs heat from it. The mixing motion continuously renews the product layer at the heat-transfer surfaces, thereby improving heat transfer. This principle is particularly suitable for sensitive, toxic, solvent-containing, dusty or hygienically demanding products.

The standard design is a double jacket on the cylindrical mixing chamber and, where applicable, on the conical or domed base. A heating or cooling medium circulates in the jacket. Warm water, hot water, steam or thermal oil are used for heating tasks. Warm and hot water are suitable for moderate temperature ranges and readily controllable processes. Steam enables high heat flows and short heat-up times, but requires a suitable pressure design, condensate management and safety assessment. Thermal oil is preferred where higher temperatures or a wide temperature range are required. Cooling water, glycol-water mixtures or brine are mostly used for cooling tasks. For temperatures below the freezing point of water, a specially designed refrigeration system may be required.

The vessel jacket can be designed as a conventional double jacket, channel jacket, half-pipe coil, or dimple jacket. Which design is appropriate depends on the permissible pressure of the temperature-control medium, the required heat output, the temperature range, the vessel geometry and the mechanical design. Half-pipe coils are suitable, for example, for high pressures on the medium side. Channel or dimple jackets can allow targeted flow guidance. However, heat-transfer performance is not determined by the jacket design alone. The medium, flow velocity, temperature difference, fouling tendency, product movement and the contact time of the mixed material with the vessel wall are also essential.

A vessel jacket is often sufficient where the product is well circulated, a moderate heating or cooling rate is adequate, and no very tight temperature tolerances need to be maintained. For poorly heat-conducting, highly viscous, pasty or strongly insulating products, however, the jacket area alone can be too small. In these cases, it can be worthwhile to additionally temperature-control the mixing shaft and mixing tools.

A temperature-controlled mixing shaft is designed as a hollow shaft through which a heating or cooling medium flows. Mixing arms, helical tools or screw flights can likewise be made hollow and designed for temperature control. This means that heat is introduced or removed not only via the vessel wall but additionally within the product itself. The active heat-transfer area increases and long heat paths through the bulk material are shortened. This can be particularly advantageous for vacuum contact drying, for pasty or crystallising products, for viscoplastic intermediate phases, and for reactions with high heat release or tight temperature specifications.

Temperature-controlled mixing shafts and tools are supplied via suitable rotary unions. These must be designed for the temperature-control medium, pressure, temperature, rotational speed, tightness requirements and maintenance needs. With steam, thermal oil or very cold media, material selection, thermal expansion, seal loading and safety aspects are particularly important. Temperature-controlled internal tools improve heat transfer, but they also increase technical complexity, cleaning effort and investment cost. Their use should therefore be decided on the basis of the actual heat-transfer requirement and product trials.

Additional temperature-controlled surfaces can be worthwhile for special process requirements. A temperature-controlled lid, for example, can prevent vapours from condensing on a cold lid surface, drops falling back into the product, or local build-up forming. Whether a heated lid is required depends mainly on product temperature, vapour pressure, vacuum level, lid geometry and vapour offtake routing. Discharge elements, product connections, filters, dosing lances or other internals can likewise be temperature-controlled. This is particularly relevant for melting or solidifying binders, fat-containing formulations, crystallising media and viscous liquid additions. The aim is to avoid critical cold spots and to keep viscosity or flowability within the desired range.

Electric jacket heaters, heating tapes or heating mats are an alternative where only a moderate heating capacity is needed, or where the plant is to operate independently of a central heat-transfer-medium circuit. They are suitable for pure heating tasks but do not provide a direct cooling function. At high capacities, high temperatures or fast temperature changes, water-, steam- or thermal-oil-based temperature-control circuits are often more controllable and more economical.

Vacuum can usefully complement temperature control. Under reduced pressure, the evaporation temperature of water or solvents decreases. Combined with a heated vessel jacket and, where applicable, temperature-controlled mixing tools, this enables contact and vacuum drying at a comparatively low product temperature. This requires a vacuum-rated, sufficiently tight mixer, suitable shaft and lid seals, a vapour filter, a vacuum system and, for solvent-containing products, a suitable condensation and recovery unit.

Vacuum lowers the boiling temperature, but it does not itself supply the energy needed for evaporation. This must be introduced into the product via the vessel jacket, mixing shaft, mixing tools or other heat-transfer surfaces. Particularly with powders, which conduct heat poorly and contain a lot of entrapped air, the combination of good product circulation and sufficiently large temperature-controlled surfaces is decisive for effective drying.

For precise temperature control, temperature measuring points are used in the product as well as in the flow and return of the temperature-control medium. For critical processes, additional sensors on the shaft, lid, filter or discharge element can be worthwhile. Cascade control can guide the product temperature via the temperature or mass flow of the heating or cooling medium. For exothermic reactions or thermally sensitive products, temperature limits, alarm functions and a defined safety response to failure of cooling, the mixing tool or the energy supply are part of the process concept.