One of the key processes in the food industry – and its greatest consumer of energy – is in heating and cooling products. Products are heated for a wide variety of reasons including:-
- To dissolve solid ingredients
- To boil off water or other components
- To sterilise or pasteurise
- To dry wet materials
- To develop flavours
In almost all cases products do not come into direct contact with the heating medium, drying being a possible exception but even in that technology there are cases where heating is indirect.
Because of the nature of food ingredients heat transfer surfaces almost inevitably become fouled – coated with degradation products – which greatly reduce the efficiency of heat transfer and create a potential contamination risk and microbiological hazard.
Surface fouling results in plant having to be cleaned frequently to restore performance and reduce potential contamination with resulting loss of production, loss of materials, high water and energy consumption and effluent disposal issues. This has generally been assumed to be “inevitable” and little original thinking has been applied to the problem. However, ever rising costs of ingredients, utilities and services mean that this view has to change
The primary approach should of course be to re-think our manufacturing processes to examine whether we really need all that heat energy which not only has to be purchased, but also disposed of through complimentary cooling and refrigeration processes.
Many manufacturing processes in the food industry are based on taking raw materials, adding a lot of water and then removing most of it again. This approach is frequently “the way it has always been done” and may indeed contribute to the unique flavour or texture characteristics of the product. However modern analysis and understanding of the process and product can frequently indicate an alternative approach which will reduce the energy required. Unfortunately, whilst rethinking the process may well reduce basic energy requirements there will still remain some requirement to heat and this element may even be made more challenging.
If the heat transfer surface can be eliminated, however, the problem of fouling is eliminated and immediate improvements in efficiency and reductions in cost can be achieved. There are several ways in which the heat transfer surface can potentially be eliminated
Microwave and RF heating have been tried and are used in certain applications where it is appropriate, but power limitations, control issues, equipment cost and the difficulty of applying it to large volumes or flowing systems mean that it is rarely practical for general heating duties.
An interesting approach which is currently rarely used but has much potential is to use intensive mixing to generate heat directly though mechanical work. Although the prime energy supply is electricity which may be seen as an expensive form of energy, the system is efficient when combined with good process design and has many potential advantages particularly as primary energy sources change in response to availability, cost and the political pressure to reduce carbon emissions.
As long as the product structure is homogenous a well designed ultra high turbulence mixer can be used to design a simple and efficient system not only for heating liquid systems but also for dissolving and melting solids. Because electric induction motors, the prime mover, are highly efficient and built in quantity the technology is straightforward and fairly cheap. Control – by varying the speed and hence energy input – is fast in response and conventional.
Comparing this “direct” approach with conventional predissolving and evaporation can show reductions in energy of up to 80%. Whilst in the food industry it is frequently difficult to justify replacing a plant which is operational, the potential savings should make this approach very attractive for new investments, particularly when the other savings which result are considered:-
- Faster control response
- No boiler required
- No steam and condensate system to install and maintain
- Reduced cooling and condensing requirements
- Great flexibility
A particular example where this technology can be particularly advantageous is in producing syrup for sugar and sugar free panning systems. Rather than having to produce and store batches of syrup at different solids and recipes to supply the coaters syrup can be produced on line and the solids and recipe easily altered to the plant’s requirements.
At present only few companies can develop processes and manufacture custom designed equipment of this type because the concept is new. Probably the leader in this field is Lipp Mischtechnik based in Mannheim.