Reasons for drying range from extending shelf life extension and preventing spoilage, to quality preservation or as part of food processing, such as concentrating or dehydrating products. As a result, a number of different drying systems have been developed, with the main technique being spray drying. Other less common techniques include tunnel drying and drum drying, but all these methods are designed to move air over the product to dry it without damage. Other specialist techniques including freeze drying and vacuum drying may also be employed in certain situations.
Drying rates will vary with the method used, but general considerations include:
- The nature of the material, including its physical and chemical composition, moisture content, etc.
- The size, shape, and arrangement of the materials to be dried
- The relative humidity, or partial pressure of water vapor, in the air being used for drying
- Air temperature
One thing that most drying processes have in common is the use of direct or indirect heat to drive moisture from the product. This is frequently inefficient in terms of energy use, requiring high inputs and taking a relatively long time to achieve the required level of moisture reduction. The Carbon Trust has therefore recommended considering alternatives to conventional process heating, such as, microwave drying (which is expensive and not suitable for many products) or using lower temperature processes, such as evaporation. There is also an increasing overlap between using evaporation and drying techniques, and a combined approach is becoming more common.
Evaporation to complement drying
By using evaporation to reduce the water content of fluid products before they are sent to a spray drier, the overall amount of energy used by the process can be significantly reduced, improving efficiencies and lowering both the cost and environmental impact of the process. In some cases using a combined technique can double the output of an existing spray dryer. Low temperature evaporation combines the use of a vacuum to reduce the boiling point of the liquid to be removed, together with traditional high temperature evaporators based on heat exchanger technology.
Selecting the right heat exchanger
The type of heat exchanger used will depend on the nature of the products being treated. For materials with low or medium viscosities, such as brines, sugar solutions or evaporated milk, using the HRS K Series as an evaporator module provides high heat transfer rates with good resistance to fouling. For more challenging and viscous materials, such as purees, malt extracts, liquid cheeses and whey protein concentrates, the HRS Unicus Series contains a self-cleaning scraper mechanism which reduces fouling and maintains heat transfer rates (and therefore operational efficiency).
As can be seen, for non-solid materials with low water content, such as gels, jellies and some emulsions, low temperature evaporation can significantly reduce running costs compared with other drying systems. Where you have a process which requires the removal of water from a food product it may be worth speaking to a heat exchange specialist before you commit to traditional drying methods.
Matt Hale is the International Sales and Marketing Director for HRS Heat Exchangers.