Scraped Surface Heat Exchangers are double pipe elements where coolant is provided in the annulus running countercurrent to the process fluid flow. SSHE used in the Food and Chemical Industries may differ in their materials of construction and use in a number of ways.
You may remember making Ice Cream using a jacketed wooden pail where ice is packed into the space between the outside and inside of the pail. Once the inside was filled with delicious fresh Milk, a vane or rotor was then cranked or turned by hand, and as the temperature of the Milk decreased, the viscosity rose, and the milk turned into cream and then a thick paste until the ice cream was formed. To do this, the scraping mechanism will rotate up to 250 RPM, whereas a typical speed of an SSHE used to make Crystals in the Chemical or Wax production will be around 25 RPM to produce easily separable crystals and to avoid producing agglomeration and producing anything like a paste.
For decades, scraped surface heat exchangers have replaced slower, less efficient batch operations with more uniform, controllable, and repeatable continuous processing. Typical applications are heating and cooling cheese sauce, deserts, caramel, deboned meat, pet food, and fruit products.
Materials of construction of SSHE for use in the Food Industry are typically required to be “Food Grade,” and these may require a Sanitary or polished finish. These SSHE are typically made of Stainless Steel and polished. They are honed internally so they may be cleaned and prepared for other products to be processed without leaving any trace of foreign materials or fluids previously processed by the same equipment.
Not all Scraped Surface Heat Exchangers are used for Crystallization, and not all Crystallizers are Scraped Surface Heat Exchangers. Other types of crystallization include evaporative crystallization (think Sea Salt) and vacuum crystallization, where the solubility curves do not allow for crystals to precipitate out using continuous cooling crystallization. Some crystallization involves heating the process fluid when, for example, the solubility curve of the process fluid is inverse, and crystals form by heating, as is the case with certain inorganic salts.