Scraped surface or scraped wall heat exchangers have been used in one form or another for around 100 years. The oldest one the author has encountered was probably made in the late 19th or early 20th Century and consisted of a large half Vat, similar to a Wine Barrel, and was used for processing candle wax. The scraper mechanism looked like a rudder from an old sailing ship. The movement caused an agitation which removed wax from the wall and served as an agitator. Most early manufacturers have long since ceased production. Now, Armstrong-Chemtec Group is the largest manufacturer of Scraped Surface Heat Exchangers used for numerous processes including the separation and crystallization of both organic and inorganic mixtures.
The Scraped Surface Heat Exchanger works by a continuously controlled cooling of a process stream. If the application is for crystallization this enhances crystal growth. Crystals will preferentially adhere to the inner face of the inner pipe within a double pipe element. When Crystals adhere to the inner pipe wall, they inhibit heat transfer from taking place. Blades, held by springs, which are attached to a rotating centrally positioned shaft, scrape off the crystals at a preset speed without causing damage to them which allows more crystals to form. The crystals exit the scraped surface heat exchanger as a slurry in solution. The slurry is then put through a separation device, depending on the fragility and morphology of the crystals, such as a centrifuge, a press, or a vacuum filter. The crystals are then removed, either as a high purity, high-value product or as a waste product (such as crystallization of inorganic salts from wastewater).
SCRAPED SURFACE HEAT EXCHANGERS
Scraped surface heat exchangers (SSHEs) are often used in process industries to remove fouling and enhance heat transfer, for optimal crystallization. They are ideally suited for the food industry where products are viscous, sticky, and may contain particulate matter. Since these characteristics describe many processed foods, SSHEs are especially suited for pumpable food products. During operation, the product is brought into contact with a heat transfer surface which is rapidly and continuously scraped, thereby exposing the surface to the untreated product. In addition to maintaining high and uniform heat exchange, the scraper blades also provide a degree of simultaneous mixing and agitation. Heat exchange for viscous foods such as salad dressings, margarine, chocolate, peanut butter, ice cream, meat substitutes and shortenings are made possible using SSHEs. High heat transfer coefficients are achieved because the heat transfer surface is continuously exposed to the incoming new process fluid. As the product is in contact with the heating surface for a short time, high-temperature gradients may be used without the danger of causing undesirable reactions.
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