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scraping surface then becomes the hardest material in use and can accommodate any of the common blade materials. The sole drawback is that the chrome is sensitive to salt and acid attack.

Bimetallic cylinders offer some advantages over those of any other material, whether nickel, chromed nickel, or stainless. Produced from two different materials, the bimetallic cylinder usually is made by centrifugally casting a hard, corrosion-resistant alloy inside a tube that has high tensile strength and thermal conductivity approaching nickel. While the lining is not quite as hard as chrome, it is hard enough to withstand abrasion from both stainless-steel and plastic blades. One disadvantage other than a slight loss in heat transfer is that the inner alloy coating is susceptible to strong acid corrosion.

The selection of a cylinder with a compatible blade material is compounded by the products to be processed and the cleaning procedures used. For example, on slush freezing applications or where media temperature is below freezing, plastic blades can be abraded within hours as hard ice is scraped away. And as already noted, despite being the hardest cylinder material in use, chrome plated nickel is subject to attack by common acid CIP solutions and does not stand up well to salty products over extended periods of time. Table 2 shows the difference in corrosion for various materials and acids, and it is worth noting that it is the cleaning regimen rather than the product that causes problems in some cases. A change in CIP procedures may well extend the life or allow the use of nickel or chrome nickel cylinder material. In effect, much thought and experience must go into selecting the best cylinder and blade combinations—a selection best left up to the equipment manufacturer.

Cylinder Jackets—Media

The three common media used in a SSHE are water, steam, and refrigerants such as ammonia and Freon. Each requires a slightly different cylinder jacket design for optimum performance.

The water jacket is designed with a small spacing between the jacket and heat exchange cylinder. This induces high liquid media velocities that cause turbulence, which in turn, improves heat-transfer efficiency. It also minimizes fouling on the media side. Normally, countercurrent flow is recommended between product and media. In many cases, however, because of the high flow rate of media over product and the high temperature differences between media and product, the difference in performance between countercurrent and cocurrent flow is small.

On steam units, media enters the jacket via a header that distributes the steam over the entire length of the cylinder. Condensate runs to the bottom of the jacket, where it is collected and removed by a steam trap. Direction of product flow is immaterial.

When ammonia or Freon is used, the refrigerant is handled differentially by various manufacturers. Most exchanger designs are of the flooded type with media being fed at the bottom of the jacket and boiling occurring within the jacket. Since the key to efficient heat transfer is to

Table 2. Corrosion of SSHE Cylinder Metals

Material corrosion at 140°F

Table 2. Corrosion of SSHE Cylinder Metals

Material corrosion at 140°F

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