Basic Considerations

The scraped-surface heat exchanger (SSHE) is a specialized piece of heat-transfer equipment that was patented around 1926 by Clarence Vogt in an effort to develop a more efficient freezer for making ice cream. The design incorporated a scraping action to prevent buildup of frozen ice cream on heat-transfer surfaces. The concept was successful. Thermal efficiency was improved and production capacity greatly increased.

Since that time, the SSHE has become essential for numerous processes in the dairy and food industries. Many of these applications are similar to the ice cream problem in that, as the product is heated or cooled, it fouls heat-transfer surfaces and reduces efficiency. Such applications, for example, include cooling peanut butter or plasticizing shortening and margarine. In each instance, fat crystals that form are like the ice crystals found in ice cream. Without the scraping of the heat exchanger sweeping the solidified fats away from the surface, heat-exchange efficiency would fall off very rapidly. Conversely, in heating applications where a product tends to "burn on," the scraping action removes product from the surface before fouling can occur. Typical heating examples are aseptic cheese sauces and aseptic puddings where processing temperatures approach 300°F (150°C).

The modern SSHE also is capable of processing products containing particulates. Currently, standard units can handle 1/2-in. (13-mm) particles without excessive breakage while special designs are available to accommodate 1-in. (26-mm) particulates.

While more expensive than other types of heat exchanger, the SSHE is the best and only thermal approach in hundreds of applications where high viscosity, large particulates, crystallization, and burn-on are problems that must be considered.

A scraped-surface heat exchanger (Fig. 1) basically consists of a jacketed cylinder fitted with a rotating shaft on which scraper blades are mounted. Product is pumped through the cylinder while a heating or cooling medium is circulated in the annular space between the cylinder and

Figure 2. Model 2HD-648 SSHE with water flush seals.

the jacket. The blades are fixed to pins that allow them to swing freely. No springs are necessary since centrifugal force holds the blades in position against the inside of the cylinder wall as product constantly is swept away from the heat-transfer surface and new product exposed to treatment.

The standard horizontal exchanger generally has from one to three independently functioning jacketed cylinders mounted on a heavy steel base (Fig. 2, 3). A stainless-steel casing covers the cylinders, base and drives to form a completely enclosed system. Vertical units also are available for use when floor space is at a premium (Fig. 4).

Many processes require more than one unit, and in such cases where multiple cylinders are used, product always should be piped in series. Heat transfer will be higher because of the higher flow through each cylinder. Parallel

Product in

Figure 1. Cutaway of horizontal SSHE.

Steam in

Condensate Steam 'acket out

Figure 3. A pair of scraped—surface heat exchangers provide an added dimension to an aseptic system by permitting the processing of fluids containing particulates.

Figure 1. Cutaway of horizontal SSHE.

Product in

Steam in

Condensate Steam 'acket out

Figure 4. Vertical Model VExHD-884 SSHE.

flow arrangements fail because there is no way to ensure equal flow to each cylinder unless individual pumps are used for each circuit. It should be noted that a scraped surface heat exchanger does not do any pumping. A pump is required to move product through the unit.

Dashers

The shaft which carries the scraper blades is called a dasher by some manufacturers and a mutator by others— the "dasher" terminology being derived from early days when the exchanger initially was used as an ice cream freezer.

Dashers are engineered to achieve high heat-transfer coefficients with minimum power consumption and are supported by heavy-duty bearings located outside the product contact zone. Three standard designs (Fig. 5a, b, 6) provide product flow spaces of different sizes to accommodate different product viscosities, dwell time, level of blending, and size of particulates. For margarine or plas-ticizing applications, dashers with internal water circulation reduce adhesion of product to the dasher surface.

(a)

Figure 5. Dasher—blade assemblies cutaway, (a) Series 55; (b) Series 45.

Figure 5. Dasher—blade assemblies cutaway, (a) Series 55; (b) Series 45.

Typical dasher speeds vary from 60 to 420 rpm with standard motors providing a choice of three drive methods—direct-driven hydraulic, belt-driven electric, or direct-driven gearhead.

Scraper Blades

Designed to promote the rapid removal of product from cylinder walls while enhancing product agitation and mixing, scraper blades are available in a selection of materials and configurations. Most common materials are stainless steel and plastics since the blade is the wearing part and as such, must be softer than the cylinder wall or lining.

Blade selection generally is determined by product temperature, pressure, and formulation as well as by the cylinder material and service media being used.

Heat-Exchange Cylinders

To provide optimum performance and economy of operation, heat-exchange cylinders are available in a selection of sizes and materials of construction.

Figure 6. Series 30 dasher for viscous products or particulates.

The most common diameter for the scraped SSHE cylinder is 6 in. (152 mm) with lengths established at 48 in. (1220 mm) and 72 in. (1830 mm). There are, however, other sizes available. Some are 4 in. (102 mm) in diameter with lengths of 60 in. (1520 mm) (Fig. 7) and 120 in. (305 mm) while others are of 8 in. (203-mm) diameter and lengths to 84 in. (2130 mm).

The most common materials of construction are stainless steel, nickel with or without chrome plating, and, more recently, a bimetallic combination.

Since the cost per square foot of heat-transfer surface is higher for a SSHE than other types of heat exchangers, it is essential that cylinder material with the highest feasible heat-transfer coefficient be selected. This selection, however, must be tempered by consideration of the compatibility between cylinder and scraper blade materials and the susceptibility of these materials to acid attack and corrosion.

As charted in Table 1, nickel exhibits the best thermal conductivity while stainless steel is the least conductive. Even so, nickel is not suitable for all applications. It is relatively soft when compared to typical metal scraper blade materials and would wear rapidly if this combination were to be used. Since SSHE always are designed to make the blades rather than the more expensive cylinders the wearing part, plastic blades must be used with nickel cylinders if only for reasons of economics and reduced downtime. For the same reasons, only plastic blades are run on stainless-steel cylinders.

To retain the superior heat-transfer characteristics of nickel while benefiting from the extended durability of steel blades, nickel cylinders may be chrome-plated. The

Figure 7. Four-barrel vertical scraped-surface heat exchanger is designed to cool meat gravies from 190 to 70°F. Unit capacity is 5000 lb/h.

Table 1. Heat Transfer Coefficients of SSHE

Cylinder Metals

Table 1. Heat Transfer Coefficients of SSHE

Cylinder Metals

Material

K, Btu/h • °F • in.

Nickel

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