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Abrasion influence

Temperature influence

Figure 14. Temperature—wear relationship.

tions differ widely, and no one seal can handle the variety of temperatures, pressures, and product characteristics constantly encountered.

Most manufacturers offer a number of shaft seals, each designed to satisfy the demands of certain applications. The simplest and least expensive seal to maintain is the O-ring seal (Fig. 16a). This type of seal is recommended for use with products having high lubricity, eg, margarine, lard, shortening, ice cream mix, certain meat products, oils, and fats. Normally these products are not processed at temperatures above 140°F. High temperatures above 180°F shorten the effective life of an O-ring by causing it to lose elasticity. The sealing action of an O-ring seal depends on its ability to be sufficiently flexible to fill the gap between the rotating shafts and the static O-ring groove.

While the advantage of the O-ring is that it is inexpensive to replace when worn, the disadvantage is that it wears rapidly. Although daily replacement is not uncommon, this is not a problem when frequent equipment inspection is scheduled. The material of the O-ring depends on the fats or oils in the product to be processed. Typical materials are Buna N, Viton, and EPDM.

Rotary face seals are for all other applications (Fig. 16b). These can handle high temperatures and abrasive products, and offer longer wear life than O-rings. Rotary seals can be broken down into two groups—those with and without a seal water chamber.

The seal water chamber allows water or steam to flow around the edges of the seal faces and is recommended for use when handling products that tend to crystallize or dry out in the sealing area. Examples would be liquid sugars, tomato products, candies, fondants, and processed cheese. Water flushing increases the useful life of the seal. Steam is injected into the chamber for aseptic applications since FDA standards require a sterile barrier media on all rotating shaft seals in low acid aseptic processes. The steam, a sterial medium, prevents sterilized products from being contaminated should there be a seal leak. Many manufacturers build an integral water chamber into their equipment with the chamber to be used only as required.

Critical to the life of a seal is the materials used for the seal faces. The typical seal material is hardened stainless steel on carbon. Alternates for the carbon can be phenolic resins, plastics, silica carbide, or ceramics. Other more exotic seal materials to replace the hardened stainless include tungsten carbide and chrome oxide. Each has advantages and disadvantages, such as wear life, and cost. Normally, the manufacturer recommends the seal type and seal materials based on test work and field experience.

Selecting Dashers

Dashers or shafts that carry the scraper blades are available in diameters from 1-1/2 to 6-1/2 in. and are mounted within heat-exchanger cylinders that, in turn, are manufactured in diameters from 4 to 8 in. Since the most widely used cylinder has an ID of 6 in., the following is based on that model.

The best heat-transfer efficiency is provided when the annular space between a dasher and a cylinder is small. For example (Fig. 17), a 5-1/2-in. dasher within a 6-in. cyl-

Dasher rotation

Dasher rotation

Chisel edge scrapes product from wall

Scraper blade in operation

Chisel edge scrapes product from wall

Scraper blade in operation

Heel results from contact with inner cylinder wall. This heel is removed when blade is reconditioned.

New scraper blade

Worn scraper blade

Worn scraper blade

Figure 15. Wear pattern on metal scraper blades.

Figure 16. Shaft seals: (a) simple O-ring seal; (b) rotary mechanical seal with water flush or stream aseptic outer chamber.

inder provides a quarter inch annular product flow space, which causes a high axial flow velocity, induces turbulence, and offers a short product residence time. This is particularly useful in crystallizing applications such as for processing shortening and margarine since these products essentially are subcooled and allowed to crystallize after leaving the heat exchanger. Dashers for these products are water circulated.

The disadvantages of the 5-1/2-in. dasher are that it cannot handle viscous products because of high product pressure drop and cannot accommodate products containing particulates over a 1/4 in. in size. In such cases, the 4-1/2-in. dasher with its 3/4-in. annular product flow space is recommended. The smaller the dasher diameter, the larger the annular space, and the larger the particulates that can be processed. As the dasher diameter decreases, so does the effective U value for a given dasher speed.

Dasher speed is another design consideration. Typical dasher speeds vary from 60 to 420 rpm, and, in general, the higher the dasher speed, the higher the U value (Fig. 18). While this is desirable, increased dasher speed also increases the motor load, which can cause problems in cooling applications. Recalling the heat-transfer formula given previously, heat removed when cooling comes from both the product and the motor load required to turn the dasher. In some applications, the motor load actually can equal the product heat load. Therefore, in some cases a slower dasher speed nets a more beneficial end result.

The graph in Figure 19 compares horsepower and rpm involving various products. All are cooling applications performed on a 6-in. diameter by 72-in.-length unit. Note how small changes in dasher speed on marshmallow change the horsepower requirement significantly while the power requirements vary little while cooling cheese sauces at different speeds. It should be pointed out that horsepower requirements do not necessarily follow viscosity of the product. Rather, the term tenacity would best describe the measurement that would equate to the horsepower— rpm relationship. A good example is corn syrup versus a cooked starch slurry base for salad dressings. At the same viscosity, the corn syrup has much more tenacity in retarding dasher rotation than does the cooked starch.

To reduce motor loads while processing tenacious products, slow dasher speeds are required but the dasher cannot be run so slowly that the U value is reduced significantly. The properties of such products normally generate a low U value at any dasher speed, usually in the range of 120-180 Btu/h • ft2oF. At very slow speeds, the U value is

O-ring seal

"0" ring

Replaceable wear sleeve

O-ring seal

"0" ring

Replaceable wear sleeve

Seal water chamber

Water flush seal or aseptic

Figure 16. Shaft seals: (a) simple O-ring seal; (b) rotary mechanical seal with water flush or stream aseptic outer chamber.

Seal water chamber

Seal water chamber

Seal water chamber

Water flush seal or aseptic

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