Iqu

B.C.—Barometric condenser F.C.—Flash cooler

Cold water

Steam rfH^f iH-

P/H—Pasturizer/heater A Condensate out C

P/H—Pasturizer/heater A Condensate out C

H—Heater vapor recompressors have more or less the same limited flexibility as thermal recompressors. This applies particularly to the high-speed radial turbo compressor, which reaches its surge limit if the actual vapor flow is too small, thus rendering the system unstable (16). Mechanical recompressors are complicated in design, and need careful maintenance and supervision of mechanical vibration.

Refrigerant Cycle Evaporators. Figure 18 shows a schematic system of juice concentration using a technique that combines a vapor compression refrigeration cycle with the evaporation system (25). Condensing water vapor boils liquid refrigerant to form a refrigerant vapor, which is then compressed to a higher temperature level and acts as a heat source to boil the liquid being concentrated (19). The evaporator, for simplicity, is shown as a single-effect design with means for recirculating the juice. Variations may employ multiple effects. The pressure in the system can be maintained by a relatively small steam ejector or a me chanical vacuum pump. Its advantage over a mechanical vapor recompression system is that the volume of vapor being compressed is much reduced at a given evaporator temperature. However, the refrigerant must be compressed through a wider temperature range because an additional heat exchanger is required.

Auxilliary Equipment

Preheaters. After feeding the product into the evaporation plant via a float balance tank and feed pump, the first process step is to preheat the product. In general, liquid food is stored at between 5 and 20°C for quality preservation; therefore, it must be preheated to at least the evaporation temperature of the first effect, most commonly between 40 and 110°C. Three types of preheater are used: plate, spiral tube, and straight tube heat exchangers (16). If there are solid particles in the product, a plate heat exchanger should not be used, because the particles may block the narrow product passages. The same problem may

Steam

Steam

Steam

Steam

□ Stages O Effects

Juice flow Vapor flow

& Condensate C out

Flash cooler

B.C.—Barometric condenser

Figure 15. Schematic of a 4-effect, 5-stage plate evaporator. Source: Reprinted with permission from Ref. 24.

□ Stages O Effects

Juice flow Vapor flow

& Condensate C out

Flash cooler

B.C.—Barometric condenser

Figure 15. Schematic of a 4-effect, 5-stage plate evaporator. Source: Reprinted with permission from Ref. 24.

Barometric

Barometric

Figure 16. System of concentration using thermal recompression with a steam-jet compressor. Source: Reprinted with permission from Ref. 25.

also occur with protein-containing products, due to the formation of deposits if the preheating temperature exceeds the denaturalization temperature. Secondary flows developed in the spiral tube leads to higher heat-transfer coefficients than straight tube, and a reduced tendency to foul. The disadvantages are that inspection and replacement of the tube is very difficult. Straight tube preheaters are especially useful as intermediate preheaters following first step preheating with plate heat exchanger, where product of high concentration is required, and where equipment inspection is essential.

Separators. Separators are used to separate the mixture of vapor and concentrated product produced in the calandria. A separator should have high separation efficiency, small pressure loss, and short residence time. Because a fine mist of concentrate is produced during vigorous boiling and carried over in the vapor, separators should be designed to minimize entrainment. Large pressure loss is to be avoided lest extra heating surface must be installed in the next effect. The many different separators in use can be classified according to three operating principles: gravity separator, baffle separator, and centrifugal separator.

Condensers. Surplus vapor produced in evaporation can be condensed and reused where possible. Condenser cooling water is normally provided from a cooling tower or the city main; in certain cases, it may be extracted from rivers, lakes, or the sea. The tasks of a condenser are to maintain the heat balance of an evaporation plant, to stabilize the

Mechanical

Mechanical

Figure 17. A mechanical vapor recompression system. Source: Reprinted with permission from Ref. 19.

temperature in each effect, and to produce and maintain vacuum.

Berry Boosters

Berry Boosters

Acai, Maqui And Many Other Popular Berries That Will Change Your Life And Health. Berries have been demonstrated to be some of the healthiest foods on the planet. Each month or so it seems fresh research is being brought out and new berries are being exposed and analyzed for their health giving attributes.

Get My Free Ebook


Post a comment