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Food Processing And Health

Water boiling point

Figure 4. Duhring plot for sucrose for various sucrose concentrations per 100 g of water. A, 1,000 g; B, 800 g; C, 600 g; D, 400 g; E, 200 g. Source: Reprinted with permission from Ref. 12.

Water boiling point

Figure 4. Duhring plot for sucrose for various sucrose concentrations per 100 g of water. A, 1,000 g; B, 800 g; C, 600 g; D, 400 g; E, 200 g. Source: Reprinted with permission from Ref. 12.

Pressure drop increases as more vapor is formed, and hence the evaporation temperature drops. This temperature drop must be as small as possible because it leads to higher energy consumption. The flow regime and temperature profile are depicted in Figure 7.

For more viscous foods, or those that are very heat sensitive, the feed is introduced at the top of the tube bundle, and the concentrate and the vapor are leaving the tubes at their bottom. Normally the liquid is superheated when entering the tube, thus the liquid is already at its boiling point and no part of the heating surface is used for pre heating. This type of equipment is called the falling-film evaporator (Fig. 8). Only a small amount of product is in the tube, compared with the column of liquid in the climbing-film evaporator. Hydrostatic head loss found in climbing-film evaporators is absent in falling-film evaporators. Thus little temperature drop is seen during the passage through the tube (14). The gravitational force enhances the forces due to expansion of the steam, to produce high flow velocities of up to 200 m/s at the end of the tubes (12). Falling-film evaporators were developed to increase the rate of removal of water and decrease the danger of localized overheating.

A falling-film evaporator consists of the following major components: vertical heating tube bundle with surrounding heating jacket, calandria upper section with product distribution device, and calandria base section with separator. The liquid to be concentrated is fed into the upper part of the calandria where it is distributed evenly on the tube sheet, from which it flows down the interior walls of the vertical tubes as a thin boiling film. The most important factor in the trouble-free operation of this evaporator is the liquid coverage of the heating tubes, that is, the liquid flow rate required to cover each heating tube. Without enough liquid, the liquid film will break near the bottom of the tube. Dry spots develop on the tube, the heating surface becomes coated with a deposit, and eventually the tube may get clogged with burned-on product. The minimum liquid coverage depends on the type of product, required concentration, and operating time. If the coverage

Vapor outlet

Vapor outlet

Inlet (steam)

Condensate vent """" U

Infeed (dilute product)

Outlet (concentrated product)

Figure 5. Calandria evaporator. Source: Reprinted with permission from Ref. 14.

Inlet (steam)

Condensate vent """" U

Infeed (dilute product)

Outlet (concentrated product)

Mist flow

Annular flow

Plug flow Slug flow

Single-phase flow

Concentrate and vapor

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