Tubular heating surfaces

Heating medium separated from evaporating hquid by tubular heating surfaces. 

By far the largest number of industrial evaporators employ tubular heating surfaces. Circulation of liqmd past the heating surface may be induced by boiling or by mechanical means. In the latter case, boiling may or may not occur at the heating surface. 

Tartaric Acid.— The usual concentration of the filtered solution is from 30 to 70 per cent of solids, and the work is done in single-effect evaporators made entirely of hard lead with extra-heavy lead coils. The all-lead construction may be replaced by a cast-iron shell with suitable lead lining, and the tubular heating surface with vertical tubes and lead flueplates can be used instead of the lead coils. It is very important that the construction is such that tubes can readily be cleaned from the hard scale produced by the calcium sulphate in the solution. With a steam pressure of from 30 to 50 lb. in the coils, and a vacuum of 27 in., the capacity is from to 2 gal. per square foot. 

D. Steam-heated evaporators with tubular heating surfaces ... 

Most evaporators employ steam-heated, tubular heating surfaces, over which the liquid to be vaporized is circulated. The efficiency of an evaporator may be reduced considerably by the formation of scale on the heating surface, which may be removed by chemical or mechanical means. Another problem is the formation of foam which may seriously restrict the capacity of an evaporator, but this may be controlled by the use of special chemical defoaming agents or gas or air jets. 

Evaporators with tubular heating surfaces dominate the field. Circulation of the liquid past the surface may be induced by boiling (natural circulation) or by mechanical methods (forced circulation). In forced circulation, boiling may or may not occur on the heating surface. 

The epitaxy reactor is a specialized variant of the tubular reactor in which gas-phase precursors are produced and transported to a heated surface where thin crystalline films and gaseous by-products are produced by further reaction on the surface. Similar to this chemical vapor deposition (CVE)) are physical vapor depositions (PVE)) and molecular beam generated deposits. Reactor details are critical to assuring uniform, impurity-free deposits and numerous designs have evolved (Fig. 22) (89). 

For certain applications multi-pass arrangements are used. When a plate heat exchanger is used instead of the tubular unit, boiling on the heating surfaces is avoided by increasing the static head using a line restriction between the plate pack and the separator. Compared with tubular units, lower circulation rates and reduced liquid retention times are important advantages. Plate-type units are discussed further in Section 14.7.7. 

Figure 8.4. Example of tubular heat exchangers (see also Fig. 8.14). (a) Double-pipe exchanger, (b) Scraped inner surface of a double-pipe exchanger, (c) Shell-and-tube exchanger with fixed tube sheets, (d) Kettle-type reboiler, (e) Horizontal shell side thermosiphon reboiler, (f) Vertical tube side thermosiphon reboiler, (g) Internal reboiler in a tower, (h) Air cooler with induced draft fan above the tube bank, (i) Air cooler with forced draft fan below the tube bank.

An air conditioner condenser in an automobile consists of 2 of tubular heat exchange area whose surface temperature is SOT. Saturated refrigerant-134a vapor at S0°C hjf = 152 kJ/kg) condenses on these tubes. What heat transfer coefficent must exist between the tube surface and condensing vapor to produce 1.5 kg/min of condensate ... 

Figure 7 A considerable improvement as far as capacity is concerned is obtained by the use of tubular coils placed in the liquid to be evaporated. Large amounts of heating surface can be concentrated in a small space and this type of pan is still used to a large extent in the sugar and milk industry. Figure 8 shows the Eeilly...

Circulation of liquid across the heating surface is caused by the action of the boiling liquid (natural circulation). The circulation rate through the evaporator is many times the feed rate. The downcomers are therefore required to permit the liquid to flow freely from the top tubesheet to the bottom tubesheet. The downcomer flow area is, generally, approximately equal to the tubular cross-sectional area. Downcomers should be sized to minimize holdup above the tubesheet in order to improve heat transfer, fluid dynamics, and minimize foaming. For these reasons, several smaller downcomers scattered about the tube nest are often the better design. 

Special heat-transfer devices used to liquefy vapors by removing their latent heats are called condensers. The latent heat is removed by absorbing it in a cooler liquid called the coolant. Since the temperature of the coolant obviously is increased in a condenser, the unit also acts as a heater, but functionally it is the condensing action that is important, and the name reflects this fact. Condensers fall into two classes. In the first, called shell-and-tube condensers, the condensing vapor and coolant are separated by a tubular heat-transfer surface. In the second, called contact condensers, the coolant and vapor streams, both of which are usually water, are physically mixed and leave the condenser as a single stream,... 

Its chief advantage is its versatility. A single unit can be varied over a wide range of heat transfer coefficients, residence times, and temperature profiles. It consists of a stationary horizontal vessel with a tubular rotor on which are mounted the doubled-walled disks. These hollow disks provide approximately 85% of the total heating surface. It has been nsed commercially for drying ABS, PCs, polyolefins, and other polymers. 

---