Finned-surface exchangers types

Tlie fin is the most common fonn of extended surface, associated with both tubular and plate-type heat exchangers - the car radiator is a highly compact finned unit. Most heat exchangers used in gas streams have fins to improve the gas-side heat transfer. Compact units such as the plate-fin heat exchanger have fins, or secondary surfaces , between each pair of plates. Fins are used in both natural and forced convection. They are also used, when they are sometimes called ribs, to aid boiling or to provide drainage... 

Compounds having low vapor pressures at room temperature are treated in water-cooled or air-cooled condensers, but more volatile materials often requite two-stage condensation, usually water cooling followed by refrigeration. Minimising noncondensable gases reduces the need to cool to extremely low dew points. Partial condensation may suffice if the carrier gas can be recycled to the process. Condensation can be especially helpful for primary recovery before another method such as adsorption or gas incineration. Both surface condensers, often of the finned coil type, and direct-contact condensers are used. Direct-contact condensers usually atomize a cooled, recirculated, low vapor pressure Hquid such as water into the gas. The recycle hquid is often cooled in an external exchanger. 

Fins are used to increase the effective surface area of heat-exchanger tubing. Many different types of fin have been developed, but the plain transverse fin shown in Figure 12.66 is the most commonly used type for process heat exchangers. Typical fin dimensions are pitch 2.0 to 4.0 mm, height 12 to 16 mm ratio of fin area to bare tube area 15 1 to 20 1. 

These types of heat exchangers normally consist of heat-exchange surfaces obtained by stacking alternate layers of corrugated, high-uniformity, die-formed aluminum sheets (fins) between flat aluminum separator plates to form individual flow passages. Each layer is closed at the edge with solid aluminum bars of appropriate shape and size. 

Tube-fin exchangers can withstand high pressures on the tube side. The highest temperature is again limited by the type of bonding, materials employed, and material thickness. Tube-fin exchangers with an area density of about 3300 m2/m3 (1000 ft2/ft3) are commercially available. On the fin side, the desired surface area can be employed by using the proper fin... 

Water is circulated through a heat exchanger above the burner. The 15 heat exchanger is made of tubes of cast iron or copper, which resist corrosion. Both types use fins to increase the surface area in contact with water, which improves the transfer of heat. A thermostat located in the boiler causes the gas control valve to shut off when the water temperature reaches the pre-set level. 

Most air coolers are either induced-draft or forced-draft, as shown in Fig. 19.1, the more common arrangement being forced-draft. The air is moved by rather large fans. The tubes are surrounded with foil-type fins, typically 1 inch high. The surface area of the fins as compared to the surface area of the tubes is typically 12 to 1. That is why we call an air cooler an extended-surface heat exchanger. 

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