Tubes fins, condensation

Surface Condensers Surface condensers (indirect-contact condensers) are used extensively in the chemical-process industiy. They are employed in the air-poUution-equipment industry for recoveiy, control, and/or removal of trace impurities or contaminants. In the surface type, coolant does not contact the vapor condensate. There are various types of surface condensers including the shell-and-tube, fin-fan, finned-hairpin, finned-tube-section, ana tubular. The use of surface condensers has several advantages. Salable condensate can be recovered. If water is used for coolant, it can be reused, or the condenser may be air-cooled when water is not available. Also, surface condensers require less water and produce 10 to 20 times less condensate. Their disadvantage is that they are usually more expensive and require more maintenance than the contac t type. 

Air-cooled condensers employ axial-flow fans to force or induce a flow of ambient air across a bank of externally finned tubes. Finned tubes are used because air is a poor heat transfer fluid the extended surface enables air to be used economically. Several types of finned-tube construction are available the most common types are extruded bimetallic fintubes and footed tension-wound fintubes. The most common fin material is aluminum. 

It is common for air to be the condenser coolant. Air-fin condensers comprise layers of finned tubes over which air is forced by fans. There are a number of ways in which air flow may be manipulated. As shown in Figure 12.35, the pitch of the fan blades or the fan speed can be adjusted. However there are usually a large number of fans in place. While it may not be necessary to install such mechanisms on all the fans, it does become costly and, like most... 

Air-Cooled Overhead Condensers Air-cooled overhead condensers (AOC) have been designed and installed above distiUation columns as integral parts of distiUation systems. The condensers generally have incliued tubes, with air flow over the finned sur ces induced by a fan. PrevaUing wind affec ts both structural design and performance. 

Pinhole perforations were discovered in the walls of chiller condenser tubes of an air-conditioning system. Close laboratory examination of the internal surfaces of affected tubes revealed distinct patches of small pits (Fig. 12.18) and pit sites aligned along longitudinal mandrel marks and fine scratches (Fig. 12.19). In some locations, transversely oriented pit sites that were aligned with the locations of the fins on the external surface branched off the primary longitudinal pit alignment (Fig. 12.20). 

Open Tube Sections (Air Cooled) Plain or finned tubes No shell required, only end heaters similar to water units. Condensing, high level heat transfer. Transfer coefficient is low, if natural convection circulation, but is improved with forced air flow across tubes. 0.8-1.8... 

Water flows inside the tubes, and vapors eondense on the shell side. Cooling water is normally chilled, as in a cooling tower, and reused. Air-eooled surfaee eondensers and some water-cooled units eondense inside the tubes. Air-cooled condensers are usually construeted with extended surfaee fins. 

The usual applications for finned tubes are in heat transfer involving gases on the outside of the tube. Other applications also exist, such as condensers, and in fouling service where the finned tube has been shown to be beneficial. The total gross external surface in a finned exchanger is many times that of the same number of plain or bare tubes. 

D,.g = an equivalent diameter of a finned tube for calculation of condensing coefficient, ft. 

W" = tube loading for condensing, Ib/hr (ft of finned tube length). 9 ... 

Above this size, the flow of air over the condenser surface will be by forced convection, i.e. fans. The high thermal resistance of the boundary layer on the air side of the heat exchanger leads to the use, in all but the very smallest condensers, of an extended surface. This takes the form of plate fins mechanically bonded onto the refrigerant tubes in most commercial patterns. The ratio of outside to inside surface will be between 5 1 and 10 1. 

Construction materials will be the same as for air-cooled condensers. Aluminium fins on copper tube are the most common for the halocarbons, with stainless steel or aluminium tube for ammonia. Frost or condensed water will form on the fin surface and must be drained away. To permit this, fins will be vertical and the air flow horizontal, with a drain tray provided under. 

Condensed water will run down the evaporator fins to a collection tray below the coil. From there, drain pipes will take this water to a drain. If plastic pipe is used, it should be black to exclude daylight, or slime will grow inside the tube. Drain pipes passing through rooms below freezing point need to be fitted with trace heaters. 

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