Air-cooled tubing

A condenser should be chosen which has an inside diameter of at least 1.5 cm. otherwise frequent loosening of the sublimate is required to prevent clogging. A 3-ft., air-cooled tube with an internal diameter of about 2.5 cm. serves as a satisfactory condenser. 

Long, purple needles are obtained by subliming Ti( 5-C5H5)2(BH4) in vacuo at 80° into a 40-cm, air-cooled tube. Sublimation is accompanied by a substantial amount of thermal decomposition to an involatile residue which is pyrophoric in air. Sublimation is therefore not employed as a purification step during the preparation. 

The maximum power is available with air cooling up to 45kV for insulation reasons, the tubes are downrated to about 20W at 65kV. 

Most of the advantages of MCB technology can be used to make small anode-grounded metal-ceramic X-ray tubes as well. These could be water- or air-cooled and reach power ranges up to 1 kW at voltages up to lOOkV. 

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

The most dramatic evolution of a microwave power source is that of the cooker magnetron for microwave ovens (48). These magnetrons are air-cooled, weigh 1.2 kg, generate weU over 700 W at 2.45 GHz into a matched load, and exhibit a tube efficiency on the order of 70%. AppHcation is enhanced by the avaHabiHty of comparatively inexpensive microwave power and microwave oven hardware (53). The cost of these tubes has consistently dropped (11) since their introduction in the eady 1970s. As of this writing (ca 1995), cost is < 15/tube for large quantities. For small quantities the price is < 100/tube. 

Electrical Connections. Electric current is brought from the transformers by air-cooled copper busbars and close to the electrode by water-cooled bus tubes and flexible cables, connecting to water-cooled copper contact plates at the electrode. The plates are held against the electrode by hydraulic pressure. The connectors are as short and as balanced as possible to allow cancelling of magnetic fields associated with individual conductors. 

In atmospheric air-cooled finned tube exchangers, the air-film coefficient from Eq. (5-64) is sometimes converted to a value based on outside bare surface as follows ... 

Furnaces Shell and tube Air-cooled Vertical Horizontal ... 

TABLE 11-5 Overall Coefficients for Air-Cooled Exchangers on Bare-Tube Basis... 

Transverse fins upon tubes are used in low-pressure gas sei vices. The primary application is in air-cooled heat exchangers (as discussed under that heading), but shell-and-tube exchangers with these tubes are in sei vice. 

Air-cooled heat exchangers include a tube bundle, which generally has spiral-wound fins upon the tubes, and a fan, which moves air across the tubes and is provided with a driver. Electric motors are the most commonly usea drivers typical drive arrangements require a V belt or a direc t right-angle gear. A plenum and structural supports are basic components. Louvers are often used ... 

HumidiRcation Chambers The air-cooled heat exchanger is provided with humidification chambers in which the air is cooled to a close approach to the wet-bulb temperature before entering the finned-tube bundle of the heat exchanger. 

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. 

Ground area and. space requirement.s. Comparisons of the overall space requirements for plants using air cooling versus water cooling are not consistent. Some air-cooled units are installed above other equipment—pipe racks, shell-and-tube exchangers, etc. Some plants avoid such inst ations because of safety considerations, as discussed later. 

Maintenance cost. Maintenance for air-cooled equipment as compared with sheU-and-tube coolers (complete with coohng-tower costs) indicates that air-cooling maintenance costs are approximately 0.3 to 0.5 those for water-cooled equipment. 

Air-cooled condensers are used mostly in air-conditioning and for smaller-refrigeration capacities. The main advantage is avauability of cooling medium (air) but heat-transfer rates for the air side are far below values when water is used as a coohng medium. Condensation always occurs inside tubes, while the air side uses extended surface (fiusy... 

Evaporative condensers (Fig. 11-88) are widely used due to lower condensing temperatures than in the air-cooled condensers and also lower than the water-cooled condenser combined with the cooling tower. Water demands are far lower than for water-cooled condensers. The chemical industry uses shell-and-tube condensers widely, although the use of air-cooled condensing equipment and evaporative condensers is on the increase. 

Air-cooled condensers are similar to evaporative in that the service dic tates either the use of more expensive alloys in the tube construction or conventional materials of greater wall thickness. 

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. 

Tube ventilation In this system cooling tubes which work as heat exchangers are welded between the core packet and the outer frame and are open only to the atmosphere. See to Figures 1.20 (a)-(c). One fan inside the stator, mounted on the rotor shaft, transfers the internal hot air through the tube walls which form the internal closed cooling circuit. A second fan mounted outside at the NDE blows out the internal hot air of the tubes to the atmosphere and replaces it with fresh cool air from the other side. This forms a separate external cooling circuit. 

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... 

Typical film coefficients can be used to build rough overall heat transfer coefficients. This should suffice in most cases to establish that the design is within ballpark accuracy. Later, for final design, certain critical services will be checked in detail. Typical film resistances for shell and tube heat exchangers and overall heat transfer coefficients for air cooled heat exchangers are shown in Chapter 2, Heat Exchangers. 

The price of air-cooled exchangers should be obtained from vendors if possible. If not, then by coirelating in-house historical data on a basis of /ft of bare surface vs. total bare surface. Correction factors for materials of construction. pressure, numbers of tube rows, and tube length must be used. Literature data on air coolers is available (Reference 15). but it should be the last resort. In any event, at least one air-cooled heat exchanger in each project should be priced by a vendor to calibrate the historical data to reflect the supply and demand situation at the expected time of procurement. 

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