Forced-draft air coolers

In a forced-draft air cooler, cool air is blown through the underside of the fin tube bundle. In an induced-draft air cooler, cool air is drawn through the underside of the fin tubes. Either way, road dust, dead moths, catalyst fines, and greasy dirt accumulate along the lower row of tubes. As the tubes foul, they offer more resistance to the airflow. However, note that... 

Basically, there are two types of air cooler according to tube bundle location relative to air fan in the unit For forced draft air cooler, tube bundles are located at the discharge side of air fans. For induced draft air cooler, tube bundles are located at suction side of air fans. See Figure S for the difference of these two types of air cooler. Most air coolers are of forced draft type. 

The advantages of forced draft air cooler are 1) tube bundle is easier to be removed for service, 2) fan and motor are easier to be installed and accessible for maintenance, 3) easier for warm air recirculation ftireold climate service, 4) use slightly less horsepower, since air to fan is cooler. Its disadvantages are 1) poorer air distribution over tube bundle, 2) tOip of tube bundle is exposed to ixmbient weather, 3) bottom of tube bundle is hard to reach for cleaning, 4) more chance for hot exit air recirciilation, due to lower exit air velocity and no stack, 5) less cooling in case fans are failed due to tow natural draft. 

Tube bundle Hus is the heart of an air cooler. Process fluid flows through it and is cooled down there. It mainly consists of tubes and two headers (one at each ). Figure 10 shows tube bundles with two diiTerent header box design, plug header box and header box with removable cover plate. A bay consists of oik or more tube bundles with one or more fan assonblies, one plenum, and structure. An air cooler consists of one ot more bays. A typical plain view of forced draft air coolers are shown in Figure 11. [7]... 

Figure 11 A typical plain view of forced draft air coolers. [7]...

Plenum There are two types of plenum, box or transltton type, for either forced draft ot indviced draft air cooler, see Figure 8. 

Most air coolers are either induced-draft or forced-draft, as shown in Fig. 14.1. The more common arrangement being forced draft. The air is moved by rather large fans. The tubes are surrounded with foil-type... 

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.

Air-cooled exchangers are classified as forced-draft when the tube section is located on the discharge side of the fan (see Fig. 5.4). Air coolers are classed as induced-draft when the tube section is located on the suction side of the fan. 

Figure 5.4 Typical side elevations of air coolers (a) forced draft, and (6) induced-draft.

Noise can cause a serious nuisance in the neighborhood of a process plant. Care must be taken when selecting and specifying equipment such as compressors, air-cooler fans, induced and forced draft fans for furnaces, and other noisy plants. Excessive noise can also be generated when venting through steam and other relief valves and from flare stacks. Such equipment should be fitted with silencers. Vendors specifications should be checked to ensure that equipment complies with statutory noise levels, both for the protection of employees (see Chapter 9), as well as for noise pollution considerations. Noisy equipment should, as far as practicable, be sited well away from the site boundary. Earth banks and screens of trees can be used to reduce the noise level perceived outside the site. 

With air coolers, louver closure is considered a total failure (10). Upon fan failure, or a fan drive (e.g., power or steam) failure, a credit is often taken for natural convection effects. This credit is usually 20 to 30 percent of the normal duty of induced-draft condensers. Forced-draft condensers have a considerably weaker chimney effect, and the credit taken is usually 10 to 15 percent of their normal duty. The above natural draft credit may not apply if a fire occurs near the cooler. 

Fan speed or fan pitch control generally works well with induced-draft fans, but is less satisfactory with forced-draft fans. In the latter case, precipitation tends to affect condensation to a far greater degree than variations in fan speed or pitch. Where precipitation severely interferes with control action, a roof some distance above the air cooler may be desirable. 

Air cooled induced draft preferred to forced draft to minimize hot gas recirculation. Include a water-cooled trim cooler . Ensure the exit tubes are flooded so that the vapor does not bypass condenser. If extreme cold conditions are expected, allow for fan to operate in reverse to counteract the overcooling by the natural circulation of cold air. 

Texas) in 1964 and 1965. Toyo also will provide the acoustic vibration unit. In prilling, the urea melt is concentrated via vacuum evaporation to 99.8% and fed as quickly as possible into the bucket to minimize biuret formation. The liquid forms drops that then fall down a cylindrical concrete tower that has either induced, forced, or natural draft air flow. The prills solidify and are removed at the bottom by belt conveying to storage. Some plants have a fluidized-bed cooler in the prill tower bottom and others use an in-line cooler before storage. If low biuret product is to be produced, the urea melt from the last decomposition stage of the synthesis plant is first crystallized and the crystals are then melted just before prilling. 

Air is circulated by multiblade propeller-type ns that provide forced or induced drafts. Fans can be supplied with either adjustable-speed or variable-pitch blades. The fen blade pitch can be changed to vary the air-flow rate to compensate for rising or falling air temperatures. Air coolers supplied with multiple fens may be operated with some of the fens shut down. Dampers, baffles, and bypasses can also be tised to further control liquid outlet temperature. For elevated installations, platforms are generally furnished for access to header boxes and motors. 

An air-cooled exchanger generally has a fan that forces air across the tubes that contain the process fluid that is to be cooled. Air coolers in hydrocarbon service are susceptible to severe fire damage because the draft created by the fan puUs up heat from fires that occur at a lower elevation. This risk can be reduced by locating equipment with high-potential fire risk such as pumps and compressors away from overhead air coolers. 

A furnace or fired heater can be classified as natural, induced, forced, or balanced draft. The pressure inside a warm furnace is typically lower because of buoyancy differences in the cooler outside air. A natural-draft furnace can operate using this approach however, when fans are used to push or pull the air through the furnace, greater heat transfer rates can be achieved. A natural-draft fired heater is severely limited in contrast to these systems. 

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