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| Air Cooled Heat Exchanger (Horizontal Forced Draft Type) for HS Compression |
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| Air Cooled Heat Exchanger (Vertical Induced Draft Type) for HS Compression |
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| Air Cooled Heat Exchanger (Horizontal Forced Draft type) for Aker Kvaerner’s Smart 1 FPSO Project |
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| Air Cooled Heat Exchanger (Horizontal Induced Draft type) for Petronas HPU Malacca Project |
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This type of heat exchangers is generally used to remove heat that is generated from the process system.
It consists of finned-tube bundle with rectangular box headers on both ends of the tubes. Cooling air is provided by one or more fans. Usually, the air blows upwards through a horizontal or vertical tube bundle by the fan. The space between the fan(s) and the tube bundle is enclosed by a plenum chamber which directs the air.
The fans are usually driven by electric motors through speed reducer usually either V-belts, HTD drives, or right angle gears. The fan drive assembly is supported by a steel mechanical drive support system.
International Standards used for Air-Cooled Exchangers
S&W air coolers are built to Sect. VIII of the ASME Code and for use in refinery and petrochemical services most customers include API 661 (Air-Cooled Heat Exchangers for General Refinery Service) in their specifications. |
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Headers are the boxes at the ends of the tubes which distribute the fluid from the piping to the tubes.
Most headers on air-cooled exchangers are welded rectangular boxes. S&W commonly fabricates plug type headers and occasionally produces the other type i.e. removable cover plate or flange type. The latter type design is usually used in low pressure applications (below 30bar) for tube access and at times, the plug type could also be used as desired. However, for applications involving more than 30bar pressure, it is mandatory to use the plug type headers as per API requirement.
Where fouling is a potential problem and the tube bundle may require occasional internal cleaning, as the name implies, the removable plate type header is more at advantage. The cover plate is attached to the header by a set of studs or through-bolts to a flange around the perimeter of the header. The removable bonnet type header is also similar in construction to this type of header.
A plug type header is, however, a rectangular box construction with threaded holes where the plugs are tightened. It is through these holes the tubes could be welded onto the tubesheet. Plug headers are sometimes used when the process fluid is hydrogen service, this media is not suitable especially for the flange type where the gasket is susceptible to developing crack in case of leakage.
Headers are usually constructed of carbon steel or stainless steel, but sometimes more exotic alloys are used for corrosion resistance. The selection of materials is usually made by the customer.
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| Plug Type Header |
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Removable Cover Plate or Flang Type Header |
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S&W manufactures both the commonly used forced draft as well as induced draft type of construction. Forced draft units are easier to manufacture and to maintain. The tube bundle is mounted on top of the plenum, so it can be easily removed and replaced. The fan shaft is short, since it does not have to extent from the drive unit through the tube bundle and plenum to the fan, as in an induced draft design. In the Forced draft units the fan is not exposed to the hot outlet exhaust air and hence, they are a better choice in such cases. (API 661 par. 4.2.3.15 &16 offer some guidelines for this.)
However, induced draft units have some advantages, too. A common problem with forced draft coolers is accidental warm air recirculation. This happens when the hot exhaust air is pulled back in to the fans. Since a forced draft cooler has a low air velocity at the exhaust from the bundle and a high velocity through the fan, a low pressure area is created around the fan, causing the hot air to be pulled over the side or end of the bay. For this same reason, there should never be a small space between the bays of a bank of forced-draft cooler. Induced draft cooler have a high exhaust air velocity through the top-mounted fan, and a lower velocity into the face of the tube bundle below. This tends to minimize the probability of accidental air recirculation. Also an induced draft plenum does not have to support the tube bundle so some weight can often be saved in this area. |
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| Depending on the type of application, the following is a list of some of the commonly used control devices for air coolers, but in no particular order. |
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Manually or automatically operated louvers. |
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Electrically or pneumatically operated louvers. |
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Pneumatically actuated automatic variable-pitch fans. |
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Variable-frequency fan drives. |
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Vibration control switches or complete systems. |
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Warm-air recirculation systems for freezing/pour point control in cold climates. |
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