Air-cooled exchangers are used to cool fluids with ambient air. They should be considered when cooling water is in limited or expensive. Air-cooled exchangers are used for cooling and condensing.

Air-cooled exchangers consist of banks of finned tubes over which air is blown or drawn by fans mounted below or above the tubes. Air-cooled exchangers are classified as forced draft when the tube section is located on the discharge side of the fan, and as induced draft when the tube section is located on the suction side of the fan.

Advantages and Disadvantages of Induced-Draft and Forced-Draft Air-Cooled Exchanger

Advantages and disadvantages of forced and induced draft air-cooled exchangers are shown below.

Aspect of Comparison	Induced Draft Air-Cooled Exchanger	Forced Draft Air-Cooled Exchanger
Distribution of air across the section	Better than forced-draft	Poor
Possibility of hot effluent air recirculating around to the intake of the sections	Less possible. The hot air discharged upward with velocity around 1500 ft/min.	Greatly increased possibility of hot air recirculation due to low discharge velocity
Effect of sun, rain, and hail	Less effect, because 60% of the surface area is covered	Total exposure to sun, rain, and hail
Capacity in the event of fan failure	Capacity increased, because higher natural draft stack effect	Low natural draft capability on fan failure due to small stack effect
Horsepower	Higher, because fan is located in the hot air	Lower, because fan is in cold air (note 1)
Allowable effluent air temperature	Maximum 200oF, to prevent potential damage to fan blades, bearings, V-belts, or other mechanical components in the hot air streams	–
Accessibility of fan drive component for maintenance	Less accessible	Better accessibility
Limitation of application	Should not be used for process fluids above 350oF; otherwise, fan failure could subject the fan blades and bearings to excessive temperature	–

1. Horsepower varies directly as the absolute temperature

Key Design of Air-Cooled Exchangers

Air-Cooled Section

Section orientation can be horizontal, vertical, or angled. Horizontal section is the most commonly used, and generally most economical.

Vertical systems are sometimes used when maximum drainage and head are required, such as condensing service.

Angled section, are used for condensing service like vertical one. Frequently angled sections are sloped thirty degrees (30^o) from the horizontal. A-framed are usually sloped sixty degrees (60^o) from the horizontal.

Fan

• Fan size : fan sizes range from 3 ft to 28 ft diameter. However, 14 ft to 16 ft is the largest diameter normally used.
• Fan drivers : fan drivers may be electric motors, steam turbines, hydraulic motors, or gas-gasoline engines.
• Speed reducer : a speed reducer, such as v-belt or reduction gear box, is necessary to match the driver output speed to the relatively slow speed of the axial flow fan. Fan tip speed are normally 12,000 ft/min or less. General practice is to use v-belt drives up to about 30 bhp and gear box at higher power. Individual driver size is usually limited to 50 hp
• Fan coverage should be above 0.4 to improve air distribution across the face of tube section. Fan coverage is the ratio of projected area of the fan to the face of the section served by the fan.

Tube Section

• Tube length are usually 6 ft to 50 ft and bay widths from 4 ft to 30 ft. Use of longer tubes usually results in a less costly design
• Base tube diameters are 5/8 in to 1 ½ in OD. Tubes are usually triangular pitch. Matching of the tube section to the fan system and the heat transfer requirement usually results in the section having depth of 3 to 8 rows of fin tubes, with four rows most typical.

Material of Construction

• Headers : common materials for headers are firebox quality carbon steel, ASTM SA-515-70, SA-516-70
• Tubes : ASTM-SA-214 (ERW), SA-179 (SMLS), carbon steel
• Louver : carbon steel or aluminium with carbon steel
• Fins : aluminium

Header Design

A vast majority of the headers are plug type. There is a shoulder plug opposite each tube which allows access for inspection and cleaning of individual tubes.

Header with a removable bonnet (or cover plate) is also available. The front plate can be removed for access and cleaning of the header box. This type can be used for low-pressure application (below 435 psig) and high fouling.

Pipe manifold and welded bonnet headers are non-removable and mainly used for steam coils or very high pressure.

References:

[1]  Improve Air-Cooled Heat Exchanger Performance

[2] Sinnot, R.K., “Chemical Engineering Design”, Elsevier: 2005

[3] API 661 Air Cooler

[4] GPSA Engineering Data Book