7. Air Cooler Exchangers

What is an air cooler?
Air-cooled exchangers use atmospheric air on the outside
of high-finned tubes (except bare tubes are used in a few
applications) to cool or condense fluids flowing through
the inside of the tubes.
This type of exchanger is used to reject heat from a fluid
inside the tubes directly to ambient air.
To be effective, the air must flow in forced convection
to develop acceptable transfer coefficients.
Two Types of Air Coolers
Induced Draft Advantages:
Better distribution of air across the bundle.
Less possibility of hot effluent air recirculating into the intake. The hot
air is discharged upward at approximately 2.5 times the intake velocity,
or about 1,500 ft per min.
Better process control and stability because the plenum covers 60%
of the bundle face area, reducing the effects of sun, rain, and hail.
Increase capacity in the fan-off or fan-failure condition, because the
natural draft stack effect is much greater.
Disadvantages and limitations:
Possibly higher horsepower requirements if the effluent air is
very hot.
Effluent air temperature should be limited to 220°F to prevent
damage to fan blades, bearing, or other mechanical equipment
in the hot airstream. When the process inlet temperature
exceeds 350°F, forced draft design should be considered
because high effluent air temperatures may occur during fan-off
or low air flow operations.
Fans are less accessible for maintenance, and maintenance may
have to be done in the hot air generated by natural convection.
Plenums must be removed to replace bundle
Hudson states that the advantages of the induced
draft design outweigh the disadvantages.
Forced Draft Advantages:
Possibly lower horsepower requirements if the effluent
air is very hot. (Horsepower varies inversely with the
absolute temperature.)
Better accessibility of fans and upper bearings for
Better accessibility of bundles for replacement.
Accommodates higher process inlet temperatures
Forced Draft Disadvantages:
Less uniform distribution of air over the bundle.
Increased possibility of hot air recirculation, resulting from low
discharge velocity from the bundles, high intake velocity to the fan
ring, and no stack.
Low natural draft capability on fan failure.
Complete exposure of the finned tubes to sun, rain, and hail, which
results in poor process control and stability.
Air Cooler Sections in Details:
Bay is a part or entire of air cooler that may include one or several tube
bundles with at least one fan.
Based on EXXON Design Practice Section IX-I page 4: Bundles may be
fabricated in widths up to 16 ft (4.9 m) and depths up to 10 rows.The most
common tube length is 30 ft (9 m) although bundles are available in
lengths from 8 to 60 ft (2.4 to 18.3 m).
Also according to IPS-E-PR-785 item 6.6: Bundles may be fabricated in
widths to 3.65 m (12 ft) and depths to 8 rows.
IPS-E-PR-785 item 8.1:At least two fans shall be provided for each bay.
According to EXXON Design Practice Section IX-I: Tubes for air-cooled heat
exchangers may be plain or may contain fins.
The fin density varies between 7 and 11 fins/in. (276 and 433 fins/m) and fin
height varies from 5/16 to 1 in. (8 to 25 mm).
The minimum acceptable tube diameter is 1 in. (25 mm).
Fin Types
• The L-footed tension wound fin is normally used
in services where the tube wall temperature does
not exceed 350 degrees F and air side corrosion is
not extremely high.
• In high temperature applications, an embedded
process is employed to attach the fin to the tube
• The embedded fin is normally used for services
greater than 350 degrees F and less than 750
degrees F.
For applications where atmospheric corrosion is
critical, the extruded fin tube provides the best
The extruded fin tube is good for tube wall
temperature up to 650 degrees F. This is the
most expensive fin tube to produce.
Other Points
• Under normal operating conditions, air outlet temperatures should not
60°C with fans in operation.
80° C with free convection on the air side.
• Fouling factor on air side of exchangers shall be 0.35m2. K/kW
(0.002h.ft2.°F/Btu) , For fresh water cooling system the fouling factor shall
be 0.0004 hr. °C m2/ kcal.
• Design maximum ambient air temperature should be selected so that it will
not be exceeded more than 1-2 percent (sometimes 2-5%) of the total
annual hourly readings based on at least 5 consecutive years.
• Units should not be located near heat sources. Experience cautions that
units near exhaust gases from engines can raise inlet air 8°C or more
above the expected ambient
Horizontal Coil Configuration
Vertical Coil Configuration
Natural Draft Configuration
An Industrial Air Cooler
Design Procedure
Selection tube length and width
Figure 10-185