Air-Cooled Heat Exchanger Fundamentals
Air-Cooled Heat Exchangers, aka Air-Coolers, are not actually cooling the air. They function like giant radiators taking heat from a process fluid and transferring it into the air. Air-coolers typically consist of a structure, tube-bundle, fan(s), motor(s), and accessories like platforms with ladders for maintenance access.
Optimizing the design of an air-cooler is a juggling act – the tube type, bundle configuration, fan & motor details, and even the bundle distance to ground all affect the overall performance, and changing one item usually means adjusting everything else.
A forced-draft air-cooler is shown in the above and below pictures – the fans are located below the bundle and push air upwards. This is the most common type of air-cooler in our market, but there is also induced-draft air-coolers which have the fans above the bundle and pull the air up.
Air (and gases in general) don’t transfer heat very well, so we need much more surface area on the outside of the tubes than the inside of the tubes – this is why air-cooled exchangers have tubes with extended fins, whereas shell & tube exchangers typically have bare tubes.
There are three common types of fin-tubes for the air-coolers we build:
L-Fin is the most economical option, which has aluminum strip wrapped around the tube as shown above. They are limited to process temperatures at or less than 130C – above that temperature the fins will expand, lifting off the surface of the bare tube and leaving an air-gap, greatly reducing heat transfer.
Embedded-fins tubes are the next most common type and use aluminum strip that is mechanically scored into the bare tubing as it is wrapped. The fins are mechanically fixed into the tube, so they will not lift and are suitable for process temperatures up to 400C.
Extruded-fin tubes are the most costly as they utilize a finned aluminum extruded sleeve that envelopes the entire tube. They are only suitable for process temperatures up to 300C, but the advantage is that they completely protect the tube from the environment. Extruded-fin tubes are often used in offshore and coastal locations where salty ocean air would cause corrosion on a carbon steel tube.
The tube bundle consists of tubes, tube supports, side-frames, and a header box on each end of the tubes.
The red arrows show the hot process flowing into the header box nozzle, through the fin tubes, and then returning as a colder process before exiting the bottom of the header.
The tubes are mechanically attached to the tube-sheet either by ring-groove and expansion, and/or welding.
The plug-sheet will mirror the tube-sheet in quantity and layout of holes, but will be threaded so that header plugs can be installed. A plug-sheet is needed for assembly and maintenance requirements.
The partition-plate (aka pass-plate) directs the flow through the each tube-pass – in this case, two passes through the bundle.
Wrapper plates are used top and bottom, with the process nozzles `set-on’ rather than `set-in’ as with shell & tube.
Items not shown: End plates are used to complete and seal up the 6-sided header boxes. Side-frames are structural steel that run the length of the bundle on either side, tying the entire bundle together. Tube-supports span the width of the bundle, and keep the tubes well supported from sagging.
Air-Cooled Heat Exchangers have always been a fundamental component of industrial processes around the world, and going forward, they will become a much bigger part Altex Industries’ continued success. A future article will cover air-cooler recirculation (winterization) with the use of additional panels, louvres, and actuators.
Cal Kveder, P.Eng. – VP, Commercial Operations