Note : attached simulation flash file
Purpose
The purpose of this article is to provide some general information on air-cooled heat exchangers and answer some of the commonly heard questions. This is a mixture of fact and opinion. Wherever the opinion is obvious to me, I have attempted to show it by use of Italics.
Why use an air-cooled heat exchanger?
Air-cooled heat exchangers are generally used where a process system generates heat which must be removed, but for which there is no local use. A good example is the radiator in your car. The engine components must be cooled to keep them from overheating due to friction and the combustion process. The water/glycol coolant mixture carries the excess heat away. A small amount of the excess heat may be used by the car's radiator to heat the interior. Most of the heat must be dissipated somehow. One of the simplest ways is to use the ambient air. Air-cooled heat exchangers (often simply called air-coolers) do not require any cooling water from a cooling tower. They are usually used when the outlet temperature is more than about 20 deg. F above the maximum expected ambient air temperature. They can be used with closer approach temperatures, but often become expensive compared to a combination of a cooling tower and a water-cooled exchanger.
How are they constructed?
Typically, an air-cooled exchanger for process use consists of a finned-tube bundle with rectangular box headers on both ends of the tubes. One or more fans provide cooling air. Usually, the air blows upwards through a horizontal tube bundle. The fans can be either forced or induced draft, depending on whether the air is pushed or pulled through the tube bundle. The space between the fan(s) and the tube bundle is enclosed by a plenum chamber, which directs the air. The whole assembly is usually mounted on legs or a piperack. The fans are usually driven be electric motors through some type of speed reducer. The speed reducers are usually V-belts, HTD drives, or right angle gears. The fan drive assembly is supported by a steel mechanical drive support system. They usually include a vibration switch on each fan to automatically shut down a fan, which has become imbalanced for some reason.
What standards are used for Air-Cooled Exchangers?
First, almost all air coolers are built to Sec VIII of the ASME Code, since they are pressure vessels. For refinery and petrochemical services most customers include API 661 (Air-Cooled Heat Exchangers for General Refinery Service) in their specifications. This API specification is very good since it includes all the necessary information to properly specify a cooler and provides for a high level of minimum quality in the design and fabrication of the cooler. In the back it has a very good checklist where a customer can decide exactly what type of construction is needed and what options are important. These include such items as galvanizing versus painting, types of headers, maintenance walkways and platforms, controls, and external loads on the cooler. The following details refer mostly to the API specifications.
What kinds of finned tubes are used?
The tubes can be of virtually any material available, such as carbon steel, stainless steel, Admiralty brass, or more exotic alloys. The minimum preferred outside diameter is one inch. Some manufacturers sometimes use smaller tubes, but most of the process coolers have tubes, which are 1.0", 1.25", or 1.5" OD. The minimum tube wall thickness varies with the material. In some cases the design pressure and design temperature of the exchanger govern the minimum thickness.
The fins are almost always of aluminum material. The most common type of fin is the helical wrapped, L-footed type. These are used where the process temperatures are below about 350 deg. F. The API specification calls for cast zinc bands at the ends of the tubes to prevent the fins from unwrapping. Some of the better manufacturers also use cast zinc bands at the tube supports. For higher process temperatures, most customers prefer either embedded or extruded fins. The embedded fins have the highest temperature capabilities. They are made by a process, which cuts a helical groove in the OD of the tube, wraps the fin into the groove, and then rolls the upset metal from the tube back against the fin to lock it into place. The tube wall must be thicker with embedded fins because of the groove.
In some applications customers often prefer extruded fins. Extruded fins are made by putting an aluminum sleeve (sometimes called a muff) over the tube, then passing the tube through a machine, which has rollers, which squish the aluminum out to form fins. The process is similar to a thread-rolling machine. The end result is a fin, which has extremely good contact with the tube, and no crevices to allow corrosion to start on the tube OD. Extruded fins are often used in coastal locations or on offshore platforms for this reason.
Some manufacturers make some rather startling claims for their "special" finned tubes. These modifications usually involve some kind of wrinkles or cuts in the fins to enhance air turbulence. We believe this to be untrue. The cost of this extra turbulence is increased static pressure for the fan(s) to overcome. These claims are sometimes just too fantastic to be considered seriously.
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