How are Air-Cooled Heat Exchangers constructed?

Typically, an air-cooled heat exchanger for process use consists of a 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 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 either 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.

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 excess heat is carried away by the water/glycol coolant mixture. 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.

Finned Heat exchangers designed to save energy

The current environment of high and continuously rising energy costs sees an increasing demand for heat recovery from existing processes, in a bid to reduce energy consumption. Waste streams of liquids or gases may contain sufficient energy to financially justify the installation of a purpose designed heat exchanger.

Traditionally these were made of carbon steel materials, but to reduce costs and increase the life of heat exchangers, We introduced higher specification materials, such as stainless tubing and aluminium fins.

Developing expertise with these materials opened up opportunities for expansion into general heat exchanger work, such as spray drier heat exchangers in the dairy industry and heat dumps and recuperators for solid fuel heatplants.

Finned tubing has a unique application in heat exchanger design. The transfer of heat to air is not easy. Air is an excellent insulator, and notoriously difficult to get to conduct heat. It is an oxidising medium, readily coating heat transfer surfaces with insulating deposits. It requires vigorous turbulence to absorb heat from a hot surface and distribute it amongst itself.

To overcome this limitation it is necessary to increase the surface area of the side of the heat exchanger in contact with the air. This is the so called finned tube; a fin is wound onto the outside surface of a tube in a continuous helix. This fin can take many forms, ranging from an interference fit (L and LL fin), through a grooved and swaged fixing (G fin), to a continuously welded form. The fin can be in a range of materials. Fin fixing generally makes little difference to thermal efficiency (unless it has a poor or non-continuous attachment to the tube) but places strict limits on the maximum temperature the fin can be used at.

For industrial applications Windsor uses almost exclusively an aluminium G fin. This ensures the aluminium fin will not come loose on the base tube in service, and provides a high fin efficiency (a measure of the variation in fin temperature) leading to a smaller, less costly exchanger.

Most heat exchangers are purpose engineered by Windsor for the particular application. There are important issues with construction - in particular with the stainless steel welding and the differential thermal expansion of components in the heater. Lifting, transportation and support of the heat exchanger also require careful attention.

Cold Rolled Steel Strip

Cold Rolled Steel Strip is produced, as the term suggests by the further rolling of strip produced by the hot strip mills. Prior to cold rolling, the mill scale has to be removed, normally by the Pickling Process which uses mechanical manipulation (around small diameter rolls) and acid to dissolve the surface scale. The surface is then washed to remove the acid and a light oil added to prevent rusting.

Cold Rolling is undertaken to :-

Reduce the thickness
Improve the surface finish                                           
Improve the thickness tolerances
To offer a range of "tempers"
As a preparation for surface coating

Most Cold Rollers, incorporate two rolling stations. The in initial cold rolling is undertaken on a more powerful "break down" mill that is capable of heavy reductions. After annealing to remove the stress and work hardening introduced in the process, the final cold rolling or "skin-pass" takes place. This is a light reduction (typically 3%) to improve the surface finish.

Cold rolling mills are typically  reversing mills. A reversing mill is where the steel enters the rolling mill from one side, passes through the other side and then comes back through the mill again. Normally it will go left to right through the mill a number of times being rolled a little thinner each time it goes through.

Cold rolled strip can be produced in various conditions such as skin-rolled, quarter hard, half hard, full hard depending on how much cold work has been performed. This cold working (hardness) is often called temper, although this has nothing to do with heat treatment temper.

In skin rolling, the metal is reduced by 0.5 to 1% and results in a surface that is smooth and the yield point phenomenon--excessive stretching and wrinkling in subsequent operations, is eliminated. This makes the metal more ductile for further forming and stretching operations.

Quarter Hard, Half Hard, Full Hard stock have higher amounts of reduction, up to 50%. This increases the yield point; grain orientation and material properties assume different properties along the grain orientation. However, while the yield point increases, ductility decreases.

Quarter Hard material can be bent (perpendicular to the direction of rolling) on itself without fracturing. Half hard material can be bent 90º; full hard can be bent 45º. Thus, these materials can be used for in applications involving great amounts of bending and deformation, without fracturing.

Hot Rolled Steel Strip

Unlike the cold rolling sector where sales are usually directly from the producer to the user, hot rolled strip is produced largely as a downstream activity by the steelmakers. Therefore the distribution of hot rolled strip which invariably requires further processing is often distributed by service centres, in particular ex wide rolled material. The service centre may be owned by the steelmaker or be independent.

Hot rolled strip is traditionally split in to two categories, narrow strip and wide strip.

Narrow Strip mill numbers have declined in recent years due to the inherently higher costs of production. They differ from the wide strip mills in a number of ways. Firstly they offer greater flexibility in terms of the variety of grades they offer, the range of sizes they produce and the types of finishes available. Along with the capability to offer smaller quantities and specialist tolerances they are well placed to deal directly with end users. If material is required with a natural mill edge (Rather than slit)  the narrow mills are the only source. Furthermore a number of high carbon and alloy grades are only available from narrow strip mills.

Their inherent flexibility means they are able in most cases to deal directly with end users, and therefore normally they do not distribute through service centres.

Wide Strip  mills produce the largest percentage of flat rolled products. Normally the mill will have little downstream processing which means that most requires further processing such as slitting, blanking etc. before it is in a useable form. Most of their product therefore goes on to service centres for onward distribution.

For the majority of hot strip requirements the service centres will be the most likely source of material. They will normally be able to offer most standard grades of deep drawing qualities, high strength low alloy and general purpose strip for press working, blanking and fabrication work.

What is the difference between the "annealed" condition and the "dead soft" condition for stainless steel?

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The usual "condition" that stainless steel products (sheets, plates, bars, wire etc.) are supplied to is the “annealed” condition. That means that the last operation is to heat the material up a temperature where the residual stresses of manufacturing can be relieved, and the material will be in the "soft" condition. Most flat rolled products however are made in coils and when a "sheet" is cut from the coil it is usually "flattened" which does add some small amount of stress to the material. Bar products are usually straightened and that adds some small amount of stress as well. The term "dead soft" usually refers to a product where the even this small amount of stress is removed, but as a practical matter, this condition is not readily available.

What are the standard finishes for stainless steel like 2B and #4?

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"Standard" finishes are produced on an on-going basis. They are generally available off the shelf or can be obtained with a short lead-time. They are the finishes that are used for most stainless steel applications. Standard finishes are categorized as either "Mill" or "Polished".

"Mill" finishes are the least expensive finish option. There are both hot- and cold-rolled mill finishes, and, as this description implies, their appearance is produced at least in part by passing the stainless steel through special rolls or dies. The reflectivity of these finishes varies from dull to mirror-like. These finishes become less uniform in appearance with increasing product thickness or diameter.

"Polished" finishes are produced by mechanically abrading the surface with a series of gradually finer abrasives or a special rolling procedure that simulates the appearance of mechanical abrasion. The smoothest polished finishes are buffed after mechanical polishing to produce a mirror-like appearance.

"Special" Finishes include finishes that are generally used when aesthetic appearance is important and for specialized industrial applications. This category includes non-directional scratch patterns, swirls, ground circles, embossed and coined textures, and various coloring and coating methods.

What is the "annealed" condition?

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Stainless steel is usually sold in the "annealed" condition. It just means that the material is in the "soft" or annealed condition. The 300 series of stainless steels can not be hardened by heat treatment (like carbon steels) but can be hardened by cold working. This cold work can be eliminated by a heating treatment (annealing) that will restore the original soft condition.