Air-cooled exchangers

Details of the construction features of air-cooled exchangers are given by Ludwig (1965). The construction features of air-cooled heat exchangers are covered by the American Petroleum Institute standard, API 661. 

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TABLE 11-5 Overall Coefficients for Air-Cooled Exchangers on Bare-Tube Basis... 

Air recirculation. Prevailing winds and the locations and elevations of buildings, equipment, fired heaters, etc., require consideration. All air-cooled heat exchangers in a bank are of one type, i.e., all forced-draft or all induced-draft. Banks of air-cooled exchangers must be placed far enough apart to minimize air recirculation. 

The price of air-cooled exchangers should be obtained from vendors if possible. If not, then by coirelating in-house historical data on a basis of /ft of bare surface vs. total bare surface. Correction factors for materials of construction. pressure, numbers of tube rows, and tube length must be used. Literature data on air coolers is available (Reference 15). but it should be the last resort. In any event, at least one air-cooled heat exchanger in each project should be priced by a vendor to calibrate the historical data to reflect the supply and demand situation at the expected time of procurement. 

In order to improve the heat transfer characteristics of air cooled exchangers, the tubes are provided with external fins. These fins can result in a substantial increase in heat transfer surface. Parameters such as bundle length, width and number of tube rows vary with the particular application as well as the particular finned tube design. 

The choice of whether air cooled exchangers should be used is essentially a question of economics including first costs or capital costs, operating and maintenance expenses, space requirements, and environmental considerations and involves a decision weighing the advantages and disadvantages of cooling with air. 

Common to all air cooled heat exchangers is the tube, through which the process fluid flows. To compensate for the poor heat transfer properties of air, which flows across the outside of the tube, and to reduce the overall dimensions of the heat exchanger, external fins are added to the outside of the tube. A wide variety of finned tube types are available for use in air cooled exchangers. These vary in geometry, materials, and methods of construction, which affect both air side thermal performance and air side pressure drop. In addition, particular... 

As described above, in the air-cooled exchanger a motor and fan assembly forces ambient air over a series of tubes to cool or condense the process fluids carried within. The tubes are typically assembled in a coiled configuration. Air is... 

The diameter and materials, specified for the tubes and fins, depend on system requirements. The fins are commonly made from aluminum or copper, but may be fabricated of stainless or carbon steel. Tubes are generally copper, but can be made from almost any material, and they range in size from 5/8- to 1-in. outer diameter. The design of the air-cooled exchanger is such, that individual coils can be removed independently for easy cleaning and maintenance. There are several common design configurations that are commercially available. Each is briefly described below. 

Air-cooled exchangers use atmospheric air on the outside of high-fmned tubes (except bare tubes are used in a few applications) to cool or condense fluids flowing through the inside of the tubes. 

Typical Forced Draft Air Cooled Exchanged Showing Two Exchanger Sections and One Fan... 

Figure 10-175. Typical forced draft air-cooled exchanger showing two exchanger sections and one fan. (Used by permission Yuba Heat Transfer Division of Connell Limited Partnership.)...

Figure 10-183A. Fin designs for use with air-cooled exchangers.

Figure 10-185. Specification sheet for air-cooled exchangers. (Used by permission Air-Cooled Exchangers Manufacturers Association, New York (no longer in existence, 1999) Hudson Engineering Corporation, now Hudson Products Corporation.)...

Typical Transfer Coefficients for Air-Cooled Exchangers Based on Bare Tube Surface... 

FA = face area of air cooled exchanger tube bundle, length X width, ff... 

Air-cooled exchangers consist of banks of finned tubes over which air is blown or drawn by fans mounted below or above the tubes (forced or induced draft). Typical units are shown in Figure 12.68. Air-cooled exchangers are packaged units, and would normally be selected and specified in consultation with the manufacturers. Some typical overall coefficients are given in Table 12.1. These can be used to make an approximate estimate of the area required for a given duty. The equation for finned tubes given in Section 12.14 can also be used. 

Calculation of Tubeside Pressure Drop in Air-Cooled Exchangers... 

Calculate horsepower per fan from the section in this chapter, Air-Cooled Exchangers—Air Side Pressure Drop. ... 

Vessels, heat exchangers (including air cooled exchangers), and other equipment containing flammable or combustible liquids over 600°F (315°C) or their autoignition temperature, whichever is less. 

Air-moled h.Mlamxir—As used in the petroleum industry, air-cooled exchangers normally comprise two headers joined h> a horizontal bank ol tinned tubes. Usually two motor-driven fans located above (induced dralii or below (forced draft) the tubes are used lo circulate the air over the tinned surface. 

Figures 5.4 and 5.5 show typical elevation and plot plan section sketches of horizontal air-cooled exchangers. The essential components are one or more tube sections, called tube bundles, and one or more axial flow fans, all enclosed in a structural module made to anchor footings into concrete or to stand on an offshore platform. As shown, one fan may serve more than one tube bundle. A bay may be made up of multiple tube bundles and may also be served by one or more fans. Most important is the fact that one exchanger may be composed of any number of tube bundles. Tube bundles may be arranged in series or in parallel, just like shell/tube bundles.

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