Tubes air-cooler

Main pipe racks located in process or other fire potential areas that also support equipment such as fin-tube air coolers should be fireproofed with a 2- to 3-hour rating from their bases up to and including the equipment support legs and any horizontal beams transmitting the... 

Support legs for fin-tube air coolers supported from ground level... 

Loss of airflow through a finned tube air cooler bundle is a universal problem. The effect is to reduce the exchanger s cooling efficiency. To restore cooling, you might wish to try the Norm Lieberman method, which consists of reversing the polarity of the fan motor electric leads. The fan will now spin backward. Depending on the nature of the deposits, a portion of the accumulated dirt will be blown off the tubes— but all over the unit. Personnel observe this procedure from a safe distance. 

Air coolers are normally used to cool down process flmds to approximately 60 °C and for condensation units operating above such temperatures. These air coolers are at least as energy-efficient as water coolers over these temperature ranges and require some 2-3 kWh per GJ of discharged heat. Temperatures are controlled either by switching fans on or off, or by closing louvers to shade some of the cooling tubes. Air coolers require considerable space. 

Inter-coolers Coolers are primarily used to reduce the operating temperature within a compressor circuit, which allows the use of a smaller machine with fewer cylinders. These coolers may vary in size and type (e.g., shell and tube, air coolers, and U-tube) and... 

Based on different mechanical construction, the common types of heat exchanger used in industry are doublepipe or multi-tube hairpin, shell and tube, air cooler, plate and frame, and spiral. Only the tirst three types of heat exchanger will be discussed in this chapter. 

General guidelines for estimating the tube area for finned-tube air coolers are as follows ... 

Rolling mill drive motor heat exchanger (air cooler) Horizontal (tubes)... 

A rolling mill drive motor failed in service. Investigation revealed the motor was short circuited by water in-leakage. This forced a prolonged shutdown, as the motor could not be replaced on site. High humidity was initially assumed to be the problem. Upon close inspection of the motor air coolers, however, it was determined that water leaked from headboxes into the air plenum spaces. Severe corrosion at tube rolls was observed (Fig. 2.9). 

Specimen Location Tube from a heat exchanger—instrument air cooler... 

Anodic protection today allows safe and efficient protection of air coolers and banks of tubes in sulfuric acid plants. In 1966 the air cooler in a sulfuric acid plant in Germany was anodically protected. Since then more than 10,000 m of cooling surfaces in air- and water-cooled sulfuric acid plants worldwide have been protected. The dc output supply of the potentiostats amounts to >25 kW, corresponding to an energy requirement of 2.5 W per m of protected surface. As an example. Fig. 21-9 shows two parallel-connected sulfuric acid smooth tube exchangers in a production plant in Spain. 

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. 

Air coolers Tubes are 0.75-1.OOin. 00, total finned surface 15-20 sqft/sqft bare surface, U = 80-100 Btu/(hr)(sqft bare surface)(°F), fan power input 2-5 PIP/(MBtu/hr), approach 50°F or more. 

The most direct way to reject heat above ambient temperature to the environment is by the use of aircooled heat exchangers, as discussed in Chapter 151. These coolers exploit a flow of ambient air across the outside of tubes through which process fluids are flowing that require cooling. Such air coolers are very common in some industries, particularly when the plant is located in a region where water is scarce. 

Their major disadvantages are that they require more space, they have a higher initial capital cost, and the coolest temperature that can be attained is 20 to 30°F (10 to 15°C) above the ambient air temperature.1 7 For the same amount of heat transfer, an installed carbon-steel shell-and-tube exchanger will cost about one-third as much as an air cooler. This difference diminishes as more expensive materials are used.18 A modification of the air cooler, called the wet-surface air cooler, overcomes some of the above-mentioned disadvantages. It can reduce the temperature that can be attained to nearly the ambient temperature, and there are some claims... 

Air coolers often consist of two tube bundles in one frame with one set of fans. In this case one tube bundle may be adequate for the initial capacity, and the space where the other tube bundle would reside can be blocked off by sheet metal to prevent the air from bypassing the cooling section. The other tube bundle can then be purchased when more cooling capacity is needed. 

Preliminary studies showed that the relatively high ambient temperatures encountered ot the platform location would limit the effectiveness of air coolers. Furthermore the potential rist to the plotform from a gas release through impact domoge to a large array of exposed finned tubes was considered to be an unacceptable hazard. Water wos therefore selected os the preferred medium for providing process cooling. 

Air coolers are twice as expensive to purchase and install as water coolers. The great advantage of an air cooler is that it does not need cooling water. The difficult aspect of air cooling arises from the flow of air across the tubes. 

Most air coolers are either induced-draft or forced-draft, as shown in Fig. 14.1. The more common arrangement being forced draft. The air is moved by rather large fans. The tubes are surrounded with foil-type... 

In a forced-draft air cooler, cool air is blown through the underside of the fin tube bundle. In an induced-draft air cooler, cool air is drawn through the underside of the fin tubes. Either way, road dust, dead moths, catalyst fines, and greasy dirt accumulate along the lower row of tubes. As the tubes foul, they offer more resistance to the airflow. However, note that... 

Spraying water on fin-fan air coolers is generally not a good idea. It is really effective only in dry climates with low humidity. The evaporation of water by the dry air cools the surface of the fins that is, the latent heat of vaporization of the water, robs sensible heat from the tubes. 

The mechanical construction of the tubes in an air cooler create some rather nasty problems. Figure 14.4 shows the exterior appearance at either end of an air cooler. The small black circles are threaded steel plugs. They are not connected to the ends of the tubes. Allow me to rotate the air-cooler header box, shown in Fig. 14.4, by 90°, and display a cross-sectional view in Fig. 14.5. Note that the plugs are not connected to individual tubes. Unscrewing a plug just gives one access to the end of a tube, for cleaning purposes. 

Once the pass partition baffle fails, the process fluid may bypass the finned tubes, and cooling efficiency is greatly reduced. This is bad. But worse yet, during a turnaround of the cooler, there is normally no way to inspect the pass partition baffle. There is no easy way to visually verify the mechanical integrity of this baffle. A few air coolers have removable inspection ports for this purpose most do not. 

If one of the air coolers begins to experience tube-side fouling, the fluid flow will be reduced. But the tube-side pressure drop will remain the same. The pressure drop across all five air-cooler bundles, shown in Fig. 14.6, is 10 psig. 

The best way to handle the nonsymmetrical flow problem described above is to make the pressure drops in both the inlet and outlet tube bundle headers very small, as compared to the bundle pressure drop itself. Many of my clients add additional tube bundles in parallel with existing air coolers. This helps at first, but they find that the long-term benefits are quite disappointing, because of high pressure drop in the new header lines. 

Figure 8.4. Example of tubular heat exchangers (see also Fig. 8.14). (a) Double-pipe exchanger, (b) Scraped inner surface of a double-pipe exchanger, (c) Shell-and-tube exchanger with fixed tube sheets, (d) Kettle-type reboiler, (e) Horizontal shell side thermosiphon reboiler, (f) Vertical tube side thermosiphon reboiler, (g) Internal reboiler in a tower, (h) Air cooler with induced draft fan above the tube bank, (i) Air cooler with forced draft fan below the tube bank.

In such equipment the process fluid flows through finned tubes and cooling air is blown across them with fans. Figures 8.4(g) and (h) show the two possible arrangements. The economics of application of air coolers favors services that allow 25-40°F temperature difference between ambient air and process outlet. In the range above 10MBtu/(hr), air coolers can be economically competitive with water coolers when water of adequate quality is available in sufficient amount. 

Standard air coolers come in widths of 8, 10, 12, 16, or 20 ft, lengths of 4-40 ft, and stacks of 3-6 rows of tubes. Example 8.8 employs typical spacings. 

Correlations for friction factors and heat transfer coefficients are rated in HEDH. Some overall coefficients based on external bare tube surfaces are in Tables 8.11 and 8.12. For single passes in cross flow, temperature correction factors are represented by Figure 8.5(c) for example charts for multipass flow on the tube side are given in HEDH and by Kays and London (1984), for example. Preliminary estimates of air cooler surface requirements ram be made with the aid of Figures 8.9 and 8.10, which are applied in Example 8.9. 

TABLE 8.11. Overall Heat Transfer Coefficients in Air Coolers [U Btu/(hr)(°F)(sqft of outside bare tube surface)]... 

Air cooler data are based on 50 mm tubes with aluminum fins 16-18 mm high spaced 2.5-3 mm apart coefficients based on bare tube surface. Excerpted from HEDH, 1983. 

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