AIR COOLED HEAT EXCHANGERS
Water shortage and increasing costs, together with more recent concerns about water pollution and cooling tower plumes, have greatly reduced industry's use of water cooled heat exchangers. Consequently, when further heat integration within the plant is not possible, it is now usual to reject heat directly to the atmosphere, and a large proportion of the process cooling in refineries and chemical plants takes place in Air Cooled Heat Exchangers (ACHEs).
There is also increasing use of Air Cooled Condensers for power stations. The basic principles are the same but these are specialized items and are normally configured as an A-frame or "roof type". These condensers may be very large—the condensers for a 4000 MW power station in South Africa have over 2300 tube bundles, 288 fans each 9.1 m in diameter and a total plot area 500 m × 70 m.
ACHEs for process plants are normally just called Aircoolers, but should not be confused with devices for cooling air (best described as Air Chillers).
The design of an ACHE is more complex than for a Shell and Tube Heat Exchanger, as there are many more components and variables.
The principle component of an ACHE is the tube bundle, of which there may be many, normally comprising finned tubes terminating in header boxes. The fins are most commonly spirally wound aluminum strips 12.7 × 10−3 m or 15.9 × 10−3 m high and with 275 to 433 fins/m. There are two main types of wound fin which are usually known as L-fin and G-fin. There are several variations of the former type—single, overlapped, and knurled, but all suffer a high contact resistance, which increases with temperature due to differential expansion between the fin and the core tube. Embedded fins (G-fins) are wound into a groove in the core tube which is then peened back providing a mechanical bond. This gives better heat transfer but requires a thicker core tube. Integral fins extruded from an aluminum sheath are often used for more severe environments, and instead of embedded fins with expensive core tubes. When an exceptionally long life is required in aggressive environments galvanized steel fins can be the best choice, and these frequently use elliptical tubes, which also have improved airflow characteristics. Core tubes may be carbon steel, stainless steel or various alloys and are usually of 25.4 × 10−3 m outside diameter. For low pressure or highly viscous applications the tubes can be up to 50.8 × 10−3 m diameter. Tube lengths vary to suit the installation, which will often be over a piperack, but generally do not exceed 15 m.
Unlike most other pressure vessels an ACHE header box is normally rectangular in cross-section, and the most widely used type has threaded plugs opposite each tube for access. Various coverplate types may be used for low pressures, and for high pressures (up to 500 bar) manifold headers made from thick walled pipe or forged billets are needed. When there could be a large temperature drop across a multipass tube bundle, split headers may be necessary to accommodate differential expansion between passes.
The air is moved over the tubes in a single crossflow pass by axial flow fans, which may be arranged for forced or induced draught. Forced draught is suitable for most applications, has easier maintenance and is by far the more common. Induced draught gives a more even air distribution across the tubes, but requires more power as the fans are in the hot air stream. This latter point also means that induced draught is not suitable for high process temperatures, but is advisable for a close temperature approach as exit velocities are higher and hot air recirculation less likely. For induced draught installations with fan diameters greater than 2.4 m the motor and speed reducer will normally be mounted below the tube bundles, with an extended drive shaft, as shown in Figure 2 There are normally at least two fans in each exchanger bay so that significant cooling is maintained in the event of a partial failure, and it is preferable for fans to cover at least 40% of the total bundle face area.
An ACHE is a large piece of equipment compared to other types of heat exchangers, and requires free space around it for the cooling air flow. In refineries and chemical plants ACHEs are usually mounted over a piperack, saving plot space at grade and ensuring free airflow. A further advantage of this elevated mounting is shorter pipe runs for column overheads, saving both cost and pressure drop. In some cases an ACHE may be mounted on top of a column to keep pressure loss to an absolute minimum, but this can make maintenance more difficult. Rooftop mounting is sometimes used, particularly for turbine steam condensers. When no suitable supporting structure is available, or where there is ample space available, the cooler may of course be ground mounted.
A typical face velocity for the air flowing across the tube bundle is 3 m/s. Higher air flows increase both the heat transfer coefficient and the mean temperature difference, thereby reducing the surface area required, but at a higher power consumption. Increased airflow and power also mean greater fan noise, which is an increasingly important factor.
The choice of design ambient temperature is the most critical factor affecting the size of an ACHE. A dry bulb temperature that is not exceeded for 95% of the year is the usual choice, accepting that there may be a cooling shortfall on the hottest days. In some cases the plant loading may be reduced in the summer, so that a lower design air temperature is appropriate. The majority of ACHE designs have between 4 and 6 rows of tubes (in the airflow direction). This may rise to 8 rows or more if there are plot restrictions, but successive rows become less and less effective for heat transfer and costs increase. If the core tubes are of high value material, fewer rows and increased plot area will certainly be cheaper.
Small independent ACHEs can be quite expensive, and it is therefore normal practice to install two or more small units in a shared fan bay. This is particularly useful when several exchangers are to be mounted in a bank with a common tube length.
Sound pressure level limits in work areas within a plant are usually about 85 dB(A), but community noise levels need to be much lower and frequently necessitate an analysis of overall sound power levels. In Europe the sound power limits now tend to be more severe than the local sound pressure limits, and in some cases control the ACHE design.
The principal source of noise in ACHEs is the fans. Moderate reductions in noise levels can be achieved by reducing the fan speed and using more blades or wider chord blades. Very low noise designs necessitate low face velocities, with a consequent increase in surface area, so that the fans can run very slowly and still generate sufficient pressure.
The extremely low noise restrictions now being applied on some sites has led to the development of special fan designs, which are much quieter than conventional fans while maintaining a reasonable airflow.
The tubeside heat transfer and pressure drop are calculated in the same way as for Shell & Tube Heat Exchangers. For the airside heat transfer rate a number of calculation methods are available, including correlations by Briggs and Young (1963), PFR (1976) and ESDU (1986). As there is a temperature gradient along the fin the calculated heat transfer is adjusted using the concept of fin efficiency, which is the ratio of the actual heat transfer from a given surface, to the heat which would be transferred from the same surface at a uniform temperature equal to the fin root temperature—for details see Extended Surfaces Heat Transfer. The fin efficiency is in the range 0.8 to 0.9 for fin types and dimensions generally used in ACHEs.
Several correlations exist for predicting the airside pressure loss across the finned tube bank—those most commonly used are by Robinson and Briggs (1966), PFR (1976), and ESDU (1986).
Typical values of overall heat transfer coefficient for various fluids are given in ESDU (1993) and these may be used to obtain approximate sizes. This item also describes the C-value method of comparing costs for various heat exchanger types.
Several options are available for controlling ACHEs. Simply switching fans on and off is adequate in many cases, and can give quite close control if the item has a large number of fans. The addition of louvre shutters, which can be manually or pneumatically operated, will provide further improvement, and two-speed motors are sometimes used.
The best control is obtained by the use of auto-variable pitch fans or variable speed motors, both of which provide gradual airflow adjustment. Improved electronics have made variable speed much more popular in recent years, with the additional benefits of power consumption and noise always being minimized.
The method of flooding condensers as frequently employed in shell and tube heat exchangers is not practical for ACHEs, and reductions in the effective surface area can only be achieved by valving off bundles.
Large variations in ambient temperature throughout the year will have a considerable effect on the available range of control, especially if there is a close approach at the design condition. Process engineers should be aware of this and avoid building in large design margins when a high degree of turndown is required, since for most of the year the ACHE will be massively oversurfaced and a control problem created.
If there is a possibility of freezing, waxing or hydrate formation, it will be necessary to maintain a sufficiently high tube wall temperature to avoid this under all conditions. In many cases this will not be a problem, or may be easily solved by using reduced finning and/or cocurrent flow. However, in extreme cases hot air recirculation will be required. This is achieved by enclosing the ACHE in a cabin with inlet and outlet louvres, and a duct to redirect some of the exhaust air to mix with the cold inlet air. The normal arrangement is shown in Figure 3, although the recirculation duct may occasionally be at a header end (external over end).
Process side enhancement
In the majority of ACHE designs the airside heat transfer coefficient is controlling (i.e., much lower than the tube-side coefficient), and enhancement of the inside coefficient would give very little overall improvement, such that the additional cost of the enhancement device cannot be justified. However, for viscous fluids where the flow in plain tubes would be laminar, wire wound turbulator inserts are frequently used. The improved heat transfer coefficient these inserts provide can also help to avoid pour point problems, since the tube wall temperature will be closer to the bulk fluid temperature.
Tubeside fouling factors normally follow shell and tube standard practice. Airside fouling factors are sometimes specified but have little effect on the already low airside heat transfer coefficient. The restriction to airflow of fouling on the finned tubes is of greater significance, and occasional cleaning is advisable to maintain cooling efficiency. In order to avoid fin damage, particularly with wound aluminum fins, this cleaning should be carried out by specialists.
The internationally accepted standard specification for refinery ACHEs is API 661. Many user companies now base their own specifications on this standard, with their preferences given as amendments/supplements to the API 661 clauses.
Compiled by Rami E. Kremesti M.Sc.