Practical Thermal Design of Air-Cooled Heat Exchangers. Begell House 2019-01-13

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Air

Practical Thermal Design of Air-Cooled Heat Exchangers

Therefore, the trim cooler was designed for this heat duty; that is, for condensing the uncondensed vapor from 133. Limitations in plant layout Air-cooled heat exchangers cannot be located near large obstructions, such as buildings, since air recirculation can set in and affect the cooling performance adversely. The principal construction and performance parameters of this design are indicated in Table 3. This is somewhat surprising, considering that shell-and-tube heat exchangers in the same chemi- cal process industries can be designed and built with 0. While this is true, it should also be realized that the heat duty will increase directly with specific heat so that all other things remaining the same, a higher specific heat will result in a considerably higher heat duty than heat transfer coefficient! So, while the former high heat transfer coefficient will tend to reduce the first cost of the heat exchanger, the latter high pressure drop will tend to increase the operating cost of the heat ex- changer.

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AIR COOLED HEAT EXCHANGERS

Practical Thermal Design of Air-Cooled Heat Exchangers

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. All the blades of a fan should be set at the same angle for smooth operation. The large variation in the heat transfer coefficient of hydrocarbon gases is attributable to the large variation in operating pressure. Evidently, they are used where floor space is at a premium and could thus be used advanta- geously in offshore platforms as well. Heat duty It is a good idea for the thermal engineer to corroborate the heat duty since licensors oc- casionally slip in this aspect. Compliance with this requirement is strongly recommended since it yields significantly better fan performance but is not expensive to imple- ment.

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Air

Practical Thermal Design of Air-Cooled Heat Exchangers

However, such services as described above are quite rare and invariably the process inlet temperature will be fairly high, such as 212°F 100°C , whereas the process outlet temperature will be rather low, such as 113°F 45°C. The simultaneous optimization of airside and tubeside calculations is certainly not an easy task. Grooved finned tubes can tolerate process fluid temperatures of up to 752°F 400°C due to their strong fin bond usually no fin-bond resistance penalty is applied because it is e c d a b Fig. The upper limit of process fluid inlet temperature for these finned tubes is 338°F 170°C. Thus, a very important goal for a good thermal design is the best utiliza- tion of the allowable pressure drop.

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Practical Thermal Design of Air

Practical Thermal Design of Air-Cooled Heat Exchangers

Usually, this velocity is unacceptably low since it yields a very low heat transfer coefficient and therefore has to be increased. Manufacturers usually conduct static and dynamic balancing of the hub in the shop. The maximum and minimum ambient temperatures at the site were 100. Thus, at a site where the design ambient temperature is 107. The addition of louvre shutters, which can be manually or pneumatically operated, will provide further improvement, and two-speed motors are sometimes used.

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Begell House

Practical Thermal Design of Air-Cooled Heat Exchangers

In such situations, the tube length is usually 1. A proper and sound understanding of the fundamental principles and interplay of parameters is essential in order to produce an optimum design. Tube lengths vary to suit the installation, which will often be over a piperack, but generally do not exceed 15 m. For condensing services, the total heat duty is the sum of the sensible vapor cooling duty, the sensible liquid cooling duty, and the condensing duty, which is the product of the amount of vapor condensed per hour and the latent heat of condensation. Bank: A bank or battery of air-cooled heat exchangers comprises one or more sections or units arranged on a continuous structure Fig. The initial cost is evidently the fixed cost or the first cost of the heat ex- changer.

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Practical Thermal Design of Air

Practical Thermal Design of Air-Cooled Heat Exchangers

In other services, where both the inlet and the outlet temperatures of the process stream are relatively high, e. Thus, depending on the type of bond between the tube and the fin, maximum operating temperatures have been established. A special situation may call for special measures. It will be seen that there is a significant reduction in the overdesign with a lowering in the air flow rate, from 6. In recirculating cooling water systems which are the norm , the outlet warm water is cooled by a cooling tower so as to eliminate this increase in temperature of the discharge water with its associated adverse effect on aquatic life. The fins are most commonly spirally wound aluminum strips 12.

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AIR COOLED HEAT EXCHANGERS

Practical Thermal Design of Air-Cooled Heat Exchangers

Obviously, then, the designer has to optimize the design so that while the velocity is not low enough to exacerbate fouling, the pressure drop is not ex- cessively high. This is particularly useful when several exchangers are to be mounted in a bank with a common tubelength. . I will always be grateful to Cindy Mascone, ex-technical editor at Chemical Engineering Progress, and her one-time compatriot, Gail Nalven, who led me to believe that I could write a book. Induced draft disadvantages The main disadvantages are as follows: 1 The fans and drives are less accessible for maintenance, being located above the plenum chamber. The incli- nation from the horizontal is usually between 45° and 60°. More often than not, a single case is controlling from all points of consideration.

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Begell House

Practical Thermal Design of Air-Cooled Heat Exchangers

Heat release and other profiles will be discussed in detail in Chapter 8. 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. The fin efficiency is in the range 0. This is addressed later in Chapter 13. Thus, if an air-cooled heat exchanger tube bundle has 200 tubes and 2 passes, the fluid flows through 100 tubes at a time, and twice along the exchanger: first from left to right and then from right to left or vice versa. Thus, plug headers are preferred for clean and moderate-to-high pressure applications. The higher the pressure drop, the higher will be the heat transfer coefficient and thereby the lower the heat transfer area and fixed cost.

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Begell House

Practical Thermal Design of Air-Cooled Heat Exchangers

Now, this represents a direct conflict because as we shall see later on in the book, the best way to minimize fouling is to maximize velocity within limits of ero- sion, of course that will directly maximize pressure drop and thereby power con- sumption. In this book, we shall talk predominantly about thermal design. Airside fouling factors are sometimes specified but have little effect on the already low airside heat transfer coefficient. Special considerations All pertinent requirements, such as cycling, upset conditions, alternate cases of operation, and whether operation is continuous or intermittent, should all be specified so that all demands made on a heat exchanger during its expected lifetime can be taken into account for design. This situation is exacerbated when the tubeside viscosity is high, because then a higher mass velocity is required to obtain a satisfactory Reynolds number. An interesting aspect to observe here is that even for the same Reynolds number, two liquids of fairly different viscosities will yield fairly different heat transfer coefficients due to their different Prandtl numbers—the liquid having the higher viscosity will have the higher Prandtl number and thereby the higher heat transfer coefficient. This book has been written primarily for the heat exchanger thermal designer.

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