The lead article of the October 1951 edition of Modern Metals began by claiming, "The most important heat transfer development in the past quarter century was recently announced ... the successful fabrication and operation of large brazed aluminum heat exchangers ...."
|Figure 1. Long used in cryogenic applications, brazed aluminum plate-fin heat exchangers are being specified more frequently in warm applicaitons.|
Since then, brazed aluminum plate-fin heat exchangers have become the most widely used type of heat exchanger in the cryogenic industry. After 50 years of use and advancement, they are used extensively in the cryogenic separation of industrial gases, large-scale production of petrochemicals, natural gas processing (NGP) and liquefaction of natural gas (LNG). More than 40,000 brazed aluminum plate-fin heat exchangers are operating worldwide, and these heat exchangers are not used only at cryogenic temperatures, but also in warm temperature applications where large heat transfer areas are needed (figure 1). Process engineers are beginning to understand the process efficiency benefits made possible, even at warm temperatures, when the process is designed around the advantages of the heat exchanger.
|Figure 2. The core block is one of the basic components of a brazed aluminum plate-fin heat exchanger.|
Brazed aluminum plate-fin heat exchangers offer two main advantages over shell-and-tube heat exchangers:
- Surface Area Density. Brazed aluminum plate-fin heat exchangers have 10 times the surface area density (square foot of surface/cubic foot volume) of shell-and-tube heat exchangers and allow a reduced temperature approach, resulting in lower operating costs and reduced energy consumption. Therefore, the process can be designed to produce more product for the same amount of energy.
Combining Process Streams. With this design, many process streams can be combined into a single compact exchanger. This approach reduces the exchanger's installation cost and provides a mechanism to combine services to design the heat exchanger network in the optimum manner.
Figure 3. Fin designs include serrated, perforated and plain.
To fully understand where and why brazed aluminum plate-fin heat exchangers are specified, it is helpful to understand how they are made (figure 2). The heat exchanger consists of a core block constructed of alternating layers of corrugated sheets (more commonly called fins) and flat parting sheets. Each layer is bound by bars and provided with inlet and outlet distributors. Manufacturers permanently join the stacked assembly by brazing it in a large vacuum furnace. To complete the unit, headers with nozzles are welded to the bars covering the distributors or ports. The heat exchanger's size is described by the size of the rectangular core block: width, stack height and length.
|Figure 4. Three shell and tube heat exchangers can be combined into a singe brazed aluminum plate-fin heat exchanger.|
Normally, the units are mounted vertically using support angles on each side of the exchanger. The cutaway view provided in figure 2 illustrates the warm end, or top of the exchanger, with one warm stream (A) entering and two cold streams (B and C) leaving. Stream A enters the exchanger through the nozzle and header and is introduced to the exchanger block at the port. After flowing through the port, stream A is directed across the width of the exchanger via the distributor fin. Good flow distribution is needed for proper thermal and hydraulic performance. Stream A then enters the heat transfer fin section. Flow continues through the heat transfer fin, then through the outlet distributor fin, port, header and nozzle. Considering the exchanger's size, geometry, number of streams, types of fins, and number of layers for each stream, the construction of a brazed aluminum plate-fin heat exchanger allows an infinite number of design configurations. Normally, the flow arrangement is counterflow due to its efficiency, but crossflow or cross-counterflow designs sometimes are used.
|Brazed aluminum plate-fin heat exchangers commonly are installed directly inside distillation columns to eliminate costly piping.|
Compact size and versatility are typical features of a brazed aluminum plate-fin heat exchanger. The unit's compact size is due to the heat transfer fins and the aluminum material. Typical fin heights range from 0.200 to 0.380", with an average 18 fins per inch. Most of the surface is secondary surface with the parting sheet being primary surface. However, the thermal conductivity of aluminum is so high that fin efficiencies are typically more than 80%. The heat transfer fin configuration is designed to create an optimum balance between pressure drop and heat transfer performance of each stream. Serrated, plain and perforated fin designs are available (figure 3). The heat transfer fin design enhances the stream flow characteristics and increases the amount of heat transferred.
A brazed aluminum plate-fin heat exchanger is considered versatile because it can handle the flows and duties of many different types of fluids such as gas-to-gas, gas-to-liquid, two-phase or any combination thereof, with each stream optimized and balanced thermally and hydraulically. A typical shell and tube exchanger is limited to two streams, but brazed aluminum heat exchangers can be configured to optimize the combination of services in parallel or in series, allowing multistream configurations with up to 15 different streams. Figure 4 shows how a network of three shell and tube heat exchangers can be combined into a single brazed aluminum plate-fin unit.
These heat exchangers also can be packaged to meet a particular process requirement. For example, some plate-fin designs look and operate like kettle-type shell and tube heat exchangers but perform more efficiently. In this design, the tube bundles are replaced with a brazed aluminum plate-fin heat exchanger to allow multiple services to be installed inside a single shell, and at the same refrigeration level. The kettle provides both a liquid reservoir to feed refrigerant to the core block and a disengagement space to remove liquids before the refrigerant is piped back to the compressor (figure 5). Brazed aluminum plate-fin heat exchangers commonly are installed inside distillation columns to eliminate piping. If many units are required, they can be packaged in a steel cold box that provides structural support, interconnecting piping and insulation (figure 6).
To successfully specify brazed aluminum plate-fin heat exchangers, it is important to understand where they can be used. For new applications, it is especially helpful to understand where others have successfully used them. Use of these heat exchangers is limited by several factors, including:
- Only clean, noncorrosive-to-aluminum fluids can be used.
- Because aluminum has an upper temperature limit of 400°F (204°C), maximum design temperature is 400°F.
- Maximum design pressure is 1,751 psig.
- The local stream temperature difference at any point along the length of the heat exchanger must be limited to prevent excess thermal stress. A difference of 50°F (28°C) is a safe rule for all designs. A maximum of 100°F (56°C) or more is possible under appropriate conditions.
Specifying brazed aluminum plate-fin heat exchangers can be complicated, but they provide one of the best ways to optimize a heat exchanger network. Rating programs are available, but some practice and experience is needed to use them properly. The best way to proceed is to work closely with the supplier's thermal engineers early in the design phase to optimize the heat exchanger network and the process design scheme.
In the figures, the heat exchanger is shown in the horizontal orientation for descriptive purposes only.
|Figure 6. When numberous heat exchangers are required, they can be packaged in a steel cold box that provides structural support.|
Creating an Efficient Process with Brazed Aluminum Plate-Fin Heat Exchangers
Because brazed aluminum plate-fin heat exchangers can be used in applications at cryogenic levels as well as warm temperatures, it can be difficult to create an efficient process. Some points to consider when creating your process include:
- Ensure that all streams are clean and noncorrosive to aluminum.
- Employ design pressures less than 1,751 psig and desing temperatures less than 400°F (204°C).
- Combine exchanger services as much as possible.
- Optimize combined cooling curves to reduce the mean temperature difference and ensure an efficient process.
- Consider applications where brazed plate heat exchangers are used, including gas to gas, reboilers, condensers, chillers and gas to liquid.
- Work with the supplier's thermal engineers to optimize the unit and the process.
- Remembering these points will help you utilize the advantages and understand the process efficiency benefits that brazed plate heat exchangers provide.