Mahan’s is committed to ensuring that each customer we work with has the best tool for what they need, and as clear an understanding of its use as possible. As we seek to set you up with the right tool for your project, you might find that we seek to combine multiple known strategies or mechanisms into one efficient and customized solution. Such could be the case as we look at single pass and multi pass heat exchangers, the former of which may be more familiar, but the latter of which might be more appropriate for your unique needs.
Back to Basics
Heat exchangers have the potential to utilize all three types of heat exchange: conduction, convection, and radiation. Most of the operations that heat exchangers tend to capitalize on, though, are based in the principles of convection: the transfer of heat within a moving fluid.
Shell and tube heat exchangers do this with two liquids: one moving through tubes toward a desired destination, and the other in a shell serving to alter the temperature of the liquid in the tubes. Well-designed systems, with optimized tube thickness and composition and free of fouling to confound heat transfer, are essential to running this system properly.
When first learning about shell and tube heat exchangers, you’ll likely recall most explanations and simple diagrams labored under the assumption that any given exchanger worked one way. Liquids passed one another once, in a finite direction. Parallel flow exchangers passed those fluids by one another in the same direction, counterflow mechanisms did so perpendicularly, and cross flow exchangers swapped out the cooling liquid for a gas.
For an overview of heat exchangers, explore our blog on the topic and their applications.
However, as projects or desired applications get more complicated, the systems we recommend or create may be required to combine techniques. One way this is done is with hybrid flow machines, incorporating more than one of the aforementioned techniques in one exchanger. With multiple techniques comes the need for a multi pass heat exchanger, one where liquids (or gases) pass through multiple times.
How Do Multi Pass Exchangers Work?
Unlike single pass exchangers, multi pass exchangers are required to lengthen the time and surface area permitted for heating or cooling processes to take place. The number of passes refers to the tube side fluid, and the number of times it will need to move through tubes to reach the desired temperature. The amount and type of surface area needed to provide adequate heat exchange, the desired end temperature of the liquids (or gases), and space available for the mechanism in question figure prominently into the method that will make the most sense for you.
Designing for a multitude of passes can occur in two ways. The first is to facilitate multiple passes with the use of U-bends directing liquid in a snakelike formation through the heat exchanger; this is the “tube-side” solution, focused largely on lengthening the tubes for longer travel (pictured below).
The second method is facilitated on the “shell-side,” through the use of additional baffles that can control the flow of liquid through tubes. For example, in a multi pass system designed for two passes, the inlet head can guide one piece of the process while a return head facilitates the second pass before exit. At any given point, only half of the installed tubes are being used. An example of how this process works is pictured below.
These processes can be designed for two, four, or even more passes, depending on materials and ultimate use of the equipment. Because they are based in a shell and tube construction, their durability and easy serviceability make for relatively easy use in even high-pressure or environmentally demanding locations. Again, the more we know as we assess a prospective solution for you and your company, the better.
What Factors Contribute to a Need for a Multi Pass Heat Exchanger?
As we at Mahan’s consider the scope of your project and needs, several factors may come into play when recommending a multi pass heat exchanger for you. In addition to the standard questions about size, cost, and desired/maximum weight, we’ll also go deeper to determine your required efficiency, type of fluids, operating temperatures, and external temperatures. This helps us determine what sort of exchange patterns may need to be combined.