All shell-and-tube heat exchangers can be categorized into two broad categories: U-tube and straight tube. For clarity of definition, the Tubular Exchanger Manufacturers Association (TEMA) has defined the mechanical designs for front-end head types, shell types and rear-end head types by a lettering system called the nomenclature chart.
While there are many different types of U-tube and straight-tube exchangers, for the purpose of this article, we will be comparing BEU and BEM designs. These are the most commonly used and, generally, the least expensive of the options. Throughout this article, BEM designs are interchangeable with NEN and AEL designs: They have the same explored benefits and limitations, albeit with a different channel layout. Similarly, BEU designs are interchangeable with AEU designs.
BEM exchangers are the most versatile of all the shell-and-tube heat exchangers. While they might not be the most efficient option for every process case, they can be used in almost every service. Because BEU exchangers must always have tube passes in multiples of two, they are limited to services without a temperature cross. A temperature cross exists when the outlet temperature of the hot side is colder than the inlet temperature of the cold side. Because BEUs always have multiple tube passes, there can never be pure countercurrent flow, which is required to handle these design cases. Without pure countercurrent flow, the fluid being heated will instead flow through areas where it is actually being cooled down. In such cases, BEM designs must be limited to one tube pass to maintain pure countercurrent flow. In many cases, temperature crosses can be removed by running two or more exchangers in series, which would allow the use of BEU heat exchangers.
An issue that plagues many straight-tube designs is differential thermal expansion. Because the shell and tubes see different fluids, they maintain different metal temperatures. When metals see different temperatures such as with the shell and tubes, they expand by different amounts. Because both the shell and tubes are fixed to the tubesheets, if the differential expansion becomes too severe, a high stress is placed on the components, which will lead to the tube-tubesheet joints failure or worse.
To alleviate the differential thermal expansion on severe cases that can cause failure, expansion joints must installed. The expansion joint allows the shell to extend and compress at the same rate as the tubes, reducing stresses to acceptable levels. Depending on the size of the heat exchanger, expansion joints can be quite expensive. Differential thermal growth must also be rated on a per-service basis, including startup and shutdown.
Differential thermal expansion is not an issue that affects BEU designs. Because the shell and tubes of BEU designs are only fixed on one side, they are allowed to expand at different rates, removing the need for an expansion joint. This also tends to extend the life of BEU designs over BEM designs.
BEU designs also have the benefit of having removable tube bundles. BEM designs do not have removable tube bundles. Therefore, if the tube bundle needs to be replaced, the entire shell section must be cut apart and reassembled by weld or be scrapped.
Because BEU designs have removable tube bundles, inspection and cleaning of the shell side is easy. Because BEM designs do not have removable tube bundles, inspection and cleaning of the shell side must be done through nozzles and inspection openings. These may not be adequate for many services. However, because BEM designs have straight tubes and easy access to the tubesheet, cleaning of the inside of the tubes easily can be done with a wire brush or high-pressure water jet. The inside of the tubes also can be cleaned for BEU designs, but it is a little less manageable due to the U-bend.
When the shell-side inlet stream enters a BEM, the stream will instantly contact the tube bundle unless impingement protection is provided. Depending on the velocity and the nature of the fluid itself, this can cause significant erosion to the tube at the inlet nozzle. TEMA requires that impingement protection be provided at the tube bundle when the nozzle inlet ρv2 exceeds certain values, where ρ is density in pounds per cubic foot and v is linear velocity in feet per second.
TEMA requires impingement protection when the ρv2 exceeds 1500 lb/ft-sec2 for non-abrasive, single-phase fluids; 500 lb/ft-sec2 for all other liquids, including liquids at the boiling point; and in all cases for two-phase or saturated vapor fluids. In some cases, the need for impingement protection can be alleviated by simply increasing the inlet nozzle size. In other cases, impingement protection for a BEM is an absolute necessity.
Impingement protection can be provided in a variety of ways but the most common method is to weld a piece of plate to the tube bundle underneath the inlet nozzle. While an impingement plate generally is inexpensive to provide, it often requires the exchanger to increase in shell diameter because it replaces a small section of the allowable tube space. BEU exchangers normally do not have this issue.
One can alleviate the need for an impingement plate by putting the inlet nozzle beyond the tube bundle using a slightly longer shell. Because the inlet is not right above the tube bundle, an impingement plate would not be needed, allowing a full tube bundle to minimize exchanger size.
In conclusion, BEM and BEU heat exchangers are the simplest and least expensive options for straight-tube (BEM) and U-tube (BEU) shell-and-tube heat exchangers. While there are some exceptional cases, BEU designs are generally less expensive than BEM designs, assuming that the BEM option does not require an expansion joint. (Even though BEU designs typically have a larger shell than BEM designs, which normally leads to a larger cost, this often is offset with there being only one bonnet and one tubesheet.) As always is the case, pressure, diameter and overall length may dictate otherwise.
Of course, all general rules of thumb do have exceptions. There are U-tube exchangers that do have pure countercurrent flow. Also, there are straight-tube exchangers that have removable bundles. These designs generally are more expensive and have their own benefits and limitations. A BEU is incompatible with some services while for others, a BEU is the industry standard. In a case where both BEU and BEM designs are viable options, there is no wrong answer. The decision is mostly based on company preference.
This article originally was published with the title "BEM vs. BEU Heat Exchangers" in the April 2015 issue of Process Heating.
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