Choosing an exchanger bonnet style: part 1

One of the major decisions the fabricator and manufacturer have to decide on is what kind of channel box to use on the shell and tube heat exchanger.

There are many configurations shell and tube heat exchangers can come in. Choosing the right combination of styles is an important first step in deciding which exchanger is right for your business. Fabricators work with manufacturers to determine the specific needs the exchanger will address.

One of the major decisions the fabricator and manufacturer have to decide on is what kind of channel box to use on the shell and tube heat exchanger. According to Thermopedia, each exchanger has two channel boxes: the front bonnet, which is where fluid is put into the tubes, and the rear bonnet, which is where the fluid either exits the exchanger or is sent back through for another pass.

Each type of channel box, whether front or rear, has a designation, determined by the Tubular Exchanger Manufacturers Association (TEMA).Thermopedia explained there are six front bonnet designs and nine rear bonnet designs. The rear bonnet designs are divided into two categories: fixed bonnets and floating bonnets.

“Know what the exchanger will be used for to determine which qualities to focus on.”

Front channel boxes

While there are six designs to choose from, Chemical Processing said four are the most popular. Each has its own benefits and drawbacks. It’s important to know what the exchanger will be used for to determine which qualities to focus on.

For instance, if the exchanger will be used to process toxic chemicals, and especially if harmful materials will be flowing through both the tube side and the shell side, the N-type bonnet might be best. The tubes, tube sheet and shell are all welded together in this construction, which reduces the risk of leaks. However, if there is no need to have everything welded together, there is little advantage over other types. The N-type bonnet is difficult to clean and maintain. Replacing parts is also more difficult on this type than some of the others.

If cleaning and maintenance are the most important aspects to the manufacturer, an A- or B-type designation would be beneficial. Both allow for easy cleaning, as it is easy to access the tube sheet. These are the two most common channel box options, which work for most exchanger applications, according to Thermopedia. In deciding between the two, the manufacturer should consider the pressure of the fluids in the exchanger, how crucial the cleaning of the tubes is, how clean the solutions will be and cost.

“The A-type and B-type bonnets work for most heat exchanger applications.”

The A-type is easiest to clean because the tubes can be reached without disconnecting piping or taking off the bonnet. Many manufacturers like A-type channel boxes because they make cleaning the tube side easy. This allows them to use dirty tube side fluids. However, this bonnet style has two gasketed seals on the exchanger, with poses greater risk of leaks in high-pressure processes.

The B-type exchanger doesn’t have the second seal, which reduces chance of leakage, making it a more suitable option for high-pressure processes. This makes cleaning more difficult because the bonnet needs to be taken off to reach the tubing. Because of this, manufacturers who will be using clean tube side fluids, but dirty shell side solutions, will choose this option. It is also the cheaper of the two.

Finally, the C-type gives manufacturers access to the tubes without taking off the piping, though it is difficult to clean and maintain. It is good for high-pressure uses as well as with harmful substances that are on the tube side.

Thermopedia explained there are also D-type and Y-type channel boxes. The D-type is used for extremely high pressure applications. Like the C-type, it is difficult to repair because the tube bundle is attached to the bonnet. However, cleaning isn’t hard because the tubes can be reached without moving the pipes. It is the most expensive option manufacturers have for the front channel box.

The Y-type channel box isn’t recognized by TEMA, though some manufacturers like this option because it reduces need for excess pipes, thereby reducing the overall cost of the exchanger. It is used with an odd number of tubes, most commonly only one, for a single pass exchanger. If the exchanger is a part of a pipeline, this is a cost-effective option, if the industry will allow use of equipment not certified by TEMA. The Y-type channel box is versatile; it can be used as either a front bonnet or a rear bonnet.

Choosing the right bonnet is an important decision when planning out the fabrication of a shell and tube heat exchanger. While many exchangers will do well with an A-type or B-type channel box, there are some cases in which an N-type, C-type, D-type or even Y-type may be a better choice. Manufacturers should discuss the options carefully with their fabricator and know the uses expected of their machine before making their final decision.