South Africa Air Liquide starts up the worlds largest oxygen production unit.

The Diverse World of Industrial Gases*

An interview with Fred Schweighardt, National Projects Leader – Advanced Fabrication Technologies for Airgas. Fred Schweighardt is a National Project Leader and International Expert for Airgas, an Air Liquide company, based out of Houston, Texas. In this capacity, he applies his extensive welding background to industrial gas applications in fabricating and manufacturing markets.
 
^ South Africa Air Liquide starts up the worlds largest oxygen production unit.

Article By Sarah Bradley, Brittani Schroeder, and Angelica Pajkovic
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“The industrial gas industry is very diverse,” said Fred.

“Your cellphone, the metal parts of your chair, the carbonation in your soda… none of it would exist without industrial gas. We have an entire medical group that does nothing but medical gas. We have a food segment and they focus on ways that you use industrial gas in food. I focus on the ways you use industrial gas in welding.”

Fred Schweighardt has worked in the welding and maintenance industry for over 30 years, primarily in the fabrication, heating, plant and pipe industry. As such, he is no stranger to heat exchangers. “I have worked with a lot of different heat exchangers and I think they fall into a few major categories. We can break them down, for example, into shell and tube, brazed aluminum, cooling towers, or plate. The list goes on and on,” said Schweighardt. “The radiator in your car is a heat exchanger, as is a tea kettle and a refrigerator. There are heat exchangers everywhere!”

Types of Exchangers

Shell and tube heat exchangers consist of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid will run through the tubes, and another will flow over the tubes (through the shell) to transfer the heat between the two fluids. “These kinds of heat exchangers can be easy to clean and can come in varied sizes. If you need more space, you can stack the shell(s) vertically to reduce the overall size,” explained Schweighardt. Being able to reduce the footprint of a shell and tube heat is a great option for plants that are undergoing a rebuild or retrofit and will no longer be able to accommodate a large exchanger.

Brazed aluminum heat exchangers are brazed together to transfer heat between various fluids. These exchangers are commonly used in the cryogenic industry, and they are quite large in size. The brazed aluminum exchangers have a very complex internal geometry. The whole thing is filled with corrugated plates that are aligned very specifically and have small passages that the gases flow through. “What happens in the exchanger depends completely on what you are going to do with it,” said Schweighardt. “If you are going to send liquid nitrogen through the cold side, and gaseous argon through another side, the cold from the nitrogen will turn your gaseous argon into liquid argon. In this case it is essential that these gases do not mix.”

Cooling tower heat exchangers come in a variety of styles. In these units, process fluids, like water, are exposed to ambient air and large fans are used to draw the air through the water. “The action of that water breaking into fine droplets and having the ambient air pulled into it will cool it down effectively,” Schweighardt explained. “The water might be entering the heat exchanger at 120°F and could drop down to 70°F or 60°F; it all depends on the ambient air.”

Plate heat exchangers use metal plates to transfer heat between two separate fluids. As large plates are in contact with the fluid, they have a major advantage over a conventional heat exchanger because the fluids are exposed to a much larger surface area. “The way plate exchangers work is through a ‘process in and process out’ system. The large bolts hold a stack of thin plates together, which are pressed in a certain pattern so that the fluid can go in, travel through the path between the plates, and come out,” relayed Schweighardt.

Heat exchanger specifications

There are many types of heat exchangers, and they can be made with a variety of materials. Each heat exchanger is specified to its intended process, and therefore has specific temperature limits. “Heat exchangers running liquid nitrogen through them could be rated down to -320°F (196°C). Hydrogen heat exchangers could go down to -450°F, and liquid helium could go even lower,” Schweighardt said. “Those types of heat exchangers are really important because that is how we make liquid oxygen, which is used in hospitals.”

Maintenance

As long as a heat exchanger is cared for, it will continue to function as for as long as the plant is operational. “Most heat exchangers do not have moving parts, which means they are static equipment and there is very little maintenance needed. If it is designed correctly in the beginning, it will continue to meet the required flow rates and environmental conditions originally specified,” explained Schweighardt. The most important maintenance procedures for heat exchangers is ensuring that they are cleaned properly.

Keeping equipment clean

Heat exchangers require a thorough cleaning to make sure they are operating at the best possible level. “When we are cleaning the inside of the tubes of a shell and tube heat exchanger, we call it ‘punching tubes.’ When we have to clean the outside of the tubes, we pull the whole tube bundle out and clean each part because dirt will prevent the heat exchange from happening,” Schweighardt said. The tube bundles come apart with a series of big bolts, which hold the head(s) to the shell.

Plate exchangers can be cleaned as well. You can split the stack, and clean all the plates. Once everything has been cleaned, it can be put together again. “If you add longer bolts when reassembling the shell and tube exchanger, you can make it as big as you like and there is essentially a new heat exchanger.”

Brazed aluminum heat exchangers, however, require extra care. “As we use the brazed aluminum exchangers all the time, they require a great deal of care. The passages are extremely tiny, so if they get clogged and we are unable to clean them out, we basically have to replace the whole unit; there is no way to take it apart and clean the insides,” Schweighardt said.

The frequency with which the exchangers need to be cleaned depends entirely on the process it is used for. “If you are using chilled water, you probably only need to clean it once a year, or every other year. If we are talking about a boiler, you need to clean it every year to remove any deposits that have built up. An automotive radiator, however does not need to be cleaned very often as it is a closed system, and not much dirt will get into it,” Schweighardt continued. “Now, if you drive in Texas and the front of your radiator is full of bugs, that is when you will have to wash it so you can still get airflow.”

Although there are a number of chemicals that can be used to clean a heat exchanger, one of the most common things that we use is nitrogen. “We make thousands of tons of nitrogen every day. It is inert at the temperatures and pressures we are using. For us, it is relatively inexpensive, so that is what we use to blow through the pipes and Balanced Heat Exchanger (BHX),” he said. Nitrogen will blow through the pipes of the plant during a turnaround period and will sweep away any potential bits of moisture or other particles that made it into the heat exchanger.

This process is called derime. Schweighardt continued, “If moisture somehow got into the heat exchanger and, over the years, has built up and freezes during operation, the derime process will melt it all and sweep it out so that you are able to start fresh with your heat exchanger.”

Heat exchanger challenges

Although the most common challenge for heat exchangers is the cleaning and maintenance, choosing the heat exchanger best suited to the required application can be a tricky task. “There are many different designs within each major category of heat exchanger, and each design has a number of variations,” explained Schweighardt. “First, you need to look at flow rate and ambient conditions. You need to make sure it can reach the right temperatures for the specific fluid or gas that will be passing through it. The last thing you want is your heat exchanger turning into a freezer and being covered in ice,” he continued. “Ice is a really good insulator, and it will ultimately stop the heat transfer. Heat exchangers that run into that problem may not have been chosen properly for the project.”

Operating with new materials

Even though heat exchanger technology and the use of industrial devices to perform heat exchanging tasks have been around for several hundred years, there are always variations that can be made to improve the equipment. “A lot of the time, the modifications made to improve the heat exchange process focus on the materials that are being used. The abundance of developed materials that are able to withstand corrosive fluid can make the process much more effective and efficient. There are new metals, polymers and resins that can do the job properly,” Schweighardt expressed.

Final thoughts

In Schweighardt’s opinion, as long as you take care of your heat exchanger, it will last until it is no longer economical to run. “The technology used in heat exchangers is a testament to how long something can last without needing major upgrades. If you take good care and make sure that nothing gets into the heat exchanger that is not supposed to be there, it will last you forever.”

*Portions of this article were previously featured in other KCI publications.

ABOUT AIRGAS

Airgas is a leading supplier of gases, welding equipment and safety products in the United States. Headquartered in Radnor, Pennsylvania, the Airgas network is comprised of more than 1,400 locations including retail branches, cylinder fill plants, gas production facilities, specialty gas laboratories and regional distribution centers. Employing approximately 18,500 people across the nation and serving more than one million customers, Airgas supplies industrial, medical and specialty gases, carbon dioxide, nitrous oxide, welding equipment and supplies, process chemicals and more to industries including: manufacturing and metal fabrication, construction, chemicals, life sciences and healthcare, food and beverage, materials and power, defense and aerospace. 

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