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Tag Archive: Super Duplex Heat Exchangers

  1. Heat Exchanger Material Selection based on Common Criteria

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    With so many factors to consider in choosing a material for your shell and tube heat exchanger, you may have a lot of questions.

    To start, you’ll need to decide which criteria are most critical to your operation. Criteria like thermal efficiency, cost, availability, corrosion resistance, cleanability and durability. You can then weigh the pros and cons of the options that best meet your priorities, since there is typically more than one good alternative. For example, the best material for heat transfer may not be sanitary enough for your application; or the most corrosion-resistant option may far exceed your budget. In most cases, there is a heat exchanger material option that can balance most of your priorities.

    Here are a few tips and suggestions for evaluating the heat exchanger material options based on these common criteria.

    Thermal Efficiency

    Since the goal of a shell and tube heat exchanger is to transfer as much heat as possible between the product (typically in the tubes) and the utility fluid (typically in the shell), the thermal conductivity of the tube material is a key factor. Based on thermal modeling comparisons using HTRI software, copper and copper/nickel are typically the most conductive material available for exchanger tubes. Carbon steel, stainless steel and higher alloys are slightly less efficient, but all perform similarly. Plastics, graphite composites and ceramics are the least conductive materials available.

    Thermal performance can also be enhanced through modification to tubes, such as corrugating, adding external fins to the tubes, or by adding twisted tape turbulators to the inside of the tubes. However, because thermal performance among metals is so similar, this is not usually a high priority factor in heat exchanger material selection.

    Cost & Availability

    Material pricing and availability can fluctuate based on market supply and demand, the quantity required for your exchanger, and the components needed. Copper was a low-cost option a few years ago, but now due to supply, it is more expensive than stainless steel. Conversely, Titanium used to be one of the most expensive alloys but is now more reasonably priced. At the time of this article, here is the relative ranking for some of the common metal material options by price from lowest to highest: carbon steel, 304/304L SS, 316L SS, Duplex 2205, Titanium, AL6XN, Duplex 2507, Hastelloy C-276, Hastelloy C22, Monel 400, Alloy 625, and Nickel 200.

    Typically, the higher priced alloys are also in shorter supply, due to lower demand and the higher cost of carrying inventory. This directly affects the lead-time of these materials, often by 2-4 times that of more common alloys like carbon steel and stainless steel. Quantity of these higher alloys can also greatly affect price. Steel mills typically don’t run small batches of tubes or plate or they will charge for the entire mill run if they do.

    Enerquip Bundle

    U-tube water cooler built with 2205 Duplex tubes – an austenitic-ferritic stainless steel that stands up to chloride stress corrosion.

    Shell Side and Tube Side May Be a Combination of Materials

    The shell side and tube side of an exchanger can be built from different materials of construction. It is common to use a more corrosion-resistant and/or sanitary alloy for the product side of the unit, while using a lower grade alloy for the utility side of the exchanger.

    If a higher alloy is the material selection, it is more economical to use it on the tube side of the unit instead of the shell side. However, be aware that for tubing, seamless tubes are almost double the price of welded type tubes, so this should be considered as well. You may find that seamless 316L stainless tubes cost more than welded Duplex 2205 tubes, for example.

    Common parts such as fittings are less likely to fluctuate than raw materials like plate stock, tubing and forgings. There are ways to help reduce cost when using higher alloy components. Consider lap-joint flanges, where the product contact nozzle (stub end) is high alloy, while the flange is stainless or carbon steel. These also aid in ease of installation, ensuring bolting alignment with existing, mating piping or equipment. Larger parts like tubesheets, flange rings and channel covers can often be made from a lower alloy base material that is clad with a thinner layer of high alloy to reduce cost.

    Supplier Resources

    Another factor to consider when evaluating the various alloys is the number of suppliers that work with these materials, since this can affect cost and delivery. There are many fabricators that build exchangers from carbon steel and copper, so competition is high, prices are low, and lead-times are generally fast. There are fewer suppliers that work with the alloys from 304/304L SS up through the Incoloy series, but enough to keep prices and lead-times reasonable. Fabricators must have ASME compliant weld procedures for these alloys to provide pressure vessels such as shell and tube exchangers, which are classified as National Board Registered pressure vessels. Not all fabricators have these procedures for all materials, so it is wise to verify your preferred fabricator’s capabilities when considering alloy options. Since Titanium, Zirconium and Tantalum require specifically controlled environments for fabrication, supplier options are more limited. This specialty niche can be expensive with longer lead-times, so these materials are normally only used when applications require nothing less.

    Dispelling the carbon steel is cheaper myth: For most small and mid-sized exchangers up to 24 inches in diameter, it can be less expensive to upgrade carbon steel shells to a material selection of 304 stainless steel. Although the material cost is a bit higher for the stainless per foot, stainless eliminates the labor cost for priming and painting the exterior, which typically offsets the material cost difference. This, combined with reduced maintenance costs and added durability, makes stainless a better long-term value.

    Corrosion Resistance

    If corrosion resistance is critical for your application, it is best to consult a metallurgist to discuss the operating conditions and request their recommendation. Most reputable high alloy providers have metallurgists on staff. Rolled Alloys has proven to be a good resource, with responses and recommendations provided within 24 hours. Corrosion resistance charts can also be helpful for less critical applications where corrosive element concentrations are low.

    Many people are not aware that carbon steel is not only corrosive itself but can cross-contaminate stainless and higher alloys and cause them to rust. That is why it is not a good idea to use regular steel wool to clean your stainless sinks and silverware, as it will cause rouging. The same affect can be seen in heat exchangers containing carbon steel parts. Rust from carbon steel parts will attack the higher alloy parts of your equipment, potential causing premature failure. However, there are also cases where carbon steel offers better resistance to stress corrosion cracking than stainless steel, so the risks of rust corrosion need to be weighed against stress corrosion cracking during material selection.

    Copper and CuNi have moderate corrosion resistance but tend to discolor and scale easily. These softer materials are commonly used for shell and tube exchangers in utility applications, where the non-sanitary tube material and carbon steel shells do not create a product contamination issue.

    Stainless steel has become a very common heat exchanger material selection for low to moderate corrosion resistant applications. Since 316L SS is more corrosion-resistant than 304L SS, it is often selected for the tube side of an exchanger, while the shell is made from 304L SS. If additional corrosion resistance is needed, then the Duplex stainless series (2101, 2205, or 2507) is considered. Continuing up the ladder of corrosion resistance AL6XN is followed by Hastelloy alloys C-276, C22 and C2000. For higher corrosion resistance, Monel 400 and Alloy 625 are considered, before evaluating the extreme corrosion resistance of Titanium, Zirconium and Tantalum. Note: Each alloy has a specific resistance level to specific chemicals or solutions, so it is best to consult with a metallurgist during the selection process to match your process with the proper material.

    Enerquip wiped film evaporators

    This custom pair of wiped film evaporators were built for a hemp processing application. Their 304L stainless steel straight tubes were bright annealed to reduce surface oxidation.

    Cleanability

    If ease of maintenance and integrity of your product quality are high priorities, then materials that are easier to clean and maintain should be selected. The materials must stand up to your preferred cleaning regimen – whether it be chemical, mechanical or ultrasonic cleaning (or a combination). Acids, caustics and chlorides are common in cleaning solutions, but can be harmful to metals in higher concentrations or elevated temperatures. Once you decide on a material of construction, companies that provide these cleaning chemicals, like Ecolab or AFCO, can provide recommendations on concentrations that are suitable.

    Sanitary Markets Require Stainless

    In sanitary industries such as food, beverage, dairy, pharmaceutical and cannabis processing, product contact surfaces must be stainless steel or a higher alloy, and cleanable, to comply with strict guidelines such as FDA, ASME BPE or the 3-A Sanitary Standards. Because the surface finish of the material impacts its cleanability, these industries require product contact surfaces to be polished to a specific Ra (roughness average) for food, beverage and dairy, and for pharmaceutical applications. Some pharma applications also require electropolishing, which removes a very thin layer of material, aiding cleanability further. Smoother material surfaces also resist build-up and scaling on both the product and utility side of an exchanger. Therefore, many companies avoid carbon steel and copper, which can become more porous as they corrode, encouraging scaling and fouling.

    Durability

    For situations where durability is not a high priority, low cost, catalog type heat exchangers constructed from copper and carbon steel may be appropriate. Many HVAC applications fall into this category, where replacing parts from time to time is normal and acceptable. However, in production facilities, where process equipment is pushed to its limits on an ongoing basis, durability is much more critical. Equipment failures can contaminate product, stop production and even be dangerous.

    It is important that the material selected for your heat exchanger meets ASME Code requirements and can operate for an extended time at your operation’s design pressures and temperatures. The materials must withstand your cleaning regimen and environmental factors such as moisture, dust, and temperature extremes.

    In situations where dissolved solids and high tube velocities can take their toll on heat exchanger parts, it makes sense to use materials that are erosion resistant. When carbon steel and copper exchangers are designed, ASME Code requires a corrosion allowance be added to the material thickness. This is added to account for anticipated corrosion and loss of material thickness over time. These exchangers may also include anodes that corrode away before the exchanger parts do. These can be monitored and replaced periodically. When higher alloys are used in place of carbon steel and copper, these corrosion allowances are no longer required, resulting in thinner, lighter materials in a more durable piece of equipment. Stainless and higher alloys do not require painting to protect their exterior finish from corrosion, so there is no flaking and peeling paint to contend with, decreasing maintenance time and expense.

    The experts at Enerquip are happy to assist you by providing options for your shell and tube heat exchanger materials of construction. Since they work in many alloys, they can provide suggestions and feedback to help you narrow down your choices based on your priorities. For more unique cases, they will refer you to credible metallurgists who can help you finalize your heat exchanger material selection.

    Click here to learn more about Enerquip’s custom exchangers.

    Article Author: Ron Herman, Director of Business Development

    Ron Herman, Enerquip Director of Business Development

  2. Zeron 100: The super duplex stainless steel that stands up to saltwater

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    In seaside communities, cities should consider water treatment system materials’ reactions to environmental conditions.
    In seaside communities, cities should consider water treatment system materials’ reactions to environmental conditions.

    One of the most popular stainless steel alloys used in process equipment fabrication is 316L. It’s able to withstand high heats, resists corrosion and rouging fairly well, and is durable and long-lasting. While generally suitable for many applications, this alloy and others like it aren’t perfect. In some applications, it may give way to corrosion due to environmental conditions, reactions with the substances introduced to the equipment and other factors.

    To address these shortcomings, there was a push in the 1970s and 1980s to develop new alloys with higher levels of chromium and molybdenum, which have been found to be particularly helpful in protecting against corrosion in offshore drilling operations and other circumstances where seawater is a major concern.

    One of these new alloys and one of the first super duplex stainless steel was Zeron 100, according to Stainless Steel World. Originally created specifically to be used in seawater applications, it has a high molybdenum content (3.5 percent, as compared to 316L’s 2.2 percent by weight) and incorporates nitrogen. Zeron 100 also contains a good amount of nickel, which is known for its corrosion resistance, though also for its costliness.

    “One of the first super duplex stainless steel developed was Zeron 100.”

    Zeron 100’s strength allows for thinner walls

    Zeron 100 contains 8 percent nickel by weight in its cast iron form and 7 percent in its wrought iron form, but its strength and temperature resistance allow for thinner walls than would be needed when using most other stainless steel alloys, which offsets the higher price.

    The super duplex alloy’s strength was the reason engineers working to install a membrane filtration system at the Syd Arne reservoir, an offshore oil field off the coast of Denmark, chose Zeron 100 for its pipework, according to Offshore Magazine.

    The engineers wanted to control the weight of the system, and their ability to incorporate thinner pipes, smaller diameters and reduced valve sizes made this possible. Since Zeron 100 can handle high pressures and velocities, it was ideal for the system, which required high pressure pumps and a constant flow of seawater through the system.

    Nitrogen contributes added benefits

    The ability to control nitrogen levels in alloys is a relatively recent development in metallurgy, according to the British Stainless Steel Association. This newfound technique was a major contributor to the development of new alloys and helped give way to the introduction of duplex and super duplex stainless steels.

    The nitrogen incorporated into Zeron 100 has several benefits. Nitrogen helps prevent pitting and corrosion, increases strength and makes the metal more weldable – a recurring concern with other stainless steels, which tend to lose corrosion resistance after welding, according to the Water Research Foundation.

    Zeron 100 has a pitting resistance equivalent number of greater than 40, which many engineers likely would be happy to see, considering other popular alloys like 316L and 904L have PRENs of 24 and 35, respectively. While it’s not the highest PREN around – Hastelloy C-276, for example, comes in at 67 – fabricators often find that Zeron 100’s ability to fight against pitting combined with its cost-effectiveness offers the best of both worlds.

    Zeron 100’s ability to resist corrosion in saline or brine environments is what brought the alloy to the attention of the City of Hollywood, Florida. Unfortunately, the city discovered these benefits after it had already installed a well system primarily composed of 316L stainless steel.

    Not long after the system was implemented, it began to rust and pit, according to Water Quality Products. Within a year, the pipes had more than 20 leaks, primarily caused by anodic action and microbiological organisms. The pipes’ J-bends sustained the bulk of the damage, where water was allowed to collect and wear down on the metal over time. Sitting water can commonly lead to crevice corrosion, which is exactly what happened in Hollywood, Florida.

    It didn’t take long for the city to reevaluate its decisions to get to the bottom of the problem. City officials determined that the biggest mistake they made was not considering other types of stainless steel; 316L can’t withstand the high chloride levels present in the water treatment system, which is necessary to prevent biofouling.

    Zeron 100 has been shown to withstand high chloride levels. Stainless Steel World noted that it doesn’t show any signs of chloride stress corrosion cracking when tested with 3 percent sodium chloride solution at 250 degrees Celsius. Stainless steels in the 300 series commonly cannot compete with this. Hastelloy C-276 is another contender, though it comes at a higher price point.

    The City of Hollywood identified that a super duplex stainless steel would the answer to their problems, and began testing Zeron 100 to replace the highly corroded J-bends. It determined that an all-out replacement of the system wouldn’t be prudent right away, but intends to incorporate more Zeron 100 as parts wear out. It noted in Water Quality Products that, in hindsight, starting out with Zeron 100 could have made the project much more successful.

    When it’s time to upgrade equipment or bring in additional functions to your operation, it’s crucial to consider the materials used for fabrication. Choose the wrong one, and you could wind up with ineffective equipment and escalating maintenance costs.

    Every situation, application and environment is different, so there’s no one-size-fits-all metal alloy that suits everyone’s needs. As such, it’s important to confer with an experienced engineer who knows the pros and cons of various metals and can tell you which options would be best for you. If you’re considering bringing a new shell and tube heat exchanger into your operation, reach out to the helpful engineers at Enerquip to determine which materials best suit your needs.

  3. Duplex stainless steel makes an excellent choice for manufacturers

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    Designing a shell and tube heat exchanger involves many decisions that must be made carefully and intentionally. One crucial choice that’s important to every exchanger is the material from which it’s made.

    Engineers have a wide variety of options when choosing the right material from which to build a shell and tube heat exchanger. Steels and alloys come in various compositions, each with their own unique properties, benefits and disadvantages.

    What makes one material more suited for an exchanger over another largely depends on how the equipment will be used – including what chemicals will be introduced to it, the temperature and pressure settings it will be subjected to, and what kind of environment it will be housed in. In addition, the composition material must be cost-effective and easily acquired by the manufacturer.

    Many manufacturers find duplex stainless steel to be an ideal material for shell and tube heat exchangers in a wide variety of industries, including the pharmaceutical, oil and gas and biotechnology industries. Duplex stainless steels are composed of ferrite and austenite, and thus can withstand high-stress applications.

    Ferrite and austenite

    Stainless steels come in many forms. Austenitic stainless steels are the most popular, as they are the most versatile. They are very weldable, according to the American Welding Society, but they do have a tendency to crack when overheated or under too much pressure.

    “Duplex stainless steels can withstand high stress applications.”

    Ferritic stainless steels are another good option for a variety of applications. Though they aren’t as durable as austenitic steels, they have high resistance to corrosion and are fairly formable, making them a versatile solution.

    When combined to create duplex stainless steel, the best properties of both the austenite and ferrite come out. Austenite lends its strength while ferrite resists corrosion and cracking.

    Duplex stainless strength
    Since duplex stainless steels are so strong, engineers have found they can reduce the weight and thickness of materials made from it while still maintaining durability and corrosion resistance. According to Process Heating, duplex stainless steels usually have double the strength of austenite stainless steel. Thinner tubes mean less material is needed, reducing the building cost of the equipment. The exchanger may also be able to be more efficient because of the lower weight.

    In addition to having a strong, efficient exchanger, manufacturers seek out those that can resist corrosion best. Corroded or pitted tubes become weak in time and can eventually spring a leak. A cracked tube is never a good thing, and can cause:

    • Fouling
    • Cross contamination
    • Damaged tube sheets, shells or other important features of the exchanger.

    Damaged tubes are also difficult and expensive to replace.

    Corrosion can occur anytime a substance is introduced to the exchanger that naturally reacts with the chemical composition of the tubes or shell. Chlorine is a common cause of pitting, and can be a major concern for manufacturers operating in a chlorine-heavy environment with an austenite stainless steel heat exchanger. Duplex stainless steels, on the other hand, resist the ill effects of chlorine well and are sought after in industries that work with chlorine frequently.

    Duplex stainless steels’ ability to withstand high temperatures also make them a prime choice for shell and tube heat exchangers. When chloride and tensile stresses are both at play, it only takes a temperature of 150 degrees Fahrenheit to endanger austenitic stainless steel, according to the International Molybdenum Association. When duplex stainless steels are in use, however, temperatures can rise to around 250 degrees before they become risky.Rouging is another problem that some industries need to fend off. Rouging refers to the oxygenation of the metal, represented through a rainbow of colors including red, blue, gold, gray and dark brown. However, IMOA noted that the causes of rouging aren’t always completely understood, but can be greatly affected by the grade of steel equipment is made from.

    When rouging builds up on equipment in the pharmaceutical and biotechnology industries, it’s imperative that it gets cleaned to prevent product contamination. The biggest challenge in this is the time and money that goes into this process. Choosing a rouging-resistant material to build equipment is ideal. IMOA explained that the 316L grade of austenitic stainless steel is a favorite among the pharmaceutical industry because it is resistant to rouging to a certain extent. However, duplex stainless steel has also been found to be just as resistant to it, if not more so.

    Cost benefits

    In addition to information about how well one alloy performs in operation compared to another, manufacturers also need to know whether the material is available to them, how difficult it is to obtain it and how much it costs. Process Heating noted that duplex stainless steels have lower nickel and molybdenum contents compared to austenitic stainless steels, bringing down their price point. It also protects the alloy from a volatile raw materials market, which can cause alloy and metal prices to swing upward dramatically at times.

    “Lower nickel and molybdenum contents bring down the price point of duplex stainless steel.”

    Additionally, since the stronger duplex stainless steel can be made durable with less material than an austenitic or ferritic steel, the cost to build it can be reduced.

    Manufacturers also need to look at cost comparisons over the long term. For example, a material that costs less upfront but has weak properties will eventually cost more in the long run through maintenance and replacement costs. Since duplex stainless steel can stand up to higher temperatures and pressures, as well as more corrosive materials, than austenitic or ferritic stainless steels, less maintenance would theoretically be needed to keep the exchanger in working order.

    Using equipment that is made from the proper elements is crucial to having a healthy and productive manufacturing operation. If you’re in the market for a long-lasting shell and tube heat exchanger,

    speak to the engineers at Enerquip, where we’ll be able to identify the best materials and configuration for your operation.