Heat Exchanger Material Selection based on Common Criteria
With so many material options available, how do you select the most appropriate shell and tube heat exchanger material? Most fabricators can help you with suggestions, but most won’t give a definitive recommendation due to potential liability. Since you are responsible for operating the exchanger, and are most familiar with your process, the burden will most likely fall back on you to decide on the best material of construction.
Selection criteria typically include thermal efficiency, cost, availability, corrosion resistance, cleanability and durability. You will need to decide which criteria are the most critical to your operation and prioritize accordingly. Then you can 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.
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, CuNi, Duplex 2205, Titanium, AL6XN, Duplex 2507, Hastelloy C-276, Hastelloy C22, Monel 400, Alloy 625, Inconel 800, 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.
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 selected, 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.
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 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.
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 material of preference 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, Alloy 625 and Inconel 800 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.
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.
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.
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 helpful heat exchanger 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. To learn more about custom shell and tube heat exchangers, click here.