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
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.
Condensing economizers can help recover latent heat.
Boilers involve a lot of energy and heat, much of which can be wasted if a system isn’t designed to prevent this. One way to capture and reuse lost heat is through installing a condensing economizer. This item captures water vapor and heat, turning it into usable energy.
Condensing economizers have a place in virtually every industry. Food processing plants, greenhouses, pulp and paper mills, hospitals and even district heating systems all utilize boilers and have the potential to use economizers. Investing in one can reduce energy requirements and cut operational costs.
However, condensing economizers typically don’t come in a one-size-fits-all format. They often require highly specialized design and are fabricated to meet the specific needs of one facility’s operations.
How Condensing Economizers Work
In most natural gas-fired boilers, it’s tough to cool flue gas much lower than 250 degrees Fahrenheit, according to the U.S. Department of Energy. Beyond this point, the gas can begin to condense, and the resulting vapor can become corrosive, posing a threat to the integrity of the stack or stack liner. However, for the flue gas to be captured and used as boiler feedwater, it needs to be cooled below its natural dew point of 135 degrees Fahrenheit.
Condensing economizers collect flue gas and cool it to the appropriate temperature, allowing the economizer to reclaim sensible heat (in the form of flue gas) and latent heat (in the form of flue gas water vapor). The amount of latent heat that’s recoverable depends on the type of fuel used, the style of condensing economizer used and other factors.
Types of Condensing Economizers
There are two types of condensing economizers that plant managers or other decision makers can choose from: direct and indirect.
A direct contact condensing economizer includes a vapor-conditioning chamber that’s connected to a countercurrent spray chamber, the Department of Energy explained. Inside the spray chamber, droplets of cool fluid come into contact with the hot gas to facilitate the heat exchange. The droplets cool the stack gas. This option provides a high heat transfer rate and allows for relatively easy water recovery, though this method can only heat water to about 140 degrees Fahrenheit. This style requires a mist eliminator, which prevents droplet carryover.
An indirect contact condensing economizer utilizes a shell and tube heat exchanger to capture the flue gas. This style offers greater heating and cooling capacity; water can be heated to as much as 200 degrees Fahrenheit and cooled to as low as 75 degrees Fahrenheit.
One example of an indirect contact condensing economizer successfully helping reduce costs and energy requirements was detailed by the Department of Energy. A food processing facility wanted to reduce its deaerator steam requirements, which were 5,000 lbs per hour. The company installed the economizer, which served to lower flue gas temperatures from 300 degrees Fahrenheit to 120 degrees Fahrenheit. The process allowed them to capture 2 million British Thermal Units per hour, and effectively cut its deaerator steam requirements down to 1,500 pounds per hour.
Considerations for Condensing Economizers
For both direct and indirect contact condensing economizers, the water exiting the system is usually very acidic and should be neutralized before being used as process water or directed to a sewage system. External heat exchangers, membranes or pH control systems are all viable options for this.
Choosing the right materials of construction is an important aspect of designing a heat recovery system with an economizer. Because the water vapor can be highly acidic or otherwise corrosive, the fabrication materials should be resistant to wear. Stainless steel and similar alloys typically hold up well when exposed to water vapor that comes as a result of hydrocarbon fuel combustion, which is the case with natural gas-fueled boilers.
The space in which the economizer will be used should also be carefully analyzed. For operations where most of the deaerator steam comes from blowdown heat recovery, there is limited potential for cost savings with an economizer. Additionally, operations where there isn’t much need for cold or low-temperature water, a direct contact condensing economizer won’t be especially helpful.
Working with heat exchanger experts such as those at Enerquip can help you determine your needs and the potential for heat recovery at your facility. Enerquip’s engineers will help you design a shell and tube heat exchanger that suits your specific operation. Request a quote, today.
GENERON is the world leader in the design and manufacture of custom process air and gas separation systems including nitrogen generators for onshore and offshore platforms, floating production, storage and offloading units and transport tankers for the oil & gas market. GENERON has an expanded product base which includes primary compression, instrument air and post compression packages.
GENERON can design and manufacture standard systems or custom engineered packages. For over 40 years, GENERON has provided thousands of systems worldwide to the oil and gas, marine, and industrial service industries that meet stringent customer and third-party society specifications. GENERON® systems are designed for all areas of classification, from Safe to Hazardous Areas, Class I Division 2, Zone 1 and 2, potentially explosive atmospheres, CE / PED, as well as other European standards.
GENERON has a wide variety of clients that require nitrogen generation systems, including drilling and service contractors like Schlumberger, Weatherford, and Halliburton; engineering companies like Alliance Engineering, Wood Group, McDermott, Fluor, and Petrofac; and major oil companies like Exxon-Mobil, Shell, Chevron, Total, and British Petroleum.
High Expectations
GENERON’S dedicated research and development team in California is constantly working to improve product offerings.
Most recently, the GENERON® Dehydration Hollow Fiber Membrane, was re-developed to reduce the weight and size of overall systems, while maintaining instrument quality air. Innovations like this, along with the complete GENERON® product line, continue to elevate the standards and expectations of clients.
GENERON’s facilities in Houston, Texas and Pittsburg, California allow the hands-on monitoring of quality control while delivering the most cost effective products. Both are certified by certified by DNV to ISO-9000 standards, the American Society of Mechanical Engineers, the Pressure Equipment Directive, GOST, and Underwriters Laboratories and the Canadian Standards Association. High quality and high standards are expected from not only their company, but the companies they partner with.
For nearly a decade, GENERON has trusted Enerquip to provide stainless steel shell and tube heat exchangers for these systems. GENERON turns to Enerquip multiple times a year to fulfill the needs and expectations of a growing customer base. The shell and tube heat exchangers Enerquip develops are integral in the nitrogen generation and natural gas compression and processing packages. GENERON relies on the high standards and integrity of Enerquip’s products and services to fulfill this need.
Industry Standards
GENERON relies on Enerquip’s commitment to meet all necessary compliance standards. Enerquip produces shell and tube heat exchangers that are code compliant and follow the regulations set by the Tubular Exchanger Manufacturers Association (TEMA’s) Classes B, C and R; the American Society of Mechanical Engineers (ASME); the Pressure Equipment Directive; the Ministry of Manpower; 3-A; as well as the codes set forth by the American National Standards Institute. Enerquip also fabricates to American Petroleum Institute (API) and Canadian Standards Association (CSA) standards. Enerquip is able to produce heat exchangers that are customized to GENERON customers’ specific needs. Many have unique requirements for various sizes, models and capacities. While many other shell and tube heat exchanger suppliers provide standard pieces of equipment, Enerquip is able to tailor each product to the unique applications for which it will be used. This is because Enerquip has in-house engineers who develop solutions for GENERON’s clients’ needs.
“We often require more customized equipment,” explained Sergio Gonzalez, the Americas Sales Director at GENERON.
“That’s why we turn to Enerquip. They have the engineering and manufacturing capabilities and facilities.”
Enerquip can produce shell and tube heat exchangers ranging from two inches to four feet in diameter, and it has access to a variety of alloys with which to create the equipment. Using the right material is important to GENERON’s clients to ensure the longest lifespan of the equipment as possible. Using the wrong metal can cause corrosion or won’t be able to withstand the pressure or other conditions of the operations.
Quick Turn-Around and On-Time Delivery
Delivery time is another key factor GENERON appreciates. Enerquip prides itself on providing fast deliveries to clients for whom time is a critical factor. GENERON clients sometimes need to put in rushed deliveries for various systems that GENERON provides. However, the company cannot deliver unless it works with a supplier that can provide them with the necessary equipment in a short period of time. Gonzalez explained that even when GENERON clients need a system to be expedited, Enerquip is eager to accommodate the short time frame whenever possible.
GENERON also values the time Enerquip takes to answer questions and give feedback about various products and orders. Gonzalez explained the contact person he has at Enerquip, Shane Viergutz, is always available to talk and is helpful.
“Every time I call them, even if it’s after hours, he answers the phone or returns my calls,” Gonzalez said.
Sometimes GENERON’s customers need a heat exchanger but don’t need a full system for gas compression, production or processing. Other times, they’ll indicate they need an exchanger or system for an application that GENERON doesn’t specialize in. In these instances, Gonzalez explained, he steers them directly to Enerquip. This is because he knows Enerquip’s engineers will be able to work with them to create the right solution for their needs.
“I’ve recommended Enerquip to some of my clients when they only need the heat exchanger, or when it’s not our market,” he said. “When they are working on a different application that we are not directly involved with, I send them directly to Enerquip.”
In getting connected with GENERON’s trusted supplier, the clients know GENERON is looking out for their best interests and will help them succeed in the future.
GENERON plans to continue working with Enerquip for years to come. The company knows Enerquip and its engineers are dependable, efficient, and will work hard to create the best solution for the many systems GENERON provides to its clients.
Simply put, said Gonzalez, “Overall, Enerquip gives good service to us, good products, and good quality.”
Before choosing a shell and tube heat exchanger for your operation, it’s crucial to know the strengths and weaknesses of the many alloy options available to you. Different applications will have different requirements in terms of resistance to corrosion, heat or pressure. Learning about these traits is imperative to choosing an alloy that will best meet your needs.
Alloys are created when specific metals are combined to create a new material. Each element has particular properties that change the characteristics of the resulting alloy. For example, nickel is typically used to increase strength and the alloy’s ability to harden, but without sacrificing ductility, Manufacturers’ Monthly explained. Nickel-based alloys are optimal for operations that require equipment that has high resistance to stress corrosion cracking.
There is a whole rainbow of nickel-based alloy options that are durable and corrosion resistant. One of the most popular is Hastelloy C-276, which is composed of:
Nickel
Molybdenum (which reduces brittleness)
Chromium (which improves ductility and wear resistance)
When choosing an alloy for a shell and tube heat exchanger, it’s not enough just to choose a material that meets your needs. Making sure it meets government regulations and industry standards is also critical. These guidelines are set by a wide variety of entities. One such organization is ISO, which creates international standards for industries ranging from agriculture to technology.
ISO 15156 is a standard that corresponds to the petroleum and natural gas industries, and includes recommendations and requirements for the materials that help create equipment used in highly corrosive environments. This standard lists five nickel-based alloys that are ideal for use in hydrogen sulfide-rich environments, Manufacturers’ Monthly reported. The five alloys are categorized on their chemical composition as well as the method in which they were formed. Two ways to create alloys are through solution annealing and cold-working.
According to ISO, annealing is the process of heating the material to a particular temperature and holding it at that temperature until the metal becomes a solid solution. Once cooled, it becomes easier to cut and work with, and isn’t as hard.
Cold working refers to the manipulation of the material at a temperature below the recrystallization point, according to Total Materia. This can improve strength, though it can make the metal harder to work with. To offset this effect, cold-worked metals are often intermittently annealed.
Reducing the hardness is important in some applications, Manufacturers’ Monthly pointed out. Cold-worked nickel alloys can make great tubular structures as a part of larger pieces of equipment, as long as the hardness is lower than 40 on the Rockwell hardness scale.
Standing up to hydrogen sulfide
ISO 15156 also offers recommendations on how to address areas where hydrogen sulfide is abundant. This chemical compound is commonly found in natural gas and crude oil, and tends to collect in spaces with little air flow, according to the Occupational Safety and Health Administration. As such, it’s highly abundant in offshore drilling applications.
“Hydrogen sulfide can wreak havoc on the wrong material.”
Additionally, hydrogen sulfide can be especially abundant in sour reservoirs, or those where abiotic and biotic reactions begin to occur. Manufacturers’ Monthly reported that sour reservoirs in the northern Caspian Sea can have hydrogen sulfide contents as high as 20 percent.
Hydrogen sulfide can wreak havoc on the wrong material, and as ISO 15156 points out, equipment failure due to chemical corrosion can pose great health and safety risks to those in the vicinity of the operation, as well as the environment. Noting how resistant to hydrogen sulfide corrosion a material is should be a key priority to manufacturers working in these industries.
Hastelloys 825, 625, and C-276 that are solution-annealed can work efficiently in high-hydrogen sulfide environments, Manufacturers’ Monthly pointed out. Hastelloy C-276 is particularly useful and able to withstand higher pressures of hydrogen sulfide than the 825 and 625 alloys.
Acidic Environments
In addition to hydrogen sulfide, plenty of chemical compounds can induce corrosion or other weakening or undesirable reactions. Acidic environments are particularly damaging to many alloys. Hastelloy C-276, for example, doesn’t hold up well to nitric acid, Corrosion Materials noted. However, it has shown considerable resistance to negative effects caused by other common acids and compounds, including:
Hydrochloric acid.
Sulfuric acid.
Acid chlorides.
Phosphoric acid.
Acetic and formic acids.
Hypochlorites.
Wet chlorine gas.
Acetic anhydride.
Given the ability to withstand a host of potentially dangerous substances, Corrosion Materials commented that Hastelloy C-276 is one of the most corrosion-resistant materials on the market today.
Heating up Hastelloy
As metals warm up, their physical properties begin to change. It’s important to know what limitations your alloy has under certain temperature conditions. The nature of the operation, as well as environmental factors, must be taken into consideration before choosing an alloy. For example, temperatures around oil drilling operations can rise quickly, regardless of climate. But an oil rig near the equator will likely have different requirements than a rig in Alaska or Russia.
Hastelloy C-276 has a melting point of between 2,415 and 2,500 degrees Fahrenheit, according to Corrosion Materials. As such, it can withstand incredibly high temperatures:
At 2,000 degrees Fahrenheit, it maintains oxidation resistance.
At 1,900 degrees Fahrenheit, it continues to fight pitting, corrosion and cracking.
At 1,600 degrees Fahrenheit, it can still carry loads.
At 1,000 degrees Fahrenheit, it has a thermal conductivity of 11 Btu/ft•h•°F.
Choosing a powerful alloy for your shell and tube heat exchanger is a critical decision to make. Depending on where you are operating, you may have very specific needs. In many cases, Hastelloy C-276 has the ability to withstand the harshest environments while continuing to work efficiently. However, it’s best to speak with a knowledgeable professional before making any concrete decisions.
If you’re looking to invest in equipment that is highly corrosion resistant, durable and long-lasting, give consideration to a nickel-based alloy. For more information about how your operation can benefit from incorporating Hastelloy C-276, reach out to the engineers at Enerquip. These professionals are among a select few in the shell and tube heat exchanger space that have experience incorporating Hastelloy C-276 in process equipment.
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