Stainless Steel boiler tubes - The Home Machinist!

Post by Bill Shields » Tue Aug 28, 8:28 am

Contact us to discuss your requirements of stainless steel heat exchanger tube. Our experienced sales team can help you identify the options that best suit your needs.

Making such a statement just confirms the belief of anyone who may have previously thought you were foolish.



Even first year mechanical engineering students go through the exercises (or used to) of measuring thermal conductivity in the lab.

Heck, I remember doing basic experiments with thermal conductivity in junior high school science class (8-9th grade).

Without such BALONEY, the world as we know it would not exist, so PAY ATTENTION and MAYBE learn something:

Thermal coductivity is measured in the engineering world (by real engineers, not just people who think they know more), in a coefficient called K, defined as:

Heat transfered / feet (thickness of material)*time* temperature differential degrees.

Depending on the units you work with, it is:

1 W/(m.K) = 1 W/(m.oC) = 0. kcal/(hr.m.oC) = 0. Btu/(ft.hr.oF)

It is (as stated) a function of material thickness and to a certain extent, the temperature differential from 'hot' to 'cold' side.

Conductivity DOES NOT become irrelevent once things get up to temperature.

The amount of heat you 'extract' from the gas as is passes through the tubes is still controlled by the K value for the material.

For a given tube configuration and gas flow, the gas leaving a copper tube bundle will always be cooler than the gas leaving a stainless steel tube bundle (unless the copper tubes are VERY THICK - as in INCHES thick).

The K value for different materials is (in the English BTU scale):

COPPER is 200+ -> VERY HIGH

Stainless steel is on the order of 17 (depends on the grade) - considered VERY LOW

Carbon Steel is 43 (varies a bit with grade)

Duplex Stainless 8 - 10 depending on the temperature differential, but still VERY LOW. (From Sandmeyer steel, Philadelphia and a supplier in the UK). If you make this stuff 1/2 the thickness of 304 / 316L, you get it's K value UP to that of 'normal' stainless steel, but still nowhere near steel or copper.

Since something like 90% of the heat transfer in a loco boiler is from the firebox, the actual choice of tube material is not all that critical. Get the tube sizes / number correct and you will have a good steaming boiler, irrespective of the tube material. HOWEVER, given a choice, I will always go for something that gives me more heat into the water than something that I know provides less heat. Why choose something else?

Would I recommend welding tubes into a boiler? I don't -> but then I have worked with large ASME code boilers and heat exchangers for most of my life, and know what a good, rolled tube is, and the lack of need to weld (except for a seal weld in a boiler).

I prefer to roll and use copper since it is pretty much a lifetime installation.

Getting a welded tube (of any material) OUT of a boiler or exchanger is a major problem, and usually requires a LOT MORE work than removing a rolled tube. For people who weld, this may not be an issue, but for many, it is (should be) a major concern.

I have own a small set of tube cutters / peelers that allow me to remove copper or steel tubes from a boiler / exchanger without damaging the tube sheet. SOMETIMES this allows retubing of a boiler without major surgery.

Note that I am not getting into the question of safety and corrision pitting / cracking with these materials. While it is a documented problem, I really don't think that it is a big deal with our small boilers who may only run a few hours in their life.

Would I confidently / comfortably ride behind your loco with the SS tubes?

Sure, why not?

Even if there is a crack, the likelihood of there being a major failure where someone was hurt is less than worrying about being hit by a meteorite. If / when the tubes go bad, you and I will most likely long be pushing up daisies and your son can worry about it (hope he can weld).

As for Doug A. and JFN's book chapter regarding stainless steel boilers, I can only comment that:

1> I wish that chapter had never been written / published
2> Just because it WAS written / published, does not make it correct from an engineering standpoint. When I showed the chapter to my father, who was a lifetime boiler expert / consultant to Hartford Steam Boiler, only commented "hope he runs it on distilled water".
3> It is interesting that despite all the claims made for the material in that chapter, nothing is stated regarding the cracking problem, which was well known by the ASME at the time (I was taught about it in engineering school, so I know it was general knowledge).

Well Dave, here is some of the BALONEY that the entire engineering world has been operating on for the past 200 years.Making such a statement just confirms the belief of anyone who may have previously thought you were foolish.Even first year mechanical engineering students go through the exercises (or used to) of measuring thermal conductivity in the lab.Heck, I remember doing basic experiments with thermal conductivity in junior high school science class (8-9th grade).Without such BALONEY, the world as we know it would not exist, so PAY ATTENTION and MAYBE learn something:Thermal coductivity is measured in the engineering world (by real engineers, not just people who think they know more), in a coefficient called K, defined as:Heat transfered / feet (thickness of material)*time* temperature differential degrees.Depending on the units you work with, it is:1 W/(m.K) = 1 W/(m.oC) = 0. kcal/(hr.m.oC) = 0. Btu/(ft.hr.oF)It is (as stated) a function of material thickness and to a certain extent, the temperature differential from 'hot' to 'cold' side.Conductivity DOES NOT become irrelevent once things get up to temperature.The amount of heat you 'extract' from the gas as is passes through the tubes is still controlled by the K value for the material.For a given tube configuration and gas flow, the gas leaving a copper tube bundle will always be cooler than the gas leaving a stainless steel tube bundle (unless the copper tubes are VERY THICK - as in INCHES thick).The K value for different materials is (in the English BTU scale):COPPER is 200+ -> VERY HIGHStainless steel is on the order of 17 (depends on the grade) - considered VERY LOWCarbon Steel is 43 (varies a bit with grade) Duplex Stainless 8 - 10 depending on the temperature differential, but still VERY LOW. (From Sandmeyer steel, Philadelphia and a supplier in the UK). If you make this stuff 1/2 the thickness of 304 / 316L, you get it's K value UP to that of 'normal' stainless steel, but still nowhere near steel or copper.Since something like 90% of the heat transfer in a loco boiler is from the firebox, the actual choice of tube material is not all that critical. Get the tube sizes / number correct and you will have a good steaming boiler, irrespective of the tube material. HOWEVER, given a choice, I will always go for something that gives me more heat into the water than something that I know provides less heat. Why choose something else?Would I recommend welding tubes into a boiler? I don't -> but then I have worked with large ASME code boilers and heat exchangers for most of my life, and know what a good, rolled tube is, and the lack of need to weld (except for a seal weld in a boiler).I prefer to roll and use copper since it is pretty much a lifetime installation.Getting a welded tube (of any material) OUT of a boiler or exchanger is a major problem, and usually requires a LOT MORE work than removing a rolled tube. For people who weld, this may not be an issue, but for many, it is (should be) a major concern.I have own a small set of tube cutters / peelers that allow me to remove copper or steel tubes from a boiler / exchanger without damaging the tube sheet. SOMETIMES this allows retubing of a boiler without major surgery.Note that I am not getting into the question of safety and corrision pitting / cracking with these materials. While it is a documented problem, I really don't think that it is a big deal with our small boilers who may only run a few hours in their life.Would I confidently / comfortably ride behind your loco with the SS tubes?Sure, why not?Even if there is a crack, the likelihood of there being a major failure where someone was hurt is less than worrying about being hit by a meteorite. If / when the tubes go bad, you and I will most likely long be pushing up daisies and your son can worry about it (hope he can weld).As for Doug A. and JFN's book chapter regarding stainless steel boilers, I can only comment that:1> I wish that chapter had never been written / published2> Just because it WAS written / published, does not make it correct from an engineering standpoint. When I showed the chapter to my father, who was a lifetime boiler expert / consultant to Hartford Steam Boiler, only commented "hope he runs it on distilled water".3> It is interesting that despite all the claims made for the material in that chapter, nothing is stated regarding the cracking problem, which was well known by the ASME at the time (I was taught about it in engineering school, so I know it was general knowledge).

Stainless steel is a commonly used material in applications ranging from medical instruments or chemical storage to transport or power generation because of its high corrosion resistance, hygiene, and strength. While there are more than 3,500 grades of steel, not all stainless steel grades are created equally.

With so many options, how can you ensure you&#;re selecting the appropriate grade for your specific needs?

1. Choose the Right Metal for Your Operating Environment

To determine which grade of steel will uphold best in a given environment, think about the conditions your final product will face. Extremely low pH, high stresses and high temperatures, and crevice corrosion negatively impact stainless steel performance. Steels in the austenitic T3XX series, like the common types 316 and 304 alloys, retain their strength, toughness, and corrosion-resistant properties over the broadest temperature range.

Corrosion resistance is the main reason for choosing austenitic stainless grades. Type 316, with its molybdenum addition, even resists chloride ions found in marine and chemical processing applications. With any steel grade, high-quality structural design is the best defense against corrosion.

2. Prioritize Strength, Ductility, and Toughness

Next, consider these three top mechanical qualities:

• Strength: The stress a metal can withstand before it fractures or deforms
• Ductility: The ability of a material to have its shape changed, such as being drawn out into a wire or thread, without losing strength or breaking
• Toughness: The metal&#;s ability to deform and absorb energy before fracture

Stainless steel contains 10&#;30% chromium as its alloying element, which is what helps it resist corrosion. The nickel addition in austenitic grades provides the highest toughness and ductility among stainless grades. Grades high in chromium, molybedenum, and nickel are the most resistant to corrosion.

Alloy content is not the only aspect to consider when choosing a grade of stainless steel; the material&#;s processing also affects the mechanical response. The duration of time steel is held at different temperatures as part of its cooling process, as well as the total speed at which it is cooled can affect its overall quality.

Related links:
What is the introduction of steel made structure?

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While the hardness of carbon steels can be increased by heat treatment, austenitic stainless is hardened by cold working operations like rolling, bending, swaging, or drawing at temperatures below the recrystallization temperature. Be aware that increased hardness by cold working operations decreases other properties like elongation and impact resistance.

3. Factor in Form and Process

Austenitic stainless steel is widely available in bar, wire, tube, pipe, sheet, and plate forms; Most products require additional forming or machining before they can be used for their specific application.

Stainless steel tubing, for instance, may need bending or coiling, re-drawing, machining, welding, or end forming. If your stainless steel will see machining processes like CNC machining, drilling, reaming, bevel cutting, chamfering, knurling, or threading, choose a machining rate that mitigates the risk of work hardening or select a &#;free-machining&#; grade containing sulfur.

When welding any stainless steel parts, embrittlement in the weld area is a top concern. Choose a lower carbon grade like 304, 304L, or 316L to reduce carbide formation.

4. Consider Your Customers&#; Preferences

Many designers choose stainless steel for its aesthetic appearance, whether that appearance is a shiny, electropolished &#;bright&#; finish, a dull &#;pickled&#; finish, a matte surface polished to a specific RMS, or a light-absorbing black oxide coating. Austenitic stainless steel grades can take any of these finishes plus the common addition of passivation.

Customers may also need certification for application-specific specifications. For instance, ASTM A213 and A249 should be used for boiler, superheater, and heat-exchanger tubes, while ASTM A908 should be used for hard-drawn austenitic stainless steel industrial needle tubing. There are more than 12,000 ASTM standards, and each addresses a specification so customers know the technical standards tested for chemical composition, heat treatment or temper, and other physical and mechanical attributes.

5. Manage Material Cost and Availability

Although high-performing austenitic stainless steels are the most expensive stainless steels upfront, they are well worth the investment. Choosing a corrosion-resistant material well-suited to its application reduces maintenance, downtime, and replacement costs. Life-cycle costing methods can quantify current and future costs and create an &#;apples-to-apples&#; comparison of different materials.

Selecting a Stainless Steel Supplier

At Eagle Stainless we can help you prioritize your material requirements and guide you toward choosing the perfect stainless steel grades for your application. With quality management certifications in place since , our commitment to quality is evident at every step of the process and designed to help you make the best steel selection for your industrial application.

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