Selecting the Right Sheath Material

There are many different sheath materials available. The most important factor is the material or fluid that will be in direct contact with the heater. In many situations, different sheath materials could be used. But in some instances, you don’t have a choice to what sheath material needs to be used. (Example: Hydrofluoric Acid requires PTFE “Teflon” coated heat elements) If the specs calls only for stainless steel, make sure which one (Example: 304, 310, 316 or 321; see below) you need.


Applications
Stainless steel sheath materials:

1. 304 stainless steel is used in food cooking applications. It should not be used in corrosive environments where the sheath temperature is above 900°F. It is less costly than Incoloy. It is not recommended to be used in water heating as it will corrode
2. 316 stainless steel and Carpenter stainless steel: It generally has the same features as 304 material but because of its molybdenum content it is more corrosive resistant to some chemicals. It is used for corrosive immersion heating.
3. 310 stainless steel (better corrosion resistance at higher temperatures, fossil fuel gases)
4. 321 stainless steel: similar to 304 SS, but contains Ti, well suited to long exposure to air and combustion like jet engine parts, aircraft exhaust & manifolds.

Other sheath materials used:

Aluminum:
Mainly used for comfort heating. Fins are used to increase surface area. Maximum recommended watt density in air is 30 WSI with fins, 6 WSI without fins.
Inconel 600
This material was the most widely used for air heating applications, but has been replaced by Incoloy 800 because of its cost. It still finds use in areas of extremely high temperatures 1700°F + and corrosive environments. It is composed of nickel, chromium, plus trace elements.

Titanium:
This material is used primarily in immersion heating of highly corrosive materials such as acids. It is the most costly of the sheath materials mentioned.