Moisture Is Bad For Your Electric Heater. What You Can Do About It

Moisture can have a very negative effect on process heaters and their applications, sometimes leading to violent heater failure. It’s important to know effective solutions to safeguard your equipment, as discussed in the latest Ask Ian. 

A typical heater won’t fare well in a moist environment. They’re not normally built to withstand the ingress of water or other liquids. The standard seal on most heaters is porous, and most high-temperature lead wire insulation may actually wick moisture into a heater. This applies to any type of heater. 

In order to prevent the ingress of water, whether in liquid or vapor form, a moisture seal is required for both the heater itself and the lead wire insulation. An obvious solution for the lead wire insulation is to avoid mica or fiberglass insulation and switch to Teflon, PVC, or rubber (silicone) insulation. While these alternatives don’t have temperature ratings as high as mica insulation, with careful routing of your wires, they can be effective. Additional lead wire protection, such as stainless steel armor, can help prevent wires from touching hot spots in the application, but care must still be taken. 

Be aware that not every type of lead wire insulation can be used on every type of heater we make. Most lead wire insulations can be built into a cartridge heater, but it’s much more difficult to do so with certain band heaters because it’s challenging to keep the lead wire termination area cool enough for the lead wire insulation material to survive. When it comes to the heater itself, you don’t want a porous material that will allow moisture to enter. Typical seals like cement or some other ceramic and mica or fiberglass insulated leads are used by default because of their high temperature rating. 

Alternate materials that are good in moisture situations are epoxy, RTV, or Teflon. In the cases of epoxy and RTV (which is a silicone compound), these materials are poured into the open end of the heater around the leads as thick liquids and allowed to set and harden. Teflon can be swaged into high-temperature cartridge heaters, providing its seal that way. Again, be aware that not all seals can be used on all types of heaters, so be sure to inquire if what you’re looking for is possible. 

As stated earlier, the temperature ratings of all these sealing materials are not as high as the default mica insulated leads with a cement seal. When a sealed heater is required, there needs to be an unheated section or length at the seal area so that the seal won’t get too hot. Too much heat can degrade the seal, defeating its purpose. 

 Here are the temperature ratings of the materials mentioned: 

 LEAD WIRES:  • Mica Insulated Leads: 450°C (≈840°F)   • Teflon Leads: 200-250°C   • PVC Leads: 105°C  • Rubber Leads: 90°C 

SEALS: • Cement: 2600°F (≈1425°C)  • Teflon: 500°F  • RTV: 400°F  • Epoxy: 300-450°F

One of the most surprising places for moisture to leak into a heater is along the inside cover of a lead wire, be it Teflon, PVC, or rubber, where the moisture travels between the conductor strands. It sounds unbelievable but it’s fairly easy to demonstrate. Take a piece of Teflon insulated wire, of any gauge, cut it to about 6” long, bend it in half, and hang it over the wall of a container of water so that one of the cut ends of the wire is submerged. The other end should be allowed to hang in open air outside the container. Leave it undisturbed for a few months and eventually, you’ll see small crystals starting to form on the end of the wire that’s outside the container. Those crystals form from moisture traveling between the conductors of the wire and evaporating at the other end of the wire. In most situations, the heat produced during use keeps this moisture away, but be aware that it is a possible moisture path. 

Besides the lower temperature ratings of making a heater resistant to moisture, it’s important to consider if the material is compatible with the environment it will be in. Not all methods of sealing can be used everywhere. For example, using PVC insulated lead wires when they will be exposed to oil is not a great idea. Contact with oil will lead to the removal of plasticizers in the PVC, causing the wire to become brittle and crack. Not good. 

With the careful selection of materials and ensuring they won’t get too hot, it’s possible to make several varieties of heaters resistant to the ingress of moisture. This helps prolong heater life and maintain efficiency, ensuring reliable operation in challenging environments. By understanding the limitations and capabilities of different sealing and insulation materials, you can make informed decisions to protect your equipment and enhance its performance.