Tubular Heater

Choose the sheath for the medium, not for the price

Most elements that die early were not worn out; they were corroded. A stainless 304 sheath runs for years in ordinary mains water and lasts months in heavily chlorinated water or an acidic solution. Stainless 304 or 321 is enough for water and steam, stainless 321 for hot oil and mazut, Incoloy 800 for high temperatures and corrosive conditions, and copper or titanium for demineralised water and plating baths.

Watt density matters more than total wattage

This is the most common mistake when ordering. Two elements of the same wattage but different lengths have completely different lives. Watt density comes from:

Watt density (W/cm²) = power ÷ (π × sheath diameter × heated length)

Put the diameter and length in centimetres. If the answer is above the safe figure for your medium, you have two options: make the heated length longer, or split the power across more elements. Reducing the power is usually not an option, because then the machine never reaches its working temperature.

Figures used across the industry: circulating water up to about 12, still water around 8, moving air 3 to 5, still air 2 to 4, and oil 1 to 3 watts per square centimetre. Oil is the strictest. Too high a watt density chars the oil onto the sheath, and that carbon layer acts as insulation and cooks the element from the inside.

Keep the cold end out of the hot zone and out of the spray

The two ends with no resistance wire do not heat up. They need to sit so that neither stays inside the hot zone nor has its terminal exposed to steam and splashing. A wet terminal is the single most common cause of earth leakage and a tripped RCD.

Never run an element out of its medium

An element designed for water, switched on in air, burns out within minutes. Water carries its heat away; air does not. On tanks where the level changes, fit a level switch or a protective thermostat. That part costs less than any element it saves.

Bend radius on a formed element

If the element is bent, the minimum bend radius is usually around twice the sheath diameter. Anything tighter packs or thins the magnesium oxide in the bend and makes a hot spot there. A hot spot is where the next failure happens.

Looking after it

In hard water, descaling on a schedule is the main maintenance job. One millimetre of limescale on the sheath cuts heat transfer enough to send the sheath temperature sharply up. If the machine is heating more slowly than it used to, the problem is usually scale rather than a weakening element.

Before any restart, measure the element’s resistance and its insulation resistance to the body. Low insulation resistance means moisture has reached the powder; drying the element in a low oven sometimes brings it back.