Cartridge Heaters in Liquid Heating Applications – Immersion and Flow-Through Systems

Cartridge heaters are best known for heating solid metal cavities, but they are also used a lot to heat liquids. Immersion applications, like industrial parts washers, laboratory water baths, and food processing equipment, put special demands on cartridge heaters. The heater must move heat into a liquid quickly while also avoiding corrosion, contamination, and unsafe working conditions.

In immersion applications, a cartridge heater is usually put into a thermowell or directly into the liquid. The thermowell keeps the heater from coming into direct contact with corrosive or abrasive fluids while still letting heat pass through the wall. If the heater is going to be directly immersed, the sheath material must be able to work with the fluid. Stainless steel 316 is a frequent choice for water and mild chemicals. For harsh chemicals or fluids that are very hot, Incoloy® is the best choice. For food use, the sheath must be easy to clean and meet health regulations.

In immersion applications, the watt density needs to be lower than in solid metal applications. If the watt density is too high, localised boiling can happen in liquids, which have lesser heat conductivity than metals. When you boil something, it makes a layer of vapour that keeps the heater warm, which makes the sheath temperature rise. Film boiling is a condition that can ruin a heater in a matter of minutes. For water immersion, a safe watt density is usually between 5 and 10 watts per square centimetre. For oils, which don't carry heat as well, the safe range is 3 to 5 watts per square centimetre.

Another set of problems comes up with flow-through systems. In some cases, a pipe or manifold holds the cartridge heater, and liquid runs past it. The circulating fluid helps heat transfer, which lets you use larger watt densities than static immersion. But changes in flow can cause complications. If the flow stops while the heater is on, the liquid around it can get too hot and stay that way. High-limit protection flow switches or temperature sensors are important safety features.

The end of immersion heaters must be kept dry and safe from water and splashes. Even a small amount of moisture that gets into the lead exit can soak into the MgO insulation and cause a short circuit. Sealed terminations, which are generally made of potting compound or moulded rubber boots, should be used on heaters for immersion applications. The leads should be able to handle the temperature and any chemicals that might be in the air. If the heater is above the liquid level, the leads can be routed up to keep the liquid from running down into the termination.

Experience reveals that scale and fouling are prevalent issues when heating liquids. Mineral deposits form on the heater's surface when the water is hard. These deposits insulate the heater, which makes it run hotter because they stop heat from moving. The heater eventually breaks down because it gets too hot. To keep scale from building up, you need to clean or treat the water on a regular basis. Heaters with polished sheaths or non-stick coatings don't let deposits build up in applications where scaling is likely to happen.

If you use cartridge heaters to heat liquids, the most important thing is to match the heater to the fluid and the flow conditions. A heater that works well in a system with moving water could not work at all in an oil tank that isn't moving. To make a heater last a long time, you need to choose the right sheath material, limit the watt density, protect the termination, and keep an eye out for scale. When these things are taken care of, cartridge heaters can heat a wide range of liquids quickly and reliably.