Avoiding Common Installation Errors with Fastened Cartridge Heaters

In maintenance departments, a common scene is when a cartridge heater that has been installed fails soon after it is put in. A nut or a flange held the heater in place. The screws were tight. But the heating still broke down. People typically think that the heater was broken. But in most cases, the true problem is a mistake made during installation that might have been avoided. Knowing about these typical faults will help make sure that your heaters last as long as they should.

Overtightening the fastener is one of the most common mistakes. If you turn a nut too hard, it can change the shape of the heater body. The distortion may not be noticeable to the naked eye, but it puts stress on the heater. The magnesium oxide insulation can break, the resistance wire can move, or the sheath can get a little bulge. Any of these problems can cause localised overheating and early failure. The right way to do it is to tighten the nut to a certain torque number, not as tightly as you can. A torque wrench makes sure that things are always the same. If you don't have a torque specification, a reasonable rule of thumb is to tighten the heater until it fits snugly and then turn it a quarter turn.

Another typical mistake is not lining up the heater and the cavity properly. People typically think that a heater that is fastened would self-align, but it won't. The fastener pushes the heater into a bent position if the cavity isn't precisely straight. The heater may be straight when it is outside of the cavity, but when it is bolted down, it bends to fit the angle of the cavity. This bending puts stress on the parts inside. Before putting it in, use a gauge pin to check the straightness of the cavity. If the cavity is bent, it needs to be fixed or the heater will break, no matter how well it is attached.

The third mistake has to do with where the fastener is in relation to the chilly zone. The fastener should be in the part of the heater that isn't heated, usually within 20 to 30 millimetres of the lead exit. If the fastener is put over the hot zone, it will get hot. The metal in the fastener can get softer, which makes it less strong. The fastener can also operate as a heat sink, pulling heat away from the heater and making a cold spot. You should talk to the manufacturer to make sure the fastener is in the right place.

Using the improper type of fastening material is the fourth mistake. A regular stainless steel nut on an Incoloy® heater that gets really hot will gall and lock up at temperatures over 400°C. You can't get the nut off without cutting it. Sometimes, the nut comes free or cracks because the variable thermal expansion rates. The material of the fastener and the heating sheath should be the same or work well together. Incoloy® or Inconel® fasteners are needed for high temperatures.

A fifth mistake is not paying attention to the leads when you tighten the fastener. As you turn the nut, the leads can get twisted or pinched. A lead that is twisted puts stress on the crimp connection within. A pinched lead might get hurt on the inside without showing any signs. When tightening, the leads should be kept out of the way. Check the leads for twists or kinks after tightening them. A service loop in the lines helps relieve strain and keeps tension from getting to the heater.

Experience suggests that the sixth mistake is omitting to put anti-seize compound on the threads. If you don't use anti-seize, threads can gall, especially when connecting stainless steel to stainless steel. Galling makes the surface rough and jagged, which traps the threads together and makes it almost impossible to take them apart. Nickel-based or ceramic-based paste is an example of a high-temperature anti-seize compound that stops galling and makes it easy to take apart again. Before putting it in, it should only be used on the threads.

To avoid mistakes when installing affixed cartridge heaters, you need to pay attention to torque, alignment, the position of the fasteners, the compatibility of the materials, lead protection, and the use of anti-seize. A heater that is properly secured will stay in place, move heat well, and last for thousands of hours. One that is put in with any of these typical faults will fail early. Taking a few extra minutes to get the installation right will save you time and make your heater last longer. In important situations where every minute of output counts, taking the time to carefully place attached heaters is worth it.