Engineering Insights

By Ian Renwick

The words “heat” and “temperature” are sometimes used interchangeably, but they shouldn’t be. They’re quite different and should be thought of as such.

Let’s start with temperature because we should all be familiar with it. If you’re told that something has a temperature of 300°F, you know it’s hot. You know not to touch it, whether it’s the size of a marble or the size of a battleship. Don’t touch something that hot! Temperature is always measured with a temperature scale like °F, °C, K, or some other exotic scale, and is independent of any of the properties of the material that has that temperature.

Temperature refers to the average energy of motion of the particles that make up a substance. It’s an average— and that’s a big point. The higher the average energy, the higher the temperature. You might think of that energy as the average speed of the particles in a substance, whether they’re whizzing around like in a gas, swirling around in a liquid, or vibrating in place in a solid.

Heat, on the other hand, is the total energy of the motion of the molecules of a substance. We’re not talking about averages anymore, but the total of all the energy of the motion of all the particles that make up a substance. The bigger an object, the more particles there are, the more heat energy it holds, and the more heat energy it will take to change its temperature.

Imagine a little cartridge heater the size of your finger creating 500 watts of power. It’s going to get very hot; hot enough to glow, probably. That’s over 1000°F. The same 500 watts coming out of a ceramic strip heater 180″ long wouldn’t even be noticed. The heat of an object is not only dependent upon the size of the object and the number of particles that make it up but also the properties of those particles, like their size and how they’re locked together in the overall object. Properties like Specific Heat and Thermal Conductivity help describe those characteristics so we can calculate how much heat it’ll take an object to get from temperature X to temperature Y.
From a different angle, imagine both heaters at 1000°F. It might take only 500 watts and a few minutes for the little cartridge heater to get to that temperature. The big, long ceramic strip, however, might take as much as 10 to 12 kilowatts and an hour to achieve the same temperature. So, both objects are at the same temperature, but it’s very easy to see which one has more heat.

Understanding the distinction between heat and temperature is crucial, especially when considering their real-world applications. While temperature gives us an indication of how hot or cold something is, heat tells us about the energy required to change that temperature. This distinction is not only fundamental in science but also in designing heaters used in any process that involves temperature changes. By grasping these concepts, we can better appreciate the subtleties of thermal dynamics and how they impact the work that we do.

As the name implies, a temperature controller’s purpose is to
control a heating device so that a desired setpoint temperature (the temperature you want to achieve) is reached and maintained. A few examples of temperature controllers around your home are your central heating, refrigerator, freezer, oven, and water heater. These may be of the PID variety or could just be simple on-off controllers, otherwise known as thermostats.

For the purposes of this document, we’ll use the example of a
temperature controller being used to control heating something to a certain temperature. A PID controller is more advanced than an old-fashioned wall-mounted thermostat and performs more calculations when determining how to control a process.

In the name ‘PID’, ‘P’ stands for Proportional, ‘I’ stands for Integral, and ‘D’ stands for Derivative. Does that bring anyone back to calculus class? They are all calculated to determine how to adjust the temperature of a system in order for it to achieve the setpoint temperature and hold it there. Those variables and how they are adjusted are dependent upon the current temperature being measured, the setpoint temperature, and the current rate of change of the temperature. More on that later.

A temperature controller has a temperature sensor built into it or attached to it (to sense the temperature elsewhere, away from the actual controller), that senses the temperature of the surroundings or of the process/application being heated. In the controller interface, it also has a place for the desired setpoint temperature to be entered. The difference between these two numbers determines what the controller will do, as in, turn a heater on or off.

The proportional section of a temperature controller is the easiest to understand. If the current temperature is below the desired setpoint, it turns the heater on so things can get hotter. If the current temperature is above the desired setpoint, it turns the heater off. That’s the behavior of an on-off controller when it just compares two numbers and turns things on or off. Pretty basic. That’s how an old mechanical thermostat works. There’s an issue with that type of control. As the temperature of the system approaches the setpoint temperature, the distance between the two (called the error) will decrease in size, meaning that the controller will call for less and less heat. The temperature of the system will approach the setpoint slower and slower, never really quite getting there. It will get there within a reasonable margin, but it’ll take a while to get there.

The next two methods of control (Integral and Derivative) are used to help control the temperature better and bring things where they need to be more quickly, but they introduce other issues.

Integral
The Integral part of the control reacts to the difference between the actual and setpoint temperatures as well as the time that they’re separated based on the past history of the system. You could say that this part of the control gets the system to temperature faster by looking at this error over time, but it introduces two new issues. One of the errors is overshoot, where the temperature goes beyond the setpoint temperature. The other error is hysteresis, which is a fancy word for bounce or wobble. It occurs when the temperature bounces up and down on either side of the setpoint temperature over time, which can take a while to settle down.

Derivative
The Derivative part reacts to the rate at which the other two parts of the control cause the temperature to change, regardless of how close or far away the actual temperature is from the setpoint. This can be restated as a reaction to the steepness of the temperature climb and making sure it doesn’t get too out of hand. The derivation part of the control watches the rate of change of the error of the system. It does impede the rate of change of the process (slows it down), but with the other proportional and integral components of the control, it balances out nicely, thereby reducing the overshoot and hysteresis.

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:

When it comes to describing cartridge heaters, some details can be more complex than others. This article aims to resolve any difficulties you may encounter when describing the length of heaters with elbows, right-angle leads, bushings, or other terminations.

Here’s the bottom line: If you can see the sheath, measure it, and nothing more!

Specifying the length of a cartridge heater with leads protruding straight out of the end hardly requires discussion. The heater length is simply the heater length – it couldn’t be simpler.

But what if you add a bushing? Do you include the threaded bushing length in the measurement? How about the hex head? The answer to both questions is no. Even for a center hex bushing, the principle remains the same. Measure only the visible sheath length up to where it enters the threads – this is what you specify as the insert length. There’s no need to specify anything else regarding the heater length.

The same principle applies to heaters with a right-angle copper elbow. Don’t concern yourself with the length of the copper elbow; focus solely on the visible sheath length. Once again, you’re specifying the insert length of the heater.

For heaters with a right-angle lead wire exit, where the leads emerge through a hole in the side of the heater sheath, things become slightly more complicated, but not overly so. In this case, specify the overall length of the heater from one end of the sheath to the other. The position of the hole for the lead wire exit determines the insert length of the heater, which you don’t need to specify. However, if you choose to specify the insert length, ensure it’s consistent with the typical value. For instance, a 5/8” diameter heater that’s 6” long (overall) would typically have a 5 3/8” insert length.

There’s one exception to the rule: heaters with stop rings or flanges. In such cases, it’s advisable to specify both the insert length to the underside of the flange and the overall length, especially if there are clearance issues in your application.

In any case, if you’re unsure about specifications or if you have any questions, don’t hesitate to contact us. We’re here to ensure you get the right heater for your needs.

Please refer to the image below for visual clarification on the concepts discussed here.

One of our readers recently wrote in to ask why heaters have an upper temperature limit. Great question. While there are many factors that determine a heaters upper temperature limit, understanding temperature limitations in a heater is essential for maximizing heater performance and ensuring safety.

Electric heaters are limited in temperature by the properties of the materials they’re built from. Materials’ properties change with temperature, and they can only take so much abuse (heat) before their properties are not able to behave in the manner intended. There are two types of changes in temperature that happen to materials in the way of degradation: excessive oxidation or dielectric breakdown. Very quickly put, the metallic parts of a heater fail due to oxidation. That’s when materials either break, as in snap or separate, or allow a path for electricity to get from the heating circuit to a component that should be electrically insulated, like the heater sheath. Dielectric breakdown occurs when an insulating material does not insulate as it once did, resulting in a short from the circuit to the sheath or between live components.

A lot of the design of a heater has to do with preventing electricity from flowing where it shouldn’t, like from the circuit to ground. That’s one definition of heater failure. It’s one thing to burn yourself from touching an energized heater (silly person), but it’s another altogether if you were to get electrocuted. That’s why there are safeties in place in our homes and businesses to protect us, mostly as ground-fault circuit interrupters (GFCI) or Residual Current Device (RCD).

The dielectric strength of an insulating material, which is its ability to prevent electricity from flowing through it, decreases with temperature. That’s just the nature of the beast and there’s nothing we can do about it. We’re talking about ceramic materials, magnesium oxide powder which is used as an electrical insulator in cartridge heaters, and mica or aluminum oxide used in band heaters. Insulating materials don’t become full-on conductors when they get too hot but they can allow too much current to flow through them, resulting in a short, usually to ground. That leakage current can initially be as small as a few milliamps, but it is still dangerous. That decrease in dielectric strength at higher temperatures is something that limits the upper temperature of a heater. The change in the insulating properties of a material depends on the material itself, often expressed in volts/mil, and its temperature.

Oxidation occurs when oxygen in the atmosphere combines with metal components altering the material. A shiny new nail that’s been exposed to the elements over many years becomes an old rusty nail. That rust is the oxidation the nail has been subjected to where the iron in the nail turned into iron oxide, giving it that characteristic dark orange rust color. Iron oxide is not iron and it doesn’t have the same properties as iron. If you were to hammer an iron nail and a stainless steel nail into a fencepost and leave them alone for several years you would return to find that the iron nail had rusted and was darker in color and that the stainless steel nail was as shiny and new looking as the day it was placed there. That’s great. Stainless steel doesn’t rust. Or does it? At room temperature stainless steel doesn’t rust. Get it hot enough though, and it will indeed oxidize and degrade. Oxidation occurs faster at higher temperatures, even with materials that are designed to resist oxidation, making things challenging for electric heating elements. There are other, even higher oxidation-resistant materials like Incoloy and Inconel that oxidize too, but it takes a few more hundred degrees (°F) to get them to oxidize. That shady straw color that appears on a heater even if it’s only reached 500 or 600°F is a thin oxide layer. At that level, things aren’t too bad. It’s when you get to the 1100°F or 1200°F range (and above) that things start to get a bit dicey. Oxidation on resistance wire can cause it to break. Oxidation on resistance wire and the inside of a heater sheath can create a conductive path between the two. The first failure mode occurs when the oxidation degrades the integrity of the resistance wire, allowing it to break. The second failure mode occurs when the oxide layer gets so thick, both growing inward from the sheath and outward from the resistance wire, that the two surfaces get close enough to each other to allow electricity to flow. That’s a failure to ground.

Oxidation is not always bad. It does provide an airtight seal over the material that was originally oxidizing, thereby preventing it from oxidizing any further. The oxide actually becomes an airtight protective layer. If a heater is left on and held at a fairly constant temperature it’s a lot better than cycling it between hot and cold many times. With cycling, that oxide layer can crack, exposing the inside unoxidized material to air, meaning it will oxidize further when it next gets to a high temperature.

There are many varieties of resistance wire alloys, and many of them are developed for resistance to the formation of oxides. Chromium and aluminum are common alloying agents that create tough oxide layers that are less prone to cracking from thermal cycling.

The best thing to do in order to lessen the occurrence of these types of failures, be it from dielectric breakdown or oxidation, is to not operate a heater as hot as it can. A heater in a large heatsink in a tight fitting hole or clamped tightly around a large mass (and retightened once at temperature to take up the slack from thermal expansion) will help pull heat away from the heater quickly, getting it to where it needs to go away from the heater and putting less stress on it. As counterintuitive as this may sound, one of the worst things for a heater is heat. Operating a heater as coolly as possible will prevent early heater failures as discussed here and prolong heater life.