Designing Heating Elements
The first step in designing a heating element is to understanding exactly how it will be used and in what application. Where and how is a heating element going to be used? That's the first thing to consider when you think about what kind of heating element you need. Some everyday examples of the heating element to be use are:
- A simple coiled element like in a stove-top heater
- Two coils in a ceramic stove plate
- Two coiled elements in a basic space heater with reflectors to "beam" their heat into the room
- Ribbon elements in a hair dryer with a fan to blow their heat forward
- A heated rearcar window is essentially a ribbon-type heating element bonded to toughened glass.
The design considerations in the last type include making sure the element doesn't block the driver's view, sticks permanently to the glass, doesn't damage the glass when it heats up, is powerful enough to melt frost and snow relatively quickly, and can be powered from the vehicle's battery or electricity supply.
All this makes heating elements sound very simple and straightforward, but there are, in fact, many different factors that electrical engineers have to consider when they design heating elements. There are roughly 20 to 30 different factors and parameters that affect the performance and use of a typical heating element, including obvious things like the voltage and current, the length and diameter of the heating element, the type of material used, and the operating temperature of the heating element. There are also many specific factors that need to be considered for each different type of heating element based on their use and application. For example, with a coiled element made of a round metal alloy wire, the diameter of the metal alloy wire and the form of the coils including its diameter, length, pitch, stretch, and so on, are among the many things that critically affect the performance of the heating element. With a ribbon heating element, the ribbons' thickness and width, its surface area, and weight all have to be factored in when designing the heating element.
And that's only part of the story of designing heating elements, because a heating element doesn't work in isolation. Electrical engineers have to consider how it will fit into a bigger appliance and how it will behave during use or when it's used, or abused, in different ways. How, for example, will the heating element be supported inside the appliance by insulators? How big and thick will they need to be and will that affect the size of the appliance being made? For example, think about the different kinds of heating elements you'd need in a soldering iron, the size of a pen, and a large convector heater. If you have an element "draped" between supporting insulators, what will happen to it as it gets hotter? Will it sag too much and will that cause problems? Do you need more insulators to stop that happening, or do you need to change the material or the element's dimensions? If you're designing something like an electric fire with multiple heating elements close together, what will happen when they're used individually and in combination? If you're designing a heating element that has air blown past it, in something like a convector heater or a hair dryer, can you generate enough airflow to stop the element overheating and dramatically shortening its life? All these factors have to be balanced against one another to make a product that's effective, economical, durable, and safe.
Does a heating element need a high or a low resistance?
You might think a heating element would need to have a really high resistance, after all, it's the resistance that allows the material to generate heat. But that's not actually the case. What generates heat is the current flowing through the element, not the amount of resistance it feels. Getting the maximum current flowing through a heating element is much more important than forcing that current through a large resistance. This might seem confusing and counterintuitive, but it's quite easy to see why it is (and must be) true, both intuitively and mathematically.
To perform as a heating element the tape or wire must resist the flow of electricity. This resistance converts the electrical energy into heat which is related to the electrical resistivity of the metal, and is defined as the resistance of a unit length of unit cross-sectional area. The linear resistance of a length of tape or wire may be calculated from its electrical resistivity. As a heating element, tape offers a large surface area and therefore, a greater effective heat radiation in a preferred direction making it ideal for many industrial applications such as injection mould band heaters.
Suppose you made the resistance of your heating element as big as you possibly could, infinitely big in fact. Then Ohm's law (voltage = current × resistance or V = IR) tells us the current flowing through your element would have to be infinitely small, ir., if I = V/R, I approaches zero as R approaches infinity. You'd have a whopping great resistance, no current, and therefore no heat produced. Right, so what if we went to the opposite extreme and made the resistance infinitely tiny. Then we'd have a different problem. Although the current I might be huge, R would be virtually zero, so the current would zip through the element like an express train without even stopping, producing no heat at all.
What we need in a heating element is therefore a balance between the two extremes: enough resistance to produce heat, but not so it reduces the current too much. Nickel-chrome alloy is a great choice. The resistance of a nichrome wire is roughly 100 times higher than that of a wire the same size made from copper which is an excellent conductor, but only a quarter as much as a similar-sized graphite rod which is a fairly good conductor and maybe only a million trillionth that of a really good insulator such as glass. The numbers speak for themselves: nichrome is an average conductor with only moderate resistance, and not remotely an insulator!
We can reach exactly the same conclusion with math. The power produced or consumed by a flow of electricity is equal to the voltage times the current (watts = volts × amps or P = VI). We also know from Ohm's law that V = IR. Eliminate V from these equations and we find the power dissipated in our element is I2R. In other words, the heat is proportional to the resistance, but also proportional to the square of the current. So the current has much more effect on the heat produced than the resistance. Double the resistance and you double the power (great!), but double the current and you quadruple the power (fantastic!). So the current is what really matters. It's easy to calculate that the resistance of the filament in a typical incandescent lamp is a few hundred ohms.
We often refer to electrical heating—what heating elements do—as "Joule heating" or "resistance heating," as though resistance is the only factor that matters. But, in fact, as I explained above, there are dozens of interrelated factors to consider in the design of a heating element that works effectively in a particular appliance. The resistance isn't always something you control and determine: it's often determined for you by your choice of material, the dimensions of the heating element, and so on.
An important characteristic of these electrical resistance alloys is their resistance to heat and corrosion, which is due to the formation of oxide surface layers that retard further reaction with the oxygen in air. When selecting the alloy operating temperature, the material and atmosphere with which it comes into contact must be considered. As there are so many types of applications, variables in element design and different operating conditions for element design comeinto play.
Causes of Failure of Heating Elements
The main causes of failure of heating elements are given below:
1. Formation of Hot Spot:
Hot spots are the points in the heating element which are formed at higher temperature. One of the reasons of formation of hot spot in heating element is high rate of local oxidation that may reduce the element cross-section, thereby increasing the resistance at that spot and produces more heat locally and causing breakdown the element.
The other causes are wrong fuse material, may result in sagging and wrapping of the material.
2. Contamination and Corrosion:
Gases of the controlled atmosphere prevailing in annealing furnace or fumes from flux used in brazing furnaces or oil fumes caused by heat treatment of components contaminated with lubricant contaminate the elements and produce dry corrosion.
Article courtsey of Explain That Stuff