This article identifies different types of electric heating elements, highlights important considerations when determining a heater solution and finally looks at some of the less obvious costs. Examples from experience as well as in-house engineering white papers are referenced throughout the article.
What is a Heating Element? A heating element is a component composed of both electrically conductive as well as insulating material, designed to serve a heating purpose. Let's break that down.
Component: A heating element is more than the heating alloy alone. It is an assemblage of parts that includes a framework of insulating material as well as lead connectors. In the case of an open coil heater for example, the heating alloy is typcially held or suspended by a mica or ceramic insulator. Wire terminals safely connect the heater coils to the circuit.
Electrically Conductive: The fundamental core of an electric heater is the heating element alloy within that turns electrical energy into heat energy when subjected to a current. It is the part of a heater where the electrical load occurs. When heat is produced in this way we call it Resistive Heating. It is also known as Joule Heating.
Designed to Serve a Purpose: A heating element is more than its material makeup. It is a product of design. Alloy and insulator must be manipulated to become a useful component that serves a heating purpose. The multi-talented craftsman who determines the alloy and gives the heater it's form is the heater design engineer.
What are they made of?
The material at the heart of a heater is commonly metal in the shape of wire, ribbon or a design etched from a metal foil. A heater may also contain ceramic, plastic or silicone impregnated with a conductor. Choosing the best materials for the job includes a thorough understanding of material properties as well as knowing where to source the best supplies for the particular application.
Metal Wire and Ribbon Alloys
All metal heating elements have physical, thermal, electrical, and metallurgical properties. These material propeties are necessary considerations when choosing the best solution for an application. Temperature dependent differences such as electrical resistance and thermal expansion will vary depending upon the material. Many heater design challenges arise as properties tend to change based on conditions.
Heating elements found in common appliances are made from metallic resistance alloys such as Fe-Cr-Al and Ni-Cr(Fe). They have the ability to produce temperatures hot enough to get the element to glow red hot, in the neighborhood of 1112°F (600°C) and above. Heaters that operate below this range can be made from a much wider range of materials. Elements such as Copper, Nickel, Aluminum, Molybdenum, Iron and Tungsten as well as alloys containing combinations of these elements are used.
Resistance heating alloys contain varying proportions of chemical elements depending upon the wire you order and who makes it. A Nickel base alloy we commonly use is 80 Ni, 20 Cr (80% Nickel, 20% Chromium). The proportions of its composition are different from those of 60 Ni, 16 Cr (60% Nickel, 16% Chromium), The different properties these two alloys exhibit are significant. A clever engineer capitalizes on material properties to achieve better efficiency, reliability, performance, cost and safety in your application.
A specific resistance alloy from one heating element manufacturer will not necessarily exhibit the same properties when supplied by a different manufacturer. These seemingly similar products may have trace elements in addition to their namesake elements, which can drastically affect alloy properties.
Trace elements come in two varieties, contaminates and enhancements.
- Contaminates have an undesirable effect such as shorter life and limited temperature range.
- Trace element enhancements are added on purpose by the manufacturer. Improvements include increased oxide layer adhesion, greater ability to hold a shap and longer wire life at higher temperatures.
An experienced design engineer will compare alloy properties, filter through tradeoffs and arrive at the best heating element alloy and material dimensions for the job. He will then work with production to manipulate the material into a shape and orientation that produces the best outcome for your application. A good custom heater shop is going to understand how wire and ribbon alloy producers stack up. They have their eye on the market, good vendor relationships and favorably negotiated material prices.
Flexible Thin Film Heaters
Flexible thin film heaters (also known as flex heaters) are surface heaters that contain a thin foil of metal on either one surface or embedded inside of multiple pieces. The metal foil is etched by way of a chemical process to create the heating element shape. Flexible heaters have great dialectic strength and are resistant to many chemicals. As the name suggests they are flexible and can bend. Tutco-Farnam offers two types of flex heater, Silicone Rubber and Polyimide Film.
Silicone rubber heaters contain one or more heating elements sandwiched within two pieces of vulcanized silicone rubber. The rubber is electrically insulating yet thermally conductive. The elements within are thin film alloys that have been etched. They can also be made with wire though this is increasingly less common. Silicone rubber heaters are durable and versatile products offering up to 30 watts per square inch with temperatures up to 250°C. They can be cut into any shape. Their ability to flex makes them suitable for many applications with curved and oddshaped surfaces. You can learn more about manufacturing silicone rubber heaters in our Engineer Talk whitepaper.
Polyimide heaters aka. Kapton are similar to silicone rubber heaters in that they contain thin etched foil heating elements. While their max temperature limit is a bit lower than silicone rubber they have the ability to dial in very precise temperatures and can do so rapidly. Their ultra-thin profile is appealing for applications in electronics, optics, laboratory, medical, aerospace and anywhere things need to be very small and lightweight.
PTC Heaters (Positive Temperature Coefficient Heaters) contain trace amounts of electrically conductive material such as carbon black mixed with an electrically insulating yet thermally conductive material such as silicone rubber. Two leads buried within this material do not physically touch. The ratio of electrically conductive vs electrically insulating material is carefully controlled during manufacturing. The most notable property of PTC heating elements is that they increase in electrical resistance as they get hotter. Engineers make good use of this property by designing PTC heaters to cap-out at a specific temperature and thus become self limiting.
A PTC air heater designed for aircraft environmental control. It contains several PTC heating elements in one package. Temperature limiting means it can never get too hot.
A PTC heater piggybacks this silicone rubber heater acting as a small temperature limiter without the need for bulkier control options. The small evaporator heater design saves the client cost and valuable space within their enclosure.
A cartridge heater contains an electric coil surrounded by an insulating powder (typically magnesium oxide) and packed within a tapered sheath. This type of heater is commonly inserted into a cylindrical hole. The size and shape of the bore as well as the heating element is extremely important. There must be a tight uniform fit in order to achieve safe and efficient conductive heat transfer. In some cases a cartridge heater is used to heat a fluid and may be equiped with fins. They can also be made in shapes other than round such as square and triangular.
A strip heater is a relatively flat rectangular heater made from a strip of mica that has been wrapped with ribbon wire. That assembly is sandwiched between two more pieces of mica and then encased in a metal sheath. Many terminal styles exist for this heater.
If you bend a strip heater into the shape of a ring then you have a band heater. They are clamped around pipes, barrels and the base of kettles. They are used to heat fluids and to assist in the melting of solids. The later is very common in the plastics processing industry where plastic pellets must be warmed to a sufficient temperature. This does not in itself melt the plastic but prepares the material for the mechanical process that actually does the melting. Sufficient heat is required for the retraction of the auger.
Determining a Solution
In order to arrive at a heating solution ideally suited for your particular application, it is useful to understand how a heater will fit into and support a larger system. When engaged in design discussions with a client we ask questions in order to understand the application and to build tenable requirements from which we make design decisions. Initially we’ll want to know some of the more fundamental requirements.
- What are we heating? A fluid? A solid?
- What is the max operating temperature?
- What is the available power?
Essentially were defining the problem we inted to solve with the heater. Every project is different and has its own unique heating needs. Decisions regarding dimensions, choice of alloy and overall heater design will be based on your unique project requirements. There may be any number of hidden requirements that will effect the direction of a design so we want to dig deeper whenever possible.
We want to know start and finishing temperatures, flow rates, cycling frequency, ramp speed, peak temperature, electrical power, thermal controls and physical space. Each project will have its own unique application circumstances such as environmental contaminants, tolerances, safety, factory assembly and budget to name a few. When faced with a thorough list of well thought out requirements, proper design choices can progress.
Power & Temperature
The application will need to supply enough power to run the heater. We're going to want to know the available power and any limitations.
We want to know the minimum amount of power necessary for the application to function properly? A heater does not require the same power all of the time. There are moments in time when a heater will require more power than others. We want to know the most power that will ever be demanded of that heater. In some applications max power happens when the heater is fired up and making its way to temperature. In other applications peak is demanded while maintaining an operating temperature. Whichever one of these is higher is a minimum power requirement.
We want to know how much power is needed to successfully heat the thing we are heating within the required amount of time. We could be heating a block of steel, a box of air, a tank of oil or water flowing in a pipe. Each of these scenarios is simple to estimate if you are willing to forgo some precision. You can see calculations with examples in Ian Renwick’s Engineer Talk article, How Much Wattage do I Need? Jerry Sain addresses this specifically for heater coils in his paper, Heater Coil Design.
Our heater design will not only need to safely and reliably handle the power required but it also needs to deliver the heat. We can narrow down our material and dimension choices for many heater shapes with textbook heat transfer calculations. An approach for estimating the temperature produced by a spiral wound wire coil in a flow of gas is less obvious. Dexter Diepholz outlines an approach for this in his Engineer Talk paper, Estimating the Resistance Wire Temperature for an Open Coil Heating Element.
Watt Densiy is another useful way to quickly compare materials. Measured in Watts/in2 or Watts/mm2 watt density is the total watts of the heating element divided by the surface area that generates the heat. You can learn more in Dexter's white paper, Why Does Watt Density Always Come Up When Discussion Heating Elements?
Heating Elements and their Environment
Different materials will react differently depending upon their environment. It is useful to know if there will be a high concentration of a particular gas, significant humidity or alloy harming contaminates in the space where the heating element is being energized.
Ammonia, sulfur, zinc, chlorine and boron will bring an early end to a heater with a poorly matched alloy. For example, chloride contaminates are generally bad for Iron base alloys while sulfides are harmful to Ni-Cr. Process air, industrial cleaners, municiple water supply and even oil from an installers fingers may be a source of alloy eating contaminates.
You can learn more about extending heater life in Patrick Laws Engineering Talk article, Watts Killing your Heater.
Designing for Preexisting Equipment
It is common for us to design a heater for equipment that has already been designed or even manufactured. It is also the more limiting option. Any opportunity to be involved early in the product design process is going to result in a better product with a better heating solution at a reduced cost.
The amount of allowable space for the heater as well as the shape of the space is often the culprit. If your product requires an open coil heater yet we can't get proper airflow across the heater coils then that is going to be a problem. Not only does a preexisting product limit your heater design options it can become cost prohibitive or even impossibly difficult to engineer. We have produced thousands of designs and as such we can spot many of the classic pitfalls before they happen.
While early involvement is ideal, we also recognize that such a luxury is not always possible and we are very happy to work with you to engineer a solution at any stage of your design process. Designing innovative heaters to fit difficult requirements is something we have become quite good at doing. The following are projects that showcase our engineering prowess when faced with pre-existing limitations.
- Evaporator Heater
- Project required a submersable heater with temperature regulation to fit a very tiny space.
- Miniature Heat Torch For Blood Analyser
- Precision field-replaceable air heater with 1" profile to heat a flow of .4 lmp to 380°F within 3.5 seconds.
- Patient Warming Blanket Heater
- Tight space, controlled ramp-up and safety were principle concerns for this surgical room product.
- Custom Heating Element for Medical Storage Device
- This retrofit heater merged the mounting brackets for the fan into the heating element housing.
- Internal Heating System for Medical Imaging
- Turnkey solution including element, blower, sensor and controller offers even regulated temperature inside the awkward shape of a CT scanner.
Custom Heating Elements
An off-the-shelf solution is often a first consideration as it will be the easiest short-term path if something suitable exists. This does not mean it will be the best long term value. Product performance, durability and efficiency are costs not necessarily obvious at the time of purchase.
For products with complex requirements, it may be difficult to find an existing heater that makes the grade. A new piece of lab equipment requiring a fast and controlled ramp-up, a high cycling frequency and an unusually shaped space is probably going to benefit from a custom heating element solution. In the hands of experience you have the best opportunity to improve product performance, increase reliability and reduce cost.
The Real Cost
A lower cost alternative can have some expensive side effects such as repair costs and brand erosion. At Farnam-Tutco we have seen OEMs choose the low bidder only to switch over to us later after discovering the TRUE cost of the cheaper heater. Our products are not the most expensive nor are they the cheapest. We strive to provide quality products that have a high lifetime value.
Good choices in design and material will improve heater life while poor material-to-application matches and other bad design choices can result in costly field replacements, product damage, safety issues and unhappy customers.
All resistance heating elements eventually burn out. Oxidation, changes in electrical resistance, damage and deformation are all factors that limit longevity. An experienced heater design engineer can help you avoid classic mistakes and achieve long heater life for your particular application.
Resistance heating alloys form an oxidation layer at higher temperatures. The layer grows quickly at first as the alloy is easily able to interact with oxygen in the air. As the layer grows it becomes a protective layer inhibiting access to oxygen until eventually preventing further oxidation.
The amont that the heating alloy expands when heated (referred to as an alloy's coefficient of thermal expansion) is going to be different than that of the oxide layer. That difference in thermal expansion as well as the strength of adhesion (the oxide layer's adhesion to the alloy) has a strong correlation with a heating elements longevity.
An oxide layer that remains stongly adhered to the alloy without cracking and spawling will continue to protect the alloy. A heating element with a high coefficient of thermal expansion and poor oxide layer adhesion will not last long in an application with rapid low-to-high temperature cycling.
Sometimes lowering the temperature of the heating element is the best solution. In a recent example we met with a client who manufactures warming cabinets. A competitor's heater was causing dramatic failures. Our calculations found the watt density to be higher than advisable. We introduced our crossflow blower style heater where we were able to fit more wire in the same space and lower the watt density. This in turn lowered coil temperature and extended heater life. Good design and attention to detail helped the client avoid those dramatic failures.
Installation & Assembly
The ease in which a heating element integrates with an application affects cost. Difficult and time consuming product assemblies will burdon a manufacturer with labor, unnecessary part inventories and fewer units going out the door. Field installations and replacements will take longer and may require higher skilled workers.
A heating element designed for a particular product should result in superior integration with that product. This will yield better performance as well as faster field installation and product assembly. In some cases additional part costs may be removed too. The following are specific examples where we saved the client cost and hastle with install and assembly.
During a visit with a long time customer we were walking the factory floor. We asked an assembler on the floor how we could make the heaters easier to install. All he could comment on was how much of a pain it was to snake 48” leads through his gantry. And then when they failed, heaters are like light bulbs in that they do fail, this customer did not like having to string the lead wires. To solve this problem we now offer a 3 pin connector directly on most of our heat torches. Easy install and easy replacement.
An Eligant Solution
Farnam has a customer that makes pump houses for the oil and gas industry. Silicone Rubber heaters are used to help the pumps to start in regions where temperatures fall below -40. Their biggest challenge is that they had 480V 3-phase running to it. They had to figure out how to go from 480V 3-phase to 120V single phase on these silicone rubber heaters. They were breaking a leg, daisy chaining heaters and cobbling stuff together. These were not elegant solutions.
Tutco-Farnam replaced it with a 480V 3-Phase silicone rubber heater with a PSA adhesive. No more worrying about changing your voltage. No more daisy chaining and putting multiple heaters on there. One heater to do the trick. Saved the client time on install and it’s a much more attractive solution.
Add-on Saves Time & Hassle
Many of our dehumidification heating units use jumpers for joining together sections of open coil. One particular customer preferred to wire them up in-house. They wanted to be able to do their own custom configurations.
As it turns out, the product assemblers were cutting their own jumpers. This seemingly small step was taking a long time thus slowing down production. The assemblers were not pleased with this extra step either.
Tutco-Farnam offered to produce the jumpers and ship them with a line of heaters that we were already making for the client. We zip-tied a full set of jumpers to each unit with easy accessible for the assemblers. This saved them oodles of time!
The value added was so successful that the inclusion of jumpers is now shipped standard with all of their dehumidification heaters. We added value to an existing product, we saved the client time and the purchasing agent looks like a hero.
Value-Add w/ Packaging
One day while visiting a customer we noticed a shelf being stocked with our heaters. The employee opened the box, pulled out the product dividers and then, one by one, put the heaters on the shelf. On the opposite side an assembler grabbed a few heaters and set them up on the bench for assembly.
Our solution: We made the packaging a smidgen thicker. This small change allows the dividers to stand up with the heaters facing the assembler. This eliminated the step of moving individual heaters to and from the shelf. The entire packaging is placed on the shelf and the assembler can just pull, pull, pull as heaters are needed. At Tutco-Farnam we go the extra mile to create value for our customers. That is what we call a Higher Degree of Performance.
Cost Savings Retrofit
A customer of ours was using an old screw flange style light bulb to heat their pneumatic delivery systems for moisture prevention and freeze protection. They evolved from that solution to a cartridge heater that had a funky bottom on it that screwed in. It was very expensive as was the base assembly. The customer was frustrated with the accelerating cost.
Tutco-Farnam vulcanized a silicone rubber heater to a piece of angle and matched the mounting holes on the face plate. The costly sheathed element AND the socket it fit into were eliminated. Using two screws the field tech attaches the silicone rubber enclosure heater, hooks up two wires in a quick connect and the job is done. Field installation couldn't be simpler.
The result is a super simple field installation with dramatic cost savings. Because of the retrofit, the old heater and base assembly could be removed entirely. The new silicone rubber heater cost less than the base assembly alone, not including the old heater cost. By understanding our customer’s needs we were able to offer a cost and time savings solution.
No More Production Headaches
We learn that one of our customers was sourcing a minimum order of 3,000 fans at a time with a 20 week lead time from China. They hold those while they bring in a plate, 4 bolts, 4 lock washers, 4 nuts, another bracket for a thermostat and the actual thermostat from all of these different vendors. They have vendor relationships, part numbers and inventory to manage, while deciding on a monthly basis if they are going to make the assembly in-house or have someone else do it.
Tutco-Farnam provided a custom solution completely assembled in a box with instructions. No more inventory and assembly headaches. By listening to one recent client we were able to help reduce assembly cost, increase the rate of production and simplify their inventory as well as their purchasing.
The Cost of Quality
We had a customer in the plastic process industry that had left us because of pricing. They were happy with our quality and our delivery but they wanted a price that we couldn’t meet and still be profitable. Five years pass and we get a phone call. Heaters are failing in the field and they are tired of the poor quality they are getting from their current supplier.
Consider what this is costing them. One of their units requires ten heaters. They get installed and tested. Three fail. Those get yanked out. Then the company has to have their customer service people request an RMF and send it back. When the replacements come back they have to reinstall them and test again. Now they’re late on their units. In addition they had product failing in the field. It is failing for their customers and they're paying a field technician to go out and fix their units.
This company started realizing those costs and that is why they came back to us. We modified the heater to perfectly meet their needs. We cost more than the cheaper option upfront but when they realized everything involved they realized that the Tutco-Farnam solution is a massive cost savings and we’re also saving their reputation.
Conversations Lead to Understanding
A conversation between specialist and client is where the real value begins. It takes a willingness to listen and engage. It's also your best chance to discover any pitfalls and hidden gems.
At Tutco-Farnam we are both application and customer committed. An engineer looks at every opportunity that comes in. We listen and we apply what we've learned. Customers often see something on our website and say, 'hey, that looks like what I want.' We then work with them to understand the requirements and guide them towards something that is going to work best.
The following are examples where conversations lead to understanding, which subsequently resulted in marked improvements for our clients.
While visiting a long term customer in the plastic process / resin drying business we went to lunch with our buyer and the service manager. The service manager was recently back from a local call and was complaining about fiberglass making him scratchy. At the time we were wrapping all of their heaters in fiberglass insulation to reduce heat loss and keep operators from burning themselves on the outside of the heater. As we ate we determined that something non fiberglass, reusable and robust would be ideal. From there we developed our insulation blankets. No more scratchy fiberglass for them again!
The flexographic industry heavily uses our Flow Torches. Spending time on production floors showed us that most customers were modifying our heaters to the point where they were voiding the warranty. We assembled all the modifications we were seeing and now offer most of those modifications as standard options for our Flow Torch Family. This would include reducers, both with and without NPT threads, flanges, v band connectors, raised junction boxes for higher inlet temps and plugs for lead wires or junction boxes.
Safety Stock Program
During a customer meeting with a heat staking client we were trying to avoid the rush-rush orders they were constantly placing. It seems their service department was constantly robbing from production and vice versa and it was not until the next week they would realize they were out. Like most of our customers they were also being pressed to reduce inventory. To avoid this we established a safety stock program at Farnam for them so their heaters were never more than a day or two away. No more rush-rush orders.
Tire Refill Station
Nicer convenience stores and truck stops have vacuums for cleaning your vehicle and air compressors to fill your tires. Those units require heat to keep the electronics from freezing. Heat can also be used to keep the tip of the tire refill nozzle from freezing while sitting in it’s holder.
Traditionally you’d use a strip heater. The downside of those include rising cost (due to their material makeup) and bulk. Open coil air heaters are not a great solution because they suck air in from the outside where there is dust, rain and pollution.
Our Solution: Tutco-Farnam created a silicone rubber heater that is a 20%-25% savings for the client. It mimics the shape of that thin strip heaters being replaced with the exception that it is a lot thinner. As older units get refurbished the customer pulls out the old strip heater and puts in our new custom enclosure heater.
At Tutco-Farnam we go the extra mile to understand the needs of our customers. For this client it allowed us to provide an elegant solution that best suited the application while saving the client money.
A customer that makes chocolate enrobing equipment for the confectionery industry was seeking a better solution for melting chocolate. Foods that are coated with chocolate by enrobers include nuts, ice cream, toffee, biscuits and cookies. Enrobing equipment replaces the task of hand-dipping these items. Traditionally air is drawn across a light bulb and the chocolatier fluctuates the variables to keep the chocolate flowing with the right consistency. Tutco-Farnam replaced the lightbulb within the enrobing machine with a custom air heater that includes the fan and assembly. We also made it retrofittable across the board. Tutco-Farnam created a more efficient and consistent solution with finer temperature control. It is also easy to install.
Small Container A/C
We have a customer that makes air conditioners for small containers. These are enclosures that have to be kept cool. At home you’d normally have an air conditioner that is running and water that drips out the back, outside of the house. We don’t want that water dripping within the enclosure. The client had it going into a small tray and was hoping that it would naturally evaporate. That wasn’t happening quick enough and they were getting leaks. Tutco-Farnam came back with a flexible waterproof heater with a pressure sensitive adhesive. The client is now able to go back into the field and easily install the heater inside of every single drip tray. Our small heater helps the water evaporate faster. No more water running on the floor.
Getting Started with Tutco-Farnam
Tutco-Farnam is a heating element manufacturer specializing in custom heaters large and small. We have used a variety of electrically resistive metal and intermetallic alloys in over 2,000 custom designs. You can find examples of our heating elements on our custom heaters page.
Contact us about your unique application needs and see what Tutco-Farnam can do for you.