Other types of metal heating elements include resistance wire, which are commonly used in things likes toasters, hair dryers, furnaces and floor heating. Additionally, etched foil, which is also made from similar substances as resistance wire and commonly used in precision heating applications. PTC heating elements, which are made by conducting PTC rubber increase resistivity exponentially with increasing temperatures.
These elements work with heaters that produce large amounts of power in the cold. As a result, these heat up quickly and maintain a constant temperature. In composite heating elements, tubular or sheathed elements create a fine coil of nichrome resistant heating alloy wire.
The composite heating elements can be incorporated into appliances, such as a toaster oven, as a straight rod. Conversely, composite elements can be bent and incorporated into appliances, such as electric stoves, ovens or coffee makers.
Many heating elements contain their part number on the actual element. This helps identify the part, which aids in replacement. For example, knowing the exact part helps technicians address any issues with heating elements in particular, in a furnace. All furnaces list the model and serial number in a visible area to assist in finding replacement parts. The electrically conductive heat generating material is embedded within an electrical insulating yet thermally conductive material.
A cartridge heater contains an electric coil surrounded by an insulating powder typically magnesium oxide and packed within a tapered sheath. All terminals exit the same end. This type of heater is commonly inserted into a cylindrical hole. The size and shape of the bore as well as the size and shape of the heating element is extremely important.
There should be a firm uniform fit when energized to achieve safe and efficient conductive heat transfer. Not too tight or the heater may need to be drilled out when it expires. In some cases, a cartridge heater is used to heat a fluid instead of a block of metal and will have fins to increase surface area. 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. Strip heaters can be outfitted with fins and they can also be specially manufactured for extreme environments. Many terminal styles exist for this heater. Cut-outs and other shape modifications are possible. 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 a large auger used in many plastic manufacturing processes. Tubular heaters have an electric coil surrounded by a ceramic insulating powder encased within a metal sheath.
The terminals exit opposite ends of the heater. This type of heater typically has a round cross-section though it can be manufactured into other shapes like square or triangular. They are often manufactured with curves and bends to best support an application.
A common location to find a tubular element is inside an electric kitchen oven. 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. Essentially were defining the problem we intend 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 affect the direction of design so we want to dig deeper whenever possible.
We want to know the 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.
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. 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 may be less obvious.
Watt Density is another useful way to quickly compare materials. 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, municipal water supply, and even oil from an installer's fingers may be a source of alloy eating contaminates. It is common for us to design a heater for equipment that has already been designed or even manufactured.
It is also a 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. An off-the-shelf solution is often the 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. You can see examples of custom heating elements on our custom heaters page. Take notice of what a wide variety of shapes and styles result from the needs of a custom application. 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 amount 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 heating element longevity. An oxide layer that remains strongly 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 temperature cycling. Sometimes lowering the temperature of the heating element is the best solution.
An example of this can be illustrated with one of our clients who manufacture 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. The ease in which a heating element integrates with an application affects cost. Difficult and time-consuming product assemblies will burden 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 hassle 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.
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. 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 Their biggest challenge is that they had V 3-phase running to it.
They had to figure out how to go from V 3-phase to V single phase on these silicone rubber heaters. They were breaking a leg, daisy-chaining heaters, and cobbling stuff together. These were not elegant solutions. No more worrying about changing your voltage. No more daisy-chaining and putting multiple heaters on there. One heater to do the trick. Some of our dehumidification heaters use jumpers for joining together sections of open-coil elements.
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 access 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. 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. 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 faceplate.
The costly sheathed element AND the socket it fits 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.
We learn that one of our customers was sourcing a minimum order of 3, 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. With more heat, the resistance will decrease further resulting in instability of operation. Since the material of the heating elements has to have convenient shapes and sizes, it should have high ductility and flexibility. The material of the heating element should possess high mechanical strength of its own. Usually, different types of alloys are used to get different operating temperatures. With the passage of time, every heating element breaks open and becomes unserviceable.
Some of the factors responsible for its failure are:. Write a short note on the Requirement of a Good Heating Element. State the factor responisible for the failure of the heating element with the passage of time. Distribution Determination of M. Branch : Electrical and Electronics Engineering.
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