How to Design for Effective Heat Transfer

Industrial electric heaters are useful for delivering heat when you need it and controlling temperature at a steady rate that you have determined. However, poor design through improper placement of sensors or the heating system itself, will fail to produce desired results and can even lead to heat failure.
 
By understanding how heat transfer works, you can increase the likelihood of your success in any project and achieve your goals in designing an industrial heater. Heat is ultimately energy and follows thermodynamic laws of physics.
 
In designing a heater, there are five essential rules of thermal transfer that can be applied to the design to minimize energy loss, increase efficiency and offer better results.

#1 Providing a Direct Path

Heat travels will radiate from its source on a direct path. Any obstacle placed in the way, diffuses that energy and can block it almost entirely. In designing an electric heater, you must take into consideration all avenues of heat loss, ranging from holes and slots to surfaces that lack a conforming material. All of these areas can prevent heat from traveling where you want it to go and can result in heat loss and a less efficient heater.
 
If you are presented with a complex design without alternative options, it is advisable to seek out the help of a heating expert. Certain designs can consist of (for example) a flexible heater 0.125 inches in length that has a thermocouple attached across anywhere along the length that is heated. In this scenario, you can reduce the energy necessary to heat up the tool by decreasing its mass.

#2 Designing To Preserve Bore and Groove Integrity

When you design for fit, you increase the longevity of the heater and also save energy. In order to increase and get the best performance out of your electric heater, you will need to provide contact between the heater and the system it is linked to for a tight fit, preserving bore and groove integrity. For example, for a high wattage cartridge heater, you can use a heater borer hole with a±0.001” tolerance away from the nominal position, using a DIN norm g6 heater body.
 
You can also push the electric heater into the tool, increasing heat transfer and offering a higher surface watt density. Pressing the heater forward must be done carefully to avoid compressing the heater itself. While some deformation may occur and is acceptable, compression is not. It can be helpful to check with a thermal solutions partner to make sure you are installing the heater properly.
 

#3 Controlled Energy is the Only Useful Energy

Heat transfer is affected by thermocouple placement or the location where temperature is sensed and the amount of material mass that is heated.
 
For example, if the sensor is placed too far away from the heater, the response can be out of control and if it is too close, the reading can be inaccurate. Your goal is to have the temperature that you are measuring be the same value as the working surface temperature. When you have the right thermal placement, you can lower the Δ or the temperature differential between the setpoint and the current working temperature.
 
The amount of material mass that is heated is also important. If the thickness of the material is too small, heat does not radiate from the heater evenly. A “rib” of heat is created. If the material is very dense, efficiency is reduced. A mass and web thickness can affect the distribution of heat. When the temperature of the working surface is the same as the thermocouple temperature, the process is well controlled. On the other hand, a rib of heat can be created if the mass is too thick, the temperature of the thermocouple is not the same as the surface, resulting in a less than ideal heat distribution and a process that is not controlled.
 

#4 Improved Design Prevents Problems Caused by “Bandaid” Approaches

As heaters warm-up and cool down, components inside the electric heaters are affected. Placing screws inside as a bandaid approach can keep things in place, however screws create problems such as blocking heat transfer and stress failures on resistance wires. Instead of using makeshift approaches like this that can create new, bigger problems, it is better to design a better electric heater initially – one that is designed for fit to preserve the integrity of the bore and groove. If you find that you need to secure the heater to the equipment body, then use a flange welded to the body of the heater. The flange is held in place by screws but does not damage or affect the heater’s performance.

 #5 Increasing Wattage Is Not Always The Right Choice

Most energy is consumed during the initial increase from room temperature. After reaching a steady state, the heater will only require about 20 percent of the initial wattage to maintain a setpoint. Increasing wattage will increase the initial rate of rise to bring the heater to the steady state but after that does not offer any value. A setback from applying too much wattage initially is ‘warpage’. Increasing power does not necessarily improve performance with a faster heat transfer as too high of wattage initially can also decrease the life of the heater overall.
 
In review, designing an electric heater requires understanding of the proper way to control and distribute heat, following the basic rules of heat transfer. Using a design review and FEA or CFD modeling can aid in exploring different scenarios. Even small changes can affect the life of the heater, quality of performance and speed.
 
Effective thermal management is essential in designing an electric heater that functions well, is efficient and long-lasting. Working with a thermal solutions partner can provide you the necessary engineering expertise to achieve this.

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