5 Essential Rules for Effective Heat Transfer Design
These basic rules of thermal transfer design will increase your project’s efficiency, decrease energy loss and optimize your process.
Electric heaters can deliver heat just where you need it and, when teamed with proper thermocouple placement, keep your process in control and at a steady state. But, poor heating system design or sensor placement will result in less-than-ideal results, increased scrap and premature heater failures.
Understanding the basic principles of heat and heat transfer is vital for the success rate of projects and to address common challenges before they occur. Heat transfer properties are much like a sound wave. A tool that has heat traveling around a corner, through threads or jumping across non-machined surfaces is not a realistic expectation for any real-world system. Heat is energy and, therefore, must conform to the laws of physics.
With some basic design rules in your heat transfer toolbox, you will be more likely to achieve your desired goals and easily avoid costly mistakes. These basic rules of thermal transfer design will increase your project’s efficiency, decrease energy loss and optimize your process.
Give Heat a Straight Path
Just as when the cat lies on the radiator in the winter and the room gets cold, you cannot expect heat to get from one point to another by magically going around an obstacle. Holes, slots, pockets and interface surfaces without a conformal material are all opportunities to stop heat from getting to where it needs to go.
To illustrate this, consider an application where the location of the thermocouple in a set of packaging jaws blocks heat transfer to the sealing surface (figure 1). In the finite-element analysis (FEA) image, the thermocouple hole is the smaller hole on the right above the heater. In this case, only 66 percent of the seal face is within a 6°F (3.3°C) temperature band.
Wattage distribution in the heater can help even out this profile (figure 2). Simply by changing the watt density at the ends of the heater, 86 percent of the seal face is held within the 6°F (3.3°C) temperature band. The hole is still present, but heater design and layout can compensate with increased wattage to recover the sealing face.
Sometimes, there are just difficult designs with no other functional alternative. When faced with one of these challenges, a heating expert can provide options you might not have otherwise considered.
Figure 3 illustrates one such challenging heat transfer design. If you cannot fit a standard heater in a tool, there are heaters that can conform to the shape you are heating and avoid being blocked. This example uses a 0.125” flexible heater that can include a thermocouple anywhere along its heated length.
This heating method also has the benefit of reducing the energy required to heat up the tool by reducing its mass. The minimum thickness of the tool can be 0.375” thick using this type of heater. In some cases, you can use even smaller heaters that go down to 0.055” (1.4 mm) for lesser masses.
In the case of a packaging jaw, converting to a focused-energy design can dramatically reduce the power required. Using advanced FEA software, the heater designer can evaluate the thermal needs of the process and design the heating solution for better uniformity, faster cycle time and lower power consumption. The design in figure 4 shows a 0.125” heater as a face-to-jaw insert that is much smaller than the original design. This heater is designed to be backed up with a low thermal conductivity material to isolate the heat. In this example, a 1.5-by-2-by-10” jaw would require 650 W to reach 500°F (260°C) in 30 minutes. Changing to the focused heater design reduces the power required to 125 W — yielding substantial savings, lower material costs and ease of heater replacement from seized cartridge heaters.
Design for Fit to Preserve Bore and Groove Integrity
An excellent fit means increased energy savings and longer life. By definition, electric heat is 100 percent efficient. What is not 100 percent efficient is the process of heat transfer from the heater to the process or product that needs to be heated.
To get the best performance from an electric heater, the heater must have intimate contact with the process. For a high watt-density cartridge heater, that can mean a heater bore/hole held to ±0.001” tolerance from nominal, and a heater body designed with a DIN norm g6 undersize condition. These design conditions will ensure a tight, sliding fit.
A better solution, however, may be to press the heater into the tool. This allows for optimal heat transfer to the process and a higher surface watt density on the heater for longer life. If you decide to press the heater in, be careful that you are not further compressing the heater. Keep in mind that compressing is different than deforming. High watt-density heaters are already compacted; however, some deformation during installation is acceptable. What they cannot be is further compressed. If you plan to press the heater into the tool, consult with the heater manufacturer and share the groove geometry of the tool into which the heater is being installed. A thermal solutions partner can offer suggestions on proper installation techniques to ensure long heater life.
Energy Is Only Good If It Is Controlled
Both where the temperature is sensed and how much material mass is heated affect heat transfer.
First, thermocouple placement is important. If the sensor is placed too far from the heater and the process, temperature response can be dampened and out of control. If the thermocouple is placed too close to the heater and the working surface where the temperature must be measured, the readings may result in inaccurate controller setpoints. Ideally, you want the temperature you are measuring to be the same value as the working surface temperature (figure 5). Correct thermal placement reduces the temperature differential (Δ) between setpoint and working temperature, providing a much better understanding of the true process environment.
Second, the amount of mass to distribute the heat from the heater is equally important. If the web thickness is too small, the heat will not be able to radiate from the heater evenly. Instead, a “rib” of heat will be created. At the same time, if the web is too thick, efficiency is reduced and process recovery can be slow. Resist the urge to put a set screw — even a small one — into the side of a heater to keep things in place.
Figure 6 shows how mass and web thickness affect heat distribution. In the figure, the smaller hole is the location of the thermocouple. The holes to the left and right are cartridge heaters bores. (For illustrative purposes, the cartridge heaters have been removed from the image to isolate the effects of heat distribution.)
The left half of the image shows the heater is properly sized to the block, allowing at least 1.5 times the heater diameter in web thickness (from heater centerline). Notice the light green color (indicating temperature) on the surface of the block. It is the same light green that the thermocouple is indicating. That means the temperature of the working surface is the same as the thermocouple temperature — a sign that the process is well controlled. By contrast, the right half of the image shows an artificially thinned mass of the web. As a result, you can see the red “rib” of heat on the working surface, and you can also see that the thermocouple is not the same color as the surface. This process is not in control and the heat distribution is not ideal.
Better Design Prevents ‘Fixes’ That Cause Larger Problems
Heaters get hot and cool down, and that means things move. However, heater users must resist the urge to put a set screw into the side of a heater to keep things in place — not even a small one. Using set screws creates a two-fold problem. First, they block heat transfer. Screw threads are full of air, which is very bad for heat transfer. Second, the dimple pushed into the heater by the set screw applies pressure on the internal parts of the heater. This can create a hot spot or cause a stress failure on the resistance wire, which is in very close proximity to the sheath. If you have to work with a set screw, you have not satisfied the second key point noted in this article: Design the heater for fit to preserve bore and groove integrity.
If the heater must be secured to the equipment body, the preferred method is to use a flange welded to the heater body. The flange is screwed into place. Many flanges and alternate holding methods are offered that will not damage the heater.
More Wattage Is Not Always Better
Contrary to what you may have heard, more wattage is not better. The majority of energy consumed is during heat up from room temperature. Once the tool reaches a steady state, if a process is well designed, the heater will require about 20 percent of the initial wattage to maintain setpoint. So, while putting more watts into a tool will increase its rate of rise, it may make no difference in the actual operating characteristic at temperature. One risk of rapidly heating up a tool is warpage, and rapid heatup also can shorten heater life. Just because the heater has more power does not mean that it will transfer out of the heater to the tool faster.
A design review is a great way to learn about heat flow, and a partner that has FEA or computation flow dynamics (CFD) modeling can help with “what if” assessments quickly.
In conclusion, heat is energy. And though it is a powerful tool, it is not magic. Proper control and distribution of heat in an application can make or break the success of a project — and the operating efficiency of the system.
|See the related web exclusive, "More Wattage is Not Always Better When It Comes to Industrial Heating," to learn how and where heat loss can derail an electric heating system.|
It is important to follow the basic rules of heat transfer during the machine design phase. Small changes to a heater can significantly impact process speed, quality of the tool and heater life.
A thermal solutions partner can offer engineering expertise to ensure the end result will be a robust design that includes effective thermal management as a key strategy and help you avoid startup issues when the switch is turned on.