Achieving effective heat transfer involves good heater design and proper temperature sensor placement.

Smart temperature management and control are important in applications ranging from hot runner injection molding in the plastics industry to form, fill and seal systems for packaging, to rubber molding and to food production. Key to success in any industrial process is effective temperature control. Here are 10 tips on matching your control to your heater to ensure effective process heating in your application.

1. Define the Type of Temperature Control Required

Temperature control can have many different meanings. A packaging engineer may be looking to increase the temperature uniformity of a seal bar to improve seals in a plastic film, including zippers and gussets. A chemical process engineer may need to control temperature so that it never exceeds a certain value for safety or to prevent damage to the product. An application engineer designing systems for process water may need to limit the heater temperature for safety reasons or to prevent boiling at the surface of the heater to promote longer life. The term temperature control also can be used in reference to response time for heat to recover from variability in the process.

Keys to Effective Temperature Control

To control temperature in a process, it is essential to know what influences temperature within that process. Major influencers include environment, control system and heater design.

Environment

The position of the heater with respect to the process variability

The influence of heat sinks in the heat transfer equation

The amount of mass being heated and its thermal conductivity

Control System

The method of temperature feedback

Controller settings or how the power is being managed to the heater

Heater Design

The rate of heat flux off the heater into the process

The construction of the heater being used in the process

2. Size for the Best Thermal Dispersion

The mass being heated should be close to three times the heater diameter for best thermal dispersion.

Simulation software shows where a heater should be applied in a design to best manage the heat. It is far too common of a practice to not consider the heat aspect of their designs until the end, which leads to inefficiencies and performance compromises. When designing a process where heat transfer takes place, there are considerations in the design that have to be made.

Figure 1 shows the temperature distribution of a simple thermal FEA simulation with two different sizes of heaters and a thermocouple mounted in a block. As this image illustrates, the mass being heated is an essential component of an effective heating system. The heater and thermal mass must work together. 

Heaters are capable of providing heat to the tool, but if the mass of the tool is not correct, the heat being transferred will not be able to disperse well. This introduces thermal-management problems. 

In figure 1, the bigger hole on the right illustrates the effect of having too large of a heater in the block. In this case, the web of material between the heater and the working surface is too thin, causing a “rib” of high temperature on the surface. The effect of this is a local hotspot that could negatively influence the tool’s performance.

3. Proper Thermocouple Position Is Important

The smallest hole between the two larger holes in figure 1 is the thermocouple. The position is reporting a different temperature than the larger heater is indicating. This means the working surface would be hotter than the thermocouple is reporting, leading to significant delays in the control feedback and resulting in instability in the temperature control.

In figure 1, the smaller heater on the left side of the image shows a mass-to-heater diameter that is correctly defined. Heat transfer is well dissipated into the block, and the working surface shows a good temperature uniformity. The thermal color of the working surface is similar to the temperature around the thermocouple, so both accurately reflect the same temperature. The distance to the working surface is set to 1.5 times the heater diameter, and the thermocouple is on an imaginary arc drawn from the center of the heater bore and the working surface.

Figure 1 illustrates a cartridge heater, but it also could be a coil heater for outside-in heating. Regardless of the method of heat being applied, the concept of heater-to-thermal-mass ratio is relevant.

The control system in these examples refers to the method of reading the temperature of the process and managing voltage being sent to the heater to control the system to a setpoint. In most process systems, a thermocouple is used to measure the temperature. Common types include Type J and Type K thermocouples and resistance temperature detectors (RTDs). But, there are many others, including infrared sensors. Processes that do not use a thermocouple would not use a temperature controller and could instead use a thermostat or be self-regulating through a temperature/resistance-changing heater. 

In addition to thermocouple position, it is essential that the thermocouple is well seated to the tool to ensure accurate sensing. Similar to heat transfer of a heater to a tool, air gaps will act as pockets of a thermal barrier and reduce the accuracy of a sensed temperature.

4. Check the Thermocouple Wire

If the temperature displayed on your controller does not make sense, check the thermocouple wire.

Type J or K thermocouples are readily available with great flexibility of design and installation method. They all read temperature by contacting the surface and converting the heat to a millivolt signal through a process known as the Seebeck effect. These sensors have polarity, and the wires are color-coded to indicate proper hookup. Many, many resources are available on the web to define the color of the wires and polarity.

Thermocouples are simple to use. However, the signal is prone to corruption by electromagnetic fields (EMF) generated from nearby devices. Accuracy of the reading is subject to junction design and the calibration of the different wires used inside the sensor as well as any extension wires used. Connections that are influenced by heat can cause offsets in the reading, and the terminals need to be compensated to minimize the effect. The temperature response of the sensor is subject to the size of the junction and the enclosure in which it is fitted. The sensor (and its service temperature range) must be selected based upon the application temperature, or it can degrade the long-term service.

5. Consider an RTD

If accuracy is important and you want to minimize the potential influence of heat in the connections, you may want to consider an RTD instead of a thermocouple. These have a very small wire-wound chip embedded with a special alloy wire that changes electrical resistance based on temperature. These also come in several options. Platinum 100 Ω (Pt100) and nickel 120 Ω (Ni120) are the most common. The noted resistance is calibrated at room temperature and increases as the temperature rises. They are available in two-wire and three-wire combinations. The choice depends on if the control circuit needs a reference to remove the influence of the lead wires in the circuit resistance, increasing the accuracy. It should also be noted that whereas thermocouples often can be incorporated internal to a heater, in most cases, an RTD only can be an external sensor.

A potential drawback of RTD sensors is that the measuring temperature rating is lower than that of thermocouples, so care must be taken during the design of the control system. Because RTDs have fine wire chips installed, durability should be considered although there is a hardened version available for packaging systems. The size of an RTD is generally larger than that of a thermocouple, so latency may be possible in readings.

6. Understand the Open and Close Limits of the Thermostat

If you plan on controlling based on a thermostat, understand the open and close limits of the thermostat. Compared to industrial temperature controllers, thermostats offer less precise temperature control. In cases where a process operates well within a wider temperature range, thermostats have their place. Knowing the open and closing temperature range of your thermostat is important to evaluate the thermostat for a temperature control application and problem solving, should that be necessary.

7. Consider Fit Before You Add Wattage to a System

To consider fit before you add wattage to a system, look at the fit between the heater and thermal mass.

We have regularly had discussions with customers where they feel they need to add wattage to an existing process. In many cases, it is to a system that has operated well for a long time and now the “old wattage is not working right anymore.” It is important to look at the process as a system, including the heater, sensing, control and thermal mass. 

If a process was once highly functional, it often is worthwhile to look at the fit of the heater to the thermal mass to see if conditions are present for effective heat transfer. If, for example, the bore of a cartridge heater was modified due to difficult removal of a heater — it is more likely that the fit between the heater and bore is not providing sufficient contact to allow for proper heat transfer. Instead of adding watts, it would be better for the process effectiveness to gun drill the bores and move to the next size up bore and heater.

8. Consider Fit Before You Increase Watt Density

The higher watt density of the heater, the more important fit to the mating surface becomes. The better the fit between the heater and tool, the more efficient heat transfer will be. An electric heater delivers thermal energy to a process. For conductive heat, there must be a sufficient surface contact for the transfer of thermal energy to take place. If there are air gaps, these gaps will act as thermal barriers. More of the thermal energy will stay on the heater and less will be transferred to the tool. If you have an external thermocouple, the heater will have to generate more thermal energy in order to meet the target temperature. Often, this will lead to premature failure of the heater. If you are using a thermocouple internal to the heater, insufficient thermal transfer will result in the sensed temperature inside the heater being significantly higher than the tool temperature. The controller then will open the solid-state relay even though the tool is not at target temperature. This will result in process performance issues.

Fit between the heater and thermal mass is an essential component of heater life and optimized thermal performance.

9. Temperature, Installation and Clamping Method Affect Coil Heater Performance

Coil heaters wrap around an object and heat from the outside in. Coil heaters without an outside sheath should be slightly undersized to the tool they are heating and installed with a corkscrew motion. This ensures the best fit of heater to tool as operating temperature rises. Other methods to improve fit include a pressed-on sheath (heater would be a sliding fit) and embedding the coil heater in thermal mass. 

Recently, new methods have arisen where the coil heater is undersized, installed in a corkscrew method and a material is used as an outside sheath that has a lower coefficient of thermal expansion. This method improves fit across the entire tool and is beneficial for sensing when using an internal thermocouple.

10. Check for Compatibility at Temperature

Heated media can react with the case material of the heater at elevated temperatures.

Tubular heaters are used in process heating systems where fluids are being heated as a direct immersion-style heater. These have surface loads that are designed to limit localized boiling or higher for pressed-in applications. Some heaters are pre-bent by the heater manufacturer to a specific shape. Other versions are available that are flexible for easier installations into a tool.

Ceramic heaters are good for noncontact heating where contact would damage the product or it is not practical to design a mating surface for heat transfer. These come in options from ultraviolet to far-infrared emitters and need to be designed for the application for best results. Emitters operate over 932°F (500°C) to generate the wavelength of energy, so care must be taken in the installation of the heaters.

(Source: http://www.process-heating.com/articles/91840-tips-for-controlling-temperature-in-industrial-processes)