Control Systems
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Thermal System Basics
Heat Transfer Fundamentals
  Thermal System Basic

Thermal systems are easy to understand. A thermal system is typically made up of a workload, heater, temperature controller and temperature sensor. Figure 1 shows a simple water heating application using all four parts of the thermal system. Study it for a moment. The workload is the water being heated. The sensor is immersed directly into the water so it can accurately sense water temperature. The temperature control is controlling the water temperature by adding heat as necessary. The heater, of course, is providing the heat or thermal energy required to heat the water tank.

Note: Some thermal systems use only a workload, heater and controller. The controller only controls power flow to the heater. It does not control the workload temperature.

The workload is the material or object we want to heat up. The work load may be in the form of a solid (such as a metal block), liquid (such as water) or gas (such as air), or a combination of these. The figures below illustrate examples.

Look at Figure 3. Notice how heat generated by the heater goes immediately into the workload (water). Now look at Figure 2. To heat the aluminum casting, the heater must first heat the steel mold. Do you see any difference in this? Sure! The steel mold in Figure 2 functions as a heat transfer medium. It transfers the heat from the heater to part (aluminum casting). No heat transfer medium is required in Figure 3. The water is directly heated. A heat transfer medium transfers heat from the heater to the product in many conduction and convection applications. So actually, a workload can consist of two things:
1) The product made by the manufacturing process, and
2) The heat transfer medium (which transfers heat from the heater to the product manufactured).

An electric heater simply converts electric energy into heat or thermal energy. This thermal energy is then transferred to the workload raising its temperature. There are many types of heaters for many different applications. Some heaters heat workloads using conduction such as a cartridge heaters (Figure 2). Heat transfer to and through the workload takes place by conduction. Some heaters heat liquids or gases via convection such as an immersion heater (Figure 3). Finally, some heaters heat the workload using radiation such as a radiant panel (Figure 4).

The Temperature sensor measures the temperature of the workload. It then sends a signal corresponding to that temperature to the temperature controller. As the temperature changes, the sensor signal also changes. The controller uses this information to decide whether or not to add heat to the workload. There are various types of temperature sensors, but by far the most common type is the Thermocouple. A thermocouple sensor is usually encased in a metal tube or sheath for protection (Figure 8). Sometimes a plug or other connector is attached to the lead wire for ease of wiring.

Temperature controller is used to ensure that the workload is maintained at the proper temperature. After the controller receives the sensor signal, it compares the actual workload temperature to the desired temperature (called set point). If the workload temperature gets too high, the heater is switched off. If the workload temperature drops too low, the heater is switched on. This "on/off" switching regulates electric current to the heater and thus work load temperature. Temperature control examples are shown in the figures below.

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