Thermal systems have four elements, all of which contribute to systems control performance.
- Work (or load) is the material or product that must be maintained at a controlled temperature.
- Heat source is the device that delivers the heat used by the system, such as electric heaters.
- Heat transfer medium is the material that transmits the heat from the heat source to the work.
- Controller is the instrument that controls the heat flow on the basis of the difference between sensed temperature and the controller's setpoint.
The physical makeup of the thermal system is important, such as proper location of heat sensor and workload, appropriate heat transfer medium and reliable components.
Although in practice, thermal systems are not purely steady or variable, they usually are predominantly one or the other.
For basic system design, the following rule of thumb is helpful: where the heat demand is relatively steady, the controller's sensing element should be placed close to the heat source, but where the demand is largely variable, it should be near the work area. A complicated system may require trying several different sensing-element locations before a suitable one is found. Always remember, however, that the element should be closer to the area where a temperature change must be sensed with minimum thermal lag. Thermal lag is the delay in heat transfer from place to place in the thermal system.
Liquid and gas systems require additional considerations. Because the heat demand is basically steady, the sensing element normally should normally be located close to and above the heat source to minimize system bandwidth. Bandwidth is the total temperature variation above and below the average operating temperature measured at some point in the system.
When a close grouping of heater, sensing element and work is feasible, the result is excellent control under most conditions and is desirable when the thermal load changes frequently. The heat transfer paths from the work and heater to the thermostat are short, so that thermal lag is slight. System inertia is low because of the small mass of heat transfer medium. Rapid cycling will hasten recovery of the system from thermal upsets.
Having the thermostat between heater and load is a “general purpose” arrangement for installations where the heat demand may be alternately steady and variable. By being midway between them, the sensing element can respond to changes at the work and the heater without excessive lag in either instance.
An arrangement with the heater at load and the thermostat distant practically guarantees poor control. The sensing element is too far from either the heater or the load to respond to temperature changes from either one without excessive lag. This arrangement is presented primarily to emphasize that unless you are careful in placing the element, the controller may find it impossible to maintain even fair control.