Temperature control may be provided to a system in order to:-
The most important aspect in the design of a heat tracing system is in ensuring safe temperatures. This is achieved preferably by a stabilised design, or when that is not possible, by temperature control means. The use of temperature controls for ensuring safe temperatures are not required if inherently temperature-safe heaters or a stabilised design are possible.
A stabilised design ensures that, under the most onerous conditions, limiting temperatures are never exceeded. The limiting temperature may be, for example, the maximum withstand temperature of the heater construction, a maximum process temperature, or the Temperature Classification if the trace heating is in a hazardous location.
The most onerous conditions occur when :-
Under such ‘runaway’ conditions, a pipe being freeze protected at 5°C may typically result in a temperature of, say, 100°C, and a heater surface temperature substantially higher, for example 150°C.
In this case, should the limiting temperature be higher than 150°C, then we have a safe stabilised design. If not, stabilization may be achieved by, for example, reducing the heater power (W/m) and using more passes.
But the easiest way to achieve a safe stabilised design will be when inherently temperature-safe self-regulating heaters are available. In the above case, the maximum pipe temperature may typically be 50°C with a heater sheath temperature of 60°C.
Where inherently temperature-safe heaters or a stabilised design are not possible, then temperature control to ensure safety cannot be avoided. In this case, it is necessary that:-
It is important that the sensor of the over-temperature controller is fitted to the pipe or workpiece to limit the pipe to a temperature level at which the heater will not exceed the maximum limiting temperature. A sensor fitted to the heater is more dangerous should the sensor become detached or is not in the hottest location.
There are basically 3 types of control systems we can use