This technique was originally developed by FGH Controls Limited for the metals industry, & provides the following benefits:

Faster heating under full control Limits surface temperatures Load temperature is measured & recorded Flexible alarm criteria Saves energy Compensates for different heating rates on thin & thick sections

 Basic Principal

Conventional control of industrial ovens and furnaces relies on controlling the atmosphere temperature at a desired value, and assuming that the furnace load eventually reaches the atmosphere temperature. In a totally enclosed system with no heat loss direct from the the load this will eventually occur, but unfortunately eventually is the operative word here.

The diagram below shows a typical response for heat treatment of a large metal mass:

Loads with large thermal mass, heat very slowly.

There are two main reasons for the very long lag. Firstly, the load is usually a material with a high thermal mass compared to the thermal mass of the atmosphere in the furnace. This limits the rate at which heat can be taken up. Secondly, and far more crucially, since heat is proportional to temperature difference, the rate of heat flow and therefore temperature rise, falls rapidly as the set point is approached. The result is not a true exponential in practice, because atmosphere heating is progressive, but to a first approximation for large loads, the error is given by e^-x, where x is the number of time constants. This shows that, with a 1000 °C set point, the error will still be 18 °C after 4 time constants, and 6 time constants have to elapse before the error is within 3 °C.

Faster Rates Of Rise

In order to achieve a faster rate of rise in the latter stages of approach to set point, the temperature difference between load & atmosphere must be maintained at a appropriate, preferably large value. An upper limit must be placed on the difference to prevent damage to the surface of the load and an overall upper limit must be imposed for protection of the furnace. Finally, the temperature difference must be reduced to zero as the load approaches it's set point to prevent overshoot.

The technique of thermal head ratio control fulfills these criteria for faster rates of rise by utilising two instruments. One sensor monitors the load and calculates a Thermal Head Ratio value to control the atmosphere temperature in the usual way. The load controller calculates the atmosphere set point so as to maintain an appropriate difference or thermal head, between the load & atmosphere using the following algorithm:

SPA = SPL + R (SPL -TL)

Where SPA is atmosphere set point, SPL is load set point, R is the thermal head ratio and TL is the measured temperature

By maintaining the temperature difference between atmosphere and load, much faster heating is achieved.

As can be seen from the graphs, the thermal head ratio technique achieves a much faster, but still controlled, rate of rise by maintaining the temperature difference between atmosphere load until the load is almost up to temperature. Like any other control system the variables (in this case the the ratio setting R) must be adjusted to produce optimum results. However, unlike most systems, the response time of the load is automatically taken into account and so the system is largely independent of thermal mass changes when different sizes and types of load are processed.