The hot air dump control loop scheme, which represents the first level of recuperator protection, is shown in Loop Diagram above.
The control loop has been named TIC-200.
The type Pt-100 sensor, called TE-200, sends the electronic system a combustion air temperature signal, where it is converted to a digital value that is used by loop TIC-200.
The error used as a parameter by the control routine is calculated as follows:
error = air temperature - maximum allowable temperature
The correction determined by the control routine is, therefore, of direct action type, since, as the temperature increases, the output value increases.
The allowable temperature for the combustion air is 450 C. This is the maximum allowable temperature above which, the recuperator rapidly deteriorates. Hence, the condition that this hot air dump works as a recuperator protection.
As the combustion air temperature increases beyond the limit, butterfly valve TCV-200, located on top of the inlet (right after the recuperator) starts opening. As it opens this valves delivers hot air to the atmosphere, so that a greater volume of air circulates inside the recuperator, causing a decrease in the temperature of these pipes (metallic) walls.
If the air temperature (PV) is higher than its limit value (SP), as defined in the electronic system, this, with its PID (Proportional, Integral, Differential) type temperature control routine, determines the increment that must be given to the position of the air flow regulating valve, using as a calculation parameter, the existing error which is determined by the difference between the limit value and the value read by the thermocouple.
If the increment in the valve aperture is greater than the programmed dead range, the electronic system, through the appropriate output, sends a digital signal to the digital/P converter which in turn sends a 3 to 15 psi signal to pneumatic actuator SIGMA_F in order to open combustion air dump valve TCV-200, delivering air to the atmosphere.
On the other hand, if the air temperature is lower than the limit, the control routine sends a signal that commands pneumatic actuator SIGMA_F to the valve closed or close it if it is opened.
In case this valve happens to be completely opened, that means that it has not been possible to keep the temperature below 450 C by increasing the airflow inside the recuperator tubes only. So, it is necessary to inject dilation cool air in the smoke tube before the recuperator. A 100% open valve signal is sent to the following dilution air loop. This cool air injection shall reduce the air temperature to a value below 450 C. The dump valve starts closing and when it leaves its 100% open position, the dilution air injection in the smoke is cut.

LOOP

  PC = pneumatic cylinder

The recuperator protection loop monitors the smoke temperature before the recuperator to decide about the injection of cool air as a diluent to reduce the temperature of the smoke and, so, delay oxidation wear of the metallic tubes as shown in Loop Diagram above.
This loop drives a pneumatic cylinder that opens at once and totally, the butterfly valve located between the dilution air fan and the smoke channel. This fan is permanently on, so as to guarantee the presence of dilution air whenever necessary.
Control is of type ON-OFF, or 0-100%, since, when there is a smoke temperature increase, before the recuperator, above 900 C cool air is injected.
As soon as the smoke temperature goes down to a value below 900 C cool air is turned off. This way, injection time is minimized, once the atmospheric air contains 21% oxygen, which accelerates oxidation.
As already presented in the loop of hot air dump, this dilution is also activated whenever the dump valve reaches the 100% open position.
Additionally, if the smoke temperature before the recuperator reaches 950 C, in spite of the cool air injection, then fuel for heating zone (the largest furnace zone) is turned off, by actuating over the 3-way valve that deviates the fuel to the tank. This is the extreme measure to prevent destruction of the recuperator (which can happen in a few minutes).