Fired heater and heat exchanger temperature
A fired heater temperature loop looks like any other slow temperature until it does not. The process lag is large, the dead time through the coil is real, and the manipulated variable is fuel to a burner that has its own failure modes. Heat exchangers are the milder cousin: coolant or steam flow instead of flame, same cascade pattern, fewer ways to get hurt.
Tuning cannot fix a burner that is unstable, a fuel-air ratio that is wrong, or a tube that is fouled. Those show up as a temperature that will not hold, and they get reported as a PID problem because the PID is what the board can reach.
Cascade onto the flow, not the flame
The temperature controller should set the setpoint of a fuel-flow or coolant-flow controller, not stroke the valve directly. The inner flow loop absorbs supply-pressure changes and keeps the outer loop from seeing them. Tune the flow loop first. Then the temperature.
Direct-acting a temperature PID onto a fuel valve makes every fuel-header wiggle into a coil-outlet swing. People then add derivative to chase it. Derivative on a noisy fuel measurement is how the valve starts to chatter.
What is not a tuning problem
Flame instability, a missing or failed fuel-air ratio, a fouled coil, and a bypassed pass all produce a temperature that hunts or will not come up. Detuning the PID will make the swing slower. It will not light the burner correctly. If the inner flow loop cannot hold fuel flow, stop there.
Heat exchangers
Steam or coolant to a process exchanger is the same structure without the combustion. Cascade the process temperature onto the utility flow. Watch for a utility that saturates — steam valve wide open, cooling tower at the limit — because the temperature loop will wind up against it and then overshoot when the constraint lifts.
Questions that come up
Should I add feedforward from feed rate?
Often yes, on a heater whose duty tracks feed. A feed-rate feedforward into the fuel-flow setpoint takes the slow, repeatable part of a rate change off the temperature PID. It does not replace the cascade, and it does not fix a burner problem.
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Last reviewed 2026-09-10.