Combustion & Temperature Control for a Direct-Fired Process Heater

The figure below shows the control schemes applied to a direct-fired process heater.

From the diagram above, the heat from the burning fuel is applied directly to the tubes carrying the process fluid in similar way as the water tubes in a steam generator. This type of heater is usually used as a reboiler in columns distilling crude oil, and in these instances the process fluid, that is, the feed is always controlled to ensure maximum heat transfer to it.

The rate of flow is set such that at the same time it prevents the internals of the process heating coils from fouling with any deposits precipitated from the process fluid. Some unheated process fluid is mixed with heated process material via temperature control loop T2 to achieve the required temperature.

The Operation of the Direct-Fired Process Heater Combustion & Temperature Control System

The temperature of the heated feed is monitored by sensor/transmitter tagged TT-1 and applied as a temperature measurement to controller tagged TIC-1, which has a manual set point and whose output regulates the fuel supply to the heater.

The amount of fuel to the burners is monitored by an in-line flowmeter tagged FT-4 and applied as a flow measurement to a ratio controller tagged FrIC-4.

The ratio of air to fuel to bring about combustion is set on the ratio controller FrIC-4, and this value is finely adjusted by the amount of oxygen in the exhaust gas. The oxygen in the exhaust is measured by the oxygen analyzer tagged AT-5 and applied as the oxygen measurement to the controller tagged AIC-5 whose output is applied as the set point of controller FrIC-4 and, thus, indirectly adjusts the position of the air damper. The ratio controller FrIC-4 operates on a calculated ratio range, and its remote set point driven by the output signal from the oxygen controller AIC-5 is also termed to as the oxygen trim, which precisely corrects the air needs (including the mandatory “excess air”) for complete combustion.

The reset feedback connections to controllers TIC-1 and FrIC-4 are important to avoid saturating the integral term of these controllers, where the operator puts either FIC-3 or FrIC-4 into manual mode. We also have a square root extractor tagged FX-3 in the output from the differential pressure transmitter FT-3 in the combustion air supply, however the fuel flow transmitter FT-4 is an in-line flow instrument, which has a linear output with respect to flow.

You may also read:

• Industrial Process Temperature Control System
• The Instrumentation for Furnace Control
• The Operation of a Temperature Control System for lubricating oil
• Steam-heated Reboiler Control System