When fuel consumption in a process fired heater rises progressively, the first checks usually focus on the burners and the combustion system. However, if the heater maintains the same process load but starts requiring more fuel to achieve the same duty, the cause of the deviation may lie elsewhere in the system.

The heat recovery system, and the air preheater (APH) in particular, has a direct influence on the heater’s energy balance. If it no longer recovers the expected amount of energy from the flue gas, the combustion air reaches the burners at a lower temperature than the design value and the heater has to compensate by burning more fuel.

The equipment can remain in service and maintain production while its performance gradually deteriorates. Simply checking that the air preheater is operating therefore does not show how much heat it is actually recovering.

The air preheater recovers part of the energy available in the flue gas and uses it to raise the temperature of the combustion air.

When heat recovery decreases, the combustion air reaches the burners at a lower temperature than expected. To maintain the same process duty, more energy has to be supplied through fuel.

The deviation may develop gradually. There does not need to be a sudden failure, nor does the heater need to stop producing. The control system may continue to maintain the required process conditions while specific fuel consumption increases.
For this reason, a loss of APH performance can remain hidden for a prolonged period if the assessment is limited to production or fired-heater availability.

A loss of performance in the heat recovery system should not be assessed from a single variable. It is the combination of several parameters that indicates whether the equipment is still operating close to its design conditions.

Key signs to review include:

• An increase in specific fuel consumption at the same process load.

• Combustion-air temperature below the expected value.

• Changes in flue-gas temperatures.

• Deviations in emissions.

• Tube metal temperatures above the expected values for the same process load.

• Changes in pressure drop.

• Changes in furnace pressure or draught.

• A shift in the fans’ operating point.

None of these signs identifies the cause on its own. The diagnosis should consider the combined behaviour of the fired heater, the air preheater and the combustion system.

A loss of performance can have several causes within the heat recovery system itself.

Leakage between the air and flue-gas circuits

Unwanted communication between the two circuits changes the operating conditions of the air preheater and can affect both the heat balance and the behaviour of the combustion system.

Leak tightness should therefore form part of any APH performance review.

Deterioration of joints and seals

The condition of joints and sealing elements directly affects the equipment’s ability to keep the different flows separated.
Deterioration can introduce progressive deviations that do not necessarily cause the equipment to shut down, but can still reduce its performance.

Fouling of heat-transfer surfaces

Deposits on the heat-transfer surfaces reduce the amount of heat that can be recovered.

The effect may be visible in both temperatures and flow behaviour, so it should be assessed alongside pressure drop and the actual operating conditions.

Poor flow distribution and preferential flow paths

Non-uniform distribution of air or flue gas can create preferential flow paths, causing part of the flow to make uneven use of the available heat-transfer surface.
In these cases, the equipment may theoretically have sufficient heat-transfer surface and still operate below its expected performance because part of the bundle is not exposed to the flow conditions for which it was designed.

A loss of air-preheater performance does not always appear only as reduced heat recovery.

In some cases, pressure drop also increases. This affects the available draught and can shift the fans’ operating point.

At that stage, the issue is no longer limited to the air preheater and begins to affect the balance of the combustion system.
Looking only at combustion-air temperature can therefore give an incomplete picture. The diagnosis should relate temperatures, pressure drop, pressure, draught, oxygen content and fuel consumption.

This combined view helps distinguish whether the efficiency loss is genuinely caused by combustion or whether it originates in the system responsible for recovering energy from the flue gas.

This is one of the main reasons why the problem can be difficult to detect.

Progressive degradation of the air preheater does not have to cause an immediate loss of production. The fired heater can maintain the required duty because the system compensates for reduced heat recovery by consuming more fuel.

From an operational perspective, the plant continues to run.

From an energy perspective, however, the conditions are no longer the same.
Equipment availability should therefore not be confused with performance. An air preheater remaining in service does not necessarily mean that it is still recovering the amount of energy for which it was designed.

When fuel consumption increases or the thermal behaviour of the fired heater deviates from normal, the air-preheater review should start from actual operating data.

The main variables include:

Air and flue gas temperatures

These help determine whether heat transfer is still delivering the expected performance and identify deviations from the reference conditions.

Pressure drop

Changes in pressure drop can help identify changes in flow distribution through the equipment, fouling or restrictions that may affect the available draught.

Oxygen content

This should be assessed together with the other combustion-system parameters to understand whether operating conditions are changing.

Furnace pressure and draught

These indicate whether changes in air-preheater performance are affecting the overall balance of the air and flue-gas circuit.

Specific fuel consumption

This is one of the most relevant variables for determining whether the fired heater progressively needs more energy to maintain the same process load.

These variables are most useful when assessed together and compared with the design conditions or with historical periods of stable operation.

An increase in fuel consumption does not necessarily mean that the fired heater must be replaced or that a new technology needs to be introduced.

In some cases, there is an opportunity to improve equipment that is already part of the installation.

The air preheater is one such item.
Determining whether it is still operating in line with its design conditions helps establish whether part of the heater’s efficiency loss originates in the heat recovery system and what action may be required on the existing installation.
This review is particularly important when:

• Specific fuel consumption increases without an equivalent change in process load.

• Progressive deviations appear in air or flue-gas temperatures.

• Pressure drop increases.

• The behaviour of the draught or fans changes.

• Combustion parameters deviate without an obvious cause at the burners.

The aim is not to assess the APH in isolation, but to determine how it is affecting the overall performance of the fired heater.

The performance of a process fired heater does not depend on combustion alone, but on the combined behaviour of all the systems involved in its energy balance

The heat recovery system forms part of the installation’s heat balance and influences the amount of fuel required to achieve the necessary duty.

When efficiency deteriorates progressively, the diagnosis should therefore extend beyond the burners and flame conditions.

Temperatures, pressure drop, leak tightness, flow distribution, draught and specific fuel consumption can show whether the air preheater is still recovering the expected amount of energy or is simply remaining in operation.

Identifying that difference makes it possible to address the actual cause of the problem and assess potential optimisation or revamp work on the existing installation without assuming from the outset that the entire fired heater needs to be replaced.

AITESA designs, supplies, and develops revamping projects for heat recovery equipment used in process furnaces.

The analysis is based on actual operating conditions and a comparison of those conditions with the equipment’s expected performance, taking into account thermal variables, pressure drops, and the integration of the APH with the combustion system.

The goal is to identify where performance is being lost and to determine the appropriate scope of intervention to restore efficiency without compromising operational reliability.

If the kiln maintains production but specific fuel consumption increases, temperatures change, or deviations in draft and pressure drops occur, it may be necessary to review the performance of the heat recovery system.

You can share with AITESA any available information regarding the furnace, burners, preheater, fans, and actual operating conditions. The technical team will analyze the initial data and assess which aspects should be reviewed.