Why New Stainless Steel Process Tubes Can Reduce Fired Heater Performance Following a Retube

Replacing fired heater process tubes with stainless steel is a common strategy for improving corrosion resistance and extending service life. While the long-term metallurgical benefits are well understood, the thermal characteristics of newly installed stainless steel tubes are often overlooked during project planning.

Unlike process tubes that have operated at elevated temperatures, new stainless steel tubes have a polished metallic surface with relatively low emissivity. As a result, they initially absorb less radiant heat, temporarily reducing fired heater performance until a stable oxide layer develops during operation.

For facilities seeking to achieve full operating rates immediately following startup, understanding the relationship between tube surface emissivity and radiant heat transfer is an important consideration.

 

Surface Emissivity and Radiant Heat Transfer

Radiant heat transfer is the primary mechanism by which process tubes absorb energy within the radiant section of a fired heater. The efficiency of this process depends not only on furnace operation but also on the emissivity of the tube surface.

Carbon steel and low-alloy process tubes naturally develop an oxide layer during operation that provides a high emissivity surface capable of absorbing radiant energy efficiently.

New stainless steel process tubes, by comparison, typically have a polished metallic surface with emissivity of approximately 0.35-0.40. A metallurgy upgrade is often intended to allow higher heat flux, such as for increased throughput or process severity. However, the heater may not realize the full benefit of the new material immediately after startup. Until a stable oxide layer develops, which may take several months to years, radiant heat absorption is reduced, shifting more furnace duty to the convection section.

The result can include:

  • Higher bridgewall temperatures
  • Potential convection section limitations that may reduce production or cause excessive fouling
  • Reduced radiant heat absorption
  • Excess heat transfer to the convection section
  • Reduced operating margin during startup

Although natural oxidation gradually restores surface emissivity, the process may take several months to years, depending on operating conditions.

Why Bridgewall Temperature Is Important

When less radiant energy is absorbed by the process tubes, a greater proportion of the furnace duty passes beyond the radiant section. This increases bridgewall temperatures and exposes downstream components, including convection tubes, tube supports, and associated internals, to higher thermal loads. If the bridgewall temperature limit is reached, production must be reduced.

Maintaining efficient radiant heat absorption enables more of the furnace duty to be absorbed within the radiant section, reducing bridgewall temperatures while maintaining the intended heat distribution throughout the heater.

 

Restoring Surface Emissivity Before Startup

The temporary reduction in radiant heat transfer can be avoided by restoring surface emissivity before the heater returns to service.

Applying a thin-film ceramic coating before startup increases the surface emissivity of new stainless steel process tubes from approximately 0.35-0.40 to 0.90. This enables efficient radiant heat transfer from startup rather than waiting for a natural oxide layer to develop during operation.

Depending on project requirements, the coating can be applied in a fabrication shop before shipment or in situ. Recent advances in coating technology have eliminated the need for separate thermal curing before startup. Final curing occurs during initial startup. Eliminating this pre-startup step reduces preparation time, simplifies logistics, and lowers overall project costs.

 

 

Demonstrated Performance in Refinery Service

The benefits of restoring surface emissivity were demonstrated during a fired heater retube at a North American refinery.

New 317H stainless steel process tubes were coated before installation in atmospheric crude and vacuum heaters. Measured operating data collected over eight months were compared with engineering calculations for identical uncoated stainless steel tubes.

The coated tubes demonstrated bridgewall temperatures that were 111-113°C (200-203°F) lower and increased radiant energy absorption by the process tubes by 2.44-2.60 MW. The heaters also achieved full operating capability immediately following startup without waiting for natural oxidation to restore tube emissivity.

The project demonstrated that restoring surface emissivity before installation can eliminate the temporary reduction in radiant heat transfer normally associated with new stainless steel process tubes.

 

 

High Emissivity Coatings for Different Fired Heater Applications

High emissivity ceramic coatings can be applied to different surfaces within a fired heater, with the application selected according to the operational objective.

  • For new stainless steel process tubes, the coating restores surface emissivity before startup, allowing the heater to achieve its intended thermal performance immediately following a retube.
  • For existing process tubes, the coating maintains a clean, high emissivity surface by preventing oxidation and carburization. This can help operators increase throughput at the same fuel consumption or reduce fuel consumption at the same production rate, while extending tube life.
  • For refractory surfaces, high emissivity coatings improve the efficiency of radiant heat transfer from the refractory to the process tubes. Depending on the heater configuration, this can increase heater capacity, reduce fuel consumption, or lower bridgewall temperatures without major modifications to the fired heater.

Although these applications use the same underlying technology, they address different operational challenges and are evaluated independently to determine the most appropriate solution for each fired heater.

 

 

Find the Right Solution for Your Fired Heater

High emissivity coatings can improve fired heater performance in different ways, depending on where they are applied. Explore our solutions for new and existing process tubes, as well as refractory surfaces.

 

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