API 560 Refractory Thickness Calculation

Refractory Thickness Calculation

API 560 Considerations for efficient refractory design


In fired heaters, refractory lining plays a critical role in protecting the metal casing from high temperatures and ensuring efficient heat transfer. This article discusses the process of calculating the required refractory thickness inside a fired heater according to the guidelines provided in API 560. It also includes relevant conductive heat transfer calculations.


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1. Introduction

API 560 sets the standards for the design of fired heaters, emphasizing the importance of refractory linings for energy efficiency and operational safety. Refractory thickness needs to be optimized to balance between heat conservation and adequate protection of the steel shell. An appropriate refractory thickness ensures the heater casing does not exceed specified temperatures, maintaining structural integrity while minimizing heat loss.

2. Overview of Refractory Materials

The choice of refractory material is influenced by the nature of the process and operating temperature. Common materials include:

  • Alumina-Silica Refractories: Suitable for temperatures up to 1600°C
  • Silicon Carbide: High thermal conductivity and abrasion resistance
  • Magnesia: Offers superior resistance to chemical attack

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3. Thermal Conductivity and Heat Transfer Principles

Refractory thickness calculation hinges on the conductive heat transfer equation, which can be expressed using Fourier's law:

q = k * A * (T_hot - T_cold) / L


  • q: Heat transfer rate
  • k: Thermal conductivity of the refractory material
  • A: Surface area
  • T_hot: Temperature of the inner refractory surface
  • T_cold: Temperature of the outer refractory surface
  • L: Refractory thickness

The thermal conductivity is usually temperature-dependent and is provided by the material manufacturer.

4. Calculation Steps for Refractory Thickness

Step 1: Determine Process Parameters

  • Operating Temperature: Expected maximum temperature on the inner refractory surface.
  • Steel Shell Temperature: Maximum allowable temperature for the steel casing, often around 200°C.

Step 2: Select Refractory Material

Choose a refractory material based on process requirements and obtain its thermal conductivity data.

Step 3: Apply Fourier’s Law to Calculate Refractory Thickness

The equation can be rearranged to solve for the thickness:

L = k * (T_hot - T_shell) / q


  • Thermal Conductivity: 1.5 W/(m·K) (typical for alumina-silica refractories)
  • Heat Transfer Rate: Approximate heat flux through the refractory

Step 4: Check API 560 Requirements

Verify that the calculated thickness meets the requirements of API 560:

  • Refractory Lining Thickness (API 560):
    • Wall Lining: 2-4 inches (50-100 mm)
    • Floor Lining: 3-6 inches (75-150 mm)

In this example, the calculated thickness of 2.4 m is excessive. Thus, the heat flux assumption must be revisited to align with practical expectations.

5. Effect of Temperature on Refractory Thermal Conductivity

The thermal conductivity of refractory materials changes significantly with temperature. As temperature rises, most refractory materials exhibit an increase in conductivity, which impacts the overall heat transfer rate. This relationship is crucial when designing the refractory lining thickness to ensure accurate heat transfer calculations.

The graph below shows the variation of thermal conductivity with temperature for different refractory materials:


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6. Reduced Efficiency and Safety Concerns with Poor Refractory Design or Maintenance

Inadequate refractory design or poor maintenance practices can significantly impact the efficiency and safety of a fired heater. Key concerns include:

  • Heat Losses: Insufficient refractory thickness can result in excessive heat loss, reducing the thermal efficiency of the heater and increasing operational costs.
  • Shell Overheating: A poorly designed refractory can lead to excessive casing temperatures, compromising the structural integrity of the fired heater shell and creating a significant safety hazard.
  • Fouling and Slagging: Improper material selection or inadequate maintenance may result in slag accumulation, further reducing heater efficiency and risking tube failures.
  • Energy Consumption: Increased heat losses due to refractory degradation lead to higher fuel consumption, thus reducing the overall efficiency of the heater.

Therefore, following API 560 requirements and implementing a rigorous maintenance schedule are essential to ensuring optimal performance and longevity of the refractory lining.


Calculating the refractory thickness in accordance with API 560 involves understanding the process parameters, selecting suitable refractory material, and applying heat transfer equations. By optimizing the refractory thickness, operators can ensure energy-efficient fired heater operations while protecting the steel shell from excessive heat.


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