Insulation Refractory

Features and Insulation Principles of Lightweight Insulating Refractory Materials

Release Time: 2025-01-23
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This article explores the five key characteristics and insulation principles of lightweight insulating refractory materials, crucial components in high-temperature industrial applications.

Lightweight refractory materials, primarily used as linings and insulation in high-temperature kilns and equipment, offer significant advantages over dense refractory materials. Their low bulk density, low thermal conductivity, low compressive strength, and high apparent porosity effectively prevent heat transfer through the enclosure, minimizing heat loss in high-temperature equipment.

PART 01: Low Bulk Density

Bulk density, defined as the ratio of the mass of a dry product to its total volume, is significantly lower in lightweight refractory materials. Typically, their bulk density ranges from 0.6 to 1.2 g/cm³, while semi-lightweight materials range from 1.2 to 1.8 g/cm³. As a key factor in evaluating lightweight materials, minimizing bulk density while meeting the required strength for kilns or high-temperature equipment is a primary objective in manufacturing.

PART 02: Low Thermal Conductivity

Thermal conductivity, a crucial indicator of a refractory material’s insulation performance, is minimized in lightweight materials. A lower thermal conductivity, while maintaining sufficient strength, helps retain heat within kilns and high-temperature equipment, reducing heat loss. The presence of numerous pores filled with insulating gas contributes to the low thermal conductivity of these materials. The size and uniformity of these pores directly affect both insulation performance and material strength; smaller pores generally result in better insulation. Lightweight refractory materials typically exhibit thermal conductivity values below 1.0 W/(k·m) (600℃×3h), while semi-lightweight materials are generally below 1.26 W/(k·m) (600℃×3h).

PART 03: Low Compressive Strength

Due to their high apparent porosity and low bulk density, lightweight refractory materials lack dense internal crystalline connections, resulting in lower compressive strength compared to dense refractories. They also exhibit poorer wear resistance and erosion resistance. Consequently, they are typically used as insulation layers behind the working lining, rather than as the working lining itself. Design considerations prioritize minimizing bulk density and thermal conductivity while ensuring adequate load-bearing capacity.

PART 04: High Apparent Porosity

Porosity is the most visually apparent characteristic of lightweight insulating refractory materials. These pores are primarily created by adding pore-forming agents that foam, burn out, or react during the manufacturing process. Pores can be classified into three types: (1) open pores, (2) closed pores, and (3) interconnected pores. Interconnected pores are the most detrimental to insulation performance, while closed pores provide the best insulation.

Apparent porosity, defined as the ratio of the total volume of pores to the total volume of the material, is typically greater than 45% for lightweight insulating refractory materials. For materials with a bulk density around 0.5 g/cm³, the apparent porosity can reach as high as 80%.

PART 05: Wide Applications

Lightweight refractory materials are widely used as linings or insulation layers in industrial kilns. Their low thermal conductivity reduces heat loss from kiln brickwork, saving energy and improving production efficiency. However, their high porosity, loose structure, low strength, and poor resistance to erosion and wear limit their use to linings and insulation in high-temperature thermal equipment. They are generally not suitable for working linings or load-bearing applications.

Insulation Principles

Heat, a form of energy, is closely related to the movement of molecules, atoms, and electrons within a material. All thermal phenomena result from the collision, transfer, and movement of internal particles. In kiln and thermal processing industries, energy loss due to heat transfer is significant, with temperature differences often exceeding 800℃. Lightweight insulating refractory materials mitigate heat transfer through their internal pores, providing thermal insulation.

These materials typically consist of solid and gaseous phases. The overall heat transfer process involves:

  • Heat conduction between solid particles.
  • Heat transfer between solid and gaseous phases.
  • Convection within the gaseous phase.
  • Radiative heat transfer between pores.

At high temperatures, radiation is the dominant factor influencing thermal conductivity, while at room temperature, its influence is less significant.

The high apparent porosity of lightweight insulating refractory materials results in a large volume of air trapped within the pores. Since air has a much lower thermal conductivity than solids, this significantly reduces overall heat transfer. While a high air content can increase convective heat transfer within the pores, the predominantly closed-pore structure of these materials minimizes this effect. Heat transfer between the solid and gaseous phases can be represented by:

This implies that higher apparent porosity leads to increased gas content and reduced solid content, thus decreasing the effective thermal conductivity between the solid and gaseous phases and improving insulation performance.

Radiative heat transfer can be calculated using the Stefan-Boltzmann Law (2) and Fourier’s Law (3):

From (2) and (3), it can be deduced that K2 is proportional to T³, meaning the contribution of radiation to effective thermal conductivity is proportional to the cube of the thermodynamic temperature. Therefore, radiation dominates thermal conductivity at high temperatures. The effective thermal conductivity of insulating materials is the sum of solid-gas heat transfer and radiative heat transfer. Research suggests that the effective thermal conductivity of porous insulating materials can be accurately represented by the following simplified non-coupled relationship:

This equation indicates that the effective thermal conductivity of porous insulating materials is directly related to the material’s porosity, the thermal conductivities of the solid and gaseous phases, and the average thermodynamic temperature. This theory effectively explains the indirect effects observed during the use of insulating materials.

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