Heat Transfer Design Considerations For Refractory Linings – Part 1
The heat transfer analysis of multi-layer refractory concretes in both transient and steady state conditions is particularly important for many pyro-processing industries. Knowing the temperature profile through the different layers is important for design and operational trouble shooting. It is clear that the design of refractory linings is becoming increasingly important both structurally and in energy efficiency terms. Also industry “fitness for service” (FFS) guidelines for pressure vessels require that the plant is safe for personnel and the public. FFS assessments will include refractory linings and the practice of simply changing the refractory design without proper engineering approval could leave companies exposed.
Heat transfer theory is well understood and it is possible to predict temperature profiles under various conditions with reasonable accuracy, where accurate thermal property and convective/radiative boundary conditions are known. Refractory linings are generally composed of multiple layers of varying insulating materials and a dense abrasion resistant hot face layer. For steady conditions, when the layer is flat or thin (thickness < 5% of the radius of curvature), conduction heat flow, Q, through each layer is well defined by Q = /Δx A ΔT, where is the average thermal conductivity of the layer material, Δx its thickness, A the area for heat transfer and ΔT the temperature difference across the layer. Convection is generally applied to one or both sides by using some simplified correlations to account for wind and other flow effects.
In many cases, simplifying assumptions are made when undertaking a heat transfer analysis. Incorrect assumptions can lead to poor or the wrong selection of materials which in turn can lead to excess heat loss, refractory failure, overheating of vessels, too low shell temperatures or poor lining design with increased capital costs.
This paper discusses the problems associated when undertaking heat transfer analysis. Specifically, the effect of taking simplified assumptions and the problem of large variations in refractory thermal conductivity values. The effect of refractory material conductivity values and air gaps at the interface in a 1D analysis is provided. It is shown that using a 1D heat transfer model to predict temperature profiles in refractory systems can result in serious errors.