The lifetime and efficiency of any technical furnace plant depend largely on a suitable selection, the quality and the correct installation of the refractory furnace lining. The furnace designer, furnace builder, refractory manufacturer, and the engineer responsible for the operation are well aware of the relationships between these factors.
A suitable selection of the furnace lining can only be made with an accurate knowledge of the properties of the refractory materials, and of the stresses on the materials during service.
The relationship between different thermal operational stresses in industrial furnaces and the important properties of refractory bricks during service forms the basis for the classification of the properties of refractory materials and the test methods. These methods are also of importance for quality control and development of new products.
Thermomechanical properties are determined using high-temperature test methods with external forces causing stresses on the tested material. The stress-strain behaviour of refractories at high temperatures is very complicated because not only reversible elastic strain occurs, but also non-reversible non-elastic time-dependent deformations.
Therefore, the thermomechanical behaviour of refractories must be considered as the interrelation of four variables:
|Type of stress||Important operational properties|
|thermal and thermomechanical|
refractoriness under load (RUL)
creep in compression (CIC)
hot modulus of rupture (HMOR)
thermal shock resistance
Refractoriness under load (RUL) is a measure of the resistance of a refractory product to subsidence when it is subjected to the combined effects of load, rising temperature and time. The range in which the softening occurs is not identical to the melting range of pure raw materials, but it is influenced by the content and the degree of distribution of low melting point fluxing agents.
No single test method can objectively measure refractoriness under load under all the possible combinations of the many factors involved, including the duration of exposure.
Certain limitations must be accepted, in order to have a single standard test method. Such a method is described in ISO 1893, Refractoriness under load (RUL; differential - with rising temperature).
Creep in compression (CIC, according to ISO 3187) refers to the percent of shrinkage of a refractory test piece under a constant load and exposed to a constant high temperature over a long period of time.
Measuring the modulus of rupture of refractories at elevated temperatures has become a widely-accepted means of evaluating materials at operating temperatures. Many companies base their specifications on this type of test. It is a very important parameter for quality control which, together with other thermophysical properties, gives information about the behaviour of refractory materials used for furnace linings.
The modulus of rupture is defined as the maximum stress that a rectangular test piece of specified dimensions can withstand when it is bent in a three-point bending device; it is expressed in N/mm2 or MPa.
Thermal conductivity is of special importance when refractory materials are used for the lining of industrial equipment. To a considerable extent thermal conductivity has a determining influence on the important parts of the construction. Low values are required if heat losses are to be kept to a minimum (insulation), while high values are necessary in materials in areas where heat transfer is important (control of hot face temperatures). Thermal conductivity, λ, is defined as a density of heat flow rate divided by temperature gradient; the units are W/(m.K).
Several methods have been developed for the determination of thermal conductivity of refractory and heat insulating materials at elevated temperatures. The only internationally standardized method is the hot-wire method (ISO 8894).
The hot-wire method is a dynamic, absolute method based on the measurement of the temperature increase
- of a linear heat source (hot wire)
- cross-wire-technique -
- at a certain location at a specified distance from a linear heat source
- parallel-wire-technique -
Both, hot wire and thermocouple are embedded between two test pieces which form the test assembly. The increase in temperature as a function of time, measured from the moment the heating current is switched on, is a measure of the thermal conductivity of the materials of which the test pieces are made.
A further variation called "Platinum Resistance Thermometer Technique", or "T® Technique" is described in ASTM-C 1113. Here an integral temperature measurement is carried out over the length of the hot wire between the voltage steps. This means that the hot wire itself functions as both, heat source and temperature sensor. The temperature increase of the hot wire is determined from its change in resistance; the calculation of the thermal conductivity is the same as that of the cross-wire technique.
The NETZSCH TCT 426 operates according to the hot-wire method using all three techniques described here.