A refining element is generally formed with a pattern of bars and intermediate grooves. The bars and grooves are formed in different ways, depending on which fibrous material is worked and the degree of refining desired and, thus, in the case of lignocellulosic material, the pulp quality which is desired. The bars have an upper surface and side surfaces so that longitudinal edges are formed between the upper surface and respective side surface. The bars can be, for example, continuous or discontinuous, and can be arranged in different patterns. The working of the fibrous material is substantially carried out by the bars of the refining elements. The refining gap is formed so that the fibrous material, seen in the radial direction, passes from the inside outwardly. Farthermost inward in the refining gap the refining elements are normally formed so as to bring about a first disintegration of the material and to advance the material outwardly in the refining gap. A certain defibering, i.e. separation of the fibers of the lignocellulosic material, also takes place in the inner portion of the refining gap where the distance between the refining surfaces is the greatest. Thereafter, that distance decreases outwardly in order that the desired working or refining of the fibrous material shall be obtained.
During the refining of fibrous material of high concentration, and above all at high energy inputs, it has been found necessary to place flow restrictions, so-called dams, in the grooves of the refining elements in order to prevent unworked material from passing out through the refining gap. These dams, however, form an obstacle for the steam developing in the refining gap during the refining. A high steam pressure is thereby created in the refining gap. This high steam pressure has a negative effect on the capacity and operational stability of the refiner. It also implies a limitation on the possible energy input. The developed steam, thus, will be forced by the flow restrictions upwardly out of the grooves, and will disturb the material flow through the refining gap.
One way of solving this problem would be to supply dilution water to the refining gap in order to thereby condense the steam. This, however, results in reducing the material concentration to a low level, and thus in a deteriorated pulp quality.
During the working or refining of fibrous material with low concentration no steam development takes place, and the material is partially transported by the liquid flow out of the refining gap. Also in this case dams are usually used to prevent unworked material from passing out through the refining gap. It can imply, however, that the flow through the refining gap will be much too low.