Designing a band for a detachable unit of an induction channel furnace

Abstract

In modern induction channel furnaces (IAK-25/2.1 and IAK-40 3.5 types) and lining of the channel sections, which is made from refractory concrete containing a high-alumina filler bonded with 18% water glass, experiences temperature variations from 330 to 1050~ The detachable induction units consist of thick-walled cylinders that are heated irregularly over their radius. The temperature drop AT equals 200~The detachable unit has a U-shaped channel section (see Fig. 1) consisting of longitudinal channels 1, 2, and a transverse channel 3, which, upon installation of the unit in the furnace, encompasses a toroidal inductor, including a magnetic circuit 4 and a coil 5. The longitudinal and transverse channels each have two layers of lining: the internal layer 6 made of refractory concrete on which is laid, with a tolerance (before the installation of the unit) in the lon#tudinal channel, a metal cylinder 7 made of nonmagnetic material; and an external layer 8 applied to the cylinder 7 and made of nonsintering, heatinsulating refractory fiber (kaolin wool). Both layers are contained in frames 9 and 10, made of nonmagnetic material. By means of the refractory gasket 11, the unit is fitted to the furnace frame 12, using bolt connections; it operates according to the principles of a channel furnace.When the furnace is working, liquid metal, heated in the channels by the induced currents, creates monolateral heating of the lining of the longitudinal and transverse blocks. This causes, in an annular direction of the lining 6, compression of the internal, and tension of the external, layers. The temperature field is symmetrical relative to the axis of the cylinder, and constant over its length.It is established [1, 2] that cracks develop on the outside of the concrete cylinder, and these are caused mainly by the normal tensile stresses which reach magntidues close to the tensile strength of the refractory concrete used to line the channels of the induction furnace. It is known that the refractory of the lining has a low tensile strength (4-6 MPa). These tensile stresses in the danger zone of the lining can be eliminated by using a band in the form of a cylinder made of nonmagnetic material, placed on the concrete cylinder with a certain clearance.The design-calculation for the detachable unit with the band consists of solving two problems: determining the main stresses in the double-layer composite cylinder that develop during pressing of the layers (normally by means of hot fitting); and determining the main stresses in the double-layer, nonuniform cylinder during its unequal stationary, assymmetrical heating.Working out the first problem with the well-known Lam6 and Gadolin equations [3] does not take into account the axial stress that develops during the coupling of the cylinders with the clearance, and also during tightening of the detachable unit with a screw device. The resulting axial stresses may be of the same order as tangential and radial stresses. Ignoring axial stresses in designing the component tubes leads to...