Thermal criteria for tundish nozzle or taphole blockage
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i N many m e t a l s p r o c e s s i n g operations molten m e t a l s a r e poured through nozzles or o r i f i c e s , the walls of which a r e i n i t i a l l y below the f r e e z i n g t e m p e r a t u r e of the m e t a l that is being p r o c e s s e d . In o r d e r to prevent the f o r m a t i o n of a solidified c r u s t on the inside walls, which could lead to blockage at any point within the s y s tem, the net r a t e of heat t r a n s f e r f r o m the metal s t r e a m must exceed the r a t e at which heat is conducted into the w a l l s - - f r o m the inside s u r f a c e . 1 In p r a c t i c a l t e r m s this m e a n s that adequate s u p e r h e a t must be provided for the m e t a l to be poured, the nozzle walls must be p r e h e a t e d and e x c e s s i v e s u r f a c e to volume r a t i o s , (i.e., s m a l l nozzle d i a m e t e r s ) have to be avoided. While guidelines have evolved for meeting the c r i t e r i a for avoiding blockage, by and l a r g e through p r a c t i c a l e x p e r i e n c e , t h e r e e x i s t s no a n a l y t i c a l t r e a t m e n t of this p r o b l e m . The work to be d e s c r i b e d in this paper was undertaken with a view of providing an a n a l y s i s of taphole or nozzle blockage (as caused by t h e r m a l effects) together with an a p p r o p r i a t e computational t e c h nique which would allow the r e a d y t r e a t m e n t of a range of p r o b l e m s that fall in this g e n e r a l c a t e g o r y . There a r e both p r a c t i c a l and fundamental r e a s o n s for doing a n a l y t i c a l work in this g e n e r a l a r e a . The p r a c t i c a l motivation is provided by the fact that t a p hole and nozzle blockage a r e as yet incompletely und e r s t o o d . Many o p e r a t o r s do face p r o b l e m s caused by tundish nozzle blockage; 2 f u r t h e r m o r e , the design c r i t e r i a for s m a l l nozzles or d i s c h a r g e o r i f i c e s in pilot plant o p e r a t i o n s a r e not too well developed. The fund a m e n t a l i n t e r e s t in these p r o b l e m s is due to the fact that blockage of a pipe due to f r e e z i n g of the fluid has been l i t t l e studied, notwithstanding the v e r y wide range of potential applications.
JULIAN SZEKELYis Professorof Chemical Engineeringand Director, Center for Process Metallurgy,State Universityof New York at Buffalo, Buffalo, N.Y. 14214. S.T. DiNOVO is with Battelle-Columbus Labs, Columbus, Ohio. 43201. Manuscript submitted March 30, 1973. METALLURGICALTRANSACTIONS
Previous Work At f i r s t sight the extensive a n a l y s e s of solidification in continuous c a s t i n g p r o c e s s e s s'4,5 could be thought i m m e d i a t e l y r e l e v a n t to the blockage through solidification p r o b l e m s to be d i s c u s s e d h e r e . On c l o s e r examination t h e r e a r e , however, m a j o r differences b e tween these two p r o c e s s e s from both the p h y s i c a l and the m a t h e m a t i c a l viewpoints. In continuous casting the solidified shell and t
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