One promising approach to improving fuel efficiency in high-temperature equipment that employs flame heating (such equipment traditionally has lower fuel efficiency characteristics) is to arrange for controlled heat exchange in the work ing space of the unit. This may include the use of cyclic (pulse) regimes for the equipment’s heating [1]. To save fuel (natural gas), provide greater flexibility in controlling the heating process, and improve the preheating of ladles for service in open-hearth shops, the Azovstal’ Metallurgical Combine has developed a pulse heating (PH) regime for the stands that are used to dry and preheat 220-ton steel-pouring ladles. The lining of walls of the ladle has a fireclay-brick reinforcing layer with a thickness δ rfc = 65 mm and a working layer with δ wkg = 160‐180 mm. The working layer is composed mainly of tamped quartz-clay refractory MKG-1 (or MKG-2) with an initial moisture content W i = 8‐12%. The combine also uses ladles in which the working layer of the walls is made of brick (fireclay). The bottom of the ladle has a reinforcing layer ( δ rfc = 195 mm) and a working layer (δ wkg = 65 mm) made of fireclay brick. The stand, equipped with gas burners, has a removeable metal cover ( δ cr = 50 mm). GARSY-type injection burners are installed at the center of the cover. The use of PH improves fuel efficiency and the efficiency of the heating operation has a whole. In addition, the hottest region (the flame) continuously “moves” relative to the lining. This makes the heating of the lining more uniform and reduces the likelihood of the formation of local hot spots, which could lead to collapse of the lining during the drying operation. The heat flux on the inside surface of the ladle lining is determined by the outcome of a complex radiative heat transfer process that takes place within the system “fuel combustion products (flame) ‐ lining of the ladle walls ‐ lining of the bottom ‐ cover (surrounding medium).” The heat flux is also influenced by forced convection of the combustion products in the ladle as they leave the cavity through the gap under the cover (ladle mouth). Heat transfer inside the lining takes place due to unsteady heat conduction and the removal of heat from the metal shell of the ladle to the surrounding medium by radiation and convection. Here, we made the following assumptions: the temperature of the combustion products at the outlet of the unit is equal to the effective temperature of the gas in the ladle (“diffuse-flame” model); since the ratio of the diameters of the layers of the ladle lining <1.2, the lining was (with allowance for the heat-conduction problem) represented as an infinite two-layer plate; the phase transformation zone which exists during drying was represented in the form of an isothermal (at the phase tra nsformation temperature) frontal surface that separates the moist material from the dried material, with there being a sudden chan ge in moisture content at the front; due to the lack of reliable information on the heat transfer coefficient and the coefficient that characterizes the transfer of moisture inside the moist material during its drying [2], we introduced the concept of effective t hermal conductivity for the moist material. The value of this parameter was determined by analyzing experimental data.
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May 1, 2004
V V Semakhin + 1
