Dmitriy Kosolukin

Relevance

The most important issue facing the field of heat treatment of metal for rolling, is the problem of energy saving. Many modern units during heating for rolling, the metal consumption of fuel reaches 120kg / t, which is due, including the low-r ate of the level of 0,1-0,3. This implies that a substantial reserve of reducing specific fuel consumption is the growth rate of recuperation. Fuel in-put for given heating parameters (steel grade, thermal value, and the initial and final heating temperature) is determined by coefficient of using fuel and heat loss. To minimize the need to raise and lower coefficient of using fuel heat loses. So that the increase in can coefficient of using fuel be achieved by: - Reducing the temperature of flue gases from the chamber, can be achieved by increasing heat transfer from gases to the heated material (increasing the luminosity of flare and increased convective heat transfer). - Increase the recovery rate (increase of heat transfer surface and heat gain) Physical heat of air, gaseous fuel increases the coefficient of using fuel, as it is obtained by cooling the flue gases before releasing them into the atmosphere, ie, for heating the components of combustion not spending additional chemical energy of fuel. This method of increasing coefficient of using fuel was named: heat recovery flue gas recuperators or regenerators. Total heat capacity of the furnace is the only external and the primary source of energy for operation of the furnace (primary energy).

Scientific Significance

The paper proposes several approaches to improve the accuracy of calculations of thermal energy recovery and justify the selection of rational design of heat exchanger in the reconstruction or creation of new furnaces. The practical value of the work: Most of the heating furnaces of domestic enterprises are characterized by low recovery rate, which determines the high level of specific fuel consumption. This technique allows the calculation of any heat exchanger into this cavity and the furnace flue to assess the effectiveness of heat exchanger unit

The main part

Heating metal before rolling is a very important stage in the formation of quality and prices of finished goods. The process of heating metal requires considerable expenses. Currently acute problem of energy-saving technologies of metal heating before rolling, as energy prices have continued grow high. Reducing fuel consumption for heating 1ton of metal in the heating furnaces is a key indicator that affects the cost of finished products.

To analyze the efficiency of the heat of furnaces, we make use of the coefficient of efficiency, Which represents a fraction of the chemical energy of fuel, internalized heated material? and utilization of fuel, the corresponding proportion of the chemical energy of fuel Left in the chamber (this energy goes into heating the material and heat treatment of the working chamber). Often used the coefficient of fuel utilization. There are the following measures to increase the utilization of fuel: - Reducing the temperature of flue gases from the chamber, which can be achieved by increasing heat transfer from gases to the heated material (increasing the luminosity of flare and increased convective heat transfer); - Increase the recovery factor, which is a fraction of the heat of the combustion products leaving the oven chamber, returned to it with the heated air in the recuperator (achieved by increasing the heat transfer surface and heat gain).

One of the most powerful levers of increasing the coefficient of fuel used at the present stage of development of metallurgical heat engineering is the growth coefficient recovery. Heated air is realized by means of special heat exchangers regenerative or regenerative type. In practice, we use the following form recuperators: ceramic, steel plaintube, needle, spiral, etc. Work on intensifying the process of convective heat transfer and create the most cost-effective technological exchange equipment has led recently to a significant improvement of designs of heat exchangers for various industries. Work BP Tebenkova, DD Kalafati, VA Smirnov and other scientists in this field suggest that any heat exchanger has a peak performance for heat removal, then there exists an optimum at which the ratio of the heat recovery to the cost of energy for pumping coolant has a maximum value. Today is the actual scientific and technical problem of finding the optimal design and technological parameters of existing and emerging types of energy recovery furnaces.

Material for metallic recuperators are chrome and nickel-type curves were 18Cr10NiTi H25N20S2 with permissible temperature of the smoke need recuperator 1100-1200 ° C, or chrome steel type with X17 admits the reversible temperature smoke no more than 1000 ° C. The thickness of the steel wall S = 2-3 mm. Heating temperature of air (or gas) in the existing recuperator does not exceed 400-500 ° C. Ceramic recuperators made of a carbonrundum and fireclay octagonal tube length 300-350 mm and wall thickness S = 10-12 mm, and less chamotte hollow blocks. In the ceramic recuperator heated air only because of their leakage. Maximal temperature of the smoke before the recuperator 1250-1300 ° C, hot air - 800-850 ° C. Higher temperatures - the only advantage of ceramic recuperators, their shortcomings are: - Leaking pipes connections among themselves, through which occurs the air leakage, which varies in the service process and violates the normal combustion; - Large dimensions because of the high thermal resistance of the ceramic wall, and therefore the heat transfer coefficient is 4-5 times smaller, and the surface heat transfer as many times greater than that of the metallic recuperator. For small furnaces, recuperators type "fuser", consists of a bundle of pipes, cast iron, filled with the stopped channels between the rows of tubes. Through the pipes pass heated air or gas, and through the channels in cast iron, placed perpendicular to the beam pipe - flue gases. The disadvantage of thermal blocks is their large relative mass per unit of heat transferred. Thermal efficiency of two-sided needle tube is much higher than the one-sided needles, but they are less heat-resistant and easier to clog. Most effectively establish needle recuperators so that flue gases pass through tubes in a vertical direction, and themselves recuperators are readily accessible for inspection and repair. The circumstances of studies on heat transfer and aerodynamic resistance of needle surfaces of various types were held AK and Scriabin A. Kuznetsov. Results of studies often had significant differences, however, BP Tebenkov conducted studies aimed at obtaining the thermal and aerodynamic characteristics of the needle of rekuperator pipes.

Ways to increase the coefficient of heat recovery: a) The impact on the flow becomes turbulent inserts. This method is largely similar to the way of influence on the flow of working medium corrugated shape of the surface heat transfer. Known to insert a slave medium mixing flow in the wall region (circular insert, inserts, providing a swirl flow at the entrance to the channel, and the entire length of the channel). b) Increasing the surface area of heat transfer by means of its fins (developed surface). c) Combined methods of intensifying heat transfer. When used simultaneously in several ways intensification in some cases achieved a greater increase in heat transfer coefficient than using each method separately. Example: the pipe with rough walls with tape swirlers flow, pipe tubes with internal fins and with inserts in the form of tape swirler; tubes with external fins exposed to vibration, the application of the pulsations.

A method for analyzing the thermal efficiency of the heat exchanger, which allows to make universal pre-design issue recuperator choice for the reconstruction of an existing unit or create a new, as follows: - Set the volume of the cavity to accommodate the heat exchanger; - For analysis is taken a few design heat exchanger (or conceptual model created) on the basis of specific surface determined by limiting heat transfer surface of each of the designs, which can be arranged in a given cavity; -Selected values of heat transfer coefficient for each of the analyzed energy recovery from a range of typical values, possible pre-calculation of these values, taking into account the thermal performance of the unit to which the "try on recuperators; , With complex relationships developed for dynamic calculation of the heat exchanger to pre-define each of the design value of the recovery value of the fuel use and fuel consumption for given parameters of heating the material; - Specifies the value of the coefficients of heat transfer on the basis of a calculated pre-regeneration process; - Finally to compute the heating of air in the recuperator; - Using the standard tools of economic analysis to compare the technical and economic characteristics of each type of energy recovery.

Conclusion