MANUFACTURE AND PERFORMANCE OF GAS FURNACE

Transcription

1 International Journal Metallurgical & Materials Science and Engineering (IJMMSE) ISSN Vol. 3, Issue 1, Mar 2013, TJPRC Pvt. Ltd. MANUFACTURE AND PERFORMANCE OF GAS FURNACE EMAN J. ABED Lecturer, Department of Materials Engineering, University of Kufa, Najaf, Iraq ABSTRACT This research is centered on the manufacturing of a gas furnace using locally sourced materials.the manufacturing philosophy is to eliminate the use of heating elements requiring electric power which is poorly supplied in the country. Design drawings were produced and galvanized sheet steel was used for the fabrication of the furnace, while the other components needed for the design were selected based on functionality, durability, cost and local availability. The furnace was assembled by lining the inner wall of the casing. Experimental tests have been performed to evaluate the performance of the furnace. It was observed that the furnace has a fast heating rate nearly 25C /min and less gas consumption rate to attain temperature in the burning zone could be as high as 1000 C in the burning chamber and 700 C in the inner pot. It was also observed that the furnace has good heat retaining capacity; can be easily maintained and safe for use. A melting gas furnace can be used to melt down metals like tin, lead, brass, copper and aluminum. KEYWORDS: Fabrication, Gas, Refractory, Efficiency, Furnace INTRODUCTION A furnace is an equipment used to melt metals for casting or to heat materials to change their shape (e.g. rolling, forging) or properties (heat treatment) [1,2]. Heating furnaces are usually classified according to (1) the purpose for which the material is heated, (2) the nature of the transfer of heat to the material, (3) the method of firing the furnace, or (4) the method of handling material through the furnace [3]. Furnaces are refractory lined vessels that contain the material to be melted and provide the energy to melt it [4].Based on the method of generating heat, furnaces are broadly classified into two types namely combustion type (using fuels) and electric type. In case of combustion type furnace, depending upon the kind of combustion, it can be broadly classified as oil fired, coal fired or gas fired [5]. Modern furnace types include, electric arc furnaces (EAF),induction furnaces, cupolas, reverberatory, and crucible furnaces. Furnace choice is dependent on the alloy system and quantities produced. Most aluminum foundries use either an electric resistance or gas heated crucible furnaces or reverberatory furnaces. Furnace design is a complex process, and the design can be optimized based on multiple factors. Furnaces in foundries can be any size, ranging from mere ounces to hundreds of tons, and they are designed according to the type of metals that are to be melted. Also, furnaces must be designed around the fuel being used to produce the desired temperature. For low temperature melting point alloys, such as zinc or tin, melting furnaces may reach around 327 C. Electricity, propane, or natural gas is usually used for these temperatures. For high melting point alloys such as steel or nickel based alloys, the furnace must be designed for temperatures over The fuel used to reach these high temperatures can be electricity or coke [4]. The fuel is a substance which when once raised to its ignition temperature continues to burn if sufficient oxygen or air is available. The main constituents of any fuel are carbon and hydrogen. These constituents are called combustibles. The calorific value of a fuel is amount of heat liberated by its complete combustion. For solid and liquid fuels, calorific value is expressed in kj/kg, where as for gaseous fuels it is expressed as kj/m 3 [6].

2 110 Eman J. Abed Furnaces are generally rated in either British Thermal Units (BTUs) or in Watts/Kilowatts (W/kW). These are two ways of expressing the same thing - the amount of heat energy the unit can produce in one hour. One BTU is the amount of heat energy needed to raise one pound of water by one degree Fahrenheit at sea level. It's also approximately the amount of energy found in a match. For the sake of comparison, 1,000 Watts (W) = 1 Kilowatt (kw) = 3,413 BTUs [7]. The basic design factors which determine furnace capacity are grate area and furnace volume. The required grate area depends upon the selected burning rate, which varies between 60 and 90 lb/(ft 2.h) of refuse in practice. Conservative design, with reasonable reserve capacity and reasonable refractory maintenance, calls for a burning rate between 60 and 70 lb/ft 2 of grate area. Furnace volume is a function of the rate of heat release from the fuel. A commonly accepted minimum volume is that which results from a heat release of 20,000 Btu/(ft 3.h). Thus, at this rate, if the fuel has a heat content of 5,000 Btu/ lb, the burning rate would be 4 lb/(h.ft 3 ) of furnace volume. A conservative design, allowing for some overload and possible quantities of refuse of high heat content, would be from 30 to 35 ft 3 /ton of rated capacity [3]. Furnace ideally should heat as much of material as possible to a uniform temperature with the least possible fuel and labor. The key to efficient furnace operation lies in complete combustion of fuel with minimum excess air. Furnaces operate with relatively low efficiencies (as low as 7 percent) compared to other combustion equipment such as the boiler (with efficiencies higher than 90 percent. This is caused by the high operating temperatures in the furnace [1]. This project is centered on the fabrication of a gas fired furnace using locally sourced materials. The design philosophy is to eliminate the use of heating elements requiring electric power which is very expensive cost. Furnace is designed for melting lead, aluminum, tin, zinc, type metal, scrap and all other low or medium-fusion metals and alloys. NATURAL GAS AND ITS CHARACTERISTICS Natural gas is one of the major combustion fuels used throughout the country [8].Natural gas consists of a high percentage of methane (generally above 85 percent) and varying amounts of ethane, propane, butane, and inerts (typically nitrogen, carbon dioxide, and helium). CH 4 = 70.9%, C 2 H 6 = 5.1%, H 2 = 3%, CO + CO 2 = 22% The average gross heating value of natural gas is approximately 1,020 British thermal units per standard cubic foot (Btu/scf), usually varying from 950 to 1,050 Btu/scf. [6]. FURNACE EFFICIENCY The efficiency of a furnace is the ratio of useful output to heat input. The furnace efficiency can be determined by both direct and indirect method [9]. Furnace efficiency measures the amount of heat produced compared to the amount of fuel burned. The percentage of fuel that a furnace turns into actual heat is called Annual Fuel Utilization Efficiency (AFUE.) A furnace is then labeled as low-, mid-, or high-efficiency. Low-efficiency models, under 78% AFUE, are older, and not generally for sale. Mid-efficiency models start at 78% and go up to 90% AFUE. Anything above that level is considered high-efficiency. In 1992, the US Department of Energy set the minimum AFUE level on a new furnace at 78% [10]. Direct Method Testing The efficiency of the furnace can be computed by measuring the amount of fuel consumed per unit weight of material produced from the furnace.

3 Manufacture and Performance of Gas Furnace 111 The quantity of heat to be imparted ( Q) to the stock can be found from the formula Q= mc. px t2 t1... eq. ( ) 2 Where: Q = Quantity of heat in kcal m = Weight of the material in kg C = Mean specific heat, kcal/kg C p t 2 = Final temperature desired, C Indirect Method t 1 = Initial temperature of the charge before it enters the furnace, C [1,8]. The furnace efficiency can also be determined through the indirect method, similar to the evaluation of boiler efficiency. The principle is simple: the heat losses are substracted from the heat supplied to the furnace. Typical thermal efficiencies for common industrial furnaces are given in the Table 1 [1,9]. Table1: Efficiencies of Common Energy Conversion Devices [11]. Energy Conversion Device Energy Conversion Efficiency % Electric heater Electricity/Thermal 100 Hair drier Electricity/Thermal 100 Electric generator Mechanical/Electricity 95 Electric motor (large) Electricity/Mechanical 90 Battery Chemical/Electricity 90 Steam boiler (power plant) Chemical/Electricity 90 Home gas furnace Chemical/Thermal 85 Home oil furnace Chemical/Thermal 65 Electric motor (small) Electricity/Mechanical 65 Home coal furnace Chemical/Thermal 55 Steam turbine Thermal/Mechanical 45 Gas turbine (aircraft) Chemical/Mechanical 35 Gas turbine (industrial) Chemical/Mechanical 30 Automobile engine Chemical/Mechanical 25 Fluorescent lamp Electricity/Light 20 Silicon solar cell Solar/Electricity 15 Steam locomotive Chemical/Mechanical 10 Incandescent lamp Electricity/Light 5 EXPERIMENTAL WORKS MATERIALS AND METHODS The materials utilized for the manufacturing of a gas furnace: 3mm thick galvanized sheet steel, burner, and castable type refractories.

4 112 Eman J. Abed DESIGN Furnace Casing The sheet selected is a galvanized sheet steel it was selected for the fabrication of the furnace casing because of its light weight, good strength, good corrosion resistant, excellent formability, weldability, availability, and low cost of purchase. The furnace casing all the components of the furnace including: the refractory lining, the burner, and cover. The design was made taking into consideration that the size of the burner was utilized in the design of the burner nose inlet. The dimensions of the furnace are presented below: - Height of the casing = 80 cm - Diameter of the casing = 50 cm - Diameter of the hole burner = 8 cm - Dimension of desk 80*55*45 cm 3 - Dimension of the reinforcing plates = 5 * 2.5 * 0.5 mm FABRICATION PROCEDURE OF FURNACE COMPONENTS Furnace Casing Rolling galvanized steel sheet of 3mm thickness as shown in figure 1 by using a three high mill to a diameter of 50 cm as shown in figure 1. Figure 1: Rolling of Galvanized Steel Sheet The rolled plate welded by manual metal arc welding (MMAW) to a length of 80 cm as shown in figure 2 Figure 2: Welding of the Rolled Plate

5 113 Manufacture and Performance of Gas Furnace Drilling After welding a circle plate at the bottom of cylinder to make the base of the furnace, making a hole of 8cm diameter in the below of the wall at a distance of 4cm above the base for entering the burner and a 56 cm diameter and 3cm thick ring is made around the wall at the base for reinforcement, also drilling two venting holes of 1cm made on the wall of the furnace for gas escaping as shown in figure 3. Figure 3: Furnace during Fabrication Then build up the lining. Set 8 to 12 lengths of wire or old hacksaw blades vertically at the center of the lining for reinforcement. Reinforcing plates made of alloy steel as shown in figure Figure 4: Reinforcing Plates The reinforcing plates (used to maintain the mixture of the refractory material at the inner wall of the cylinder) welded on the wall in a different place as shown in the figure 5. Figure 5: Positions of Reinforcing Plates Refractory Lining The refractory lining: Build the refractory lining inside a sheet sheet-metal metal 50 cm diameter, and 80 cm height. Also really wanted to try building a furnace using some commercial refractory instead of a homemade version. We looked

6 114 Eman J. Abed around for refractory at a lot of places online (refractory is basically just called "caster" in Japanese). Finally found castable type refractories (alumina cement, silica dust + others called Carath concrete) in Kufa cement factory (Zamzam Company for engineering and technology services ltd).the build a cylindrical furnace, with an internal diameter of 50 cm. For this design a wall thickness of about 7 cm will suffice for a furnace used to melt aluminum with gas firing. This thickness will also provide adequate strength when using either castable type refractories. Also this thickness will provide enough insulation to get up to melting temperature in a reasonable time. DESCRIPTION MATERIAL Carath Concrete Carath 1890-D0-6mm as shown in figure 6: the refractory lining for the furnace was made by blending of refractory material compound of alumina cement + silica dust + others called Carath concrete (table 2) and water in the ratio 12:4:3:1 by volume percent. The thoroughly blend mixture was cast into cylindrical shape. High-alumina cement: Lining of furnace sections can be constructed by casting refractory insulating concretes. The concretes contain high-alumina cement. Figure 6: Locally Sourced Refractory Materials (Carath Concrete) Table 2: Carath Dense Refractory Castable with Regular Cement Content [12]. STEPS PERPETRATION OF THE REFRACTORY LINING Mixing Fill the mixer with whole content of a bag. Mix only with drinking water using the indicated amount. Initially add 3/4 of mixing water, than cautiously add the rest.

7 Manufacture and Performance of Gas Furnace 115 Carath concretes must be processed with paddle mixers, light refractory concretes can be processed with gravity mixers also. The mixing time for Carath concretes is generally 6 min. Mixers and tools must be clean forming. Forming Shuttering must be tight and coated with release regents such as mould wax, oil or grease. Pot life approx. 30 min after mixing, less for reactive concretes. Vibrated concrete must be placed by internal or external vibrators. The Carath concrete poured on the inner wall of the cylinder at a thickness of 7cm and leaves it for 24hr to solidify (see the figure 7). Figure 7: Pouring the Carath Concrete Flooring This is done by first lining round the internal casing wall with refractory to create support and filling the centre of the base with dry sand. The surface of which made wet with water for setting after the walls of the furnace has been lined. Follow-Up Treatment No trowel smoothing after the end of forming. Striping after 6-24 hr, depending on component size. Keep the placed concrete wet for hr. DRYING AND FIRST-HEAT -UP Drying The assembly is then allowed to dry for two weeks and any breach or cracks observed are repaired as the clay dries. The clay contracts and cracks during drying which necessitates proper repair to enhance the working efficiency and life span of the furnace. First-Heat-Up The first fire is the moment when a furnace or another heating device (usually for industrial use such as metallurgy or ceramics) is first lit after its construction. The refractory of the furnace walls should be as dry as possible and

8 116 Eman J. Abed the first fire should be done slowly with a small flame as the refractory of the still unfired furnace has a minimal amount of moisture. Gradually or during subsequent firings, the flame or heat source can be turned up higher. Desk Manufacturing A desk made of from low carbon steel has a dimension of 80*55*45 cm 3. Burner Burner made of from stainless steel as shown in figure 8.The burner is placed in the hole at the horizontal plane. The burner is placed in such a way that the flame is directed to the bottom of the pot at some distance away from it. Figure 8: The Burner Natural Gas The furnace burner is fired with natural gas. The Furnace Cover The cover is made from the same plate and refractory material with 50cm diameter and 4cm thickness. Assembly The parts to be assembled include: the casing with the refractory lining, the burner, natural gas, regulator of gas and desk. The burner is placed in the position inside of the hearth, and threads the pipe through the burner hole. Finally, the furnace joined over the desk which is containing the gas bottle as shown in figure 9. Figure 9: The Gas Furnace during Testing EXPERIMENTAL MEASUREMENTS The test carried out, an Aluminium scrap was melted in the furnace operating. The burner was regulated to fire at 30% gas pressure for the first and 50%second test. The flame should never be directed against the crucible, but always

9 Manufacture and Performance of Gas Furnace 117 enter the furnace barrel tangentially in order that the flame and heat take a spiral path around the crucible. With this setting the temperature in the burning zone could be as high as 1000 C in the burning chamber and 700 C in the inner pot.the total time of melting, in-furnace temperature, and the quantity of gas consumed were recorded. The in-furnace temperature was measured and the inside and outside surface walls were measured using infra-red thermometer. COST ANALYSIS The entire materials and equipment used for the furnace design are presented in table 3.The materials and equipment used in the design are locally sourced; and the overall cost of designing the furnace is approximately 350$. The furnace is cheap in comparison to similar designs from abroad. Table 3: The Materials and Equipment Used for the Furnace Design CONCLUSIONS On completion and testing, it was observed that the furnace has a fast heating rate 25 C /min to attain maximum melting temperature in the burning zone as high as 1000 C and 700 C in the inner pot. Also observed that the furnace very good gas fuel economy consuming. A furnace s efficiency increases when the percentage of heat that is transferred to the stock or load inside the furnace increases. It was also observed that the furnace has good heat retaining capacity; can be easily maintained and is safe for use. ACKNOWLEDGMENTS The author would like to acknowledge the Kufa cement factory of Iraq for assistance with the experimental work. The author would like also to express their appreciation to the University of kufa/college of Engineering /Materials Engineering Department for its support of this work. REFERENCES 1. Rajiv Garg," Thermal energy equipment: Furnaces and refractories", Energy efficiency guide for industry in Asia, "Furnaces",materialrulz.weebly.com/uploads.pdf 3. Martin D. Schlesinger, Klemens C. Baczewski,and Glenn W. Baggley" Fuels and Furnaces ", "Types and Classification of Different Furnaces", 6. Types of Fuels and their Characteristics ",

10 118 Eman J. Abed 7. Furnace Types and Styles", 8. Natural gas combustion ", 9. Energy Performance Assessment of Furnaces", Efficiency of Energy Conversion",