Pharos University جامعه فاروس Faculty of Engineering كلية الهندسة Petrochemical Department قسم البتروكيماويات PE 330 ENERGY CONSERVATION LECTURE (8) FURNACES 1-INTRODUCTION:  A Furnace is a combustion chamber containing the heating system and the material being heated, which is often referred to as the load.  Dryers are combustors that typically operate at lower temperatures and are used to remove moisture from materials.

 Furnaces are refractory-lined chambers usually considered to be higher temperature combustors used for heating and melting solids.  Kilns (pronounced “kills”) are combustors used to bring about physical and chemical changes in materials and are a type of furnace usually associated with thermal processing of nonmetallic solids such as the ceramic, cement, and lime.  The chemical changes are often through calcination that may involve water evaporation, volatile evolution, and even partial combustion of the load material.  Heaters are typically refractory-lined chambers usually considered to be lower temperature combustors (see Chapter 10) used to heat solids and liquids.

2-Types of Furnaces : The choice of the proper furnace type depends on the specific application, as there are many available possibilities. One important consideration is the processing method and whether materials will be processed in batch or continuous mode. Another important consideration is the kind of fuel that will be used. Other considerations include the furnace geometry, type of processing material, and required temperature range.

3-Heat Transfer in Furnaces: In simple terms, heat is transferred to the stock by:  Radiation from the flame, hot combustion products and the furnace walls and roof.  Convection due to the movement of hot gases over the stock surface.  At the high temperatures employed in reheating furnaces, the dominant mode of heat transfer is wall radiation. Heat transfer by gas radiation is dependent on the gas composition (mainly the carbon dioxide and water vapour concentrations), the temperature and the geometry of the furnace.

4-Firing Method: There are two main methods used to heat load materials that are usually referred to as direct and indirect firing. The method chosen is normally dictated by the process requirements. If the material being processed must not come into contact with the combustion products, then indirect firing is used. If there are no such requirements, direct firing is preferred as it is normally more efficient and straightforward.

:4.1 Direct Firing In a direct-fired furnace, there is nothing between the combustion products generated by the burners and the load. This is the predominant type used in most industrial heating applications and essentially all of the high-temperature processes. In one respect, the heat transfer analysis of direct-fired furnaces is simpler compared to indirect fired furnaces as there is no intermediate heat-exchanging surface between the flames and the load. However, in another respect, the analysis of the heat transfer to the load is more complicated as convective heat transfer between the flame gases and the load, and therefore fluid flow, must be included.

4.2. Indirect Firing: In an indirect-fired furnace, there is some intermediate heat transfer surface between the combustion products and the load. That surface is commonly some type of ceramic due to the high temperatures, although metals are used in some cases. The surface is designed to prevent the combustion products from contacting the load and reducing the quality of the finished product. Another type of indirect-fired furnace is where the flame gases are separated from the load for either transport or safety reasons. For example, in a process fluid heater, the fluid is transported through metal tubes located inside a furnace. The metal tubes separate the combustion products from the fluid.

One reason for the tubes is to transport the fluids at an elevated pressure through the heaters that are fired at approximately atmospheric pressure. Another reason is related to safety, where many of the fluids being heated are hydrocarbons that are potentially explosive if overheated and exposed to enough oxygen. Two methods are commonly used to separate the combustion products from the load. One is to use open- flame burners but to have a separator, sometimes referred to as a muffle, across the entire combustion space between the flames and the load

Some of the challenges of this method include supporting the separator because of the high temperatures, maximizing the heat transfer from the flames to the separator to optimize the thermal efficiency, and getting a good gas seal around the perimeter of the separator due to thermal expansion. The second method commonly used in industrial combustion applications to separate the exhaust products from the load is to use radiant tube burners. In that method, the flame from each individual burner is contained in a ceramic tube. This often improves the overall heat transfer to the separator because of the improved forced convection inside the tube. As with the muffle, supporting long radiant tubes can be a problem as well as the seal between the typically metal burner and ceramic tube.

5- Proper Heat Distribution Furnace design should be such that in a given time, as much of the stock could be heated uniformly to a desired temperature with minimum fuel firing rate. Following care should be taken when using burners, for proper heat distribution: i) The flame should not touch any solid object and should propagate clear of any solid object. Any obstruction will deatomise the fuel particles thus affecting combustion and create black smoke. If flame impinges on the stock, there would be increase in scale losses. ii) If the flames impinge on refractories, the incomplete combustion products can settle and react with the refractory constituents at high flame temperatures.

iii) The flames of different burners in the furnace should stay clear of each other. If they intersect, inefficient combustion would occur. It is desirable to stagger the burners on the opposite sides. iv) The burner flame has a tendency to travel freely in the combustion space just above the material. In small furnaces, the axis of the burner is never placed parallel to the hearth but always at an upward angle. Flame should not hit the roof.

v) The larger burners produce a long flame, which may be difficult to contain within the furnace walls. More burners of less capacity give better heat distribution in the furnace and also increase furnace life. vi) For small furnaces, it is desirable to have a long flame with golden yellow colour while firing furnace oil for uniform heating. The flame should not be too long that it enters the chimney or comes out through the furnace top or through doors. In such cases, major portion of additional fuel is carried away from the furnace.

6-HEAT TRANSFER MEDIUM: Another factor that has a significant influence on the heat transfer in the furnace is the medium used to transport the energy from the burners to the load. In industrial combustion, this medium is most commonly a gas, but liquids and solids are also used. In some heat-treating processes, no medium at all is used (vacuum furnaces). 6.1 Gaseous Medium: Certain gases (CO2, H2O, SO2, CH4, etc.) transfer heat to the load and to the furnace walls by gaseous radiation. This radiation is dependent on the wavelength, gas temperature, pressure and composition, and on the path length from the gas to the surface being heated.

6.2.Vacuum : Some heat treating furnaces have a vacuum atmosphere to protect the material being heated from reacting with any gaseous materials. Vacuum heat-treating furnaces rely solely on thermal radiation to heat the parts being treated. The walls of the furnace are heated and those walls radiate to the load. Then, the radiant heat transfer is determined by the temperature and emissivity of both the furnace walls and the load, and on the geometrical view factor between the walls and the load. There will be thermal conduction and radiation between the parts, but not convection as there is no fluid to transport the energy

6.3 Liquid Medium: There are several ways that liquid mediums can transport heat in an industrial furnace. One way is by convection from molten materials. 6.4 Solid Medium: There are several ways that heat can be transferred through a solid medium in an industrial furnace. One method is by luminous radiation produced by solid soot particles generated in the flame. Another method is by thermal conduction between contacting solid parts.