If the effective area of the tubes, A 1 , is only a small fraction of the total area. On the other hand, if A 1 is almost equal to the total surface area, A t , then. Figure 4. Effect of heat sink area on effective emissivity. In practice the area, A 1 , receiving radiation is a tube bank, Figure 5 , which intercepts only a fraction of the incident radiation; some passes through to the refractory wall and is re-irradiated.
For a detailed analysis see Hottel and Sarofim Figure 6 shows F as a function of B for one and two rows of tubes. Figure 6. Fraction of incident radiation intercepted by tubes.
The effective emissivity based on the projected area using Eq. This is illustrated in Figure 7 for a typical tube surface emissivity of 0. The relationship between the rate of heat transfer, , from the gas to the tubes and the gas temperature, T g , is obtained by combining Eqs. This in turn can be combined with Eq. An example of this procedure is described in Hewitt, Shires and Bolt The complete mathematical description of a practical furnace is complex, combining aerodynamics, chemical reactions and heat transfer, and computer programs are normally used for detailed solutions.
There are two basic types of approach; zone methods and flux methods. Zone methods are employed when the heat release pattern from the flame is known or can be calculated independently. Conceptually, the furnace and its walls are divided into discrete zones, the effective exchange areas between zones are determined, and radiative heat transfer corresponding to the prescribed heat release pattern is calculated.
In flux methods, instead of dividing the space into zones the radiation arriving at a point in the system is itself divided into a number of characteristic directions, representing averages over a specified solid angle.
Flux methods are well suited for use in combination with modern methods of prediction of fluid flow and mixing. Simultaneous solutions of the radiative heat transfer equations using flux methods and turbulent flow models are feasible. For further information on these two methods see Beer , and Afgan and Beer , where examples of their application can be found. As a first approach to the estimation of furnace performance the well stirred furnace model is relatively simple and quick to use.
One of the first versions was introduced by Lobo and Evans , and was used by Kern An improved version expressed in nondimensional terms was introduced by Hottel and Sarofim This was reveiwed by Hottel and subsequently described by Truelove , and Hewitt, Shires and Bott Figure 8a shows graphically the well stirred furnace model performance prediction for zero wall losses, and Figure 8b for the same conditions but with typical wall losses included.
Comparison of these figures shows that wall losses have a very significant effect when tube temperatures are high. As the firing rate, represented by D'd, is reduced, the efficiency, represented by , reaches a peak then falls, eventually approaching zero as the major part of the heat is lost through the walls.
Figure 8. Performance curves for stirred reactor furnace with negligible wall losses. The combustion chamber loses less heat to the outside, making the furnace cost less to run. The highest efficiency furnaces today combine sealed combustion with a second condensing heat exchanger. Finally, going with sealed combustion heating helps to solve a problem homes often encounter with gas furnaces: drier conditions. What actually happens is an atmospheric combustion chamber draws air from inside the house as it runs, causing an air deficit.
Air from the outside—which is usually drier in the winter—then rushes in to replace it. Although any high efficiency furnace will cost more to install than a mid-efficiency unit, the payback with energy savings often makes it worthwhile. Twitter Link. Your Browser Is Incompatible You are seeing this message because we have detected you are using Internet Explorer 10 or older to browse our site.
Hoop-tube fired heater This fire heater has tube bent like U-type with vertically oriented. In all-vapor flow, non-coking services where low coil pressure drop is desired. This design is used where the pressure drop must be very low since the path through each tube provides a design with many passes.
Application of this type is in the catalytic reformers charge heater. Hoop-tube fired heater These design guideline are believed to be as accurate as possible, but are very general and not for specific design cases. Vertical tube box fired heaters In this fire heater, tubes stand vertically along wall in the radiant section. Vertical radiant tubes are arranged in a single row in each combustion cell there are often two cells and are fired from both sides of the row.
Such an arrangement yields a uniform distribution of heat-transfer rates about the tube circumference. This heater is suitable for the large forced-draft burners. Requirement of heat input to each cell provided by burner. Horizontal tube box fired heaters The radiant and convection section in a typical of horizontal tube box in the Figure 5 are separate by a wall called bridge wall. Function of bridge wall is to create a good direction of flame and to stream the smoke in to flue stack.
Burners are firing from the floor along both sides of the bridge wall. Helical coil fired heater This heater configuration is commonly used where the duties are small. Since each pass consists of a separate winding of the coil, pressure drop options are limited. Many of these only have a radiant section, since efficiency is often not that critical, especially in intermittent services like for a regeneration heater. The design requirements must be properly addressed. Fired heater performance can be measured by a combination of operability and maintenance.
There are several factors effecting fired heater selection and design: all-liquid vaporizing service and all-vapor service. Fire heaters in all-liquid or vaporizing service Inside the tube wall coke may be formed that can interfere with heat transfer process. Fired heaters should be design to minimize coke. Incipient coke begins to form at a film temperature above about oF, usually equivalent to a bulk fluid temperature of about oF.
In other services such as visbreaking and thermal cracking, where fluid cracking is an inherent characteristic of the process, acceptable coke formation and run length can usually be attained if film temperatures do not exceed oF equivalent to a bulk fluid temperature of about oF.
For reduce the formation of coke, a high inside film coefficient is necessary to minimize the difference between bulk fluid and film temperature. The higher the speed of the mass of the heat transfer coefficient will be better. Therefore, the mass of turbulent flow must be maintaining in the tube. Fire heater in all-vapor service For this fired heater service is generally not as susceptible to the severe coking problems as those in vaporizing services because of the lighter nature of the process fluid.
When a furnace is operated properly, the furnace and its parts have a longer working life with minimum repairs. A properly run furnace is a safe furnace. Skillful handing of a furnace means safety for worker. Heat is produce by the ignition of fuel at the burner in the firebox. The tubes along the wall of the firebox are the radiant and the shock bank tubes. These tubes receive radiant heat from the burners.
The firebox wall and roof is lined with a material then reduce heat losses and radiates heat back to the tubes. The entire furnace structure must be air tight for efficient furnace operation.
Air should only enter at designed entries. An air leak reduces the efficiency of the furnace. Below are design considerations for furnace. Heaters shall be designed for uniform heat distribution 2. Multi-pass heaters shall be designed for hydraulic and thermal symmetry of all passes. The number of passes shall be minimized. Each pass shall be a single circuit 3. Average heat flux density in the radiant section is normally based on single row of tubes with two nominal tube diameter spacing.
The maximum allowable inside film temperature for any process service shall not be exceeded in the radiant, shield, or convection sections. Heaters shall be designed such that a negative pressure of at least 0.
The flue gas dew point can be predicted, and the minimum tube-metal temperature can be kept high enough to prevent condensation, if the fuel's sulfur content has been correctly stated. For estimated flue gas dew points with respect to sulfur content in fuel oil and gas 9.
Higher radiant flux means less heat transfer surface area for a given heat duty; hence, a smaller furnace. The higher the film temperature, the greater is the tendency of the fluid particularly a hydrocarbon to crack and deposit a layer of coke.
Heat-transfer fluids tend to degrade quickly at high film temperatures.
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