Furnaces efficiencies

Fig. 1. Induction furnace efficiency. Typical characteristics of a 1000 kW furnace. Example = 15% of P and = 100 kW. P = useful power ...

Heat/Solvent Recovery. The primary appHcation of heat pipes in the chemical industry is for combustion air preheat on various types of process furnaces which simultaneously increases furnace efficiency and throughput and conserves fuel. Advantages include modular design, isothermal tube temperature eliminating cold corner corrosion, high thermal effectiveness, high reHabiHty and options for removable tubes, alternative materials and arrangements, and replacement or add-on sections for increased performance (see Furnaces, fuel-FIREd). 

At EA of a shot of mass 5 g for deriving alloy with usage US by intensity of 9 W/cm there ai e enough 10 minutes of handling. The common time EA depends on furnace efficiency, and in our works compounded 15-25 minutes. 

Improve blast furnace efficiency by using coal and other fuels (such as oil or gas) for heating instead of coke, thereby minimizing air emissions. 

Natural draft is a low capital cost option, but the draft requirements tend to lead to high stack temperature with a low efficiency for the furnace. Forced draft and induced draft are higher capital cost options than natural draft but tend to lead to higher furnace efficiencies as the stack temperature can be lowered significantly. 

The profile shown in Figure 15.21 represents the furnace efficiency, if the casing heat losses are neglected. Making this assumption, the process duty plus the stack loss represents the heat released by the fuel. 

Example 15.6 For the three cases in Example 15.5, determine the furnace efficiency for an assumed stack temperature of 100°C. 

For a given stack temperature, the higher the theoretical flame temperature, the higher the furnace efficiency. However there is a minimum excess air required to ensure that the combustion is itself efficient. 

Obviously, the lower the stack temperature, the higher the furnace efficiency. As already noted, it is desirable to avoid condensation in the convection section and the stack. If there is any sulfur in the fuel, the condensate will be corrosive. There is thus a practical minimum to which a flue gas can be cooled without condensation causing corrosion in the stack, known as the acid dew point. If there is any sulfur in the fuel, the stack temperature is normally kept above 150 to 160°C. Natural gas can normally be cooled to... 

In preliminary design, the heat duty and furnace efficiency are the prime considerations. However, if the tube area needs to be specified, a preliminary estimate can be obtained from an assumed flux. In the radiant section, this usually lies in the range of 45,000 W m-2 to 65,000 W m 2 of tube surface, with a value of around 55,000 W m 2 most often used. The heat flux is particularly important if a reaction is being carried out in the furnace tubes. Overall heat transfer coefficients in the convection section are in the range 20 to 50 W m-2 K-1. 

Example 16.4 The process in Figure 16.2 is to have its hot utility supplied by a furnace. The theoretical flame temperature for combustion is 1800°C and the acid dew point for the flue gas is 160°C. Ambient temperature is 10°C. Assume A7 = 10°C for process-to-process heat transfer but A7 = 30°C for flue gas to process heat transfer. A high value for A 7 mln for flue gas to process heat transfer has been chosen because of poor heat-transfer coefficients in the convection bank of the furnace. Calculate the fuel required, stack loss and furnace efficiency. 

Reduced air preheat. Preheating the air to combustion processes increases the flame temperature and furnace efficiency (see Chapter 15). However, the increase in flame temperature increases NO formation. Thus, reducing preheat lowers the thermal NO formation. However, this lowers the furnace efficiency at the same time. 

Steam injection. Steam injection reduces combustion temperature by adding an inert to the combustion process. In principle, water or steam could be injected, but dry superheated steam is usually used. This reduces the furnace efficiency as energy is used to produce the steam, and the latent heat in the steam cannot be recovered. It is only used for moderate NO reduction. Steam injection can also be used in gas turbines. 

Also, as the flue gas cools, it sinks in the same direction as its normal flow which results in stable furnace operation. Flue gas back-mixing is avoided and the flue-gas outlet temperature is closer to the process-gas outlet for maximum furnace efficiency. 

Fixed charges for the low pressure furnace, FCf, are estimated in the same manner as the cogeneration system, at one and a half times the equipment cost. The fuel cost of producing hot water, CFUELf, is estimated using the unit cost of fuel, CF, the heat input to the water, Q, and the estimated furnace efficiency... 

Dimensionless firing density Gas-side furnace efficiency Reduced furnace efficiency Dimensionless temperature... 

For low firing rates, the exit temperature of the furnace gases approaches that of the sink i.e., sufficient residence time is provided for nearly complete heat removal from the gases. When the combustion chamber is overfired, only a small fraction of the available feed enthalpy heat is removed within the furnace. The exit gas temperature then remains essentially that of the inlet temperature, and the furnace efficiency tends asymptotically to zero. 

Equation (5-187) provides an explicit relation between the modified furnace efficiency and the effective firing density directly in which the gas temperature is eliminated. 

The asymptotic behavior of Eq. (5-189) mirrors that of the LPFF model. Here, however, for low firing densities, the exit temperature of the furnace exit gases approaches 0e = 0i - A rather than the sink temperature. Moreover, for Deff 1 the reduced furnace efficiency adopts the constant value rig = 1 — 0 = 1 + A — 0f. Again at very high firing rates, only a very small fraction of the available feed enthalpy heat is recovered within the furnace. Thus the exit gas temperature remains nearly unchanged from the pseudoadiabatic flame temperature [Te Tf,] and the gas-side efficiency necessarily approaches zero. 

FIG. 5-23 Reduced gas-side furnace efficiency versus effective firing density for well-stirred combustion chamber model. As = 0,0, = 0.0, 0.4, 0.5, 0.6,0.7,0.8,0.9. 

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