Loads preheating

Most heat recovery efforts are aimed at utilizing the waste heat exiting through the flues. Some forms of heat recovery are air preheating, fuel preheating, load preheating (Fig. 1.17), recuperative, regenerative, and waste heat boilers—all discussed in chapter 5. 

Rg. 1.17. Tool heating furnace with heatrecovering load preheat chamber. 

Fig. 4.16. Modern skelp-heating furnace with heat recovery by load preheating. Some furnaces use type H high-velocity impinging burners others use refractory radiating burners similar to type E, but with concave refractory tiles. (See fig. 6.2 for these flame types.)...

Reinforced Thermoplastic Sheet. This process uses precombined sheets of thermoplastic resin and glass fiber reinforcement, cut into blanks to fit the weight and size requirements of the part to be molded. The blanks, preheated to a specified temperature, are loaded into the metal mold and the material flows under mol ding pressure to fiU the mold. The mold is kept closed under pressure until the temperature of the part has been reduced, the resin solidified, and demolding is possible. Cycle time, as with thermosetting resins, depends on the thickness of the part and the heat distortion temperature of the resin. Mol ding pressures are similar to SMC, 10—21 MPa (1500—3000 psi), depending on the size and complexity of the part. 

X 0.0518-0.0418 VF, 0.0518-0.0113 where X = quantity of fresh air and = total air flow. Thus 75.3 percent of the air is recirculated. Load on the preheater is obtained from an enthalpy balance... 

The simplest form of recuperation is load recuperation, but this is not suited to retrofit and is incorporated at the design stage. Flue products from the highest temperature zone are used to preheat incoming stock on the... 

In many processes, load recuperation is not practicable, and combustion air is preheated in a heat exchanger by means of the outgoing flue products. Figure 19.3 gives an indication of the savings to be made for different operating temperatures. It is not normally considered economic to operate a recuperator at flue temperatures below about 750°C. 

Compare the cyclone loading with the design. If the vapor velocity into the reactor cyclones is low, consider adding supplemental steam to the riser. If the mass flow rate is high, consider increasing the feed preheat temperature to reduce catalyst circulation. 

The total heat requirement is thus around 599.98 kj, which is about 548.81 kj more than the heat available from the reaction. This calculation, however, does not take into account the inevitable heat losses due to the nonadiabatic conditions in the reactor. An estimate of these heat losses can be made by considering the industrial practice for aluminothermic chromium metal production. The charge is preheated to about 500 °C before loading into the aluminothermic crucible. This operation adds about 96.65 kj (i.e., 48.9 cal deg-1 475) of heat to the system. It, therefore, appears that around 41.84 kj (96.65 kj - 54.81 kj) of heat is lost due to radiation and convection for every mole of chromium sesquioxide reduced to the metal by the aluminothermic process. 

The reactor used for the aluminothermic reduction of niobium pentoxide is shown schematically in Figure 4.17 (A). It is a steel pipe, lined on the inside with alumina and provided with a pipe cap. The charge, consisting of stoichiometric amounts of niobium pentoxide and aluminum powder, is blended and loaded in the lined pipe, and covered with alumina. The cap is closed and the reaction initiated by placing the loaded bomb in a gas-fired furnace, preheated to 800 °C, and by raising the temperature of the furnace to 1100 °C. 

The temperature driving force will be taken as the difference between the temperature of the condensing steam and that of the evaporating water as the preheating of the solution and subcooling of the condensate represent but a small proportion of the total heat load, that is AT = (394 - 325) = 69 degK. 

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