Preheating Cold Loads

Preheating cold loads with flue gases can be accomplished in preheating chambers, in a preheat zone of a continuous furnace, or in the first part of the time cycle of a batch or shuttle furnace. (See sec. 4.3.) 

Unfired preheat vestibules take many different forms, such as (1) an elongated conveyor though a furnace extension, (2) loading cold charges down the stack of a 

At the site of a thirteenth century cathedral, a bronze bell foundry loaded their melting furnace by putting raw pig metal down the stack for preheating to save time and fuel each morning while the women of the town carried wood from diminishing surrounding forests. 

Preheating loads with waste gases has been widely practiced in the forging and hardening of tools.. . from the village blacksmith to slot forge furnaces where extra loads were placed in the slot for preheating. Their fuel efficiency may not have been so crude after all. Fuel was often scarce or dear. Necessity was the mother of invention. 

Warning In all heat recovery schemes, it is very important to minimize transport losses keep ducts and pipes (for hot flue gas, hot air, and steam) short and very well insulated. Similarly, when preheating loads, if they must be transported hot, keep the distances short and cover them with insulation while being transported. 

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When we cool a liquid off, its viscosity markedly increases. I cannot generalize—it depends on the fluid. But I can say that increasing the viscosity of a fluid from 2 to 40 cP could reduce the observed heat-transfer efficiency (U) from 100 to 25.1 know this from my experience in preheating cold, viscous, Venezuelan crude oil, off-loaded from tankers. 

Areas of high microwave flux are checked with a Pelco 36140 microwave bulb array (Ted Pella). Specimens are not placed in areas indicated by illuminated bulbs. Vials containing the specimens are placed in a cold tap water bath (50 ml) that is preheated to the required temperature. The temperature is regulated by placing the microwave temperature probe into a vial of the same solution that is present in the specimen vial. The built-in temperature probe displays the specimen temperature on the oven front panel. The wire that attaches the probe to the oven is submerged in the water to decrease the antenna effect. An additional 400 ml of static water load is placed in the oven at an optimal position determined with the microwave bulb array. This water is changed between every step. 

The combined stream is preheated to 122°C in a FEHE. A heater (HX3) is installed after the FEHE so that inlet temperature of the coolant stream in REACT2 can be adjusted to satisfy the energy balance when the exit temperature of the coolant stream is specified in this countercurrent tubular reactor. This temperature is 150°C, and the heat load in HX3 is 9.34 x 106 kcal/h. The stream is further preheated to 265°C in the tube side of reactor REACT2 by the heat transfer from the reactions that are occurring in the hot shell side of this vessel. There is no catalyst on the cold tube side, so the feed stream does not react but its temperature is increased. The stream is then fed to reactor REACT 1, which contains 48,000 kg of catalyst. This reactor is cooled by generating steam. The coolant temperature is 265°C (51 bar steam). This vessel contains 3750 tubes, 0.0375 m in diameter, and 12.2 m in length. The overall heat transfer coefficient between the process gas and the steam is 244 kcal h-1 m-2 °C 1. The heat transfer rate is 42 x 106 kcal/h. 

Incoming cold fluids should be (preheated and) brought to the required temperatures by means of heat exchange with hot outgoing fluids as much as possible. Not only will this recover heat, it can also reduce the cooling load due to the hot stream. 

The mercury removal performance of pilot-scale ICDAC and of Norit s FGD carbon were determined in a 0.236 m% (0.25 MWe) pilot plant operated by CONSOL, Inc., Library, PA. The pilot plant can simulate flue gas conditions downstream of the air preheater in a coal fired utility power plant. The flue gas mercury concentration studied (10-15 pg/m ) is typical of utility flue gas concentration. Mercury removals were evaluated in the flue gas duct, which provided a gas residence time of approximately 2 seconds, and in the baghouse, where the solids retention times can be as long as 30 min. Common test conditions were flue gas flow, 0.165 m /s flue gas wet bulb temperature, 50-53°C flue gas composition, 1000 ppmv dry SO2, 10 vol% dry O2, and 10 vol% dry CO2. All tests were conducted with a fly ash obtained from a coal-fired utility boiler firing an eastern bituminous coal. The fly ash feed rate was 4.5 kg/hr (solids loading of 90.6-104.7 gm/dcm ). Mercury removal was determined from the mercury feed rate, the solids (carbon and fly ash) feed rate, and mercury analysis of the feed and recovered solids (by combustion followed by cold vapor atomic absorption spectroscopy). Except where noted, all mercury removal results discussed in this paper include mercury removal by the carbon sorbent and the fly ash. A more detailed description of the pilot test unit is given elsewhere (27]. 

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