Gas to Economizer Heat Transfer

The Fig. 22.1 economizer s design heat transfer rate with  

Third catalyst bed = 8.46 MJ/kg mol of first catalyst bed feed gas economizer heat 100,000 Nm firstcatalystbed feed gas/h 

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Third catalyst bed exit gas to economizer heat transfer rate... 

Heat transfer from 3rd bed exit gas to economizer water This section calculates how much heat must be removed from Fig. 21.1 s ... 

Calculate the rate, MJ/hour, at which heat must be transferred from 3 rd catalyst bed exit gas to economizer water to obtain the above specified 470 K H2S04 making gas. 100 000 Nm3 per hour of lsl catalyst bed feed gas is entering the lsl catalyst bed. 

Heat transfer from 3 bed exit gas to economizer water... 

If condensation requires gas stream cooling of more than 40—50°C, the rate of heat transfer may appreciably exceed the rate of mass transfer and a condensate fog may form. Fog seldom occurs in direct-contact condensers because of the close proximity of the bulk of the gas to the cold-Hquid droplets. When fog formation is unavoidable, it may be removed with a high efficiency mist collector designed for 0.5—5-p.m droplets. Collectors using Brownian diffusion are usually quite economical. If atmospheric condensation and a visible plume are to be avoided, the condenser must cool the gas sufftciendy to preclude further condensation in the atmosphere. 

In fossil fuel-fired boilers there are two regions defined by the mode of heat transfer. Fuel is burned in the furnace or radiant section of the boiler. The walls of this section of the boiler are constmcted of vertical, or near vertical, tubes in which water is boiled. Heat is transferred radiatively from the fire to the waterwaH of the boiler. When the hot gas leaves the radiant section of the boiler, it goes to the convective section. In the convective section, heat is transferred to tubes in the gas path. Superheating and reheating are in the convective section of the boiler. The economizer, which can be considered as a gas-heated feedwater heater, is the last element in the convective zone of the boiler. 

Convection heat transfer is dependent largely on the relative velocity between the warm gas and the drying surface. Interest in pulse combustion heat sources anticipates that high frequency reversals of gas flow direction relative to wet material in dispersed-particle dryers can maintain higher gas velocities around the particles for longer periods than possible ia simple cocurrent dryers. This technique is thus expected to enhance heat- and mass-transfer performance. This is apart from the concept that mechanical stresses iaduced ia material by rapid directional reversals of gas flow promote particle deagglomeration, dispersion, and Hquid stream breakup iato fine droplets. Commercial appHcations are needed to confirm the economic value of pulse combustion for drying. 

Further, because of the nature of sohds-gas contacting, which is usually by parallel flow and rarely by through circulation, heat transfer and mass transfer are comparatively inefficient. For this reason, use of tray and compartment equipment is restricted primarily to ordinaiy drying and heat-treating operations. Despite these harsh hmitations, when the listed situations do exist, economical alternatives are difficult to develop. 

In modem, packaged horizontal FT boilers, the furnace is the most important heat-transfer component, typically providing 50 to 60% of the total heat transfer from only 30% or so of the total available heating surfaces. This level of heat transfer, coupled with the additional heat extraction obtained by the various multiple-pass designs (four passes is a practical maximum) provide efficiencies of 80 to 83% GCV. As a result, there generally is little additional benefit to be obtained from the use of economizers or air heaters, especially when using oil-fired boilers, which can operate at up to a 3% or so higher efficiency level compared to gas-fired units. 

Economizers are heat transfer tube bundles that preheat MU water or FW flowing within the tubes by extracting waste heat from the flue gas during its exit path to the stack. They typically account for approximately 10% of the total boiler heat transfer surfaces, while absorbing only 7% of the total heat generated in the boiler system. 

Economizer corrosion rates are enhanced by higher heat-transfer rates excessive heat flux may create localized nucleate boiling zones where gouging, as a result of chemical concentration effects, can occur. Air heaters are also located in the exit gas system. They do a job similar to that of economizers except that they preheat combustion air. 

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