Drafting, down

In the Davy-Powergas unit (118—120), shown in Figure 13c, the Hquids mn through a draft tube and are pumped by an impeller mnning directly above the draft tube. The dispersion flows out from the top of the mixer and down through a channel into a rectangular settler. Large units of this type are used for copper extraction (7). 

The steam generator is a balanced draft, controlled circulation, multichamber unit which incorporates NO control and final burnout of the fuel-rich MHD combustion gases. The MHD generator exhaust is cooled in a primary radiant chamber from about 2310 to 1860 K in two seconds, and secondary air for afterburning and final oxidation of the gas is introduced in the secondary chamber where seed also condenses. Subsequent to afterburning and after the gas has been cooled down sufftciendy to soHdify condensed seed in the gas, the gas passes through the remaining convective sections of the heat recovery system. 

In wetted-wall units, the walls of a tall circular, slightly tapered combustion chamber are protected by a high volume curtain of cooled acid flowing down inside the wall. Phosphoms is atomized by compressed air or steam into the top of the chamber and burned in additional combustion air suppHed by a forced or induced draft fan. Wetted-waU. plants use 25—50% excess combustion air to reduce the tail-gas volume, resulting in flame temperatures in excess of 2000°C. The combustion chamber maybe refractory lined or made of stainless steel. Acid sprays at the bottom of the chamber or in a subsequent, separate spraying chamber complete the hydration of phosphoms pentoxide. The sprays also cool the gas stream to below 100°C, thereby minimising corrosion to the mist-collecting equipment (typically type 316 stainless steel). 

Using a draft tube in the tank for solids suspension introduces another, different set of variables. There are other relationships that are veiy much affected by scale-up in this type of process, as shown in Fig. 18-22. Different scale-up problems exist whether the impeller is pumping up or down within the draft tube. 

FIG. 18-22 Typical draft tube circulator, sbown bere for down-pumping mode for tbe impeller in tbe draft tube. 

When pumping down the draft tube, flow normally makes a more troublefree velocity change to a flow going up the annulus. Since the area of the draft tube is markedly less than the area of the annulus, pumping up the draft tube requires less flow to suspend sohds of a given settling velocity than does pumping down the draft tube. 

Convection is the heat transfer in the fluid from or to a surface (Fig. 11.28) or within the fluid itself. Convective heat transport from a solid is combined with a conductive heat transfer in the solid itself. We distinguish between free and forced convection. If the fluid flow is generated internally by density differences (buoyancy forces), the heat transfer is termed free convection. Typical examples are the cold down-draft along a cold wall or the thermal plume upward along a warm vertical surface. Forced convection takes place when fluid movement is produced by applied pressure differences due to external means such as a pump. A typical example is the flow in a duct or a pipe. 

A boiler had been shut down for the repair of a forced draft fan. A blind was not installed in the fuel gas line, nor apparently was a double block and bleed in the fuel line utilized. Gas leaked into the firebox during the repair period and was not removed. A severe explosion occurred during the attempt to light of. 

For solids W hich float, or which are added from the top, a vortex action helps to draw the material down into the impeller. Often a draft tube is used to serve as a stic-don entrance for the impeller. Uniform suspensions are difficult to maintain when the tank liquid height is much greater than the tank diameter. The impeller is normally placed Vs of liquid depth off the bottom [21]. 

Performance, 387 Ground Area vs. Height, 391 Pressure Losses, 393 Fan Horsepower for Mechanical Draft Tower, 392 Water Rates and Distribution, 393 Blow-Down and Continuation Build-Up, 394 Example 915 Determining Approximate Blow-Down for Cooling Tower, 395 Pre-... 

Cross-sectional area There is a direct relationship between flue flow and the cross-sectional area. The draft is unaffected by the cross-sectional area but the frictional losses decrease as the area is increased, resulting in greater flow. Too great an area, however, will lead to a low velocity with its attendant problems of down-wash and possible condensation. 

Adverse conditions with down draft - the down draft mixes with the products of combustion and diluted products enter the room for the brief period that the downdraft persists. The draft in the primary flue (and hence the performance of the appliance) is unaffected. 

For small appliances, the down draft diverter is often incorporated into them for larger appliances, it is external. 

In larger appliances, the use of a down draft diverter can be impractical if only because of size, and a draft stabilizer may then be used. This will protect against up... 

Most burners are efficient at high fire but less so at intermediate rates and particularly at low fire. An on/off burner is therefore apparently efficient from an energy-utilization viewpoint. However, when the burner is called on to fire, in the case of forced-draft burners a purge is usually necessary which will both cool down the process and cause a delay in response, and in the case of natural draft there will be heat losses due to ventilation in the off period. 

In application they should be set below windows to offset the major source of heat loss and minimize cold down-drafts. 

A forced-draft, gas-fired burner is seated at the top of the inner tube and produces a spinning cyclonic flame that reaches down to the bottom of the furnace tube. The hot combustible gases return over the boiler shell, which is provided with heat convection fins to extract more heat before the upward flowing gases exit the boiler. The furnace tube is fitted with a top and bottom, cast-steel flame retainer. These design features act to increase flue gas residency time and provide improved structural integrity to the pressure vessel. 

Down-Draft Hood. To protect personnel from benzene vapors, it is recommended that all the pouring, container closing, and final decantation and bottling operations be performed on an efficient down-draft hood (4) To increase efficiency in inserting and removing the bolts on the steel cans, a reversible air-driven screwdriver has been suspended on a counterweight over the down-draft hood. 

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