Pressure draft

By viriue of its vertical construction, the turbo-type tray dryer has a stack effect, the resulting draft being frequently sufficient to operate the dryer with natural draft. Pressure at points within the dryer is maintained close to atmospheric, as low as 0.1, usually less than 0.5 mm of water. Most of the roof area is used as a breeching, lowering the exhaust velocity to settle dust back into the dryer. 

Draft is usually measured in inches or centimeters of water using a U-tube manometer, with one side connected to the sample point (such as the furnace section, convective-pass section or the boiler stack) and the other side open to the atmosphere. The difference in the manometer column height indicates boiler draft pressure, which may be either higher (overpressure) or lower (underpressure) than atmospheric pressure. 

Instruments include pressure gauges to measure steam drum pressure, superheater outlet pressure, FW pressure, inlet and outlet draft pressure, and fuel supply and burner pressure. 

Constituents Up-draft pressure-producer gas Down-draft producer gas ... 

Draft is a differential pressure between the outside atmosphere and the internal pressure in a furnace. Differential pressure such as units of draft are sometimes referenced as psid. However, draft pressures are ordinarily so low that smaller units are needed. Two common units are mm w.c. or in w.c. (also w.c. or i.w.c.) that is, millimeters, water column or inches, water column, respectively. For example, in order to displace a fluid from a glass to one s mouth three inches above the liquid surface via a straw, one must cause a differential pressure of 3 in. w.c. Draft pressures are often smaller. For example, a typical natural draft refinery furnace operates anywhere from 0.25 in. w.c. to 0.50 in. w.c. This seemingly small pressure difference can cause great volumes of air flow. Gas pressure and velocity are related by Bernoulli s equation ... 

Problem statement What is the maximum draft pressure at the floor of a 10-m furnace operating at 800°C Presume that the control system adjusts the pressure at the top of the furnace to a draft of 3 mm w.c. via a stack damper. 

Efficient heater operation requires that excess air entering the convection section be minimized, whieh is indicated by a very small negative pressure at the convection section inlet. To achieve this, it should have a well balanced draft pressure profile between the firebox and stack. The hot gas pushes so that the pressure is always greatest at the firewall while the stack draft pulls. When this draft is eorreetly balanced, the pressure at the bridge waU should be around 0.1-0.2 WG (water gauge). Too mueh draft allows cold air leakage into the fired box resulting in wasted fuel. 

CEN TC 54 (Unfired pressure vessels) WG E (Testing and inspection) Draft for the EN Standard"... 

Fig. 1. Typical heater configuration. BFW = boiler feed water SSH = super high pressure steam HP = high pressure and ID = induced draft..

Mercury from cinnabar ore 225 tons ore/day (95% recovery) (2) 18,0 ft. diam, 8 hearth furnaces Furnaces fired on hearths 3 to 7, inclusive retention time of 1,0 hr, furnaces are oil-fired with low-pressure atomizing air burners all air, both primary and secondary, introduced through the burners draft control by Monel cold-gas fans downstream from mercury condensers. 

H is placed at the highest available level (Note 5) and connected with the tubes 7, 7, and K in such a manner as to secure a pressure of liquid sufficient to more than balance the steam pressure (Note 6). E and F are specially constructed condensers of unusual length (160 cm. and 85 cm., respectively) and bore (40 mm.) made from large glass tubing and rubber stoppers (Note 7). I he top of condenser E is connected to a good draft chamber. 

Firebox Overpressure - The firebox of a forced-draft furnace and boiler is designed to withstand the overpressure that can be generated by the fans with dampers in their closed position. This needs to be specially checked when both forced and induced-draft fans are provided to discharge combustion products through heat recovery facilities, since higher than normal fan pressures may be used to overcome pressure drop. In the case of high-pressure process furnaces, a tube rupture could also be the cause of firebox overpressure. 

The supply airflow rate should approximately equal the exhaust flow rate. A minor difference between supply and exhaust flow rates should nor disturb the exhaust, since exhaust systems usually are operated with higher pressure differences than supply systems. If the exhaust flow rate is higher than the supply, it could result in lower efficiency due to lower exhaust flow rates and cross-drafts (see Disturbances). If the exhaust flow rate is lower than the supply flow rate, there may be fewer problems with exhaust efficiency, but this could result in a supply airflow field different from the designed one and thus result in different kinds of disturbances. 

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. 

Forced draft The forcing of air by means of a fan into a closed chamber for combustion or other purposes. The pressurization of the chamber forces the air and combustion products up a stack. 

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