Draft Chimney effect

Natural draft burners are supplied with combustion air from their surroundings and they do not allow for fhe preheating of combustion air. They utilize furnace nafu-ral draft (stack effect) and often simultaneous ejection effect of fuel streaming from burner jefs under a high speed. This is a rather frequent method of combustion air supply and is used for bofh gas fuel and liquid fuel burners (and possibly for combined burners). The burners are relatively difficult to operate under a low excess of air since the amount of supplied combustion air can only be controlled by a chimney damper or in some cases by flap valves on individual burners, which is rather demanding on the operators. Due to complicated control they are predestined to be used in operations with constant heat consumption without any sudden changes of heat supply, as it would cause product quality deterioration that is, process furnaces in chemical and pefrochemical industries [2]. 

With air coolers, louver closure is considered a total failure (10). Upon fan failure, or a fan drive (e.g., power or steam) failure, a credit is often taken for natural convection effects. This credit is usually 20 to 30 percent of the normal duty of induced-draft condensers. Forced-draft condensers have a considerably weaker chimney effect, and the credit taken is usually 10 to 15 percent of their normal duty. The above natural draft credit may not apply if a fire occurs near the cooler. 

Personnel working around hot furnaces must be protected from bums near hot flues. Best practice is to position lightweight, insulated, vertical ducts (open at both ends with a 1 ft high gap between their open bottom ends and the floor to admit cooling air) so that all poc exiting the furnace are drawn up into these ducts by their own chimney effect. This barometric damper also tends to minimize excessive draw by flues that get too hot, which could otherwise snowball into a very uneven temperature situation within the furnace chamber. Likewise, failure to clean scale or other blockages from flue entrances can cause uneven heating because nonblocked flues will get hotter and pull more draft by natural convection. 

TABLE 6.2 Draft or chimney effect at various furnace ieveis and temperatures... 

In an atmospheric spray tower the air movement - is dependent on atmospheric conditions and the aspirating effect of the spray nozzles. Natural-draft cooling tower operation depends on a chimney or stack to induce air movement. Mechanical-draft cboling towers utilize fans to move ambient air through the tower. Deck-filled towers contain tiers of splash bars or decks to assist in the breakup of water drops to increase the total water surface and subsequently the evaporation rate. Spray-filled towers depend only on spray nozzles for water breakup. Coil shed towers are comprised of a combination structure of a cooling tower installed on top of a substructure that contains atmospheric section coils. Hyperbolic natural-draft cooling towers are typically large-capacity systems. 

Draft Hood - A device built into or installed above a combustion appliance to assure the escape of combustion byproducts, to prevent backdrafting of the appliance, or to neutralize the effects of the stack action of the chimney or vent on the operation of the appliance. 

As noted, hot, expanded gases from a fire move vertically. In tall buildings the movement of heat and smoke is important. Several factors affect the vertical movement. These factors include tightness of construction, external winds, the difference between internal and external temperature, and the presence of vertical openings. Typical vertical openings are stairways, elevator shafts, and ventilation shafts. Vertical openings can create a stack effect, named after the movement of heat and smoke up a chimney or smoke stack. Some refer to this effect as the draft of a chimney or smoke stack. 

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