Forced and Induced Draft

Forced and Induced Draft The forced-draft unit, which is illustrated in Fig. 11-43 pushes air across the finnedtube surface. The fans are located oelow the tube bundles. The induced-draft design has the fan above the bundle, and the air is pulled across the finned tube surface. In theoiy, a primaiy advantage of the forced-draft unit is that less power is required. This is true when the air-temperature rise exceeds 30°C (54°F). 

Advantages and disadvantages of forced and induced draft air-cooled heat exchangers ai e shown here to aid in selection. 

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 economics of forced and induced draft cooling tower operation require a study of fan and water pump horsepower and usually dictate a fan static pressure requirement not to exceed 0.75-1.0 in. of water. For atmospheric and natural draft towers the economics of pumping water are still very important. This means that the ground area must be so selected as to keep the height dovm while not dropping the unit rates so low that performance becomes poor. This then, is a balance of ground area versus total deck height. Pritchard [16] presents an... 

This is a very versatile stoker common in shell boilers and is used with modification in watertube boilers as the traveling-grate stoker. In shell boilers, the air supply is via forced- and induced-draft fans in combination with a balanced or slightly negative condition in the furnace. 

As with furnaces, discussed in Chapter 15, the draft in boilers can be natural, forced draft or induced draft. Forced and induced draft arrangements are the most common. For large boilers requiring a significant amount of equipment in the flue to treat the exhaust gases to remove oxides of sulfur and/or oxides of nitrogen, as well as equipment for heat recovery, a combination of forced and induced draft might need to be used. 

Combustion calculations show that an oil-fired watertube boiler requires 200,000 lb/h (25.2 kg/s) for air of combustion at maximum load. Select forced- and induced-draft fans for this boiler if the average temperature of the inlet air is 75°F (297 K) and the average temperature of the combustion gas leaving the air heater is 350°F (450 K) with an ambient barometric pressure of 29.9 inHg. Pressure losses on the air-inlet side are, in inFLO air heater, 1.5 air supply ducts, 0.75 boiler windbox, 1.75 burners, 1.25. Draft losses in the boiler and related equipment are, in inH20 furnace pressure, 0.20 boiler, 3.0 superheater, 1.0 economizer, 1.50 air heater, 2.00 uptake ducts and dampers, 1.25. Determine the fan discharge pressure and horsepower input. The boiler burns 18,000 lb/h (2.27 kg/s) of oil at full load. 

Determine the types of controls to consider. There are five types of controls used for forced- and induced-draft fans (a) a damper in the duct with constant-speed fan drive, (h) two-speed fan driver, (c) inlet vanes or inlet louvres with a constant-speed fan drive, (d) multiple-step variable-speed fan drive, and (e) hydraulic or electric coupling with constant-speed driver giving wide control over fan speed. 

Figure 8-14. Forced and induced draft air cooled exchangers. 639...

Cooling towers—devices used by industry to remove heat from water. In a typical tower, a boxshaped collection of multilayered slats and louvers directs airflow and breaks up water as it cascades from the top of the water distribution system. Cooling towers are classified by the way they produce airflow and by the way the air moves in relation to the downward flow of water. Basic designs include atmospheric, natural, forced, and induced draft. 

---