Fans, capacity

This example illustrates the sensitivity of fan horsepower to small changes in tower area. The importance of designing a tower that is slightly oversize in ground area and of providing plenty of fan capacity becomes immediately apparent. 

Circulation of air at velocities of I to 10 m/s is desirable to improve the surface heat-transfer coefficient and to eliminate stagnant air pockets. Proper air flow in tray dryers depends on sufficient fan capacity, on the design of ductwork to modify sudden changes in direction, and on properly placed baffles. Nonuniform airflow Is one of the most serious problems in the operation of tray di yers. 

Finally, the mass flux of conversion gas is primarily controlled by the volume flux of primary air and conversion concepf, whereas for a gas-fired system the mass flux of gas fuel into the combustion chamber is limited by the gas fuel fan capacity and the burner design. 

A cooling tower has a cross-sectional area of 25 X 25 ft. The total heat load to the unit is 27,500,000 Btu/hr. The locality has a 5% wet-bulb temperature of 75°F. Water exits the tower with a 12° approach to the wet-bulb temperature (i.e., 87°F).The hot process water enters the tower at a temperature of 125°F, and the water equivalent to this range is 1800 gpm. The systems fan capacity is 150,000 cfm (a) Determine the number of diffusion units that the tower must be capable of performing to meet process requirements (b) the tower manufacturer provided the following data for overload and underload conditions for the tower ... 

An extremely important operating requirement for the MPF is the safe shutdown and restarting of the unit if there is a loss of the air supply for combustion, loss of exhaust fan capacity, or both. A loss of electrical power is the usual cause of such an occurrence. If this happens, there is a significant probability that the munitions in the MPF will still be vaporizing uncombusted agent when the furnace temperature drops below 1,200°F. At JACADS, the solution was to cool the furnace to 400°F with water sprays, then to back the trays into a buffer storage zone upstream of the MPF (MR E, 1996). Once the furnace was restarted, the trays were processed normally. 

V q p = exhaust at 70 F reference, i.e., 11,350 ft3/ minute Since air expands when heated, an adjustment is required to insure for correct quantities of 70 F air entering the oven. This correction factor (adjusted exhaust rate on actual fan capacity at oven temperature) can be found for the following relationship ... 

Compute the correction factors for the forced-draft fan. Commercial fan-capacity tables are based on fans handling standard air at 70°F at a barometric pressure of29.92 inHg and having a density... 

Choose the fan size from the capacity table. Turn to the fan-capacity table in the engineering data and look for a fan delivering 62,250 ft3/min at 6.3 inH20 static pressure. Inspection of the table shows that the capacities are tabulated for pressures of 6.0 and 6.5 inH20 static pressure. There is no tabulation for 6.3 inH20. The fan must therefore be selected for 6.5 inH20 static pressure. 

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