Document XzdaEwjjOwE5e7aY90LjB1EmB

American Society of Heating and Ventilating Engineers Guide, 1935 camber, and the thickness of the blades provide a considerable degree f flexibility in design, so that the peak of total efficiency may be made t occur at wide-open volume or at various percentages of that volume. 10 Another advantage of this type of axial flow fan is its low resistance t air passage when standing still. There are some installations in which such a characteristic is desirable. The straight blade (paddle-wheel) or partially backward curved blade type of fan is practically obsolete for ventilation. Its use is largely con. fined to such applications as conveyors for material, or for gases con taining foreign material, fumes and vapors. The open construction and the few large flat blades of these wheels render them resistant to corrosion and tend to prevent material from collecting on the blades. This type 0( fan has a good efficiency7 But the power steadily increases'as the "static Chapter 17--Fans and Motive Power The pressure curves begin to drop at very low capacity and continue ^ fall rapidly to full outlet opening. The steep pressure curves fthoictuations in demand occur, this type of fan is desirable to prevent wide U motors. The maximum power requirement occurs at 0- ?the maximum efficiency. Consequently a motor selected to carry a^load at this point will be of sufficient capacity to drive the fan over its ranee of capacities at a given speed. The high speed of this type adaptable for direct connected electric motor drives. The high <I\Li may necessitate somewhat heavier construction and more operating S(+ntion or service. The dimensional bulk for a given duty often is ISO per cent of that of a forward curve multiblade type fan. ^Between the extremes of the forward-and the-full backward curve blade- type centrifugal fans a number of modified designs exist, differing in the 40 50 60 Per Cent of Wide Open Volume 70 90 100 3. Operating Characteristics of a Fan-with Blades Curved Forward pressure falls off, which requires that the motor be selected, with a moder ate reserve in power to take care of possible error in calculation of duct resistance. The forward curved multiblade fan is the type most commonly used in heating and ventilating work, as it has a low peripheral speed, a large capacity, and is quiet in operation. The point of maximum efficiency for this fan occurs near the point of maximum static pressure. The static pressure drops consistently from the point of maximum efficiency to full open operation. Fig. 3 shows that this type of fan will have both a high and a low delivery for a given static pressure at constant speed. The power curve rises continually from low to peak capacity, but if reasonable care is exercised in figuring resistance there is no danger of overloading the motor. The outstanding characteristics of the full backward curve multiblade type fan are the steep pressure curves, the non-overloading power curve, and the high speed. (See Fig. 4.) This fan operates at a peripheral speed of approximately 250 per cent of the forward curve multiblade type for 286 Fig. 4. Operating Characteristics of a Fan with Blades Curved Backward angularity or in the shape of the blades. Common among these designs are the straight radial blade type, the radial tip type, and the double curve blade type with a forward angle at the heel and a slight backward angle at the tip of the blade. Characteristic curves of these types show varying degrees of resemblance to the curves of Figs. 3 and 4, according to the degree of similarity to one or the other , of the two designs of fan considered. SELECTION OF FANS The following information is required to select the proper type of fan: 1. Cubic feet of air per minute to be moved. 2. Static pressure required to move the air through the system. 3. Type of motive power available. 4. Whether fans are to operate singly or in parallel on any one duct. 5. What degree of noise is permissible. 6. Nature of the load, such as variable air quantities or pressures. 287 A