Document 5bMpovY8aNepmomYQ4bqk9Epz

American Society of Heating and Ventilating Engineers Guide, 1937 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 efficiency, but the power steadily increases as the static Chapter 17--Fans akes it adaptable for direct connected electric motor drives. The high ged may necessitate somewhat heavier construction and more operating ttention or service. The dimensional bulk for a given duty often is 150 per cent of that of a forward curve multiblade type fan. geWeen the extremes of the forward and the full backward curve blade type centrifugal fans a number of modified designs exist, differing in the 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. 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 like results. The pressure curves begin to drop at very low capacity and . continue to fall rapidly to full outlet opening. The steep pressure curves tend to produce constant capacity under changing pressures. Where wide fluctuations in demand occur, this type of fan is desirable to prevent overloading of motors. The maximum power requirement occurs at about the maximum efficiency. Consequently a motor selected to carry the load at this point will be of sufficient capacity to drive the fan over its full range of capacities at a given speed. The high speed of this type 322 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. SYSTEM CHARACTERISTICS / A given fan performs as determined by the real characteristic of the system to which it is attached. When a different performance of a fan is desired, it is necessary to either change the speed of the fan (as A to 13 or C to D in Fig. 5), or to change the system (as by moving a damper from A to C in Fig. 5). If the speed of the fan is changed, the new point of opera tion is the intersection of the constant speed static pressure--cubic feet per minute curve for the new speed with the system characteristic. If the system is changed, the new point of operation is the intersection of the constant speed static pressure, cubic feet per minute curve with the new system characteristic. Heating and ventilating systems follow the simple parabolic law quite closely but other types of systems follow some other more or less complex relation. The more complex systems can be separated into their com ponent parts whose individual characteristics are known and the sum mation of the characteristics of the several parts of a system will give the composite characteristic of the system. 323