Document jgdYyLGL6077Oje8LyQm3Da4y

822 CHAPTER 32 DeQ= 1957 Guide Fans 823 Counter-Clockwise Top Horizontal Clockwise Top Horizontal Clockwise Bottom Horizontal Counter-Clockwise Bottom Horizontal No 7-SW.S1 No 7 - DW, 01 Fig. 6. No 8 - 5W,SI No9-SW,Sl Arrangement of Fan Drives Arr. 1, SW, SI. For belt drive or direct connection. Wheel overhung. Two bearings on base. Arr. 2, SW, SI. For belt drive or direct connection. Wheel overhung. Bearings in bracket sup ported by fan housing. Arr. 3, SW, SI. .For belt drive or direct connection. One bearing on each side and supported by fan housing. Not recommended in sizes 27 in. diameter wheel and smaller. Arr. 3, DW, DI. For belt drive or direct connection. One bearing on each side and supported by fan housing. -; - Arr. 4, SW, SI. For direct drive. Wheel overhung on prime mover shaft. No bearings on fan. Base or equivalent for prime mover. Arr. 7, SW, SI. For belt drive or direct connection. Arrangement No. 3 plus base for prime mover. Not recommended in sizes 27 in. diameter and smaller. Arr. 7, DW, DI. For belt drive or direct connection. Arrangement No. 3 plus base for prime mover. Ait. 8, SW, SI. For-belt.-drive or direct connection. Arrangement No. 1 plus base for prime mover. Arr. 9, SW, SI. For belt drive. Arrangement No. 1 designed for mounting prime mover on side of base. The curves in Fig. 5 also illustrate the effect of errors which may be made in calculating the resistance of a ventilating system. For instance, if a given system requires 13,000 cfm, and the resistance to flow of the system has been computed as 1.25 in. static pressure, such a system would be represented by system characteristic curve B in Fig. 5. If a 100 per cent error had been made and the resistance were 2.5 in. instead of 1-25 in., then the system characteristic would be as shown in curve A, and would cross the fan curve at 10,000 cfm. Such an error would cause the flow of air to be decreased from a design volume of 13,000 cfm to 10,000 cfm. If the resistance to flow had been over-estimated and the resistance actually were 0.625 in., the system characteristic curve would be as shown in curve C, and the fan would deliver 16,400 cfm to the system instead of the design volume of 13,000 cfm. In this example, extreme errors have been selected to emphasize the effect the square function of the system characteristic has in maintaining the fan performance within comparatively narrow limits. In the first example, a system estimated at half what it should have been resulted in a drop of 23 percent in volume; and in the second example, a system es timated at twice what it should have been resulted in an increase of 26 percent in volume. In some instances fans may be applied to variable flow systems. In Clockwise Up Blast Coun ter-Clockwise Up Blast Counter-Clockwise Down Blast Clockwise Down Blast Counter-Clockwise Top Angular Down Clockwise Top Angular Down Clockwise Bottom Angular Up Counter-ClockwIse Bottom Angular Up Counter-Clockwise Top Angular Up Clockwise Top Angular Up Clockwise Bottom Angular Down Counter-Clockwise Bottom Angular Down Fig. 7. Designation of Direction of Rotation and Discharge Direction of Rotation is determined from the drive side for either single or double width or wngU r*w^i udet fans. (The driving side of a single inlet fan is considered to be the side opposite the inlet regarcllea of the actual location of the drive.) For fan inverted for ceiling suspension, direction of rotation ana discharge is determined when fan is resting on floor. such cases, the limiting systems may be plotted and the effect on fan per formance examined. For instance, a system might have a characteristic curve between A, shown in Fig. 5, as one limit, and B as the other limit, fhe fan performance will then fall between points X and Y on the fan curve at a point determined by the system characteristics at that particular tune. If A and B are the limiting characteristic curves of the systems, the Ian performance will never be outside the points X or Y.' . FAN ARRANGEMENTS Centrifugal fan arrangements have been standardized by the National ssodation of Fan Manufacturers. Figs. 6, 7 and 8 show the accepted csignation as to arrangement of drive, rotation, discharge and motor position, for belt drive. Axial flow fans are either belt driven or direct r, nnected, in accordance with individual manufacturer's arrangements, sually a choice of anti-friction or sleeve bearings is available. FAN CONTROL v In some heating and ventilating systems it is desirable to vary' the Ume f air handled by the fan, and this may be accomplished by a