Document MM3R1BQRgaOj672jp3noB7v57

592 CHAPTER 32 1948 Guide '' Counter-Clockwise .Top Horizontal Clockwise Top Horizontal Clockwise Bottom Horizontal : Counter-Clockwise ` Bottom Horizontal Clockwise Up Blast Counter-Clockwise Up Blast Counter-Clockwise Down Blast Clockwise Down Blasl 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 Note: Direction of Rotation is determined from the drive side for either single or double width or single ,or double inlet fans. (The driving side of a single inlet fan is considered to be the side opposite the inlet regardless of the actual location of the drive.) at various times during the day or continuously. In others an increased supply of air in summer over that needed in winter is demanded. The demands of each case will dictate which type of control is most desirable. Where noise is a factor, lowering the fan speed if possible is preferred as a control means, because, of the resulting reduction in sound level. MOTIVE POWER Heating, ventilating and air conditioning fans are usually driven by electric motors, although other prime movers may be used. The small sizes of fans, and especially those operating in the higher speed range, are equipped with direct-connected motors. For larger size.fans and those operating at lower speed V-belt drives are generally used. In selecting the size of motor for operating a fan, it is advisable to select at least the standard size next larger than the fan requirements. Direct-connected motors do not require so. great a safety factor as belted units. . J ustification for liberal power provision exists only in systems where it is possible that larger volumes of air may be required at intervals Fans 593 and made available by use of by-pass dampers, thus greatly reducing the system resistance. If such a system includes a fan with forward-curved blades, it would be necessary that the motor be sized for the maximum volume and duty. . If. such a system includes fans with backward-curved blades, the volume peak would not make it necessary to provide addi tional motor power. In selecting fans for such a system, sound ratings should be given careful consideration. Where a--.system *is constant, and has no provision for volume change that would- materially reduce, the resistance, and when the resistance calculations are reasonably accurate, there is no necessity for too liberal ' a motor allowance, even where fans with forward-curved blades are used, if the fan has been properly selected. Fig. 4 shows that the system resistance varies as the square of the volume and the fan static pressure varies approximately inversely.as the volume, thus greatly offsetting the trend toward both increase in air delivery and motor load. Reference to Fig. 4 indicates that there is. no justification for allowing large spare motor capacity, and it is generally more economical to operate motors well loaded. ATTIC FANS Attic fans are used during the warm months of the year to draw large volumes of outside air through a house and offer a means of using the comparative coolness of outside evening and night air to lower the inside temperature. Because the low static pressures involved are usually less than inof water, disc or propeller fans are generally used instead of the' blower types. The fans should have quiet operating characteristics, and they should be capable Of giving 20 to 30 room air changes per hour in northern areas. In the South the usual specification requires one air change per minute which provides appreciable air movement in addition to lowering the inside air temperatures 4- Types Open attic Jans are units in which the fan is installed in a gable or dormer of the attic and one or more grilles are provided in the floor of the attic, permitting air to flow from the hall below. - Outdoor air, which enters the house through open windows, is drawn, into the attic through the grilles, and is discharged outside by the fan. An attic stairway may be used in place of the grilles. It is essential that the roof and the attic walls be free from air leaks. Boxed-in fans are units in which the fan is installed within the attic in a box or housing directly over a central ceiling grille, or in a bulkhead enclosing an attic stair. The fan may be connected by a duct system to the grilles in individual rooms. Outdoor air entering through the windows of the rooms below is discharged into the attic space and escapes to the outside through louvers, dormer windows, or screened openings. Another version of the attic fan is the window fan for use when attic application is not feasible or no attic is available. Supplied with a perforated or expanded metal enclosure and mounted in either the upper or lower window section, this fan is easy to install or transfer. The locations of the fan, the outlet openings, and grilles should be selected after consideration of the room and attic arrangements in order to give uniform air distribution in the individual rooms served. If the outlet for the air is not on the side away from the direction of the pre-