Document Bvo6XaX0Dw7DzQ0eb2gdZwdJj

HEATINC VENTILATING AIR CONDITIONING GUIDE 1940 In exhaust ventilating systems where the air column moves toward the fan, noise due to the higher tip speeds and outlet velocities will not be so readily transmitted back through the air column to the building as when the air column is moving toward the rooms. Therefore higher outlet velocities may be used, but this will be at the expense of increased horsepower. Amply large fans should always be used for both exhaust and supply systems, as there may be and usually is leakage despite the most careful workmanship, necessitating the delivery of more air at the fans than is exhausted from or supplied through the openings in the Various rooms. Long runs of distributing ducts, heaters, and air washers require definite increments of the total pressure which a supply fan in a venti lating system must overcome. These static pressures should be con sidered when selecting the fan characteristics, speed, and power. Table 2. Good Operating Velocities and Tip Speeds for Multiblade Ventilating Fans with Backward Tipped and Double Curved Blades Static Pbessdrb Inches of Water Y. H Y Ys % 7A l IH m 2 2K 2Y 3 Outlet Yelocitt Feet rb Minute 800-1100 800-1150 900-1300 1000-1500 1100-1650 1200-1750 1200-1900 1300-2100 1400-2300 1500-2500 1600-2700 1700-2800 1800-2950 2000-3200 Tip Speed Feet peb Minutb 2600-3100 3000-3500 3400-4000 3800-4500 4200-5000 4500-5300 4800-5750 5300-6350 5750-6950 6200-7550 6650-8050 7050-8550. 7450-9000 8200-9850 Fans picked within the limits of Table 1 will operate close, to the point of maximum efficiency. No attempt has been made to select these limits for quiet operation,,since this is a relative term and varies with the type and location of the installation. The connection of a fan to a metallic duct system should be made by canvas or a similar flexible material so as to prevent, the transmission of fan vibration or noises. Where noise prevention is a factor the fan and its driver should have floating foundations. 'I Fans for Drying ' Both axial flow and centrifugal types of fans are used for drying work. Propeller fans are well adapted to the removal of moisture-laden air when operating against low resistance and when handling air at low tempera tures. Motors on these fans usually are of the fully-enclosed moistureproof types so that saturated air or air containing foreign material will not injure the motors. Unit heaters employing axial flow fans are widely used in the drying 524 CHAPTER 28. FANS field: In drying, these fans may be used with unit heaters where not too much duct work is required and where air is to be delivered against pressure, since the noise developed from the high peripheral speed of these fans is not ordinarily objectionable in process work. Centrifugal fans of the multiblade type generally are selected to supply air for drying, as they are capable of delivering large volumes of air against all pressures likely to be encountered. Belt-driven fans usually are to be preferred to direct-connected fans since efficient motor speeds do not usually coincide with efficient fan speeds. . Replacement of a standard motor is quick and easy if it is belted. Wherever drying is done throughout the year and where air require ments change as the drying conditions change, the drying can be speeded up or reduced through control of the fan capacity. This may be done by changing the fan speed or by varying the outlet area with dampers. A throttled outlet reduces the volume and reduces the power. Due to the low speeds of forward curved multiblade or paddle-wheel type fans, these can be direct-connected to reciprocating steam engines, and the exhaust steam from the engines may be used in the heating apparatus. In selecting engine-driven fans for drying processes, where a large quantity of exhaust steam is used in the heaters, a smaller fan and greater power consumption may be used, because power economy is not essential under this condition. Where static pressure in a drier varies, and where several fans must operate in parallel, fans are to be preferred which have a continuously rising pressure characteristic, such as is given by backward-curved or double-curved blades. This type of fan is well adapted for direct-con nected motors of the higher speeds. (See Chapter 36 on Drying Systems). Fans for Dust Collecting and Conveying The application of fans for handling refuse, dust, and fumes generated by machine equipment is covered in Chapter .35. Information is given regarding the methods for determining air quantities, the velocity required for carrying various materials and the method of determining maintained resistance or total static pressure at which the fan is to operate. The selection of a proper size fan is at times governed by the future require ments, of the plant. In many instances, additional future capacity is anticipated and should be provided for. Having determined the necessary volume of air and the maintained resistance or static pressure required, the proper size fan may be selected from the fan manufacturers' performance charts or capacity tables. The fan chosen should be the size that will provide the required ultimate quantities with the minimum power consumption. FAN VOLUME CONTROL Some method of volume control of fans usually is desirable. This may be done by varying the peripheral velocity or by interposing resistance, as by throttling-dampers. Both methods, since they reduce the volume of air, reduce the power required. In many installations adjustments of volume are desirable during varying hours of the day. In others an