Document jB17QGnrzn59G86xvvBeBj3eQ

American Society of Heating and Ventilating Engineers Guide ' ': for the backward or slightly backward type. A noisy fan usuall ' which is operated at a point considerably beyond maximum efficie *S For a given static pressure there is a corresponding outlet velocit^ peripheral speed wherein maximum efficiency is obtained. If a f a selected to operate at this point, the cost of operation and thP ,, an be held within control. noiSe r To aid in selecting fans as near as possible to the point of maxi' efficiency, there are listed for each static pressure, corresponding 0,,U: velocities and tip speeds which will, give satisfactory results. The poi utletl tip speed for a given static pressure varies with design of wheeplroper! number of blades or vanes in wheel. Lower outlet velocities than listed in Table 1 may be used, but carel must be exercised when fans of the forward curved type are used to avoid! selecting a fan for operation below its useful range. 1 In exhaust ventilating systems where the air column moves toward the] fan, noise due to the higher tip speeds and outlet velocities, will not bet Table 1. Good Operating Velocities and Tip Speeds for Forward Curved i Multiblade Ventilating Fans Static Pressure In Inches or Wateb X Vs X H X 1 Vs IK IX 2 2K 2K 3 Outlet Velocitt Feet pee Minute 1000-1100 1000-1100 1000-1200 1100-1300 1200-1400 1300-1600 1500-1800 1600-1900 1800-2100 1900-2200 2000-2400 2200-2600 2300-2600 2500-2800 Tip Speed ' , Feet pee Minute 1520-17J)0 ' 1760-1900 1976-2150 2225-2450 2480-2700 2660-2910 2820-3120 3162-3450 3480-3810 3760-4205 4000-4500 4250-4740 4475-4970 4900-5365 so readily transmitted back through the air column to the building. 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, air washers, etc-, usually are' parts of any ventilating system where high static pressures are needed. Under such conditions it is practicable to select fans with higher outlet velocities and peripheral speeds since the duct system itself will tend to muffle objectionable air sounds. Fans with low outlet velocities should be used regardless of the static pressure, if connected with short ducts to register faces, on account of noise. 514 Chapter 32--Fans and Motive Power nection of a fan to a metallic duct system should be made by a Cn similar flexible material so as to prevent the transmission of Oration or noises. Where fans are connected to concrete ducts such yipra _ventilation of vehicular tunnels, the connection is made for Drying ith disc and centrifugal types of fans are used for drying work. Disc ' well adapted to the removal of moisture-laden air when operating ijunst lour resistance and when handling air at low temperatures, tors on these fans usually are of the fully-enclosed moisture-proof so that saturated air or air containing foreign material will not jtUTrneitthheeamteortsoresm. ploying disc or propeller type fans are widely used in Sie drying field. In drying, disc or propeller fans may be used where not o much duct work is required and where air is to be delivered against iessure, since the noise developed from the high peripheral speed of tese fans is not ordinarily objectionable in process work of this nature. Centrifugal fans or blowers of the multiblade type generally are selected supply air for drying, as they are capable of delivering large volumes ainst all pressures with a minimum amount of power. Belt driven fans usually are to be preferred to direct-connected fans they make a more flexible and economical unit. Wherever drying is 'done throughout the year and where air requirements change as the <Jiying 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. 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 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 dryer varies, and where battery operation is required, the full backward curved fan is; recommended. This type is well adapted for direct-connected motors of the higher speeds. Fans for Dust Collecting and Conveying The application of fans or exhausters'for handling refuse, dust, fumes, etc., generated by machine equipment is covered in Chapter 25. Infor mation is given regarding the methods for. determining air quantities, velocity required for carrying various materials and method of deter mining maintained resistance or total static pressure at which the fan is to operate. The selection of a proper size fan or exhauster is at times governed by the future requirements 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 515