Document aBVMdaLmdQrj83qok3vew723R

American Society of Heating' and Ventilating Engineers Guide, 1932 aspecified capacity and pressure varies with the type of blade, and a tip speed that is excessive for the forward curved type is not necessarily so for the backward or slightly backward type. A noisy fan usually is one which is operated at a point considerably beyond maximum efficiency. For a given static pressure there is a corresponding outlet velocity and' peripheral speed wherein maximum efficiency is obtained. If a fan be selected to operate at this point, the cost of operation and the noise can be held within control. To,aid, in selecting fans as near as possible to the point of maximum efficiency',- there are listed for each static pressure, corresponding outlet velocities and tip speeds which will give satisfactory results. The proper tip speed for a given static pressure varies, with design of wheel and number of blades or vanes in wheel! '' Lower outlet velocities than listed in Table 1 may be used, but care must be exercised when fans of the forward curved type are used to avoid selecting.a fan for operation below its useful range. . . .. In exhaust ventilating systems where the air column moves toward'the fan,.,noise due to the higher tip speeds and outlet velocities, wilLnot be 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 490 Chapter 34--Fans and Motive Power 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 tb 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. ' The connection of a fan to a metallic duct system should be made by a canvas or similar flexible material so as to prevent the transmission of fan vibration or noises. Where fans are connected to concrete ducts such as are used in the ventilation of vehicular tunnels, the connection is made direct. Fans for Drying Both disc and centrifugal types of fans are used for drying work., Disc fans are well adapted to the removal of moisture-laden air when operating against low resistance and when handling air at low temperatures. Motors on these fans usually are of the fully-enclosed moisture-proof types so that saturated air or air containing foreign material will not injure the motors. - Unit heaters employing disc or propeller type fans are widely used in the drying field. In drying, disc or propeller fans may be used 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 of this nature. Centrifugal fans or blowers of the multiblade type generally are selected to supply air for drying, as they are capable of delivering large volumes against all pressures with a minimum amount of power. Belt driven fans usually are to be preferred to direct-connected fans Table 1. Good Operating Velocities and Tip Speeds for Forward Curved Multiblade Ventilating Fans Static Pressure In Inches or Water X Vs X Vs X X> '1 ix IX IX 2 2K 2M 3 Outlet Velocity Feet feb Monro 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 1520-1700 1760-1900 1970-2150 2225-2450 . . 2480-2700 2660-2910 2820-3120 3162-3450 3480-38103760-4205 4000-4500 , 4250-4740 4475-4970 4900-5365 491