Document ZBRQnXnvqbq0eK7kbEe2vdZoO

American Society of Heating and Ventilating Engineers Guide, 1935 Knowing the requirements of the system, the main points to be con sidered for fan selection are (1) efficiency, (2) speed, (3) noise, (4) size and weight, and (5) cost. In order to facilitate the choice of apparatus, the various fan manu facturers supply fan tables or curves which usually show the following factors for each size of fan operating against a wide range of static pressures: 1. Volume of air in cubic feet per minute (68 F, 50 per cent relative humidity 0.07488 lb per cubic foot). . 2. Outlet velocity. 3. Revolutions per minute. 4. Brake power. 5. Tip or peripheral speed. 6. Static pressure. The most efficient operating point of the fan is usually shown by either bold-face or italicized figures in the capacity tables. Fans for Ventilation and for Cooling Systems Two important factors in selecting fans for ventilating systems are efficiency (which affects the cost of operation) and noise. First cost and space available are secondary. The fans should be selected to operate at maximum efficiency without noise. Because noise in a ventilating system is irritating and a cause for complaint, fans must be selected of proper size in order to reduce it to a minimum. Noise may be caused by other factors than the fan, namely, high velocity in the duct work, unsatisfactory location of the fan room, improper construction of floors and walls, and poor installation. Where noise is chargeable directly to the fan, fit is caused either by excessive peripheral speeds, or the fan is of insufficient size. It should be remembered, however, that the tip speed required for a specified capacity and pressure varies with the type of blade, and that a tip speed which may be-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 is 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 in Tables 1 and 2 for each static pressure cor responding outlet velocities and tip speeds which will give satisfactory results. The proper tip speed for a given static pressure varies with the design of wheel and with the number of blades or vanes in the wheel. Lower outlet velocities than those listed in Table 1 may be employed, 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. The useful range of the fans of Table 2 extends over the full length of the per formance curve. In exhaust ventilating systems where the air column moves toward the 288 Chapter 17--Fans and Motive Power 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 X. Good Operating Velocities and Tip Speeds for Forward Curved Multiblade Ventilating Fans Static Pressure Inches or .Water K H K H 1K m m 1*A 2 2X 3 Outlet Velocity Feet per 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 per Minute 1520-1700 1760-1900 1970-2150 2225-2450 2480-2700 2660-2910 2820-3120 3162-3450 3480-3810 3760-4205 4000-4500 4250-4740 4475-4970 4900-5365 1 ClliJ, rviLiim me unuLS oi laDie 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 arid its driver should have floating foundations. 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 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 289