Document wDayN3qvL3dgQMYJ93MYEM2y6

American Society of Heating and Ventilating Engineers Guide, 1935 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 requirements 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.outletarea 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, Table 2. Good Operating Velocities and Tip Speeds for Multiblade Ventilating Fans with Backward Tipped and Double Curved Blades Static Pressure Inches or Water Yi Vi Y Vs Vi Vs 1 l% 114 1H 2 2 Ya 2H .3 Outlet Velocitt Feet pee 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 per Minute 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 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 dryer 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. Fans for Dust Collecting and Conveying The application of fans for handling refuse, dust, and fumes generated by machine equipment is covered in Chapter 21. Information is given regarding the.methods for determining air quantities, the velocity required for carrying various materials and the method of determining maintained 290 Chapter 17--Fans and Motive. Power- 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 CONTROL ---- Some method;oLvolume_controLof 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 increased supply of air in summer over that needed for winter is demanded. Experience is required in deciding whether speed-control or dampercontrol shall be used for specific cases. Where noise is a factor, it may be exceedingly desirable to reduce the speed at times, while on the other hand, any fan which has its normal speed reduced as much as 50 per cent without change in resistance will move only.50 per cent of the air. DESIGNATION OF FANS Facing the driving side of the fan, blower, or blast wheel, if the proper direction of rotation is clockwise, the fan, blower, or blast wheel will be designated as clockwise. If the proper direction of rotation is counter-clockwise, the designation will be counter clockwise. (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.)* This method of designation will apply to all centrifugal fans, single or double width, and single dr double inlet. Do not use the word "hand," but specify "clockwise" or "counter-clockwise." The discharge of a fan will be determined by the direction of the line of air discharge and its relation to the fan shaft, as follows: Bottom horizontal: If the line of air discharge is horizontal and below the shaft. Top horizontal: If the line of air discharge is horizontal and above the shaft. Up blast: If the line of air discharge is vertically up. Down blast: If the line of air discharge is vertically down. All intermediate discharges will be indicated as angular discharge as follows: Either top or bottom angular up discharge or top or bottom angular down discharge, the smallest angle made by the line of air discharge with the horizontal being specified. In order to prevent misunderstandings, which cause delays and losses, the arrangements of fan drives adopted by the National Association of Fan Manufacturers and indicated in Fig. 5 are suggested. If double width, double inlet fans are selected, care must be taken that both inlets have the same free area. If one inlet of a forward curved blade type of fan is obstructed more than the other, the fan will not operate properly, as one half of the wheel will deliver more air than the other half. Recommendations adopted by the National Association of Fan Manufacturers. 291