Document 8V6GDpam2JNgkOrq4azgQG7ey

1936American Society of Heating and Ventilating Engineers Guide, rate of air delivery with the increased static pressure, at what speed must the fan be run and what horsepower will be required? Static pressure after clogging of filter =? 1 + (0.4 -- 0.1) = 1.3 in. of water. The static pressure varies as the square of the fan speed. Therefore, if X is the fan speed after the static pressure increases: 1.3 / X \2 1 \ 400 ) X = 456 rpm. The horsepower varies as the cube of the fan speed. Therefore, if Y is the horsepower after the static pressure increases: Y_ _ ( 456 \3 3 \ 400 / Y -- 4.44 horsepower. To maintain the original rate of air delivery with the increased static pressure, the fan speed must be increased from 400 to 456 rpm, and the horsepower from 3 to 4.44. Chapter 17 FANS Performance, Fan Efficiency, Characteristic Curves, Selection of Fans, Controls, Designation of Fans IN heating and ventilating practice, fans are used to produce air flow except where positive displacement is required, in which case com pressors or rotary blowers are used. Fans are classified according to the direction of air flow as (1) axialflow or propeller type if the flow is parallel with the axis, and (2) radial flow or centrifugal type if the flow is parallel with the radius of rotation. Axial flow fans are made with various numbers of blades of a variety of forms. The blades may be of uniform thickness (sheet metal), either flat- or cambered, or may be of varying thickness of so-called aerofoil section (airplane propeller type). Where an axial flow fan is intended for operation at comparatively high pressures the hub sometimes is enlarged in the form of a disc and the fan.is known as a disc fan. Radial flow or centrifugal fans include steel plate fans, pressure blowers, cone fans, and the so-called multiblade fans. All the foregoing types have variations which may be obtained by modification of the proportions orchange in the curvature and angularity of the blades. The angularity of the blades determines the operating characteristics of a fan: a forward curved blade is found in a fan having slow speed operating characteristics, while a backward curved blade is found in a fan having high speed operating characteristics. A wide variation exists in the demands which have to be met by fan installations. A fan may be required to move large quantities of air against little or no resistance or it may be required to move small quanti ties against high resistances. Between these two extremes innumerable specific requirements must be met. In general, fans of all types in each general class can be made to perform the same duty, although mechanical difficulties, noise or lack of efficiency may limit the use to one or another type. The most common field of service for fans of the propeller type is in moving air against moderate resistances, especially where no long ducts or heavy friction must be overcome and where noise is'not objectionable, whereas centrifugal fans are commonly employed for operation at the comparatively higher pressures and where extreme quietness is necessary. PERFORMANCE OF FANS Fans of all types follow certain laws of performance which are useful in determining the effect of changes in the conditions of operation. These 309