Document bawweZGO7x1kYJvDqnQEMpVZ1

American Society of Heating and Ventilating Engineers Guide, 1934 5. Electric lights should be installed in the chamber in front of and behind the air filter. 6. Air washers should, whenever possible, be installed between the tempering and heating coils to protect them from extreme cold in winter time. 7. Filters installed close to air inlet should be protected from the weather by suitable louvers, in front of which a large mesh wire screen should be provided. REFERENCES Testing and Rating of Air Cleaning Devices Used for General Ventilation Work, by Samuel R. Lewis (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning May, 1933). Fundamental Principles in the Design of Dry Air Filters, by Otto Wechsberg (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, April, 1933). Operation and Maintenance of Air Filters, by W. G. Frank (.Heating, Piping and Air Conditioning, May, 1931). Size and Characteristics of Air-Borne Impurities, by W. G. Frank (Heating, Piping and Air Conditioning, January, 1932). Determining the Quantity of Dust in Air by Impingement, by F. B. Rowley and John Beal (A.S.H.V.E. Transactions, Vol. 35, 1929). A Study of Dust Determinators, by F. B. Rowley and John Beal (A.S.H.V.E. Trans actions, Vol. 34, 1928). Design and Application of Oil-Coated Air Filters, by H. C. Murphy (A.S H.V E Transactions, Vol. 33, 1927). Determining the Efficiency of Air Cleaners, by A. M. Goodloe (A.S.H.V.E. Trans actions, Vol. 30, 1924). Chapter 17 FANS Performance, Fan Efficiency, Characteristic Curves, Selection, of Fans, Controls, Designation of Fans, Motive Power, Electric Pouter FANS are used for producing air flow except where positive displace ment is required, in which case compressors or rotary blowers are used. Fans are classified according to the direction of air flow as (1) axial flow 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 a propeller fan is intended for operation at comparatively high pressures the hub is sometimes enlarged in the form of a disc and the fan is known as a disc fan. Radialflow 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 or ihange in the curvature and angularity of the blades. The angularity of the blades determines the speed, characteristic, the forward curve cor responding to slow speed and the backward curve to 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 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 requiring a static pressure of less than 1 in. of water, whereas centrifugal fans are more commonly employed for operation at comparatively high pressures. 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 laws apply to installations comprising any type of fan, any given piping system and constant air density, and are as follows: 1. The air capacity varies directly as the fan speed. 2. The pressure (static,-velocity, and total) varies as the square of the fan speed. 3. The horsepower varies as the cube of the fan speed. 225