Document b5paZ7Q1ZyB4m3NBzZnVK6ZOZ

908 CHAPTER 41 . 1953 Guide / found that for 360 rpm the static deflection required for a ratio of ///,, of 3 to 1 (line EF) is 2.5 in., and for a ratio of 5 to 1 (line GH) it is 7 in. For these values of deflection the only choice of material is the coil spring. This is also true for speeds up to about 700 rpm. In consideration of the transverse spring constant (so as to maintain good ratios among the various degrees of freedom) experience has shown that the spring should be designed with a working height equal to 1.0 to 1.5 times the outside diameter. A long -spring of small outside diameter has very low transverse rigidity, and therefore requires some additional means of preventing side drift of the unit, and on very sensitive applications this may tend, to destroy the isola tion efficiency. For speeds of 700 to 1200 rpm the required deflections range from 0.22 in. to 1.75 in. For these conditions rubber in shear serves as a rather satisfactory material if protected from oil. For speeds higher than 1200 rpm cork specially made for vibration damping can be applied with good results. These limitations are by no means absolute, because certain liberties may be taken without impairing the result if all possible degrees of > freedom have been taken into account in the design of the installation. * When a machine unit is properly isolated it will have a definite amount ** of movement which is determined by the ratio of the unbalanced forces to the total mass of the machine. If this resultant machine movement is too *1 great for the necessary connections or the satisfaction of the customer, it can be reduced only in two ways without destroying the quality of the isolation; first, adding mass or dead weight to the machine (such as concrete) common in the application of low speed, partially balanced machinery; second, accurately balancing (both statically and dynamically) all moving parts.so as to eliminate the vibration at the source. This latter method is the best engineering practice and is the modern trend. However, even with well balanced machinery, installed in the vicinity of quiet offices, it is usually & necessary to isolate properly the equipment to prevent the transmission of 41 vibration likely to cause complaints. Where limitation of machine movement is desired during the starting # and stopping periods, the. application of friction or hydraulic damping will serve without seriously interfering with the efficiency of the isolation. # REFERENCES * American Standard for Noise Measurement, Z24.2-1942, American Standards Association. * 1 American Standard for Sound Level Meters for Measurement of Noise and Other Sounds, Z24.3-1944, American Standards Association. * Sound Insulation of Wall and Floor Constructions ( U. S. Department of Commerce, National Bureau of Standards, Building Materials and Structures Report BMS17 ana Supplement). M 4 A.S.H.V.E. Research Report No. 1205--Determining Sound Attenuation in Air Conditioning Sys tems, by D. A. Wilbur and R. F. Simons (A.S.H.V.E. Transactions, Vol. 48, 1042, p. 207). * For coefficients of commercial sound absorbent materials see Bulletin Acoustical Materials Association, 919 No. Michigan Ave., Chicago, 111. * SoundPropagation inDucts Lined with Absorbing Materials, by L. J. Sivian (Journal Acoustical Society cf America, Vol. 9,1937-38. pp. 135-140). 7 The Absorption of Noise in Ventilating Ducts, by HaleJ. Sabine (Journal Acoustical Society of America, Vol. 12, p. 53. 1940). * Sound Absorption in Rectangular Ducts, by L. L. Beranek (Journal Acoustical Society of America, Vol. 12, pp. 228-37, October, 1040). * The Transmission of Sound Inside Pipes, by Philip M. Morse (Journal Acoustical Society of America, Vol. 11, pp. 205-210, October, 1939) 10 The Prediction of Noise Levels from Mechanical Equipment, by J. 8. Parkinson (Heating and Venti~ latino, March. 1939, pp. 23-20}. Methods of Rating the Noise from Air Conditioning Equipment, by J. S. Parkinson (A.S.H.VJ2. Jour* nal Section. Heating, Piping and Air Conditioning, July, 1940, p. 447). u The Noise Characteristics of Air Supply Outlets, by D. J. Stewart and G. F. Drake (AJ3.H.V.E. Tran*actions, Vol. 43, 1937, p. 81). CHAPTER 42 ELECTRIC HEATING Resistors, Heating Elements, Electric Heating Units, Types of . Electric Heating Systems, Equipment and Installation Methods, Heating Domestic . Water, Calculating Capacities, Induction and Dielectric. Heating, Power Problems ELECTRICITY as a source of heat represents thermal energy in a re fined form, easily applied to space heating by a variety of methods, and readily distributed and controlled. However, it usually is more expensive on a direct-heat-equivalent basis, than, heat from conventional fuels, and for economical service requires careful application in' the design of system, adaptation of building structure, choice of control devices, and in method of operation by the user. For special applications and particularly when used to ' supplement heating systems of other types, the compactness, simplicity, responsiveness, accuracy of control, safety, and other charac teristics of electric heating may carry greater weight in choice of method, than operating expense or initial investment. The basis of all electric heating methods and devices, except so-called heat pumps, is in the power-to-heat' conversion constant: 1 kilowatt equals 3413 Btu per hr. Because ratings of electric heating equipment are ex pressed in terms of watts input and volts at the terminals, heating capacity calculations are translated directly to electric power requirements without the necessity of considering ampere currents or ohm resistances. Definitions of Resistor, Heating Element, Electric Heating Unit and other terms applying to heating practices will be found in Chapter 1. It is strongly recommended that compliance with the National Electrical Code and approval by Underwriters' Laboratories Inc., or other recog nized certifying agency, be required in the specifications for all equipment, materials, and construction used in electric heating systems. RESISTORS AND HEATING ELEMENTS Electric resistors usually are composed of metal alloys such as nickelchromium wire or ribbon, or non-metallic compounds containing carbon formed into rods or other shapes. Heating elements may have, resistors either in the form of exposed coils mounted on insulators or of metallic conductors embedded in. a refractory insulating material, encased, in a protective sheath of metal. Fins or extended surfaces may be used to add heat-dissipating area. Elements are made in many forms, such as wires, strips, rings, tubes, plates and panels. Strip elements are used for clamping to surfaces requiring heat transfer by conduction,'in some types of con vection air heaters, and in low-temperature radiant heaters. Ring and plate elements are common in electric ranges and many small air heaters'. Metal or oxide conductive films on glass and ceramics have been applied, usually in the form of panels. Tubular elements may be immersed in liq-. 909