Document gq9Y0gkGGjZ0a7o33jR55KOL

............ Heating Ventilating Air Conditioning Guide 1938 5 What is meant by "arbitrarily fix the velocity in the various sections?" When using the velocity method as a basis for design, the maximum allowable velocity is fixed for the main supply duct at the fan, and this velocity is gradually decreased as each branch or outlet is taken off the main supply duct. 6 Which system of duct design is to be preferred, the velocity method or the friction pressure loss method? The friction pressure loss method can be used to advantage where no structural or building conditions limit the shape of the ducts. Where these limiting conditions exist the velocity method is to be preferred. 7 Are the grille sizes figured on the same basis as the outlets? - The. free area through the grilles is. figured the same as the outlets, and this area is increased from 20 to 50 per cent, depending on the design of the grille, to allow for the loss of area caused by the construction of the face of the grille. 8 i Where it is necessary to provide steel angle braces, how far apart should they be spaced? Angle braces for large ducts should be placed on 3-ft 0-in. centers. 9 # How much air will a 10-in. by 24-in. duct handle if it is part of a system designed on a pressure drop of 0.1 in. per 100 feet of run? 1450. cfm (Table 1 and Fig. 3). 10 # How does a splitter at a duct junction influence the volume of the air going through each branch? A splitter facing the direction of air flow cuts off the air and delivers the desired amount to the branch. 11 # Why does a wide, shallow duct offer more resistance to the flow of air than does a square duct of equal cross-sectional area? The perimeter of the wide, flat duct is greater than that of the square-section duct, so the former has the greater frictional area which increases the resistance and thus reduces the volume at any given pressure. 12 # What methods are used to keep large ducts from vibrating because of air pulsations, and from sagging because of their own weight? External bracing, such as standing seams, or structured shapes, like tees or angles, should be placed across the top and bottom. Exterior braces or cross buckling of metal sheets iirdiagonal panels may be used-for thesides of large-ducts^ 13 What velocities of air flow should be used in the trunk ducts of a venti lating system in a public buUding? From 1200 to 1600 fpm. 14 In a ventilating system in a residence, what is the recommended air velocity through supply registers and grilles? 400 fpm. 582 Chapter 30 SOUND CONTROL Decibel Defined, Apparatus for Measuring Noise, Problem of Sound Control, Acceptable Noise Levels, Controlling Vibration from Machine Mountings, Controlling Noise through Hoorn Wall Surfaces, Noise Transmitted Through Ducts, Duct Lining Factor IN ventilating and air conditioning a building or a room, the effect of the mechanical system employed must be considered on the acoustics of the space conditioned. It is important to consider also that the use of air conditioning often permits keeping the windows closed, thus giving relief from certain external noises, but at the same time increasing the necessity of providing adequate sound control. It is not assumed that the ventilating and air conditioning engineer will attempt to improve the acoustics of the space that is being con- ________ _ ditioned, but the designer should have at least enough fundamental 9 knowledge of the acoustical effects of the system which is being designed to be sure that no damaging effects occur to the existing acoustical properties. It is assumed that in a given space the architect and acovstical engineer have produced a room or rooms which are satisfactory for speech, music, or other uses. The ventilating engineer's sole function is to ventilate and air condition these rooms properly so that they will be physically comfortable without adding any acoustical hazards. UNIT OF NOISE MEASUREMENT In the United States and England the unit of noise measurement is the decibel (db). In Germany this unit is called the phon. The decibel is defined by the relation N = 10 log-J-, where N is the number of decibels by which the intensity flux Ii exceeds the intensity flux IQ. The in tensity flux is the measure of the energy contained in a sound wave and is defined in terms of microwatts per square centimeter of wave front in a freely traveling plane wave. It is usually more convenient to select an arbitrary reference intensity for I0 and express all other intensities in terms of decibels above that level. For this purpose the threshold of audibility for the average human ear at a frequency of 1000 cycles per second has been selected. This reference threshold is 10"16 watts per square centimeter or 10~10 microwatts per square centimeter. This reference level also corresponds to a pressure of 0.0002 dynes per square centimeter. A stated sound level in decibels, unless otherwise defined, will thus be related to a threshold of 10~16 watts. For example, a level of 60 db above this reference threshold is 10-10 watts. In a similar manner, when sound 583