Document 5Vkrk6eD6b69aB3qmykZ7mQ0

HEAT1NC VENTILATING AIR CONDITIONING GUIDE 1941 Example 2. Assume that the duct of Example 1 is only 12 ft long, and that a 30 db reduction is required in this length. Solution: Case 1. (No splitters). From Equation 2, Ro = 12.6 X 12 X X 0.401 ` = 12.6 db Case 2. (Two splitters, three channels). From Equation 3, Rs = 12.6 X 10 + (20 X 3) 10 + 20 29.6 db General Suggestions In some instances where high velocity air is used, a considerable amount of whistle is generated at the grille. This noise is obviously produced after the air leaves the duct and there is no treatment which can be installed in the duct that will reduce this noise. The engineer must take into con sideration the type of grille which he intends to use and provide sufficient grille area so that the velocity through the grille is reduced to a point where the grille is not too noisy. Further discussion of grille noise is given in Chapter 30, Air Distribution. Ducts serving more than one room permit cross talk between the rooms and should be lined with acoustical material. Where the rooms are close together and the ducts short, the ducts should be sub-divided to provide ample acoustical treatment. Very often in ventilajting duct work the engineer feels that it will not be necessary to line ducts if the sound is traveling against the airflow. This, however, is untrue since sound travels so much more rapidly than does the air in even high velocity systems, that it will travel as easily against the airflow as, it does with it. Sounds which are low in pitch are much harder to eliminate from a duct system than sound which is high in pitch, consequently equipment which produces low pitched sounds should be avoided as much as possible. 602 \ Chapter 33 AUTOMATIC CONTROL Purpose of Automatic Control, Types of Control, Central Fan Systems, Limit Controls, Static Pressure Control, Unit Systems, Control of Automatic Fuel Appliances, Residential Control Systems, Control of Refrigeration Equipment THIS chapter is prepared with the purpose of acquainting the engi neer with the principles underlying the use of automatic control, the general types and varieties of control equipment available and their application. Automatic control, properly applied to heating, ventilating, and air conditioning systems, makes possible the maintenance of desired con ditions with maximum operating economy. A properly designed and complete control system has the ability to interlock and coordinate the various functions of heating, ventilating and air conditioning in a manner impossible to accomplish with manual regulation. Automatic control is an integral and essential part of a heating, venti lating or air conditioning installation and cannot be regarded as an acces sory. In order to insure satisfactory results, the control should be designed with and incorporated in the heating, ventilating or air conditioning system. The control equipment should be given careful consideration in the planning of any installation in order that the entire system may operate together with satisfactory results. In order that proper selection arid application of controlling devices may be made it is important that a broad understanding exist as to the types of control available and their principles of operation. Improper selection and application of control equipment will result in unsatis factory and inefficient operation. Specific control devices and systems are described in the Catalog Data Section. PURPOSE OF AUTOMATIC CONTROL Automatic control is normally applied to heating, ventilating, or air conditioning systems: 1. To insure the maintenance of certain desired or required conditions of temperature, pressure, humidity, air motion or air distribution. 2. To serve a safety function, limiting pressures or temperatures within predetermined Points, or preventing the operation of mechanical equipment unless it may function without hazard. 3. To produce economical results and thereby insure operation of the system at a minimum of expense. 603