Document Ra3KQOMgzgjvJO9O7Z3wyw5Xn

662 CHAPTER 33 1949 Guide The Mathematical Theory of the Cottrell Electric Precipitator, by A. W. Simon (Iron and Steel Engineering, 6,1929, p. 143-6). . Progress in the Art of Electrical Precipitation Since 1900, by P. E. Landolt (Trans actions American Electrochem. Society, 51,1927). Some Factors and Principles Involved in the Separation and Collection of Dust, Mist and Fume from Gases, by Evald Anderson (Transactions American Institute of Chemical Engineers, 16,1924, Pt. 1, p. 69-86). Electrical Precipitation of Solids from Smelter Gases, by Ross B. Rathbun (Trans actions American Institute of Electrical Engineers, 41,1922, p. 815). Electrical Engineering Features of the Electrical Precipitation Process, by G. H. Horne (Transactions American Institute of Electrical Engineers, 41,1922, p. 808). Characteristics of Unit Dust Collectors, by Arthur C. Stern, Jack Baliff, Arthur E. Perina, Robert Crowley, Benjamin Feiner and Arthur A. Urbano (A.S.H.V.E. JounNAL Section, Heating, Piping and Air Conditioning, May, 1946, p. 117). CHAPTER 34 AUTOMATIC CONTROL Basic Types of Control, Types of Controllers, Actuating Devices, Actuated Controls, Residential Control Systems, Zone Control, Automatic Control Application, Control for Central Fan Systems, Panel Heating Control, Indicating and Recording Equipment THE function of automatic control, as applied to the heating, ventilat ing and air conditioning industry, may be broadly subdivided into the maintenance of temperature, humidity, and pressure, within pre-determined' ranges. ` It automatically coordinates the operation of the various con trolled devices in proper sequence to produce the desired result. BASIC TYPES OF CONTROL Available automatic control equipment may be divided into four main i groups depending on the primary source of power and can be classed as: 1. A self-actuated regulator is one in which, all the energy necessary to operate a valve or damper motor is supplied by the responsive element or bulb. Temperature changes at the bulb result in pressure changes of an enclosed fluid which are trans mitted directly to the valve or damper motor. Instruments of this type are available either with a rigid bulb or with flexible tubing from the bulb to the operating motor. The flexible tubing may be furnished in varying lengths and is generally protected by a flexible metal armor. 2. Electrically operated equipment utilizes electric current as a primary source of energy, its flow being regulated as required to operate motors, relays, or other con trolled items. 3. In pneumatically operated equipment the primary source of energy is compressed air usually at a pressure of 15 to 25 psig. The flow of this air is proportioned as re quired to operate valves, dampers, relays, or other controlled devices. 4. In hydraulically operated equipment the primary source of power is a suitable liquid at a pressure of 15 to 25 psig or higher which is handled in the same manner as compressed air. TYPES OF CONTROLLERS The basic types of controllers which are available may be classed as: Fi 1. Two position or on-off controllers are the simplest type and are clearly described by the name. With controllers of this type the valve or damper motor can assume it only two positions, either open or shut. I 2. Proportional or gradual acting controllers function to re-position the controlled \ device by small increments of travel to regulate the flow as the controller senses a slight change in the controller condition. 3. Floating controllers act to produce valve or damper movement whenever there is a deviation from the control point. Whenever the condition to be controlled is above the control point, the valve closes at a constant rate, and continues to close until the temperature returns to the control point. Below the control point the valve reverses its action and moves in the other direction until the control point is again reached. 4. Automatic reset (or proportional plus reset) controllers function to reposition a valve or damper by small increments of travel as in a proportioning controller. In addition, a mechanical device in the controller automatically and constantly resets the instrument to offset the normal drift (inherent in a proportional controller) be tween maximum and minimum load. The rate of reset is manually adjustable and. must be set to meet the load requirements of the individual system. Controllers may also be designated by types as: (1) a non-indicating controller, when it does not indicate the controlled condition and performs 663