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60S CHAPTER 42 1959 Guide transfer is controlled. For these reasons a vapor barrier should be located within the wall construction as near to the high vapor-pressure side as feasible. To be effective, a bar rier must be continuous and should be so located within,the structure that it will be protected from rupture. (See Chapter 10 Moisture In Building Construction.) BIBLIOGRAPHY R. C. Amero, J. W. Moore, and R. G. Capell: Design and use of adsorptive units {Chemical and Engineering Progress, Vol. 43, July 1947, p. 349). A full bibliography is included in this article. W. L. Ross and . R. McLaughlin: An analysis method for predicting behavior of solid adsorbents in solid sorption dehumidifiers (ASHAE Transactions, Vol. 61, 1955, p. 321). Symposium Bulletin on Dehumidification (five papers pre sented at Symposium held at 63rd Annual Meeting ASHAE February 28, 1957). G. C. F. Asker and T. H. Urdahl; Ship Dehumidification Sys tems (International Institute of Refrigeration, Nantes, France, June 20, 1957). Several references included. CHAPTER 43 AUTOMATIC CONTROL Fundamentals: Types of Control Systems and Action, System Components, Controllers and Controlled Devices, Auxiliary Control Equipment; Design Coordination: Equipment Selection and Layout; location of Controllers; Control of Steam, Water, or Air Flow; Zone Control; Control Applications: Central Fan, Temperature Control in Hot Water Systems, Zoned Steam and Water Systems, Terminal Equipment, Residential Heating and Air Conditioning RESENT-DAY standards of comfort.combined with its operation on a prearranged relationship between out Pcapacity and rapid response of modern ' heating and door temperature and heat input to the building, and room cooling equipment make automatic controls an essential part temperature has no effect on the controller. of heating, ventilating, and air-conditioning systems. These As there is no feedback, the control corrects only for automatic controls respond to variables such as temperature, those disturbances of room temperature caused by changes relative humidity, and pressure. They operate individually in outdoor temperature. or in sequence to maintain the desired conditionsthroughout the system and in the occupied space. A factor that has the subject of automatic control somewhat confuting TYPES OF CONTROL SYSTEMS Control systems are divided into five main groups accord has been the matter of terminology. Many different expres ing to the primary source of energy: sions or words have sometimes been used to convey a tingle idea or concept. This chapter, therefore, attempts to use and define the terms that are most common and suitable so that they will be readily understood. The terms and definitions used are also selected so as to conform, as nearly as posable, to automatic control terminology used by engineers in other fields of control. PART I--fundamentals of automatic CONTROL A control system. Fig. 1, consists essentially of (1) a con troller, (2) a controlled device, and (3) a source of energy. A controller is a device which measures a variable condition such as temperature, humidity, pressure, and liquid level, and produces a suitable action or impulse for transmission to the controlled devices. Thermostats, humidistats, and pressure con trollers are examples. A controlled device reacts to the impulse received from a controller find varies the flow of the control agent. It may be a valve, damper, electric relay, or a motor driving a pump, fan, etc. The control agent is the medium manipulated by the con trolled device. It may be air or gas flowing through a damper; gas, steam, water, etc., flowing through a valve; or an electric current. The controlled variable is the condition such as temperature, humidity, or presure, being controlled. Most control systems, of which Fig. 1 is typical, form a 1. A self-contained system combines the controller and con trolled device in one unit and employs the power of the meas uring system to effect the necessary corrective action. The meas uring system derives its energy from the process under control without amplification by any auxiliary source of energy, and may be of the sealed-bellows or remote-bulb type as described under Types of Measuring Elements. Temperature changes at the bellows or the remote bulb result in pressure or volume changes of the enclosed media which are transmitted directly to the operating device of the valve or damper. 2. A pneumatic system utilises compressed air, usually at a pressure of 15 to 25 psig, as a source of energy. This is supplied to the controller which in turn regulates the pressure supplied to the controlled device. 3. A hydraulic system utilises a suitable liquid under pressure as the source of energy. The pressure often is considerably higher than in a pneumatic system, but in other respects the systems are similar. Hydraulic systems find their chief use in applications where large forces are required for operation of the controlled devices. 4. An electric system utilizes electric energy, either low or line voltage as the energy source. The electric energy supplied to the controlled device is regulated by the controller either directly or through relays. 5. An electronic system also utilizes electric energy, but em ploys an electronic amplifier to increase the minute voltage variations of the measuring element to values required for operation of standard electrically controlled devices. Measuring elements usually are of the resistance type, but thermo couples are employed. Combination electronic-pneumatic systems utilize compremed air for operation of the controlled device by converting the output of the electronic amplifier into dosed loop. That is, the controller measures and responds to changes in the controlled variable and actuates the controlled device to. bring about an opposite change which is again measured by the controller. This system of transmitting information about the results of an action or operation back to its origin is known as feedback, and makes true automatic control possible. An open-loop system is sometimes employed in control circuits, but it does not provide complete control. An out door thermostat arranged to control the flow of heat to a building in proportion to the load caused by changes in outdoor temperature is an example. This system depends for y'