Control Fundamentals3CONTROL FUNDAMENTALSINTRODUCTIONThis section describes heating, ventilating, and air condition- ing (HVAC) systems and discusses characteristics and compo- nents of automatic control systems. Cross-references are made to sections that provide more detailed information.A correctly designed HVAC control system can provide a comfortable environment for occupants, improve employee productivity, facilitate efficient manufacturing, control smoke in the event of a fire, and support the operation of computer and telecommunications equipment. Controls are essential to the proper operation of the system and should be considered as early in the design process as possible.Properly applied automatic controls ensure that a correctly designed HVAC system will maintain a comfortable enviro- nment and perform economically under a wide range of operating conditions. Automatic controls regulate HVAC system output in response to varying indoor and outdoor conditions to maintaingeneral comfort conditions in office areas and provide narrow temperature and humidity limits where required in production areas for product quality.Automatic controls optimize HVAC system operation. They can adjust temperatures and pressures automatically to reduce demand when spaces are unoccupied and regulate heating and cooling to provide comfort conditions while limiting energy usage. Limit controls ensure safe operation of HVAC system equipment and prevent injury to personnel and damage to the system. Examples of limit controls are low-limit temperature controllers which help prevent water coils or heat exchangers from freezing and flow sensors for safe operation of some equipment (e.g., chillers). In the event of a fire, controlled air distribution can provide smoke-free evacuation passages, and smoke detection in ducts can close dampers to prevent the spread of smoke and toxic gases.DEFINITIONSAnalog - Continuously variable ( e.g., a faucet controlling water from off to full flow).Automatic control system - A system that reacts to a change or imbalance in the variable it controls by adjusting other vari-ables to restore the system to the desired balance.Algorithm - - A calculation method that produces a control out- put by operating on an error signal or a time series of error signals.Compensation control - A process of automatically adjusting the control point of a given controller to compensate forchanges in a second measured variable (e.g., outdoor air temperature). For example, the hot deck control point is normally reset upward as the outdoor air temperature de-creases. Also called “reset control“.Control agent - The medium in which the manipulated variable exists. In a steam heating system, the control agent is the steam and the manipulated variable is the flow of the steam.Control point - The actual value of the controlled variable (setpoint plus or minus offset).Controlled medium - The medium in which the controlled variable exists. In a space temperature control system, the controlled variable is the space temperature and the controlled medium is the air within the space.Controlled variable - The quantity or condition that is meas- ured and controlled.Controller - A device that senses changes in the controlled variable ( or receives input from a remote sensor ) and derivesthe proper correction output.Corrective action - Control action that results in a change of the manipulated variable. Initiated when the controlled variable deviates from setpoint.Cycle - One complete execution of a repeatable process. In basic heating operation, a cycle comprises one on period and one off period in a two-position control system.Cycling - A periodic change in the controlled variable from one value to another. Uncontrolled cycling is called “ hunting “.Control Fundamentals4Cycling rate - - The number of cycles completed per time unit, typically cycles per hour for a heating or cooling system. The inverse of the length of the period of the cycle.Deadband - A range of the controlled variable in which no corrective action is taken by the controlled system and no energy is used. See also “zero energy band“.Deviation - The difference between the setpoint and the value of the controlled variable at any moment. Also called “offset“.DDC - Direct Digital Control. See also Digital and Digital control.Digital - A series of on and off pulses arranged to convey information. Morse code is an early example. Processors (computers) operate using digital language.Digital control - A control loop in which a microprocessor- based controller directly controls equipment based on sensor inputs and setpoint parameters. The programmed control se- quence determines the output to the equipment.Droop - A sustained deviation between the control point and the setpoint in a two-position control system caused by a change in the heating or cooling load.Electric control - A control circuit that operates on line or low voltage and us