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Chapter 4

Chapter 4. Control System. An arrangement of a sensor, controller, and controlled device to maintain a specific controlled variable value in a building space, pipe, or duct. Sensor.

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Chapter 4

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  1. Chapter 4

  2. Control System • An arrangement of a sensor, controller, and controlled device to maintain a specific controlled variable value in a building space, pipe, or duct.

  3. Sensor • A device that measures a controlled variable such as temperature, pressure, or humidity and sends a signal to a controller • Sensor output signal may be air pressure or resistance, voltage, or current in an electrical control system • Sensors may be mounted in a duct, pipe, or room remote from the controller

  4. Controller • Device that receives the signal from the sensor, compares it to a set-point value, and sends an appropriate output signal to a controlled device • Controller set-point is adjustable • Desired accuracy is also adjustable – accuracy also referred to as the proportional band or throttling range

  5. Controlled Devices • The object that regulates the flow of fluid in a system to provide the heating, air conditioning, or ventilation effect • Dampers – a set of metal blades that are used to regulate air flow • Valves – regulate the flow of a medium, in most cases a liquid medium *** Examples-water or steam flow • Gas valves and electric heat elements – deliver regulated heating

  6. Controlled Agents • Fluid that flows though controlled devices to produce a heating or cooling effect in a building space • Control Functions – must ensure the comfort of building occupants

  7. Temperature Control • One of the most important components of comfort • Is achieved by controlling either the building space temperature, return air temperature, or air volume

  8. Building Space Temperature Control • Room controller or thermostat is connected directly to a heating or cooling controlled device • Temperature can vary in large spaces • The room controller or thermostat may be mounted where it can be influenced • Can you think of some possible influences on space temperature?

  9. Return Air Temperature Control • The return air sensor is mounted in the common return duct at the AHU • Is more accurate because the temperature is an average of the entire controlled space • Prevent tampering of the room controller or thermostat • Disadvantage is that the maintenance or service person often will be the one to make adjustments to temperature.

  10. Air Volume Control • Changes the amount of air flow over the coil (heating or cooling) • Reduces air volume therefore reduces fan speed and energy consumption • Variable speed drive may be used to achieve the proper flow If DDC is being used what kind of point would be used to control a VFD? • Safety controls are used to ensure excessively high or low temperatures

  11. Humidity Control • Proper humidity ensures comfort • The humidity level most identified to achieve comfort is 50%. • Most common method is to include a humidity sensor or controller in the building space • Controller device opens or closes a humidification device

  12. Pressure Control • Used in specialized areas such as labs • Pressure sensor is normally mounted on a pipe or duct • The pressure sensor is connected to the controller which or opens or closes a controlled device such as a damper or valve

  13. Control System Characteristics • Set-point – the desired value to be maintained by a system • Temperature • Pressure • Humidity • Light level • Dew-point • Enthalpy

  14. Control System Characteristics • Control Point – the actual value that a control system experiences at any given time – the set-point may be different from the control point • Offset – the difference between a setpoint and control point • Feedback – the measurement of the results of a controller action by a sensor or switch – Example****Temperature from a sensor

  15. Closed Loop Control • Arrangement of a controller, sensor, and controlled device in a system • Feedback occurs between the controller, sensor, and controlled device • If one part of the control system malfunctions it causes other parts to have the wrong value or position

  16. Open Loop Control • No feedback occurs between the controller, sensor, and controlled device • Example – controller cycles a chilled water pump but there is no feedback from an outside air sensor

  17. ON/OFF Control • Either on or off control either 0% or 100% • Used in open and closed loop control • Air conditioning compressors, electrical heating stages, gas valves, refrigeration compressors, and constant speed fans

  18. Proportional Control • Analog • The controlled device is positioned in direct response to the amount of offset in the system • Use a response that is a number between two values such as 0-10 volts • Used in most commercial applications use proportional control • Can be inaccurate if not set-up properly

  19. Control Systems • The process of controlling an HVAC system involves three steps. • Measuring data • Processing the data with other information • Control action • These three functions make up what is known as a control loop

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