Chapter 5 Control Principles

1. Chapter 5Control Principles

2. Control System History • Early control of the level of comfort was done manually • Example- adding fuel to the fire or letting the fire burn out

3. Control System History • Electricity became readily available in the early 1900’s • Adapted to provide automatic building environment control • Early electric controls were large, bulky, unreliable, and inaccurate by modern standards • They were also dangerous because of early electrical power quality and distribution were poor

4. Control System History • Pneumatic Control System • Came into use in the 1rst quarter of the 21rst century

5. Control System History • Direct Current (DC) • Came into common use in the 1960’s • Was adapted for use in the HVAC control of commercial applications

6. Self Contained Systems • Control system that does not require an external power source • Widely used basic control system • Power is supplied by a sealed, fluid-filled ( may be gas liquid or both) element (bulb) which is attached to the pipe containing water or refrigerant • Is a closed loop system – feedback occurs between the sensor, controller and the controlled device

7. Self Contained Systems Most common applications: • Thermostatic expansion valve (refrigerant flow) • Building space temperature control in steam or hot water heating systems Advantages • No external power source • The system is unaffected by fluctuations or loss of power to the control system – example loss of transformer doesn’t mean loss of control Disadvantages • Cannot be expanded to provide sophisticated control sequences because of only one adjustment • Poor accuracy • No diagnostic means available

8. Electric Control Systems • Control system in which power supply is either line or low voltage • Line voltage is rarely used because of the danger of occupants coming into contact, and fire and explosion in an area where flammables are present • Commonly use a mechanical device to control the flow of electricity • Most common control systems used today

9. Electric Control Systems • Advantages • Provide adequate control of temperature in building spaces • Small wires used for eases of installation in walls • Safer because of low voltages • Relatively inexpensive • Easy to install and service

10. Electric Control Systems • Disadvantages • Cannot be used in explosive areas without proper precautions • Difficult to modify • Often unable to perform complex control sequences • Not often used for proportional control • Not easily configured to allow central reporting or alarms

11. Pneumatic Control Systems • Were developed because electric control systems could not perform the flexibility and control necessary for commercial buildings Separated into four main groups • Air Compressor stations • Transmitters and Controllers • Auxiliary devices • Controlled devices

12. Pneumatic Control Systems • Air compressor stations • Power source is compressed air • Does not function without clean, dry, oil free air • Dryers, filters, and regulators help ensure these qualities of air

13. Pneumatic Control Systems • Transmitters and controllers • Sense and control the temperature, pressure, or humidity in a building space • Are connected to the main air supply • Change the pressure to the controlled devices which regulate flow of the heating or cooling medium

14. Pneumatic Control Systems • Auxiliary Devices • Normally located between the transmitters and controllers and the controlled devices • Change or reroute the air supply from the controller or transmitter before it reaches the controlled device

15. Pneumatic Control Systems Controlled devices: • Dampers • Valves • Actuators • Switches

16. Pneumatic Control Systems Pneumatic Control System Applications: • Most common in large commercial applications such as schools & hospitals • Central air handler units • Boilers • Chillers • Cooling tower systems Often changed out and replaced either on a partial or total basis when buildings are renovated.

17. Pneumatic Control Systems Advantages of Pneumatic Control Systems: • Easily provide analog or proportional control • No shock hazard • Relatively inexpensive • Rugged • Most are fairly accurate • Can be easily expanded • Can provide many different control sequences

18. Pneumatic Control Systems Disadvantages of Pneumatic Control Systems: • Air stations require routine maintenance • May require specialized tools for set-up and calibration • Technician has to be onsite to check the system.

19. Electronic Control Systems • Electronic control systems are control systems that operate on 24 VDC or less. • Are analog control system • Were originally developed as a replacement for pneumatic control systems • Often use solid state components • Common supply voltages are 10, 12, or 18 VDC • Uses transformer to drop the voltage to what is required • Sensor is wired to a resistive bridge circuit (contains four arms and four resistors

20. Electronic Control System Applications • Can be used in any HVAC application • Package units such as heat pumps or split systems • Easily expanded for different control applications

21. Disadvantages of Electronic Control Systems • May require special diagnostic tools and procedure • Actuators are normally expensive and complex • May become obsolete quickly • Are not as powerful or flexible as automated control systems

22. Automated Control Systems • Uses solid state devices • Uses on/off (binary -1/0) signals which represent numbers, set-points, and control sequences • Have become increasingly popular because of their increased reliability and increase in the power of PC’s • Use the same power supply of the electronic control systems • Use step-down transformers and rectifiers to reduce the power to system voltage • Intelligent controllers are connected to equipment for control which are independent controllers • Parameters are set in the programming of each controller and can be modified • Used in many large commercial buildings

23. Advantages of Automated Control Systems • Extremely accurate if they are set-up correctly • Can be set-up for remote data acquisition and monitoring • Can be integrated with fire and security systems and most are. • Are very versatile • Can be easily re-programmed by qualified personnel.

24. Disadvantages of Automated Control Systems • Programming may be complex • Require specialized software knowledge • May need to be upgraded for new features • Can become obsolete rapidly • Service can be expensive

25. System-Powered Control Systems • Uses duct pressure from the fan system for a power supply • Used sparingly • Developed to avoid installation cost pneumatic piping • Uses bi-metallic thermostats • Bellows inflate and deflate for control of flow • Generally used in VAV terminal boxes

26. Advantages of System Powered Control Systems • Reduce installation time • Controls are factory mounted to terminal boxes • Come pre-calibrated • Flexibility of zoning in a building • Relatively inexpensive • No external power supply

27. Disadvantages of System Powered Control Systems • Control system fails if the system air is dirty or wrong pressure • Air leaks cause improper operation • Little flexibility in application • Cannot be adapted to other control systems • No remote monitoring • Relatively inaccurate • No alarm monitoring capabilities

28. Hybrid Control Systems • Uses multiple technologies • Transducers often used as an interface between control systems • May be require different power supplies • The most common application is partial replacement of pneumatic control system to a automated control system

29. Advantages of Hybrid Control System • Take advantage of the best characteristics of each control system • Reduce cost by using existing equipment • Accuracy can be increased by using modern control system components • Remote troubleshooting and alarms are achievable

30. Disadvantages of Hybrid Control Systems • Additional knowledge is needed to service the control system • Troubleshooting may be more difficult because of system complexity • Failure of one of the control system may cause the failure of the entire system