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Objectives

Objectives. Control Terminology Types of controllers Differences Controls in the real world Problems Response time vs. stability. Motivation. Maintain environmental quality Thermal comfort Indoor air quality Material protection Conserve energy Protect equipment.

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Objectives

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  1. Objectives • Control Terminology • Types of controllers • Differences • Controls in the real world • Problems • Response time vs. stability

  2. Motivation • Maintain environmental quality • Thermal comfort • Indoor air quality • Material protection • Conserve energy • Protect equipment

  3. Basic purpose of HVAC control • Daily, weekly, and seasonal swings make HVAC control challenging • Highly unsteady-state environment • Provide balance of reasonable comfort at minimum cost and energy • Two distinct actions: • 1) Switching/Enabling: Manage availability of plant according to schedule using timers. • 2) Regulation: Match plant capacity to demand

  4. History • Process controls • Self-powered controls • Pneumatic and electro-mechanical controls • Electronic controls • Direct digital control (DDC)

  5. Terminology • Sensor • Measures quantity of interest • Controller • Interprets sensor data • Controlled device • Changesbased on controller output Figure 2-13

  6. outdoor Direct Closed Loop or Feedback Indirect Open Loop or Feedforward

  7. Set Point • Desired sensor value • Control Point • Current sensor value • Error or Offset • Difference between control point and set point

  8. Two-Position Control Systems • Used in small, relatively simple systems • Controlled device is on or off • It is a switch, not a valve • Good for devices that change slowly

  9. Anticipator can be used to shorten response time • Control differential is also called deadband

  10. Residential system - thermostat • DDC thermostat • Daily and weekly • programming • ~50 years old

  11. Example: Heat exchanger control Modulating (Analog) control Cooling coil air water Modulating Control Systems x (set point temperature)

  12. Electric (pneumatic) motor Position (x) fluid Volume flow rate Vfluid = f(x) - linear or exponential function Modulating Control Systems • Used in larger systems • Output can be anywhere in operating range • Three main types • Proportional • PI • PID

  13. The PID control algorithm For our example of heating coil: constants time e(t) – difference between set point and measured value Position (x) Differential Proportional Integral Differential (how fast) Proportional (how much) Integral (for how long) Position of the valve

  14. Proportional Controllers x is controller output A is controller output with no error (often A=0) Kis proportional gain constant e = is error (offset)

  15. Unstable system Stable system

  16. Issues with P Controllers • Always have an offset • But, require less tuning than other controllers • Very appropriate for things that change slowly • i.e. building internal temperature

  17. Proportional + Integral (PI) • K/Ti is integral gain If controller is tuned properly, offset is reduced to zero Figure 2-18a

  18. Issues with PI Controllers • Scheduling issues • Require more tuning than for P • But, no offset

  19. Proportional + Integral + Derivative (PID) • Improvement over PI because of faster response and less deviation from offset • Increases rate of error correction as errors get larger • But • HVAC controlled devices are too slow responding • Requires setting three different gains

  20. Ref: Kreider and Rabl.Figure 12.5

  21. The control in HVAC system – only PI Proportional Integral value Set point Proportional affect the slope Integral affect the shape after the first “bump” Set point

  22. The Real World • 50% of US buildings have control problems • 90% tuning and optimization • 10% faults • 25% energy savings from correcting control problems • Commissioning is critically important

  23. Practical Details • Measure what you want to control • Verify that sensors are working • Integrate control system components • Tune systems • Measure performance Commission control systems

  24. HVAC Control Example 1: Economizer (fresh air volume flow rate control) Controlled device is damper - Damper for the air - Valve for the liquids fresh air damper mixing recirc. air T & RH sensors

  25. Economizer Fresh air volume flow rate control % fresh air 100% enthalpy Fresh (outdoor) air TOA (hOA) Minimum for ventilation damper mixing Recirc. air T & RH sensors

  26. Economizer – cooling regime How to control the fresh air volume flow rate? If TOA < Tset-point→ Supply more fresh air than the minimum required The question is how much? Open the damper for the fresh air and compare the Troom with the Tset-point . Open till you get the Troom = Tset-point If you have 100% fresh air and your still need cooling use cooling coil. What are the priorities: - Control the dampers and then the cooling coils or - Control the valves of cooling coil and then the dampers ? Defend by SEQUENCE OF OERATION the set of operation which HVAC designer provides to the automatic control engineer % fresh air 100% Minimum for ventilation

  27. Economizer – cooling regime Example of SEQUENCE OF OERATIONS: If TOA < Tset-point open the fresh air damper the maximum position Then, if Tindoor air < Tset-point start closing the cooling coil valve If cooling coil valve is closed and T indoor air < Tset-point start closing the damper till you get T indoor air = T set-point Other variations are possible

  28. HVAC Control Example 2: Dew point control (Relative Humidity control) fresh air damper filter cooling coil heating coil filter fan mixing T & RH sensors Heat gains Humidity generation We should supply air with lower humidity ratio (w) and lower temperature We either measure Dew Point directly or T & RH sensors substitute dew point sensor

  29. Relative humidity control by cooling coil (CC) • Cooling coil is controlled by TDP set-point if TDP measured > TDP set-point → send the signal to open more the CC valve if TDP measured < TDP set-point → send the signal to close more the CC valve • Heating coil is controlled by Tair set-point • if Tair < Tair set-point → send the signal to open more the heating coil valve • if Tair > Tair set-point → send the signal to close more the heating coil valve Control valves Fresh air mixing cooling coil heating coil Tair & TDP sensors

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