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Objectives

Objectives. Finish DOAS Control Terminology Types of controllers Differences Controls in the real world Problems Response time vs. stability. www.doas.psu.edu DOAS with multi-split systems. Fresh air?. DOAS fresh air configurations. DOAS fresh air configurations.

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Objectives

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

  2. www.doas.psu.eduDOAS with multi-split systems Fresh air?

  3. DOAS fresh air configurations

  4. DOAS fresh air configurations

  5. Issues Related to DOAS • Split of sensible and latent load • Selection of hydronic system • Winter vs. summer operation • Winter operation with DX systems (heat pump)

  6. Sequence of operation for the control system design Adiabatic humidifier CC HC SA OA mixing RA Define the sequence of operation for: WINTER operation and: - case when humidity is not controlled - case when humidity is precisely controlled Solution on the whiteboard

  7. 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

  8. 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

  9. 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

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

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

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

  13. 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

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

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

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

  17. 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

  18. 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

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

  20. Unstable system Stable system

  21. 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

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

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

  24. 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

  25. Ref: Kreider and Rabl.Figure 12.5

  26. 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

  27. 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

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