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Objectives

Objectives. Discuss final project deliverables Control Terminology Types of controllers Differences Controls in the real world Problems Response time vs. stability. FINAL PROJECT DELIVERABLES AND GRADING DELIVERABLES 1) PROJECT REPORT: - Project statement (introduction) 2 pages

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Objectives

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

  2. FINAL PROJECT DELIVERABLES AND GRADING • DELIVERABLES • 1) PROJECT REPORT: • - Project statement (introduction) 2 pages • Explain what are you designing/analyzing and why is that important • On the second page clearly identify (bullet list) project outcomes • - Building description (geometry) 1-3 pages • Schematics that focus on your system(s) • Identify all assumptions and simplifications you introduced • - Methodology 1-3 pages • Describe methodology (equations, schematics, …) • Provide a list of assumptions used in your methodology • - Results 3-5 pages • Formatted results with comments • Tables, Charts, Diagrams, … • Analysis and Results discussion • - Conclusion 0.5-1 page • Summary of most important results • 2) PRESENTATION: • - 5 minutes (exactly) • Power point presentation (4-6 slides) • GRADING CRITERIA: • 1) Analysis approach: 60% • - Methodology 20% • - Accuracy analysis 20% • - Result analysis 20% • 2) Deliverables: 40% • - Final report 30% • - Presentations 10%

  3. 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

  4. 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

  5. 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

  6. 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

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

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

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

  10. 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

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

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

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

  14. 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

  15. 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

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

  17. Unstable system Stable system

  18. 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

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

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

  21. 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

  22. Ref: Kreider and Rabl.Figure 12.5

  23. 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

  24. 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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