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Explore the concepts of desiccant-based cooling systems, HVAC control, and PID loops. Learn about different control types and their applications in HVAC systems. Understand the importance of commissioning and optimization for energy savings.

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Lecture Objectives:

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  1. Lecture Objectives: • Discuss HW2 • Finish with desiccant systems • Learn about HVAC control • PID loop • Sequence of operation

  2. Figure 3 – A desiccant-based cooling system combined with regenerative heat exchanger, vapor compression cooling, and evaporative humidifier (hybrid system). Desiccant wheel

  3. Variation in Cycles Much more in the paper I gave you (Technical development of rotary desiccant dehumidification and air conditioning)

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

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

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

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

  12. Unstable system Stable system

  13. 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

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

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

  16. 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

  17. Ref: Kreider and Rabl.Figure 12.5

  18. 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

  19. 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

  20. HVAC Control Example : 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

  21. Relative humidity control by cooling coil Cooling Coil Mixture Room Supply TDP Heating coil

  22. 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

  23. Mixture 3 DPTSP Set Point (SP) Mixture 2 Mixture 1 DBTSP Sequence of operation(PRC research facility) Control logic: Mixture in zone 1: IF (( TM<TSP) & (DPTM<DPTSP) ) heating and humidifying Heater control: IF (TSP>TSA) increase heating or IF (TSP<TSA) decrease heating Humidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA) decrease humid. Mixture in zone 2: IF ((TM>TSP) & (DPTM<DPTSP) ) cooling and humidifying Cool. coil cont.: IF (TSP<TSA) increase cooling or IF (TSP>TSA) decrease cooling Humidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA) decrease hum. Mixture in zone 3: IF ((DPTM>DPTSP) ) cooling/dehumidifying and reheatin Cool. coil cont.: IF (DPTSP>DPTSA) increase cooling or IF (DPTSP<DPTSA) decrease cooling Heater control: IF (TSP>TSA) increase heating or IF (TSP<TSA) decrease heating

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