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Lecture Objectives:

Lecture Objectives:. Discuss HW2 Finish with HVAC control (PID loops, Sequence of operation) Learn about sorption chillers. Electric (pneumatic) motor. Position. fluid. Volume flow rate. V fluid = f(position) - linear or exponential function. Modulating Control Systems.

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Lecture Objectives:

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  1. Lecture Objectives: • Discuss HW2 • Finish with HVAC control (PID loops, Sequence of operation) • Learn about sorption chillers

  2. 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

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

  4. Unstable system Stable system

  5. 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

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

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

  8. 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

  9. Ref: Kreider and Rabl.Figure 12.5

  10. 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

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

  12. 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

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

  14. 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

  15. 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

  16. Absorption Cycle Replace compressor Same as vapor compression but NO COMPRESSOR

  17. Absorption cooling cycle Relatively simple thermodynamics with adition of mixtures (water – aminia) Rich solution of Heat H2O + NH3 H2O H2O Rich solution of H2O + NH3

  18. Mixtures(T-x diagram) Dew point curve Saturated vapor Mixture of liquid and vapor Saturated liquid Bubble point curve For P= 4 bar

  19. h-x diagram hfg for H2O hfg for NH3 Isotherms are showmen only in liquid region

  20. Composition of h-x diagram Saturated vapor line at p1 Equilibrium construction line at p1 1e Used to determine isotherm line in mixing region! Start from x1; move up to equilibrium construction line; move right to saturated vapor line; determine 1’; connect 1 and 1’. Isotherm at P1 and T1 Adding energy B A x1 X1’ mass fraction of ammonia in saturated vapor

  21. h-x diagramat the end of your textbook you will find these diagramsfor 1) NH3-H2O2) H2O-LiBr LiBr is one of the major liquid descants in air-conditioning systems

  22. Adiabatic mixing in h-x diagram(Water – Ammonia) From the textbook (Thermal Environmental Eng.; Kuehen et al)

  23. Absorption cooling cycle Rich solution of Heat H2O + NH3 H2O H2O Rich solution of H2O + NH3

  24. Mixing of two streams with heat rejection (Absorber) mixture of H2O and NH3 m3 m2 =pure NH3 (x2=1) m1 m3 m2 2 m1 Q cooling 3’ Mixture of 1 and 2 Heat rejection Mass and energy balance: (1) 1 3 (2) (3) x3 x From mixture equation: Substitute into (2) Substitute into (3) From adiabatic mixing (from previous slide)

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