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HVAC Control Systems

VIEWS: 4 PAGES: 28

  • pg 1
									                     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
        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)
          Sequence of operation for the
             control system design
                                       Adiabatic
                                  CC   humidifier   HC
             OA                                          SA
                         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
                             Economizer
                                                            % fresh air
Fresh air volume flow rate control
                                enthalpy         100%


                 Fresh
                 (outdoor)    TOA (hOA)       Minimum for
      damper      air                         ventilation




                   mixing
       Recirc.
        air




                             T & RH sensors
            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
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
                 Terminology
• Sensor
   – Measures quantity of
     interest
• Controller
   – Interprets sensor data
• Controlled device
   – Changes based on
     controller output
                               Figure 2-13
                                          outdoor




      Direct                    Indirect
Closed Loop or Feedback   Open Loop or Feedforward
• Set Point
  – Desired sensor value
• Control Point
  – Current sensor value
• Error or Offset
  – Difference between control point and set point
  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
• Anticipator can be used to shorten response time
• Control differential is also called deadband
  Residential system - thermostat
• ~50 years old        DDC thermostat
                       - Daily and weekly
                         programming
          Modulating Control Systems
 Example: Heat exchanger control
        – Modulating (Analog) control


                          Cooling coil


 air
                                          x


water




                (set point temperature)
   Modulating Control Systems
• Used in larger systems
• Output can be anywhere in operating range
• Three main types
   – Proportional
   – PI
                                                Electric (pneumatic) motor
   – PID
                                                  Position (x)

                    fluid



                      Volume flow rate
                              Vfluid = f(x) - linear or exponential function
                   control algorithm
           The PIDconstants
               time


                                                                          e(t) – difference between
                                                                                 set point and
                                                                                 measured value
Position (x)          Proportional     Integral            Differential


For our example of heating coil:

                                    K                                      d (Tset point  Tmeasured)
  x  K  (Tset point  Tmeasured)   (Tset point  Tmeasured)d  K  Td
                                    Ti                                                 d

                Proportional                   Integral                            Differential
                (how much)                     (for how long)                      (how fast)


  Position of the valve
Proportional Controllers
  x  A  K  (Tset point  Tm easured)
   x is controller output
   A is controller output with no error
      (often A=0)
   Kis proportional gain constant
   e = Tset point  Tm easured is error (offset)
Unstable system   Stable system
      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
    Proportional + Integral (PI)
                                          K
x  A  K  (Tset point  Tm easured) 
                                          Ti  (Tset point  Tm easured)d

                      K/Ti is integral gain


                                                   If controller is tuned
                                                   properly, offset is
                                                   reduced to zero



                       Figure 2-18a
     Issues with PI Controllers
• Scheduling issues
• Require more tuning than for P
• But, no offset
         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
Ref: Kreider and Rabl.Figure 12.5
        The control in HVAC system – only PI
                                          K
    x  K  (Tset point  Tm easured) 
                                          Ti  (Tset point  Tm easured)d

                Proportional                        Integral
        value


Set point
                                                                             Proportional
                                                                             affect the slope




Set point                                                                    Integral
                                                                             affect the shape after
                                                                             the first “bump”
            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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