Objectives Finish DOAS Control Terminology Types of controllers
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 OA CC Adiabatic humidifier HC 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 damper Fresh (outdoor) air TOA (h. OA) mixing Recirc. air T & RH sensors 100% Minimum for ventilation
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 – PID Electric (pneumatic) motor Position (x) fluid Volume flow rate Vfluid = f(x) - linear or exponential function
The PIDconstants control algorithm time e(t) – difference between set point and measured value Position (x) Proportional Integral Differential For our example of heating coil: Proportional (how much) Position of the valve Integral (for how long) Differential (how fast)
Proportional Controllers x is controller output A is controller output with no error (often A=0) Kis proportional gain constant e= 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/Ti is integral gain If controller is tuned properly, offset is reduced to zero Figure 2 -18 a
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 Proportional Integral value Set point Proportional affect the slope 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
- Slides: 28