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 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

Economizer Fresh air volume flow rate control % fresh air TOA (hOA) enthalpy 100% Fresh (outdoor) air TOA (hOA) Minimum for ventilation damper 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 Controller Controlled device Measures quantity of interest Controller Interprets sensor data Controlled device Changes based on controller output Figure 2-13

Direct Indirect outdoor Closed Loop or Feedback Open Loop or Feedforward

Set Point Control Point Error or Offset Desired sensor value 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 air water Cooling coil x (set point temperature)

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

The PID control algorithm constants time e(t) – difference between set point and measured value Position (x) Proportional Integral Differential For our example of heating coil: Differential (how fast) Proportional (how much) Integral (for how long) Position of the valve

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 2-18a

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 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