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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.

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Objectives

  • Finish DOAS

  • Control

    • Terminology

    • Types of controllers

      • Differences

    • Controls in the real world

      • Problems

      • Response time vs. stability


www.doas.psu.eduDOAS 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

100%

enthalpy

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

    • Measures quantity of interest

  • Controller

    • Interprets sensor data

  • Controlled device

    • Changesbased on controller output

Figure 2-13


outdoor

Direct

Closed Loop or Feedback

Indirect

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

  • DDC thermostat

  • Daily and weekly

  • programming

  • ~50 years old


Example: Heat exchanger control

Modulating (Analog) control

Cooling coil

air

water

Modulating Control Systems

x

(set point temperature)


Electric (pneumatic) motor

Position (x)

fluid

Volume flow rate

Vfluid = f(x) - linear or exponential function

Modulating Control Systems

  • Used in larger systems

  • Output can be anywhere in operating range

  • Three main types

    • Proportional

    • PI

    • PID


The PID control algorithm

For our example of heating coil:

constants

time

e(t) – difference between

set point and

measured value

Position (x)

Differential

Proportional

Integral

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

Integral

affect the shape after

the first “bump”

Set point


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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