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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

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

www.doas.psu.eduDOAS with multi-split systems

Fresh air?


Doas fresh air configurations

DOAS fresh air configurations


Doas fresh air configurations1

DOAS fresh air configurations


Issues related to doas

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

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

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

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

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

Terminology

  • Sensor

    • Measures quantity of interest

  • Controller

    • Interprets sensor data

  • Controlled device

    • Changesbased on controller output

Figure 2-13


Objectives

outdoor

Direct

Closed Loop or Feedback

Indirect

Open Loop or Feedforward


Objectives

  • Set Point

    • Desired sensor value

  • Control Point

    • Current sensor value

  • Error or Offset

    • Difference between control point and set point


Two position control systems

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


Objectives

  • Anticipator can be used to shorten response time

  • Control differential is also called deadband


Residential system thermostat

Residential system - thermostat

  • DDC thermostat

  • Daily and weekly

  • programming

  • ~50 years old


Modulating control systems

Example: Heat exchanger control

Modulating (Analog) control

Cooling coil

air

water

Modulating Control Systems

x

(set point temperature)


Modulating control systems1

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

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

Proportional Controllers

x is controller output

A is controller output with no error

(often A=0)

Kis proportional gain constant

e = is error (offset)


Objectives

Unstable system

Stable system


Issues with p controllers

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

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

Issues with PI Controllers

  • Scheduling issues

  • Require more tuning than for P

  • But, no offset


Proportional integral derivative pid

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


Objectives

Ref: Kreider and Rabl.Figure 12.5


The control in hvac system only pi

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

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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