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

mixing

CCOA

RA

SAHC

Adiabatichumidifier

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

mixing

damper

Fresh(outdoor) air

T & RH sensors

Recirc. air

% fresh air

Minimum for ventilation

100%

TOA (hOA)

enthalpy

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 outputFigure 2-13

DirectClosed Loop or Feedback

IndirectOpen Loop or Feedforward

outdoor

• 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

air

water

Cooling coil

(set point temperature)

x

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

For our example of heating coil:

Proportional Integral Differential

time

Position (x)

constants

e(t) – difference between set point and measured value

d

TTdTKdTT

T

KTTKx d

i

)()()( measuredpointset

measuredpointset measuredpointset

Proportional(how much)

Integral(for how long)

Differential(how fast)

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)

)( measuredpointset TTKAx

measuredpointset TT

Stable systemUnstable 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

dTTT

KTTKAx

i

)()( measuredpointset measuredpointset

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

dTTT

KTTKx

i

)()( measuredpointset measuredpointset

Proportional Integral

Proportionalaffect the slope

Integralaffect the shape after the first “bump”

Set point

Set point

value

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