Objectives

• Control Terminology

• Types of controllers – Differences

• Controls in the real world– Problems– Response time vs. stability

Motivation

• Maintain environmental quality– Thermal comfort– Indoor air quality– Material protection

• Conserve energy

• Protect equipment

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

History

• Process controls

• Self-powered controls

• Pneumatic and electro-mechanical controls

• Electronic controls

• Direct digital control (DDC)

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

Practical Details

• Measure what you want to control

• Verify that sensors are working

• Integrate control system components

• Tune systems

• Measure performance

Commission control systems

HVAC ControlExample 1:

Economizer (fresh air volume flow rate control)

mixing

damper

fresh air

T & RH sensors

recirc. air

Controlled device is damper

- Damper for the air - Valve for the liquids

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

How to control the fresh air volume flow rate?

% fresh air

Minimum for ventilation

100%

If TOA < Tset-point → Supply more fresh air than the minimum required

The question is how much?

Open the damper for the fresh air

and compare the Troom with the Tset-point .

Open till you get the Troom = Tset-point

If you have 100% fresh air and your still need cooling use cooling coil.

What are the priorities: - Control the dampers and then the cooling coils or - Control the valves of cooling coil and then the dampers ?

Defend by SEQUENCE OF OERATION the set of operation which HVAC designer provides to the automatic control engineer

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

HVAC ControlExample 2:

Dew point control (Relative Humidity control)filter

fancooling coil

heating coil

filter

mixing

damper fresh air

T & RH sensors

We either measure Dew Point directly or T & RH sensors substitute dew point sensor

Humidity generationHeat gains

We should supply air with lower humidity ratio (w) and lower temperature

Relative humidity control by cooling coil

TDP

Mixture

Cooling Coil

RoomSupply

Heating coil

Relative humidity control by cooling coil (CC)• Cooling coil is controlled by TDP set-point

if TDP measured > TDP set-point → send the signal to open more the CC valve

if TDP measured < TDP set-point → send the signal to close more the CC valve

cooling coil

heating coil

mixing

Fresh air

Tair & TDP sensors

Control valves

• Heating coil is controlled by Tair set-point

if Tair < Tair set-point → send the signal to open more the heating coil valve

if Tair > Tair set-point → send the signal to close more the heating coil valve

Sequence of operation(ECJ research facility)

Control logic:

Mixture in zone 1: IF (( TM<TSP) & (DPTM<DPTSP) ) heating and humidifying Heater control: IF (TSP>TSA) increase heating or IF (TSP<TSA) decrease heatingHumidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA)

decrease humid.

Mixture in zone 2: IF ((TM>TSP) & (DPTM<DPTSP) ) cooling and humidifying Cool. coil cont.: IF (TSP<TSA) increase cooling or IF (TSP>TSA) decrease

coolingHumidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA)

decrease hum.

Mixture in zone 3: IF ((DPTM>DPTSP) ) cooling/dehumidifying and reheatinCool. coil cont.: IF (DPTSP>DPTSA) increase cooling or IF (DPTSP<DPTSA)

decrease cooling Heater control: IF (TSP>TSA) increase heating or IF (TSP<TSA) decrease heating

Set Point (SP)

Mixture 2

Mixture 3

Mixture 1

DBTSP

DPTSP