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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