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HVAC Control Systems - University of Texas at Austin

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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 output
Figure 2-13
outdoor
Direct
Indirect
Closed Loop or Feedback
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
• ~50 years old
DDC thermostat
- Daily and weekly
programming
Modulating Control Systems
Example: Heat exchanger control
– Modulating (Analog) control
Cooling coil
air
x
water
(set point temperature)
Modulating Control Systems
• Used in larger systems
• Output can be anywhere in operating range
• Three main types
– Proportional
– PI
– PID
Electric (pneumatic) motor
Position (x)
fluid
Volume flow rate
Vfluid = f(x) - linear or exponential function
The PIDconstants
control algorithm
time
e(t) – difference between
set point and
measured value
Position (x)
Proportional
Differential
Integral
For our example of heating coil:
x пЂЅ K пѓ— (Tset point пЂ­ T measured ) пЂ«
Proportional
(how much)
Position of the valve
K
Ti
пѓІ (T
set point
пЂ­ T measured ) d пЃґ пЂ« K пѓ— T d
Integral
(for how long)
d (Tset point пЂ­ T measured )
dпЃґ
Differential
(how fast)
Proportional Controllers
x пЂЅ A пЂ« K пѓ— (T set point пЂ­ T measured )
x is controller output
A is controller output with no error
(often A=0)
Kis proportional gain constant
e = Tset point пЂ­ T measured 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)
x пЂЅ A пЂ« K пѓ— (Tset point пЂ­ T measured ) пЂ«
K
Ti
пѓІ (T
set point
пЂ­ T measured ) d пЃґ
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
x пЂЅ K пѓ— (Tset point пЂ­ T measured ) пЂ«
Proportional
K
Ti
пѓІ (T
set point
пЂ­ T measured ) d пЃґ
Integral
value
Set point
Set point
Proportional
affect the slope
Integral
affect the shape after
the first “bump”
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 Control
Example 1:
Economizer (fresh air volume flow rate control)
Controlled device is damper
damper
fresh
air
- Damper for the air
- Valve for the liquids
mixing
recirc.
air
T & RH sensors
Economizer
% fresh air
Fresh air volume flow rate control
enthalpy
damper
Fresh
(outdoor)
air
TOA (hOA)
mixing
Recirc.
air
T & RH sensors
100%
Minimum for
ventilation
Economizer – cooling regime
How to control the fresh air volume flow rate?
If TOA < Tset-point в†’ Supply more fresh air than the minimum required
The question is how much?
% fresh air
Open the damper for the fresh air
and compare the Troom with the Tset-point .
100%
Open till you get the Troom = Tset-point
If you have 100% fresh air and your
still need cooling use cooling coil.
Minimum for
ventilation
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 Control
Example 2:
Dew point control (Relative Humidity control)
damper
fan
fresh filter cooling heating
air
coil
coil
filter
mixing
T & RH sensors
Heat gains
Humidity generation
We should supply air with lower humidity ratio (w) and lower temperature
We either measure Dew Point directly or T & RH sensors substitute dew point sensor
Relative humidity control by cooling coil
Cooling Coil
Mixture
Supply
TDP
Room
Heating coil
Relative humidity control by cooling coil (CC)
• Cooling coil is controlled by TDP set-point
if TDP measured > TDP set-point в†’ send the signal to open more the CC valve
if TDP measured < TDP set-point в†’ send the signal to close more the CC valve
• Heating coil is controlled by Tair set-point
if Tair < Tair set-point в†’ send the signal to open more the heating coil valve
if Tair > Tair set-point в†’ send the signal to close more the heating coil valve
Control valves
Fresh air
mixing
cooling
coil
heating
coil
Tair & TDP sensors
Sequence of operation
(ECJ research facility)
Mixture 3
Set Point
(SP)
Mixture 1
DPTSP
Mixture 2
Control logic:
DBTSP
Mixture in zone 1: IF (( TM<TSP) & (DPTM<DPTSP) ) heating and humidifying
Heater control: IF (TSP>TSA) increase heating or IF (TSP<TSA) decrease heating
Humidifier: 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
cooling
Humidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA)
decrease hum.
Mixture in zone 3: IF ((DPTM>DPTSP) ) cooling/dehumidifying and reheatin
Cool. 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
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