1. Announcements Next class is the course evaluation
Final project related additional office hours
Tomorrow afternoon from 1 till 4 pm
Thursday afternoon
Friday by appointment

2. Objectives Discuss the exam problems
Talk about final project report and presentation
Learn about automatic control
PID and Control terminology
Sequence of operation
Control optimization

3. Project presentation
Timing: 10 minutes for presentation Approximately 1 PowerPoint Slides per minute
For example
Specify the project Provide some basic floor layout and input variable assumptions
Results (form calculation and manufacturer catalogs)
Disuses you system challenges and modifications related to your design solutions
3 minutes for Questions and Answers
I will need you power point before the class
You can it to me.

4. HVAC automatic control
Maintain environmental quality
Thermal comfort
Indoor air quality
Material protection
Conserve energy
Protect equipment

5. Terminology Sensor Controller Controlled device
Measures quantity of interest
Controller
Interprets sensor data
Controlled device
Changes based on controller output
Figure 2-13

6. Direct Indirect outdoor Closed Loop or Feedback
Open Loop or Feedforward

7. Set Point Control Point Error or Offset Desired sensor value
Current sensor value
Error or Offset
Difference between control point and set point

8. 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

9. Modulating Control Systems
Example: Heat exchanger control
Modulating (Analog) control
air
water
Cooling coil x
(set point temperature)

10. 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

11. The PID control algorithm
constants
time
e(t) – difference between
set point and
measured value
Position (x)
Proportional
Integral
Differential
For our example of heating coil:
Differential
(how fast)
Proportional
(how much)
Integral
(for how long)
Position of the valve

12. Proportional Controllers
x is controller output
A is controller output with no error
(often A=0)
Kis proportional gain constant
e = is error (offset)

13. Unstable system
Stable system

14. 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

15. Proportional + Integral (PI)
K/Ti is integral gain
If controller is tuned properly, offset is reduced to zero
Figure 2-18a

17. Issues with PI Controllers
Scheduling issues
Require more tuning than for P
But, no offset

18. 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

19. Ref: Kreider and Rabl.Figure 12.5

20. The control in HVAC system – only PI
Proportional
Integral
value
Set point
Proportional
affect the slope
Set point
Integral
affect the shape after
the first “bump”

21. 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

22. Practical Details Measure what you want to control
Verify that sensors are working
Integrate control system components
Tune systems
Measure performance
Commission control systems

23. HVAC Control
Example 1:
Economizer (fresh air volume flow rate control)
Controlled device is damper
- Damper for the air
- Valve for the liquids
fresh
air
damper
mixing
recirc.
air
T & RH sensors

24. Economizer Fresh air volume flow rate control % fresh air TOA (hOA)
enthalpy
100%
Fresh
(outdoor)
air
TOA (hOA)
Minimum for
ventilation
damper
mixing
Recirc.
air
T & RH sensors

25. Economizer – cooling regime
How to control the fresh air volume flow rate?
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
% fresh air
100%
Minimum for
ventilation

26. 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

27. HVAC Control Example 2: Dew point control (Relative Humidity control)
fresh
air
damper
filter
cooling
coil
heating
coil
filter
fan
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

28. Relative humidity control by cooling coil
Mixture
Room
Supply
TDP
Heating coil

29. 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
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
Control valves
Fresh air
mixing
cooling
coil
heating
coil
Tair & TDP sensors

30. Sequence of operation (ECJ research facility)
Set Point (SP)
Mixture 2
Mixture 3
Mixture 1
DBTSP
DPTSP
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 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

31. Other examples for HVAC:
Heat recovery
Dual duct system

32. Other examples
Thermal storage
UTs CHP

33. Thermal storage for adjustment production to consumption