1. Objectives Finish DOAS Control Terminology Types of controllers
Differences
Controls in the real world
Problems
Response time vs. stability

2. www.doas.psu.edu DOAS with multi-split systems
Fresh air?

3. DOAS fresh air configurations

4. DOAS fresh air configurations

5. 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)

6. Sequence of operation for the control system design
Adiabatic
humidifier CC HC SA OA
mixing RA
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

7. 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

8. 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

9. 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

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

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

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

13. 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

14. Anticipator can be used to shorten response time
Control differential is also called deadband

15. Residential system - thermostat
~50 years old
DDC thermostat
Daily and weekly
programming

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

17. 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

18. 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

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

20. Unstable system
Stable system

21. 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

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

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

25. 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

26. Ref: Kreider and Rabl.Figure 12.5

27. 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”

28. 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