1. Announcement No project presentations !
In last class (on Thursday) we will
review HVAC systems of the New Engineering Building
review course material

2. Lecture Objectives: Automatic control Heat recovery system
Sequence of operation
Heat recovery system
Desiccant systems

3. HVAC Control Example : Dew point control (Relative Humidity control)
fresh
air
damper
filter
cooling
coil
heating
coil
filter
fan
mixing
T & RH sensors

4. 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

5. 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

6. 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

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

8. Sensible and Enthalpy and Desiccant wheels

9. Sensible and Enthalpy Wheels

10. Heat recovery system in NSM building at UT

11. Desiccant wheel
Figure 3 – A desiccant-based cooling system combined with regenerative heat exchanger, vapor compression cooling, and evaporative humidifier (hybrid system).