Announcement No project presentations ! In last class (on Thursday) we will review HVAC systems of the New Engineering Building review the course material NOTE: The class will be in ECJ 4.304 (conference room across the stair door)

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

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

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

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

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

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

Sensible and Enthalpy and Desiccant wheels

Sensible and Enthalpy Wheels

Heat recovery system in NSM building at UT

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