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

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

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 email it to me.

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

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

Direct Indirect outdoor Closed Loop or Feedback Open Loop or Feedforward

Set Point Control Point Error or Offset Desired sensor value 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

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

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

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

Proportional Controllers x is controller output A is controller output with no error (often A=0) Kis proportional gain constant e = 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) 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 Proportional Integral value Set point Proportional affect the slope Set point 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 for the air - Valve for the liquids fresh air damper mixing recirc. air T & RH sensors

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 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

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) 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

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

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

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

Other examples for HVAC: Heat recovery Dual duct system

Other examples Thermal storage UTs CHP

Thermal storage for adjustment production to consumption