Presentation is loading. Please wait.

Presentation is loading. Please wait.

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

Similar presentations


Presentation on theme: "Objectives Control Terminology Types of controllers –Differences Controls in the real world –Problems –Response time vs. stability."— Presentation transcript:

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

2 Motivation Maintain environmental quality –Thermal comfort –Indoor air quality –Material protection Conserve energy Protect equipment

3 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

4 History Process controls Self-powered controls Pneumatic and electro-mechanical controls Electronic controls Direct digital control (DDC)

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

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

7 Set Point –Desired sensor value Control Point –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 Anticipator can be used to shorten response time Control differential is also called deadband

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

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

12 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 V fluid = f(x) - linear or exponential function Volume flow rate

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

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

15 Stable systemUnstable system

16 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

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

18

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

20 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

21 Ref: Kreider and Rabl.Figure 12.5

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

23 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

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

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

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

27 Economizer – cooling regime How to control the fresh air volume flow rate? % fresh air Minimum for ventilation 100% If T OA < T set-point → Supply more fresh air than the minimum required The question is how much? Open the damper for the fresh air and compare the T room with the T set-point. Open till you get the T room = T set-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

28 Economizer – cooling regime Example of SEQUENCE OF OERATIONS: If T OA < T set-point open the fresh air damper the maximum position Then, if T indoor air < T set-point start closing the cooling coil valve If cooling coil valve is closed and T indoor air < T set-point start closing the damper till you get T indoor air = T set-point Other variations are possible

29 HVAC Control Example 2: Dew point control (Relative Humidity control) filter fan cooling coil heating coil filter mixing damper fresh air T & RH sensors We either measure Dew Point directly or T & RH sensors substitute dew point sensor Humidity generation Heat gains We should supply air with lower humidity ratio (w) and lower temperature

30 Relative humidity control by cooling coil T DP Mixture Cooling Coil Room Supply Heating coil

31 Relative humidity control by cooling coil (CC) Cooling coil is controlled by T DP set-point if T DP measured > T DP set-point → send the signal to open more the CC valve if T DP measured < T DP set-point → send the signal to close more the CC valve cooling coil heating coil mixing Fresh air T air & T DP sensors Control valves Heating coil is controlled by T air set-point if T air < T air set-point → send the signal to open more the heating coil valve if T air > T air set-point → send the signal to close more the heating coil valve

32 Sequence of operation (ECJ research facility) Control logic: Mixture in zone 1: IF (( TMTSA) increase heating or IF (TSPDPTSA) increase humidifying or IF (DPTSPTSP) & (DPTMDPTSA) increase humidifying or IF (DPTSPDPTSP) ) cooling/dehumidifying and reheatin Cool. coil cont.: IF (DPTSP>DPTSA) increase cooling or IF (DPTSPTSA) increase heating or IF (TSP { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/12/3447856/slides/slide_32.jpg", "name": "Sequence of operation (ECJ research facility) Control logic: Mixture in zone 1: IF (( TMTSA) increase heating or IF (TSPDPTSA) increase humidifying or IF (DPTSPTSP) & (DPTMDPTSA) increase humidifying or IF (DPTSPDPTSP) ) cooling/dehumidifying and reheatin Cool. coil cont.: IF (DPTSP>DPTSA) increase cooling or IF (DPTSPTSA) increase heating or IF (TSP


Download ppt "Objectives Control Terminology Types of controllers –Differences Controls in the real world –Problems –Response time vs. stability."

Similar presentations


Ads by Google