Objectives - PowerPoint PPT Presentation

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Objectives

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Title: HVAC Control Systems Author: Jeffrey Siegel Last modified by: Novoselac-la Created Date: 9/4/2001 6:13:16 PM Document presentation format: On-screen Show (4:3) – PowerPoint PPT presentation

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Title: Objectives


1
Objectives
  • Discuss final project deliverables
  • Control
  • Terminology
  • Types of controllers
  • Differences
  • Controls in the real world
  • Problems
  • Response time vs. stability

2
  • FINAL PROJECT DELIVERABLES AND GRADING
  • DELIVERABLES
  • 1) PROJECT REPORT
  • - Project statement (introduction) 2 pages
  • Explain what are you designing/analyzing and why
    is that important
  • On the second page
    clearly identify (bullet list) project outcomes
  • - Building description (geometry) 1-3 pages
  • Schematics that focus on your system(s)
  • Identify all assumptions and simplifications
    you introduced
  • - Methodology 1-3 pages
  • Describe methodology (equations, schematics, )
  • Provide a list of assumptions used in your
    methodology
  • - Results 3-5 pages
  • Formatted results with comments

3
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
4
Economizer
fresh air
  • Fresh air volume flow rate control

100
enthalpy
Fresh (outdoor) air
TOA (hOA)
Minimum for ventilation
damper
mixing
Recirc. air
T RH sensors
5
Economizer cooling regime
Example of SEQUENCE OF OERATIONS If TOA lt
Tset-point open the fresh air damper the maximum
position Then, if Tindoor air lt Tset-point
start closing the cooling coil valve If cooling
coil valve is closed and T indoor air lt
Tset-point start closing the damper till you
get T indoor air T set-point Other
variations are possible
6
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
7
Terminology
  • Sensor
  • Measures quantity of interest
  • Controller
  • Interprets sensor data
  • Controlled device
  • Changes based on controller output

Figure 2-13
8
outdoor
Direct Closed Loop or Feedback
Indirect Open Loop or Feedforward
9
  • Set Point
  • Desired sensor value
  • Control Point
  • Current sensor value
  • Error or Offset
  • Difference between control point and set point

10
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

11
  • Anticipator can be used to shorten response time
  • Control differential is also called deadband

12
Residential system - thermostat
  • 50 years old
  • DDC thermostat
  • Daily and weekly
  • programming

13
Modulating Control Systems
  • Example Heat exchanger control
  • Modulating (Analog) control

x
(set point temperature)
14
Modulating Control Systems
  • Used in larger systems
  • Output can be anywhere in operating range
  • Three main types
  • Proportional
  • PI
  • PID

15
The PID control algorithm
constants
time
e(t) difference between set point
and measured value
Position (x)
Differential
Proportional
Integral
  • For our example of heating coil

Differential (how fast)
Proportional (how much)
Integral (for how long)
Position of the valve
16
Proportional Controllers
  • x is controller output
  • A is controller output with no error
  • (often A0)
  • Kis proportional gain constant
  • e is error
    (offset)

17
Stable system
Unstable system
18
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

19
Proportional Integral (PI)
K/Ti is integral gain
If controller is tuned properly, offset is
reduced to zero
Figure 2-18a
20
(No Transcript)
21
Issues with PI Controllers
  • Scheduling issues
  • Require more tuning than for P
  • But, no offset

22
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

23
Ref Kreider and Rabl.Figure 12.5
24
The control in HVAC system only PI
Proportional
Integral
value
Set point
Proportional affect the slope
Integral affect the shape after the first bump
Set point
25
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
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