Download - ASHRAE Workshop Control SamHui Part 2
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2007 ASHRAE Hong Kong Chapter Slide 1
Fundamentals of HVACControl Systems18, 19, 25, 26 April 2007
ASHRAE Hong Kong Chapter Technical Workshop
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2007 ASHRAE Hong Kong Chapter Slide 2
Chapter 3Control Valves and Dampers
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2007 ASHRAE Hong Kong Chapter Slide 3
Control Valves
Important component of fluid distribution
systems Regulate the flow of fluid to the process under control
Common types: Globe valves (for modulating)
Ball valves (less expensive) Butterfly valves (for isolation)
Valve material
Bronze, cast iron, steel
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[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]Control valve components
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2-Way and 3-Way Valve Circuits
Mixing 3-way control valve
Diverting 3-way control valve
2-way control valve
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2-Way and 3-Way Valves
2-way: for variable flow
More sensitive to high differential pressure Harder to close off against l ine pressure
3-way: for constant flow
Actuator does not need to be as powerful
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Advantages of 2-Way Valves
Less expensive to buy and install
Result in variable flow which reducespumping energy
Reduced piping heat losses and pump
energy Potentially lower costs for pumping and
distribution systems
System balancing is reduced or eliminated
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Disadvantages of 2-Way Valves
Most chillers and some boilers cannothandle widely varying flow rates
Differential pressures will increase across
control valves, reducing systemcontrollability
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Normally Open 2-Way Valve
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Normally Closed 2-Way Valve
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2007 ASHRAE Hong Kong Chapter Slide 11
[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]
Two-way globe valve application
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2007 ASHRAE Hong Kong Chapter Slide 12
Double-Seated 2-Way Valve
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2007 ASHRAE Hong Kong Chapter Slide 13
Butterfly Valve
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Ball Valve
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2007 ASHRAE Hong Kong Chapter Slide 15
[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]
Ball valve
Butterfly valve
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2007 ASHRAE Hong Kong Chapter Slide 16
Linear Valve
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Equal Percentage Valve
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Quick-Opening Valve
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Characteristics of Control Valves Valve flow characteristics
Relationship between the stem travel of a valve, expressed in
percent of travel, and the fluid flow through the valve,
expressed in percent of full flow Typical flow characteristics
Linear
Equal percentage
Quick opening
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2007 ASHRAE Hong Kong Chapter Slide 20
Typical Flow Characteristics
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2007 ASHRAE Hong Kong Chapter Slide 21
[Source: Shadpour, F., 2001. The Fundamentals of HVAC Direct Digital Control]
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2007 ASHRAE Hong Kong Chapter Slide 22
[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]
Flow coefficient:
PqAV
=
q = volume flow (m3/s)= fluid density (kg/m3)
P = static pressure loss across the valve (Pa)
For different units and locations, CVand KVare used.
For valve used in water application:
10=
PQKV
Q = volume flow (m3/h)
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Control Valve Ratings Flow coefficient
Close-off rating: The maximum pressure drop that a valve can withstand
without leakage while in the full closed position
Pressure drop: The difference in upstream and downstream pressures of the
fluid flowing through the valve Maximum pressure and temperature:
The maximum pressure and temperature limitations of fluid
flow that a valve can withstand
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2007 ASHRAE Hong Kong Chapter Slide 24
Location of Control Valves At the outlet on the top of cooling/heating
coils
Avoid coil starvation from water flow (lower pressure) Flow of water from the bottom to the top (avoid air
bubble)
Flow measuring & balancing device
should be placed after the control valve
Provide a means of shut-off to allow a
proper means for servicing
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2007 ASHRAE Hong Kong Chapter Slide 25
[Source: Shadpour, F., 2001. The Fundamentals of HVAC Direct Digital Control]
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2007 ASHRAE Hong Kong Chapter Slide 26
3-Way Mixing Valve
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3-Way Diverting Valve
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[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]
Three-way valves
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3-Way Mixing Valves
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Flow Through 3-Way Valve
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Selecting & Sizing ValvesControl valve selection depends on:
The fluid being controlled
Valve style: 2-way or 3-way
Control mode: modulating or 2-position Maximum fluid temperature
Maximum inlet pressure
Desired flow characteristic Maximum fluid flow rate
Desired pressure drop when valve is full open
Turn-down ratio Close-off pressure
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Coil Subsystem
Representative Values of
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Flow Characteristic Selection
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The desired flow characteristic is a function
of:
The heat transfer device being controlled
and its flow versus capacity characteristic
The control of fluid supply temperature The control of the differential pressure
across the valve
Capacity Versus Flow Rate
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2007 ASHRAE Hong Kong Chapter Slide 35
Authority Distortion of
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Linear Flow Characteristic
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Authority Distortion of EqualPercentage Flow Characteristic
Pump & System Curves
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With Valve Control
Control Dampers
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For controlling air distribution, such as Fire damper: A thermally actuated damper arranged to
automatically restrict the passage of fire and/or heat at apoint where an opening violates the integrity of a fire
partition or floor
Smoke damper: A damper arranged to control passage
of smoke through an opening or a duct
Volume control damper (VCD): A device used to regulate
the flow of air in an HVAC system
Control Dampers
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2007 ASHRAE Hong Kong Chapter Slide 40
Common types: Opposed blade dampers (e.g. in AHU)
Parallel blade dampers
Butterfly dampers (e.g. in VAV box)
Linear air valves (e.g. in fume hood)
Specialty dampers
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2007 ASHRAE Hong Kong Chapter Slide 41
[Source: Shadpour, F., 2001. The Fundamentals of HVAC Direct Digital Control]
Typical Multiblade Dampers
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[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]
Parallel blade damper Opposed blade damper
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[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]
Round damper Volume control damper
(opposed blade)
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[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]Typical (opposed blade) damper construction
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[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]Internally mounted electric actuator
Externally mounted
pneumatic actuator
Damper Sizing
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Typically chosen based on duct size and
convenience of location Proper selection and sizing provides the
following benefits:
Lower installation cost (damper sizes are smaller) Smaller actuators or a fewer number of them are required
Reduced energy costs (smaller damper, less overall leakage)
Improved control characteristics (rangeabil ity) because the
ratio of total damper flow to minimum controllable flow isincreased
Improved operating characteristics (linearity)
Performance Data
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Leakage ratings
Torque requirements Closing torque
Dynamic torque
Velocity ratings
Temperature ratings
Pressure ratings UL classification (fire/smoke)
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2007 ASHRAE Hong Kong Chapter Slide 49
[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]
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[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]
Control loop for
a damper system
Resistance to airflow
in actual system
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[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]
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[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]
Mixed air control system (parallel blade dampers)
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[Source: Honeywell, 1997.Engineering Manual of Automatic Control: for Commercial Buildings]Mixed air system with louvers
Selecting and Sizing Dampers
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The three basic damper applications are:
Two-position duty
Capacity control duty
Mixing duty
Fans in Parallel
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Centrifugal Fan WithInlet Vane Damper
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Characteristic Curves of ParallelBlade Dampers
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Characteristic Curves of OpposedBlade Dampers
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Flow Pattern Through Dampers
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Mixing Box Arrangements
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Pressure Drop Across Mixing Box
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Economizer With Supplyand Return Fans
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VAV System at 100 Outdoor Air andMinimum Outdoor Air
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Economizer With Relief Fan
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