hvac valves
DESCRIPTION
Hvac ValvesTRANSCRIPT
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© 2007 ASHRAE Hong Kong Chapter Slide 1
Fundamentals of HVAC Control Systems
18, 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 controlCommon types:
Globe valves (for modulating)Ball valves (less expensive)Butterfly valves (for isolation)
Valve materialBronze, cast iron, steel
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© 2007 ASHRAE Hong Kong Chapter Slide 4
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Control valve components
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© 2007 ASHRAE Hong Kong Chapter Slide 5
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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© 2007 ASHRAE Hong Kong Chapter Slide 6
2-Way and 3-Way Valves
2-way: for variable flowMore sensitive to high differential pressureHarder to close off against line pressure
3-way: for constant flowActuator does not need to be as powerful
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© 2007 ASHRAE Hong Kong Chapter Slide 7
Advantages of 2-Way Valves
Less expensive to buy and installResult in variable flow which reduces pumping energyReduced piping heat losses and pump energyPotentially lower costs for pumping and distribution systemsSystem balancing is reduced or eliminated
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© 2007 ASHRAE Hong Kong Chapter Slide 8
Disadvantages of 2-Way Valves
Most chillers and some boilers cannot handle widely varying flow ratesDifferential pressures will increase across control valves, reducing system controllability
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© 2007 ASHRAE Hong Kong Chapter Slide 9
Normally Open 2-Way Valve
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© 2007 ASHRAE Hong Kong Chapter Slide 10
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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© 2007 ASHRAE Hong Kong Chapter Slide 14
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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© 2007 ASHRAE Hong Kong Chapter Slide 17
Equal Percentage Valve
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© 2007 ASHRAE Hong Kong Chapter Slide 18
Quick-Opening Valve
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© 2007 ASHRAE Hong Kong Chapter Slide 19
Characteristics of Control ValvesValve 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 characteristicsLinearEqual percentageQuick 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 Δ
=ρ
q = volume flow (m3/s)ρ= fluid density (kg/m3)ΔP = static pressure loss across the valve (Pa)
For different units and locations, CV and KV are used.For valve used in water application:
10⋅Δ=
PQKV
ρQ = volume flow (m3/h)
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© 2007 ASHRAE Hong Kong Chapter Slide 23
Control Valve RatingsFlow coefficientClose-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 valveProvide 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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© 2007 ASHRAE Hong Kong Chapter Slide 27
3-Way Diverting Valve
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© 2007 ASHRAE Hong Kong Chapter Slide 28
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Three-way valves
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© 2007 ASHRAE Hong Kong Chapter Slide 29
3-Way Mixing Valves
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© 2007 ASHRAE Hong Kong Chapter Slide 30
Flow Through 3-Way Valve
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© 2007 ASHRAE Hong Kong Chapter Slide 31
Selecting & Sizing ValvesControl valve selection depends on:
The fluid being controlledValve style: 2-way or 3-wayControl mode: modulating or 2-positionMaximum fluid temperatureMaximum inlet pressureDesired flow characteristicMaximum fluid flow rateDesired pressure drop when valve is full openTurn-down ratioClose-off pressure
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© 2007 ASHRAE Hong Kong Chapter Slide 32
Coil Subsystem
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© 2007 ASHRAE Hong Kong Chapter Slide 33
Representative Values of Cv
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© 2007 ASHRAE Hong Kong Chapter Slide 34
Flow Characteristic Selection
The desired flow characteristic is a function of:The heat transfer device being controlled and its flow versus capacity characteristicThe control of fluid supply temperatureThe control of the differential pressure across the valve
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© 2007 ASHRAE Hong Kong Chapter Slide 35
Capacity Versus Flow Rate
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© 2007 ASHRAE Hong Kong Chapter Slide 36
Authority Distortion ofLinear Flow Characteristic
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© 2007 ASHRAE Hong Kong Chapter Slide 37
Authority Distortion of Equal Percentage Flow Characteristic
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© 2007 ASHRAE Hong Kong Chapter Slide 38
Pump & System CurvesWith Valve Control
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© 2007 ASHRAE Hong Kong Chapter Slide 39
Control Dampers
For controlling air distribution, such asFire damper: A thermally actuated damper arranged to automatically restrict the passage of fire and/or heat at a point where an opening violates the integrity of a fire partition or floorSmoke damper: A damper arranged to control passage of smoke through an opening or a ductVolume control damper (VCD): A device used to regulate the flow of air in an HVAC system
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© 2007 ASHRAE Hong Kong Chapter Slide 40
Control Dampers
Common types:Opposed blade dampers (e.g. in AHU)Parallel blade dampersButterfly 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]
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© 2007 ASHRAE Hong Kong Chapter Slide 42
Typical Multiblade Dampers
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© 2007 ASHRAE Hong Kong Chapter Slide 43
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Parallel blade damper Opposed blade damper
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© 2007 ASHRAE Hong Kong Chapter Slide 44
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Round damper Volume control damper(opposed blade)
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© 2007 ASHRAE Hong Kong Chapter Slide 45[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Typical (opposed blade) damper construction
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© 2007 ASHRAE Hong Kong Chapter Slide 46[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Internally mounted electric actuator
Externally mountedpneumatic actuator
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© 2007 ASHRAE Hong Kong Chapter Slide 47
Damper Sizing
Typically chosen based on duct size and convenience of locationProper selection and sizing provides the following benefits:
Lower installation cost (damper sizes are smaller)Smaller actuators or a fewer number of them are requiredReduced energy costs (smaller damper, less overall leakage)Improved control characteristics (rangeability) because the ratio of total damper flow to minimum controllable flow is increasedImproved operating characteristics (linearity)
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© 2007 ASHRAE Hong Kong Chapter Slide 48
Performance Data
Leakage ratingsTorque requirements
Closing torqueDynamic torque
Velocity ratingsTemperature ratingsPressure ratingsUL 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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© 2007 ASHRAE Hong Kong Chapter Slide 50
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Control loop for a damper system
Resistance to airflowin actual system
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© 2007 ASHRAE Hong Kong Chapter Slide 51
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
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© 2007 ASHRAE Hong Kong Chapter Slide 52
[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Mixed air control system (parallel blade dampers)
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© 2007 ASHRAE Hong Kong Chapter Slide 53[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Mixed air system with louvers
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© 2007 ASHRAE Hong Kong Chapter Slide 54
Selecting and Sizing Dampers
The three basic damper applications are:Two-position dutyCapacity control dutyMixing duty
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© 2007 ASHRAE Hong Kong Chapter Slide 55
Fans in Parallel
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© 2007 ASHRAE Hong Kong Chapter Slide 56
Centrifugal Fan WithInlet Vane Damper
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© 2007 ASHRAE Hong Kong Chapter Slide 57
Characteristic Curves of Parallel Blade Dampers
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© 2007 ASHRAE Hong Kong Chapter Slide 58
Characteristic Curves of Opposed Blade Dampers
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© 2007 ASHRAE Hong Kong Chapter Slide 59
Flow Pattern Through Dampers
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© 2007 ASHRAE Hong Kong Chapter Slide 60
Mixing Box Arrangements
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© 2007 ASHRAE Hong Kong Chapter Slide 61
Pressure Drop Across Mixing Box
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© 2007 ASHRAE Hong Kong Chapter Slide 62
Economizer With Supplyand Return Fans
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© 2007 ASHRAE Hong Kong Chapter Slide 63
VAV System at 100% Outdoor Air and Minimum Outdoor Air
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© 2007 ASHRAE Hong Kong Chapter Slide 64
Economizer With Relief Fan