Fundamentals of HVAC Control Systems -...
Transcript of Fundamentals of HVAC Control Systems -...
© 2007 ASHRAE Hong Kong Chapter Slide 1
Fundamentals of HVAC Control Systems
18, 19, 25, 26 April 2007
ASHRAE Hong Kong Chapter Technical Workshop
© 2007 ASHRAE Hong Kong Chapter Slide 2
Chapter 3Control Valves and Dampers
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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
© 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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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 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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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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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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Normally Open 2-Way Valve
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Normally Closed 2-Way Valve
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[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Two-way globe valve application
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Double-Seated 2-Way Valve
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Butterfly Valve
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Ball Valve
© 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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Linear Valve
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Equal Percentage Valve
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Quick-Opening Valve
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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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Typical Flow Characteristics
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[Source: Shadpour, F., 2001. The Fundamentals of HVAC Direct Digital Control]
© 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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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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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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[Source: Shadpour, F., 2001. The Fundamentals of HVAC Direct Digital Control]
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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 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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Coil Subsystem
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Representative Values of Cv
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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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Capacity Versus Flow Rate
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Authority Distortion ofLinear Flow Characteristic
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Authority Distortion of Equal Percentage Flow Characteristic
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Pump & System CurvesWith Valve Control
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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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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
© 2007 ASHRAE Hong Kong Chapter Slide 41
[Source: Shadpour, F., 2001. The Fundamentals of HVAC Direct Digital Control]
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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)
© 2007 ASHRAE Hong Kong Chapter Slide 45[Source: Honeywell, 1997. Engineering Manual of Automatic Control: for Commercial Buildings]
Typical (opposed blade) damper construction
© 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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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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Performance Data
Leakage ratingsTorque requirements
Closing torqueDynamic torque
Velocity ratingsTemperature ratingsPressure ratingsUL classification (fire/smoke)
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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]
Control loop for a damper system
Resistance to airflowin actual system
© 2007 ASHRAE Hong Kong Chapter Slide 51
[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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Mixed air system with louvers
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Selecting and Sizing Dampers
The three basic damper applications are:Two-position dutyCapacity control dutyMixing duty
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Fans in Parallel
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Centrifugal Fan WithInlet Vane Damper
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Characteristic Curves of Parallel Blade Dampers
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Characteristic Curves of Opposed Blade 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 and Minimum Outdoor Air
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Economizer With Relief Fan