ashrae workshop control samhui part 2

5/27/2018 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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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 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



2-Way and 3-Way Valve Circuits

Mixing 3-way control valve

Diverting 3-way control valve

2-way control valve



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



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



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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[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


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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 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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Typical Flow Characteristics


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[Source: Shadpour, F., 2001. The Fundamentals of HVAC Direct Digital Control]


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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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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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3-Way Mixing Valve


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3-Way Diverting Valve


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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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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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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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Typical Multiblade Dampers


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Parallel blade damper Opposed blade damper


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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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Control loop for

a damper system

Resistance to airflow

in actual system


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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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ashrae workshop control samhui part 2

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