fundamentals of hvac control systems -...

Fundamentals of HVAC Control Systems

18, 19, 25, 26 April 2007

ASHRAE Hong Kong Chapter Technical Workshop

Chapter 3Control Valves and Dampers

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

[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 flowMore sensitive to high differential pressureHarder to close off against line pressure

3-way: for constant flowActuator does not need to be as powerful

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

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

Normally Open 2-Way Valve

Normally Closed 2-Way Valve

Two-way globe valve application

Double-Seated 2-Way Valve

Butterfly Valve

Ball Valve

Ball valve

Butterfly valve

Linear Valve

Equal Percentage Valve

Quick-Opening Valve

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

Typical Flow Characteristics

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

Flow coefficient:

PqAV Δ

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=

ρQ = volume flow (m3/h)

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

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

3-Way Mixing Valve

3-Way Diverting Valve

Three-way valves

3-Way Mixing Valves

Flow Through 3-Way Valve

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

Coil Subsystem

Representative Values of Cv

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

Capacity Versus Flow Rate

Authority Distortion ofLinear Flow Characteristic

Authority Distortion of Equal Percentage Flow Characteristic

Pump & System CurvesWith Valve Control

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

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

Typical Multiblade Dampers

Parallel blade damper Opposed blade damper

Round damper Volume control damper(opposed blade)

Typical (opposed blade) damper construction

Internally mounted electric actuator

Externally mountedpneumatic actuator

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)

Performance Data

Leakage ratingsTorque requirements

Closing torqueDynamic torque

Velocity ratingsTemperature ratingsPressure ratingsUL classification (fire/smoke)

Control loop for a damper system

Resistance to airflowin actual system

Mixed air control system (parallel blade dampers)

Mixed air system with louvers

Selecting and Sizing Dampers

The three basic damper applications are:Two-position dutyCapacity control dutyMixing duty

Fans in Parallel

Centrifugal Fan WithInlet Vane Damper

Characteristic Curves of Parallel Blade Dampers

Characteristic Curves of Opposed Blade Dampers

Flow Pattern Through Dampers

Mixing Box Arrangements

Pressure Drop Across Mixing Box

Economizer With Supplyand Return Fans

VAV System at 100% Outdoor Air and Minimum Outdoor Air

Economizer With Relief Fan

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