Temperature Control Loop The purpose of the temperature control loop is to maintain a constant off-coil temperature. The control loop senses the actual

Temperature Control Temperature Control LoopLoop

The purpose of the The purpose of the temperature control loop is temperature control loop is to maintain a constant off-to maintain a constant off-

coil temperature.coil temperature.The control loop senses the actual off-coil The control loop senses the actual off-coil

temperature and compares it to a temperature and compares it to a setpoint. The controller sends an output setpoint. The controller sends an output signal to a valve actuator which adjusts signal to a valve actuator which adjusts

the valve stem position which affects the the valve stem position which affects the waterflow through the valve and the coil waterflow through the valve and the coil

which impacts the off-coil air temperature.which impacts the off-coil air temperature.


Off-Coil TemperatureOff-Coil Temperature

DDC Control CalculationsDDC Control Calculations

Signal to Valve ActuatorSignal to Valve Actuator

Position of Valve StemPosition of Valve Stem

Water flow through valve / coilWater flow through valve / coil

Air FlowAir FlowAir FlowAir Flow

CHWSCHWS

CHWRCHWR

DDC ControllerDDC ControllerSetpoint and otherSetpoint and otherTuning ParametersTuning Parameters

ValveValveActuatorActuator

Cooling CoilCooling Coil

Off-coil AirOff-coil AirTemperatureTemperature

SensorSensor



Control Loop SetpointControl Loop Setpoint


Control Loop SetpointControl Loop SetpointSetpoint of control loop is fixed by Setpoint of control loop is fixed by building engineer. Choosing too high building engineer. Choosing too high

of a setpoint will result in:of a setpoint will result in: lower chiller energylower chiller energy higher fan energyhigher fan energy reduced system capacityreduced system capacity higher RH in space higher RH in space


Control Loop SetpointControl Loop SetpointToo low a setpoint may cause the loop Too low a setpoint may cause the loop

to saturate meaning no control.to saturate meaning no control.

Common practice is to fix off-coil Common practice is to fix off-coil temperature setpoint betweentemperature setpoint between

10°C and 14°C.10°C and 14°C.


Chilled Water ValvesChilled Water Valves


Chilled Water ValvesChilled Water ValvesThe chilled water valve should be The chilled water valve should be mounted in the chilled water supply mounted in the chilled water supply

pipe.pipe.

If the chilled water valve is in the If the chilled water valve is in the return valve, then when the valve is return valve, then when the valve is closed the coil is full of cold water closed the coil is full of cold water

and condensation will result and condensation will result increasing maintenance problems.increasing maintenance problems.


Packing nutPacking guidePackings

Bonnet (Centerpiece)

Stem

Disk (Integral w/Plug)

Plug

Cage (Integral Seat)

Body

Normally Open Cage Trim Globe Valve BodyNormally Open Cage Trim Globe Valve Body

Normally Open Flanged Globe Valve BodyNormally Open Flanged Globe Valve Body


StemStemPacking nutPacking guidePackingsPacking boxCenterpiece

Disk holderDiskPlugSeat (screwed in)

Plug guide

Body

Resilient Seat Butterfly Valve BodyResilient Seat Butterfly Valve Body


Body

Valve System

Stem Bushing,Stem Seal

Stem Retaining Ring

Valve Seat

Disc

Neck


100100

% Flow% Flow

% Valve Stem Lift% Valve Stem Lift00 2525 5050 7575

Equal Percentage

Quick Opening

00

5050

7575

100100

2525

Linear

Quick Opening

LinearEqual Percentage

Valve Valve PlugPlug

DesignDesign


Use quick opening plug for Use quick opening plug for two position applications.two position applications.

Linear plug is typically used Linear plug is typically used for industrial applications.for industrial applications.

Equal percentage plug Equal percentage plug typically used for HVAC typically used for HVAC

applications. Why?applications. Why?


100100

% Design Capacity% Design Capacity

% Design Flowrate% Design Flowrate00 2525 5050 7575100100

% Flow% Flow


5050

7575

100100

2525

00

5050

7575

100100

2525

Valve Characteristic + Coil Characteristic =Valve Characteristic + Coil Characteristic =


100100

% Design Capacity% Design Capacity


5050

7575

100100

2525

A roughly linear system characteristicA roughly linear system characteristic

Temperature Control Temperature Control LoopLoopAll valves experience uncontrollable All valves experience uncontrollable

flow when almost closed.flow when almost closed.

UncontrollableUncontrollable flowflow


RangeabilityRangeabilityA high rangeability means good A high rangeability means good control under low load conditions.control under low load conditions.

RangeabilityMaximumFlow

MinimumControllable Flow

There are different standards to defineThere are different standards to define““Minimum Controllable Flow”Minimum Controllable Flow”


CavitationCavitationAs water goes through the restricted area As water goes through the restricted area

between the plug and the seat, the between the plug and the seat, the velocity increases and static pressure velocity increases and static pressure decreases. If the static pressure drops decreases. If the static pressure drops below the vapour pressure, air bubbles below the vapour pressure, air bubbles

form. After passing the restriction, form. After passing the restriction, velocity decreases and static pressure velocity decreases and static pressure increases. As the static pressure rises, increases. As the static pressure rises,

the bubbles implode causing shock the bubbles implode causing shock waves which result in pitting on the waves which result in pitting on the

inside of the valve.inside of the valve.


CavitationCavitation

PressurePressure

VelocityVelocity

PressurePressure VelocityVelocity

Liquid Vapour Liquid Vapour PressurePressure

Plug to Seat Plug to Seat ClearanceClearance

ImplosionsImplosionsFlowFlow ***


Valve AuthorityValve Authority

Cooling CoilCooling CoilPPvv

PPtt

AuthorityP

Pv

t

05.


Valve SizingValve SizingUndersized valves cannot deliver Undersized valves cannot deliver sufficient flow for maximum load sufficient flow for maximum load

conditions.conditions.

Oversized valves must operate at the Oversized valves must operate at the end of their stroke making tuning end of their stroke making tuning

more difficult.more difficult.


Valve SizingValve SizingFor 2 position valves, select the valve For 2 position valves, select the valve

according to line size.according to line size.

For modulating valves, use the design For modulating valves, use the design pressure drop and flow rate to pressure drop and flow rate to calculate the flow coefficient.calculate the flow coefficient.


Valve SizingValve SizingFlow Coefficient (CFlow Coefficient (Cvv) is the USGPM of ) is the USGPM of

60°F water that will flow through a 60°F water that will flow through a fully open valve with a 1 psi drop fully open valve with a 1 psi drop

across it.across it.


Valve SizingValve Sizing

CQ

Pvv

CCvv = Valve flow coefficient = Valve flow coefficientQ = Flow in USGPMQ = Flow in USGPMPPvv = Pressure difference between = Pressure difference between inlet and outlet in psiinlet and outlet in psi


Valve SizingValve Sizing

ExampleExample

Q = 90 USGPMQ = 90 USGPM

PPvv = 5 psi (typical) = 5 psi (typical)

CCvv = 90/2.24 = 40 = 90/2.24 = 40


Valve SizingValve SizingValves are only manufactured with Valves are only manufactured with

specific flow coefficients.specific flow coefficients.

After calculating the desired flow After calculating the desired flow coefficient, select the valve with the coefficient, select the valve with the

next lowest coefficient.next lowest coefficient.

Most systems are oversized so slightly Most systems are oversized so slightly undersizing the valve will not undersizing the valve will not

compromize system performance, but compromize system performance, but it does improve system controllability.it does improve system controllability.


Three Way Mixing Valve BodyThree Way Mixing Valve Body

Normally OpenNormally Open

NormallyNormallyClosedClosed

CommonCommon


Three Way ValvesThree Way Valves

CoolingCoolingCoilCoil

PumpPump

N.O.N.O.N.C.N.C.CC

RR

SS

CoolingCoolingCoilCoil

N.O.N.O.CC

N.C.N.C.

SS

RR

Three way valve in mixingThree way valve in mixingapplication piped N.C. to coilapplication piped N.C. to coil

Three way valve in bypassThree way valve in bypassapplication piped N.C. to coilapplication piped N.C. to coil


Three Way ValvesThree Way ValvesThree way valves are used for Three way valves are used for

constant flow systems. Constant flow constant flow systems. Constant flow systems are not popular because systems are not popular because

they waste pump energy.they waste pump energy.Constant Flow to BuildingConstant Flow to Building Variable Flow to BuildingVariable Flow to Building


Valve ActuatorValve ActuatorThere are two criticalThere are two criticalissues when selectingissues when selectingthe valve actuator:the valve actuator:• Close off pressureClose off pressure• Spring returnSpring return• End switchEnd switch


Valve ActuatorValve ActuatorFor a two way valve, the close off pressure For a two way valve, the close off pressure

must be sufficient to overcome the pump must be sufficient to overcome the pump head.head.

For a three way valve, the close off pressure For a three way valve, the close off pressure is twice the pressure drop across the valve is twice the pressure drop across the valve

plus the pressure drop across the coil.plus the pressure drop across the coil.

If spring return in provided, the valve will If spring return in provided, the valve will return to a known position if the control return to a known position if the control

signal is removed or during a power signal is removed or during a power failure.failure.


Temperature SensorsTemperature Sensors


An averaging temperatureAn averaging temperaturesensor is mounted insensor is mounted inductwork in front of coolingductwork in front of coolingcoil to measure off-coilcoil to measure off-coiltemperature.temperature.

Temperature SensorsTemperature Sensors


Temperature SensorsTemperature SensorsResistance Temperature Detectors Resistance Temperature Detectors (RTD) measure temperature using (RTD) measure temperature using the resistance property of metals the resistance property of metals

such as nickel or platinum.such as nickel or platinum.

The resistance of a thin wire varies The resistance of a thin wire varies linearly over a limited temperature linearly over a limited temperature range. Low current is used to avoid range. Low current is used to avoid

self-heating.self-heating.


Temperature SensorsTemperature SensorsDifferent metals have different Different metals have different

sensitivity to heat. Thermal sensitivity to heat. Thermal sensitivity is measured in sensitivity is measured in

ohms/ohm/Deg C.ohms/ohm/Deg C.

For example, a sensor with a sensitivity For example, a sensor with a sensitivity of 0.003 ohms/ohm/Deg C and a base of 0.003 ohms/ohm/Deg C and a base resistance of 22°C will change from resistance of 22°C will change from 10031003to 1006to 1006as the temperature as the temperature

varies from 23°C to 24°C.varies from 23°C to 24°C.


Temperature SensorsTemperature SensorsTwo common metals used for RTD areTwo common metals used for RTD are

Nickel and Platinum.Nickel and Platinum.

Both type of sensors are available Both type of sensors are available withwith

a base resistance of 1000a base resistance of 1000Platinum Platinum sensors are also available with a sensors are also available with a

base resistance of 100base resistance of 100

Base resistance is not as important as Base resistance is not as important as sensitivity and linearity.sensitivity and linearity.


Temperature SensorsTemperature SensorsNickel sensors have a high sensitivityNickel sensors have a high sensitivity

(.00343 (.00343 C).C).

Nickel sensors have a slight non-linearityNickel sensors have a slight non-linearityat extreme temperatures and software at extreme temperatures and software

correction may be required undercorrection may be required underthese conditions.these conditions.

Nickel sensors are low cost and can beNickel sensors are low cost and can beused for point or averaging elements.used for point or averaging elements.


Temperature SensorsTemperature SensorsPlatinum sensors have a low sensitivityPlatinum sensors have a low sensitivity

(.00214 (.00214 C).C).

Platinum sensors are very linear for thePlatinum sensors are very linear for therange of temperatures found in HVAC range of temperatures found in HVAC

applications.applications.

Platinum sensors are higher cost and Platinum sensors are higher cost and areare

not available as averaging elements.not available as averaging elements.


Temperature SensorsTemperature SensorsPoint Element Point Element : Appropriate: Appropriatefor Return Air Temperaturefor Return Air Temperatureor Room Temperatureor Room Temperature(well mixed air)(well mixed air)

Averaging Element Averaging Element : Used: Usedfor Off-Coil Temperature orfor Off-Coil Temperature orSupply Air TemperatureSupply Air Temperature(turbulent/stratified air)(turbulent/stratified air)


Temperature SensorsTemperature SensorsCalibrate a temperate sensor by:Calibrate a temperate sensor by:

Disconnect RTD element.Disconnect RTD element. Replace RTD element with a precisionReplace RTD element with a precision

1000 1000 resistor. resistor. Adjust offset in DDC Controller to readAdjust offset in DDC Controller to read

base temperature (this compensates base temperature (this compensates forfor resistance of wiring). resistance of wiring).

Replace RTD element.Replace RTD element.

Documents

Temperature Control Loop The purpose of the temperature control loop is to maintain a constant off-coil temperature. The control loop senses the actual