static pressure control loop the purpose of the static pressure control loop is to maintain an...

Static Pressure Control Static Pressure Control LoopLoop

The purpose of the static The purpose of the static pressure control loop is to pressure control loop is to maintain an optimal static maintain an optimal static pressure in the ductwork.pressure in the ductwork.The control loop senses the static The control loop senses the static

pressure in the ductwork and compares pressure in the ductwork and compares it to a setpoint. The controller sends an it to a setpoint. The controller sends an output signal to the VSD which controls output signal to the VSD which controls the fan speed. Adjusting the fan speed the fan speed. Adjusting the fan speed

impacts the static pressure in the impacts the static pressure in the ductwork.ductwork.


Duct Static TemperatureDuct Static Temperature

DDC Control CalculationsDDC Control Calculations

Signal to VSD ControllerSignal to VSD Controller

Fan SpeedFan Speed

Air flow through ductworkAir flow through ductwork


Air FlowAir FlowAir FlowAir Flow

DDC ControllerDDC Controller Tuning ParametersTuning Parameters

Cooling CoilCooling Coil

Static PressureStatic PressureSensorSensor

VariableVariableSpeed DriveSpeed Drive


Control Loop SetpointControl Loop Setpoint


Control Loop SetpointControl Loop SetpointSelecting the correct setpoint is very Selecting the correct setpoint is very

important for system performance.important for system performance.

The optimal setpoint cannot be The optimal setpoint cannot be calculated in advance. It must be calculated in advance. It must be

determined in the field. The current determined in the field. The current practice of fixing the setpoint wastes practice of fixing the setpoint wastes

a lot of energy.a lot of energy.


Control Loop SetpointControl Loop SetpointIf the setpoint is too low, some boxes If the setpoint is too low, some boxes

will not be able to get enough air to will not be able to get enough air to provide comfort.provide comfort.

Some boxes will have their damper Some boxes will have their damper 100% open but the actual flow will 100% open but the actual flow will still be less than the desired flow. still be less than the desired flow. These boxes are “starved” and the These boxes are “starved” and the VAV box will not be able to achieve VAV box will not be able to achieve the space temperature setpoint. the space temperature setpoint.


Control Loop SetpointControl Loop SetpointIf the setpoint is too high, fan energy If the setpoint is too high, fan energy

is wasted.is wasted.

The higher the static pressure The higher the static pressure setpoint, the harder the fan must setpoint, the harder the fan must

work to maintain the setpoint.work to maintain the setpoint.


Control Loop SetpointControl Loop SetpointIf the setpoint is too high, the noise of If the setpoint is too high, the noise of

the system will increase.the system will increase.

If the static pressure setpoint is If the static pressure setpoint is high, all of the VAV box dampers high, all of the VAV box dampers

will only need to open a little bit to will only need to open a little bit to achieve the desired airflow. Airflow achieve the desired airflow. Airflow

going through a small opening going through a small opening generates a lot of noise.generates a lot of noise.


Control Loop SetpointControl Loop SetpointIf the setpoint is too high, VAV box control If the setpoint is too high, VAV box control

may go unstable.may go unstable.

With a high static pressure setpoint, the With a high static pressure setpoint, the throttling range of the damper to go throttling range of the damper to go

from minimum flow to maximum flow is from minimum flow to maximum flow is decreased. A small control adjustment decreased. A small control adjustment to damper position has a large effect to damper position has a large effect

on airflow. The flow control loop is very on airflow. The flow control loop is very sensitive and may cycle, depending on sensitive and may cycle, depending on

the control loop parameters.the control loop parameters.


Control Loop SetpointControl Loop SetpointDuring air balancing, use a high During air balancing, use a high

setpoint to make sure that none of setpoint to make sure that none of the boxes are starved.the boxes are starved.


Control Loop SetpointControl Loop SetpointChoosing the optimal setpoint:Choosing the optimal setpoint:

1.1. Enter a low setpoint so that Enter a low setpoint so that some of the some of the VAV boxes are VAV boxes are starved with damper starved with damper 100% 100% openopen

2.2. Increase the static pressure Increase the static pressure setpoint setpoint until all of the VAV until all of the VAV boxes are in control; boxes are in control; damper less damper less than 100% openthan 100% open


Control Loop SetpointControl Loop SetpointStatic Pressure Reset Energy Saving Strategy:Static Pressure Reset Energy Saving Strategy:

1.1. Every five minutes, check the position Every five minutes, check the position of of all VAV box dampers.all VAV box dampers.

2.2. If the damper which is open the most If the damper which is open the most is is more than 90% open, increase the more than 90% open, increase the

static pressure setpoint by 50 Pascal.static pressure setpoint by 50 Pascal.

3.3. If the damper which is open the most If the damper which is open the most is is less than 80% open, decrease the less than 80% open, decrease the static static pressure setpoint by 50 Pascal.pressure setpoint by 50 Pascal.


Zone AZone A““Hot”Hot”

Zone BZone B““Warm”Warm”

Zone CZone C““Cool”Cool”

Static Pressure Setpoint = 100 PascalStatic Pressure Setpoint = 100 PascalFan Fan Speed Speed of 40%of 40%

FlowFlowDD

400400DamperDamper

100%100%FlowFlowAA

300300

FlowFlowDD

250250DamperDamper

70%70%FlowFlowAA

250250

FlowFlowDD

150150DamperDamper

30%30%FlowFlowAA

150150

FlowFlowDD = Desired Flow Flow = Desired Flow FlowAA = Actual Flow = Actual Flow



Control Loop SetpointControl Loop SetpointLow SetpointLow Setpoint

Fan runs at very low speed and uses Fan runs at very low speed and uses little energylittle energy

Zone “A” box is starved, even fully Zone “A” box is starved, even fully open the box cannot supply the open the box cannot supply the

desired air flowdesired air flow

Zone “A” will get hotter because not Zone “A” will get hotter because not enough cool air is being supplied enough cool air is being supplied






FlowFlowDD

400400DamperDamper

95%95%FlowFlowAA

400400

FlowFlowDD

250250DamperDamper

50%50%FlowFlowAA

250250

FlowFlowDD

150150DamperDamper

20%20%FlowFlowAA

150150




Control Loop SetpointControl Loop SetpointCorrect SetpointCorrect Setpoint

Fan runs at low speed and uses Fan runs at low speed and uses some energysome energy

Zone “A” box is almost fully open Zone “A” box is almost fully open but able to supply the desired air but able to supply the desired air

flowflow

All zones will be comfortable as All zones will be comfortable as enough cool air is being supplied enough cool air is being supplied






FlowFlowDD

400400DamperDamper

65%65%FlowFlowAA

400400

FlowFlowDD

250250DamperDamper

30%30%FlowFlowAA

250250

FlowFlowDD

150150DamperDamper

10%10%FlowFlowAA

150150




Control Loop SetpointControl Loop SetpointHigh SetpointHigh Setpoint

Fan runs at medium speed and Fan runs at medium speed and wastes some energy (still better wastes some energy (still better

than CAV)than CAV)

All zones will be comfortable as All zones will be comfortable as enough cool air is being supplied enough cool air is being supplied

Zone “C” will be noisy and it will be Zone “C” will be noisy and it will be difficult to maintain stable controldifficult to maintain stable control


Damper PositionDamper Position0%0% 100%100%

AirAirFlowFlow


Air flow through VAV box increasesAir flow through VAV box increasesquickly as damper starts to openquickly as damper starts to open



AirAirFlowFlow

200 Pascal200 Pascal



Static PressureStatic Pressure


Characteristic curve of VAV box changesCharacteristic curve of VAV box changeswith the system static pressure.with the system static pressure.



AirAirFlowFlow





Minimum FlowMinimum Flow

Maximum FlowMaximum Flow

VAV box controller modulates airflowVAV box controller modulates airflowbetween minimum and maximum airflow.between minimum and maximum airflow.



0%0% 100%100%

AirAirFlowFlow







50%50%

With system static pressure of 200 Pascal,With system static pressure of 200 Pascal,damper is modulated 2% to 50%.damper is modulated 2% to 50%.



0%0% 100%100%

AirAirFlowFlow







15%15%

With system static pressure of 500 Pascal,With system static pressure of 500 Pascal,damper is modulated 1% to 15%.damper is modulated 1% to 15%.



0%0% 100%100%

AirAirFlowFlow







With system static pressure of 100 Pascal,With system static pressure of 100 Pascal,VAV box can never achieve maximum flow.VAV box can never achieve maximum flow.



Control Loop SetpointControl Loop SetpointIn this example, the VAV box flow In this example, the VAV box flow

control loop is three times as control loop is three times as sensitive when the static pressure is sensitive when the static pressure is 500 Pascal as compared to when the 500 Pascal as compared to when the

static pressure is 200 Pascal. An static pressure is 200 Pascal. An overly sensitive control loop will overly sensitive control loop will

cycle.cycle.


Control Loop SetpointControl Loop SetpointDoes dynamically resetting the static Does dynamically resetting the static pressure setpoint really save energy as pressure setpoint really save energy as compared to having a fixed setpoint?compared to having a fixed setpoint?

FanFan SizeSize OperationsOperations SavingsSavingsAHU-1AHU-1 125 hp125 hp 24 hours 24 hours 42% 42%AHU-2AHU-2 100 hp100 hp 24 hours 24 hours 37% 37%AHU-3AHU-3 50 hp50 hp OccupiedOccupied 19% 19%AHU-4AHU-4 25 hp25 hp OccupiedOccupied 20% 20%

Source : ASHRAE Journal April 1993Source : ASHRAE Journal April 1993


Variable Speed DriveVariable Speed Drive


Variable Speed DriveVariable Speed DriveThe Variable Speed Drive (VSD) The Variable Speed Drive (VSD)

Controller is typically provided with Controller is typically provided with the fan and accepts a setpoint signal the fan and accepts a setpoint signal

(0-10VDC or 4-20mA).(0-10VDC or 4-20mA).

The VSD provides voltage free The VSD provides voltage free contacts indicating a tripped motor.contacts indicating a tripped motor.


Variable Speed DriveVariable Speed DriveVoltageVoltage

FrequencyFrequency

100%100%

NominalNominalFrequencyFrequency

Variable Voltage Variable FrequencyVariable Voltage Variable Frequency

Based on the control signalBased on the control signalinput, the VSD will adjustinput, the VSD will adjustthe voltage and frequencythe voltage and frequency

to the fan motor.to the fan motor.

VSD reacts to changes inVSD reacts to changes insetpoint by changing thesetpoint by changing thevoltage and frequency.voltage and frequency.

Changes are applied slowlyChanges are applied slowlybased on acceleration andbased on acceleration anddeceleration parameters.deceleration parameters.


Static Pressure SensorStatic Pressure Sensor


Static Pressure SensorStatic Pressure SensorThe conventional approach requires The conventional approach requires

the static pressure sensor to be the static pressure sensor to be located at the point of minimum located at the point of minimum static pressure in the ductwork.static pressure in the ductwork.

For one long run of ductwork, this For one long run of ductwork, this pointpoint

is 1/2 to 2/3 of the way along.is 1/2 to 2/3 of the way along.


Static Pressure SensorStatic Pressure SensorConventional ApproachConventional Approach

StaticStaticPoint of Lowest

Pressure inSystem


Static Pressure SensorStatic Pressure SensorIf the ductwork has two branches, the If the ductwork has two branches, the

static pressure in each branch is static pressure in each branch is taken and the lower of the values is taken and the lower of the values is

used to control the fan.used to control the fan.


Static Pressure SensorStatic Pressure SensorConventional ApproachConventional Approach

Low Signal SelectLow Signal Select


Static Pressure SensorStatic Pressure SensorFor example, if branch “A” serves the For example, if branch “A” serves the

east side of the building and branch east side of the building and branch “B”serves the west side of the “B”serves the west side of the

building then in the morning, the fan building then in the morning, the fan would be controlled by sensor “A” would be controlled by sensor “A” and in the afternoon, the fan would and in the afternoon, the fan would

be controlled by sensor “B”.be controlled by sensor “B”.


Static Pressure SensorStatic Pressure SensorIf the static pressure setpoint is If the static pressure setpoint is

continuously reset based on VAV damper continuously reset based on VAV damper position:position:

Energy will be saved as fan only works asEnergy will be saved as fan only works as hard as is necessary hard as is necessary

Placement of static pressure sensor doesPlacement of static pressure sensor does not matter - as long as airflow is not not matter - as long as airflow is not turbulent : away from fan, 5 duct turbulent : away from fan, 5 duct diametersdiameters away from bends, dampers, reducers, etc. away from bends, dampers, reducers, etc.


Converts differential Converts differential pressure betweenpressure between

H and L into a H and L into a signal.signal.

When H is inside When H is inside duct and L is duct and L is

outside duct static outside duct static pressure of duct is pressure of duct is

measured.measured.

Static Pressure SensorStatic Pressure Sensor


Static Pressure SensorStatic Pressure SensorCalibrate a static pressure sensor by:Calibrate a static pressure sensor by:

Disconnect static pressure sensor.Disconnect static pressure sensor. Inject a 2.00 VDC voltage and verify Inject a 2.00 VDC voltage and verify

thatthat the DDC Controller reads zero. the DDC Controller reads zero.

Inject a 10.00 VDC voltage and Inject a 10.00 VDC voltage and verify thatverify that the DDC Controller reads maximum. the DDC Controller reads maximum.

Replace static pressure sensor.Replace static pressure sensor.

static pressure control loop the purpose of the static pressure control loop is to maintain an...

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