The Consultants Role

Solutions: The Role of the Consulting Engineer

Temperature Control Systems – Why Don’t They Work?

ASHRAE CRC 2006

Why Temperature Control Systems?

Early Temperature Control System


• The controls aren’t working!

• I’m hot

• I’m cold

• These controls cost too much!

• Why are the doors are standing open!

What does the Owner do Wrong?

• Modifies programming or uses overrides without performing necessary diagnostic work and/or without fully understanding the system and problem.

• Fails to train staff on proper operation / maintenance.

• Owner fails to define site standards.

What does the Control Contractor do Wrong?

• Fails to implement the Sequence of Operation as specified.

• Fails to train techs to conform with site standards.

• Implements programming that is wrought with logic problems.

• Fails to properly tune control loops.• Perform programming and prepare graphics on

site rather than during shop drawing preparation.

What does the Consultant do Wrong?

• Sequence of Operation (SOO) does not exist or is poorly defined and/or cannot possibly work.

• SOO is used to define work scope outside of Temperature Controls Contractor’s Scope.

• Specified components are too much or too little to accomplish the task.

• The Owner’s needs are ignored.• Interoperability requirements are poorly defined.

The Role of the Consulting Engineer

Create construction documents that communicate the following:

• Define the System to be controlled.• Define hard-wired safeties.• Define the Sequence of Operation.• Develop an Input / Output Summary (Points List)

– Define requirements for alarming.– Define requirements for trending.

• Define requirements for graphics.• Identify locations of Control Components, power wiring,

interoperability• Write specifications

Example: Fraternity House

Step 1: Define the System to be Controlled

• Create a list of all mechanical systems that need to be controlled.

• Draw a flow diagram of all of the controlled systems.• Show all components that automatically control (check

valves, backdraft dampers, control valves and dampers, etc)

• If the operation of other systems are related, show enough to communicate intertwining systems.

• Consider showing everything that would make sense to show on a graphic.


What systems need to be controlled?• Heating water system• Chilled water system• Domestic hot water system• Unit Heaters• Fan coil units• VAV boxes• Air handling units• Exhaust fans• Sump Pump• Exterior lighting

Step 2: Define hard-wired control and safeties.

• Identify controls that are hard-wired.

• If all else fails, what keeps the system from damaging itself? Identify failure points:– Freezing air coils– Over / Under pressurizing ductwork– Under pressurizing piping (domestic water pumps)– Over temperature (domestic hot water)– Smoke Detection– Fan Discharge Damper Control

Step 2: Define hard-wired control and safeties.

Step 2: Define hard-wired safeties.

Specification:

• Communicate locations of hard-wired equipment on the flow diagram.

• Describe how the safeties function.– This can be done with a written description or– This can be done with a drawing.


SUPPLY FAN

STATUS

FREEZESTAT

Step 2: Define

hard-wired safeties.

Step 3: Define the Sequence of Operation.

• Identify how each component of the system is controlled.• Do not confuse hard-wired logic with programmed logic.• Identify time schedule for operating equipment.• Determine trigger point for starting and stopping logic (ie supply fan

status).• Consider what happens when equipment is offline.• Consider part load scenarios• Attempt to not rely solely on outside air temperature / relative

humidity for decision making.• Keep it simple (supply fan pressure reset, equal runtime, supply

temperature reset).• Sequence to eliminate possibility of simultaneous heating and

cooling.• Avoid control modes (ie summer / winter modes)


MAU-1 Sequence of Operation

• Supply Fan:Supply fan shall be commanded to start whenever the unit occupancy is on. The occupancy schedule for MAU-1 shall be defined in cooperation with the Owner.

• Heating Valve Control:The heating valve shall be commanded open whenever outside air temperature is less than 45°F (deadband of 2°F) or Face and Bypass Damper command is greater than 5% (deadband of 2%). On Safety Alarm trip, the heating valve shall be commanded open.

• Discharge Air Temperature Setpoint Control:If outside air temperature is less than 40°F at fan startup, fan discharge air temperature shall be ramped from 85°F down to 55°F over a period of 15 minutes (adjustable). For instances in which outside air temperature is greater than 40°F, discharge air temperature shall be 55°F.


MAU-1 Sequence of Operation

• Chilled Water Valve and Face and Bypass Damper Control:A software based PID control loop shall be utilized to control the chilled water valve and face and bypass damper in sequence to control unit discharge air temperature. The input to the control loop shall be the discharge air temperature sensor located at the fan discharge. The loop setpoint shall be a calculated setpoint (discharge air temperature setpoint). On rise in discharge temperature above setpoint, the face and bypass damper shall modulate from full open (full face) to full closed. On further call for cooling, the chilled water valve shall modulate from full closed to full open. On fall in temperature below setpoint, the sequence reverses.

When fan status is off, the cooling valve shall be commanded closed and the face and bypass damper shall be commanded to full face.


Consider simulating performance to catch logic errors.

• Use spreadsheet

• Use software designed for the purpose (ie P-Spice, Matlab (Simulink))

• Consider specifying Sequence in a graphical format.

Step 4: Develop an Input / Output Summary (Points List)

Using the Sequence of Operation and Hardwired Safety’s List, prepare a list defining control points required.

• Identify control point type– Safety– Analog Input or Output– Digital Input or Output– Network Input or Output

• Define sensor/actuator quality and type– Temperature and Pressure Sensors: Identify Range– For pressure / temperature switches: Identify manual or automatic reset

• Define Alarming requirements:– Avoid alarming every control point.– Alarm failure to start– Alarm high / low discharge temperature– Consider making alarm setpoint relative to controlled setpointIdentify time delay

for alarm reporting


Point Name Description Type Field Device

SAFETIES

OAD Outside Air Damper SafetyDamper and

120V Actuator

LOPRESS Low Pressure Switch SafetyManual Reset Air Press. Switch,

Set at 2"

HIPRESS High Pressure Switch SafetyManual Reset Air Press. Switch,

Set at 4"

ENDSWITCH Damper Endswitch SafetyDamper Endswitch. Set at approximately 75% Open

FRZSTAT Freezestat SafetyManual Reset Low Limit Switch,

Set at 40°F

FIREALARMFire Alarm System

InputSafety

Contact Provided and Mounted by Div 16

EXISTING POINTS TO BE NETWORKED

OATOutside Air

TemperatureNetwork

InputUse Campus Sensor

OAHOutside Air Relative

HumidityNetwork

InputUse Campus Sensor


Point Name Description Type Field Device

POINTS REQUIRED FOR CONTROL

DATSupply Fan Discharge

TemperatureAI Flexible Averaging Thermistor

CHWVChilled Water Valve

CommandAO

Valve with Electric Spring Return Actuator

FBDMPRFace and Bypass

Damper CommandAO Electric Spring Return Actuators

HWVHeating Valve

CommandDO Electric 2-position Actuator

SFANSSSupply Fan Start/Stop

CommandDO Relay Contactor

SFANST Supply Fan Status DI Current Sensing Relay

SAFETYAHU Safety Status

AlarmDI Relay (refer to interlock drawing)

MONITORING POINTS

OADOutside Air Damper

PositionDI

Contact Closure from Damper Endswitch

HEATTEMPHeating Discharge

TemperatureAI Flexible Averaging Thermistor

FILTERDP Filter Pressure Drop AIAir Differential Pressure Sensor,

0 - 2"


Point Name Description Alarm?Alarm Rule and Delay

Trend? Period

POINTS REQUIRED FOR CONTROL

DATSupply Fan Discharge

TemperatureX

5°F > or < than setpoint

X 15 Min

CHWVChilled Water Valve

Command X 15 Min

FBDMPRFace and Bypass

Damper Command X 15 Min

HWVHeating Valve

Command X 15 Min

SFANSSSupply Fan Start/Stop Command

XChange of State

SFANST Supply Fan Status XFan Failure, 1

MinX

Change of State

SAFETYAHU Safety Status

AlarmX 1 Min X

Change of State

MONITORING POINTS

OADOutside Air Damper

PositionX

Fan Off and Damper Open

for 4 Min

HEATTEMPHeating Discharge

Temperature X 15 Min

FILTERDP Filter Pressure Drop X 15 Min

Step 5: Define Graphics Requirements

• After Updating the flow diagram with all point data, add a note similar to the following:

“Create Operator Workstation Graphic similar to flow diagram. In addition to displaying all input and output data, display Time and Date, Outside Air Temp, Outside Air RH and Discharge Supply Temp Setpoint.”

• For VAV boxes or Fan Coil Units, consider requiring contractor to build a spreadsheet to view the data.

• Consider specifying that floor plans be provided with dynamic links between control zones and the controlled equipment.

Step 5: Define Graphics Requirements

Step 6: Identify locations ofControl Components

• Consider showing all controlled equipment on floor plans (without showing ductwork, piping, etc)– Show thermostat locations, pressure sensor locations,

outside air temperature sensors, etc.– Use these drawings to further detail requirements for

graphics.– Detail proposed locations for control panels– Where lighting is controlled, show location of lighting

contactors.

Step 6: Identify Locations ofControl Components

Step 7: Write specifications• Consider using ASHRAE Guideline 13 Guide Specs.• Consider how the system will be accessed by the

operators.– Web-Enabled and Operator Workstation– Operator Workstation Only– Computer Terminal and Modem Interface– Modem Interface– No interface

• Specify the sensors, control valves, dampers, actuators, etc. Be sure all components displayed in Input/Output Summary are specified.

• Where interoperability is desired be sure to coordinate specifications of all parties.

• Require submittal of programming and graphics.

CO

ST

Interoperability

Specifications should include the following:1. Define what is to be communicated between

Temperature Control System and 2nd party equipment.2. Specify implicitly that control contractor coordinate with

2nd party equipment to ensure system is successfully integrated.

3. Specify implicitly that the 2nd party equipment supplier coordinate with the temperature control contractor. Make sure that the 2nd party equipment supplier knows that high level communications between the contractor and supplier is required (put it in the temperature control spec and the equipment spec).

4. Try to avoid specifying exactly how this intercommunication occurs.

Solutions – The Role of the Consulting Engineer

In Conclusion• The consultant needs to stay current with

the technology to good job of specifying.

• Engineers need to better understand how to write sequences of operation. This could be a semester long class…

• Specification in the way described improves obtaining “apples to apples” bids and reduce cost on bid day.

The Consultants Role

Design

Transcript of The Consultants Role