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.

Step 2: Define hard-wired safeties. SUPPLY FAN STATUS FREEZESTAT

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

Step 4: Develop an Input / Output Summary (Points List) EXISTING POINTS TO BE NETWORKED Use Campus Sensor Network Input Outside Air Temperature OAT Use Campus Sensor Network Input Outside Air Relative Humidity OAH Contact Provided and Mounted by Div 16 Safety Fire Alarm System Input FIREALARM Manual Reset Low Limit Switch, Set at 40°F Safety Freezestat FRZSTAT Damper Endswitch. Set at approximately 75% Open Safety Damper Endswitch ENDSWITCH Manual Reset Air Press. Switch, Set at 4" Safety High Pressure Switch HIPRESS Manual Reset Air Press. Switch, Set at 2" Safety Low Pressure Switch LOPRESS Damper and 120V Actuator Safety Outside Air Damper OAD SAFETIES Field Device Type Description Point Name

Step 4: Develop an Input / Output Summary (Points List) Contact Closure from Damper Endswitch DI Outside Air Damper Position OAD Air Differential Pressure Sensor, 0 - 2" AI Filter Pressure Drop FILTERDP Flexible Averaging Thermistor AI Heating Discharge Temperature HEATTEMP MONITORING POINTS Relay (refer to interlock drawing) DI AHU Safety Status Alarm SAFETY Current Sensing Relay DI Supply Fan Status SFANST Relay Contactor DO Supply Fan Start/Stop Command SFANSS Electric 2-position Actuator DO Heating Valve Command HWV Electric Spring Return Actuators AO Face and Bypass Damper Command FBDMPR Valve with Electric Spring Return Actuator AO Chilled Water Valve Command CHWV Flexible Averaging Thermistor AI Supply Fan Discharge Temperature DAT POINTS REQUIRED FOR CONTROL Field Device Type Description Point Name

Step 4: Develop an Input / Output Summary (Points List) Fan Off and Damper Open for 4 Min X Outside Air Damper Position OAD 15 Min X Filter Pressure Drop FILTERDP 15 Min X Heating Discharge Temperature HEATTEMP MONITORING POINTS Change of State X 1 Min X AHU Safety Status Alarm SAFETY Change of State X Fan Failure, 1 Min X Supply Fan Status SFANST Change of State X Supply Fan Start/Stop Command SFANSS 15 Min X Heating Valve Command HWV 15 Min X Face and Bypass Damper Command FBDMPR 15 Min X Chilled Water Valve Command CHWV 15 Min X 5°F > or < than setpoint X Supply Fan Discharge Temperature DAT POINTS REQUIRED FOR CONTROL Period Trend? Alarm Rule and Delay Alarm? Description Point Name

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 of Control 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 of Control 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.
COST

Interoperability
Specifications should include the following:
Define what is to be communicated between Temperature Control System and 2 nd party equipment.
Specify implicitly that control contractor coordinate with 2 nd party equipment to ensure system is successfully integrated.
Specify implicitly that the 2 nd party equipment supplier coordinate with the temperature control contractor. Make sure that the 2 nd 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).
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

by ASHRAE Region VI

Accessibility

Categories

Upload Details

Usage Rights

Statistics

The Consultants Role Presentation Transcript