Sequences & Control Schemes for Air & Water Systems

Sequences and Control Schemes

for Air and Water Systems

Presented to the Illinois Chapter ASHRAETuesday, March 9, 2010

By Steve Calabrese, BSEEControl Engineering Corp.

Sequences & Control Schemes for Air & Water Systems March 9, 2010

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• Item 1...

• Mechanical Contractors...?

• Consultants...?

• Controls Contractors...?

• Electrical Contractors...!

Background & Experience

‘How The Heck Did I Get Here?’


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Presentation Summary

1) Controls: History & Background

2) Specifying Controls

3) Common Routines

4) Systems & Sequences

5) Control Devices

1) And In The Beginning...

‘Early form of temperature control’

Methods of Control

• Simple electrical control

• Pneumatic control

• Electronic control

• Direct Digital Control (DDC)


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Simple Electrical Control

• Two-position (on/off)

• Thermostatic control (on/off)

• Simple proportional control (resistance)

• Ladder/relay logic

• Crude but reliable

Pneumatic Control

• Compressed air is the control medium

• Proportional control inherent

• Strong actuation capabilities

• Safe for use in hazardous environments

• Ongoing maintenance is an issue

Electronic Control

• Integrated circuit-based (solid state)

• Gray Box Controllers (application-specific)

• Inherent stability, accuracy, & consistency

• Needed to be supported w/add’l controls

• Once implemented, little flexibility tochange


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Direct Digital Control

• Microprocessor-based

• Expandable I/O

• Configurable / Programmable

• Extremely flexible

• De-centralized (distributed logic)

• Networkable

And So It Was Written...

Χλοσεδ λοοπχοντρολ οφ αχονδιτιοναππλιεδ

διρεχτλψ αταππλιχατιονυσινγ α διγιταλλοοπ ωιτηφεεδβαχκ τοσενσε χηανγεσιν χονδιτιον.

ΔΔΧ

ASHRAE Definition of DDC

Components of DDC

• Closed Loop Control – controlling a process,and at the same time sensing the process.

• Digital Loop – microprocessor-basedcontrol loop.

• Feedback – from above, the sensing of theprocess, in order to determine subsequentactions to be taken on the process.


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Origin of the word ‘Digital’

• Extension of the word ‘digit’

• Piece (bit) of electronic information (0 or 1)

• Voltage absence (0) or voltage presence (1)

• Convert real-world analog value to digital

• Represented in the digital domain as 0s & 1s

Other Aspects of DDC

• The term has been broadened...

• Synonomous with computer-based control

• Networked DDC = BAS

• Centralized DDC (old school, prior to Y2K)

• Distributed DDC – logic is distributed amongde-centralized controllers.

Analogy of a Control Process


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Block Diagram of a Controller

Block Diagram-Digital Controller

Equipment Level Digital Controller


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Wiring Diagram

Points List

Logic Diagram


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Equipment Graphic

Simple Mechanical System

Now for the Big Questions...

• How should it operate?

• How should it be controlled to operate?

• How should it be specified to be controlled?

• Who determines, and how???


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Simple Mechanical System

2) Specifying Controls• Construction Specifications Institute (CSI)

• The Sweet Sixteen (Divisions 1 - 16)

• Division 15 – Mechanical (TC included)

• Revamped in 2004

• Now 50 available division numbers

• Division 23 – Mechanical (TC included)

Section Headings

• Automatic Temperature Controls

• Building Automation System

• Direct Digital Control Systems

• HVAC Instrumentation & Controls

• Temperature Control Systems

• Controls...


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Major Parts of the Section

• Part 1 - General

• Part 2 - Products

• Part 3 - Execution

• Part 4 – Sequence of Operation & Points

• Sometimes Part 5 - Points

• Sometimes no Part 4 or Part 5

Part 1 - General

• Description of Work / Scope

• Related Documents (Related Sections)

• Work Included (Work By Others)

• Furnished But Not Installed...

• Coordination With Other Trades...

• Submittals/Warranty/As Builts/Training...

Part 2 - Products

• Acceptable Manufacturers

• Controllers/Operator Workstation/Software

• Sensors & Transmitters

• Switches & Two-state Controllers

• Safeties & Limits

• End Devices

• Miscellaneous Devices


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Part 3 - Execution

• Project Management

• Coordination

• Wiring

• Installation Requirements (Devices)

• Startup & Commissioning (Validation)

• Demonstration & Acceptance

Part 4 – Sequence of Operation

• Outline form – found in Part 4 of the spec.

• or...on the mechanical plans

• or...in an addendum

• or...on a cocktail napkin

• or...nowhere!

Sequences & Points Lists -Benefits

• Provide detailed information required tobid the project.

• Provide the technical information requiredto design the project.

• Provide the functional information requiredto operate the installed systems.


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Taken from the Pages of Div. 23

...συβµιτ αχοµπλετε

δεσχριπτιον οφοπερατιον οφτηε χοντρολσψστεµ,ινχλυδινγ

σεθυενχεσ οφοπερατιον...

Taken from the Pages of Div. 23

...τηε ωορδινγοφ τηε χοντρολσεθυενχεσ ιντηε συβµιτταλσηαλλ µατχηϖερβατιµ τηατινχλυδεδ ιν τηεχονστρυχτιονδοχυµεντσ...

What’s Wrong With That???

• Nothing!!!

• Engineering firm with in-house capabilities...

• ...generates an all-encompassing sequence.

• Controls contractor to bold any deviations.

• Establishes a dialog between consultantand controls contractor (at least it should)


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So What’s the Problem???

• Best of all worlds, but not always the case.

• More often (to some extent), the task is leftup to the controls contractor (for good or bad)

• + Forces the contractor to understand thedesign (mech. plans, details, schedules)

• + Allows the specifying engineer toevaluate the contractor’s comprehensionof the mechanical design intents.

Attributes of a Well-writtenSequence• Together with the Points List, very powerful

tool for specifying a DDC control system.

• Outlines the CE’s basic intentions on howthe mechanical systems should operate...

• How they were designed to operate...

• And how they should be controlled to meetthat end.

Attributes of a Well-writtenSequence (cont’d.)• Direct reflection of the mechanical design.

• Serves as the final record of systemoperation, to be included in the as-builts.

• Writing good sequences is an art; inaddition to knowing the in’s & out’s of thesystems design, need to be a good writer.

• Clear, concise, complete, & unambiguous.


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Shortcomings

• Inaccurate and/or incomplete

• Copy-paste of previously written sequences

• All equipment not accounted for

• Inclusion of equipment not part of the project

• Contradictions with the drawings

• Contradictions within the sequence itself

Shortcomings (cont’d.)

• Too long (too much detail)

• Too short (not enough detail)

• Mfg’s product spec occasionally filters in

• Subject to substantial change throughoutthe course of the project.

• Archaic spec items (pneumatic references)

Sequence Styles - Outline

1a. Programmed time-of-day schedulesignifies Occupied Mode.

1b. Rooftop unit supply fan starts and runscontinuously during Occupied Mode.

1c. Rooftop unit heating and cooling arecycled as required to maintain occupiedspace temperature setpoints.


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Sequence Styles - Paragraph

When the programmed time-of-dayschedule signifies Occupied Mode, therooftop unit’s supply fan starts and runscontinuously, and the heating & coolingare cycled as required to maintain theoccupied space temperature setpoints.

Sequence Styles – Short Form

Generator room exhaust fan to becontrolled by a space-mounted thermostat.

Sequence Styles – Long Form

Space-mounted thermostat controls theoperation of the generator room exhaustfan. Upon a rise in temperature above thesetpoint of the thermostat (80 degrees F.adj.), the exhaust fan is energized. Oncethe space temperature falls back belowsetpoint and through the differential of 2degrees f. (adj.), then the exhaust fan de-energizes.


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What a Sequence Should Do

• Get to the intents, first and foremost

• Include all equipment specific to the project

• Include safeties and limits

• Include alarm limits and failure modes

• Include scheduling (if possible)

What a Sequence Should Do

• Use accepted industry terms to specifycertain descriptions of control

• Strike a balance between what’sfundamentally important, and operationaldetails that will ‘come out in the wash’

• Pay attention to detail, so as not to leaveout important items, or leave ininapplicable content

What a Sequence Should Not Do

• Belabor basic control fundamentals, asalready accepted by the industry

• Detail the operational characteristics ofpackaged equipment

• Use exact setpoints (baselines areacceptable, e.g., 55 deg. F adj.)

• Include verbiage for equipment that is nota part of the project (diminishes credibility)


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Reading a Sequence

• Know how to read a sequence

• Know how to read between the lines

• How to get into the engineer’s head

• Thoroughly understand the mech systems...

• Selection parameters, performance criteria

• Align your interpretation of the sequencewith the engineer’s basic design intents

Writing a Sequence

• A form of art, a learned skill

• We in the HVAC biz typically are not writers

• A challenge that is not for everyone

• You can utilize an online service...

• Or you can develop your own...

• Account for equipment, generate framework

• Draw from libraries or past projects

Typicals, Customs, From Scratch

• Boilerplate descriptions of control (RTUs,VAVs, EFs...).

• One whose operation is generally thesame or similar in all applications.

• General descriptions that can be usedover and over again.

• Beware of copy-paste errors!


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• Larger and/or more complex systems andequipment.

• Common enough to where ‘base’descriptions can be created.

• Customized to fit the application at hand(boiler plant).


• No explanation needed!

Points & Points Lists

• A shift in importance from specifyingsequences, to specifying what componentsare required (pre-procurement).

• The old days (pre-DDC) vs. Nowadays...

• Sequences vs. Points Lists


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Points & Points Lists

• Point – term to describe a control operation.

• A sensing action or a controlling action.

• Point Count – tally of used inputs & outputs.

• Quantitative evaluation of a given system.

• Hardwired vs. virtual vs. network points

• Points List – specifies the I/O required for agiven project.

Points List for an AHU

Points Lists - Summary

• Together with the sequence, points lists arean effective means of specifying a DDC job.

• Will give upwards of 90% of what’s requiredfor a project, control-wise.

• Still need to read the sequence, formiscellaneous items that probably shouldbe in the points list, but sometimes aren’t.


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3) Common Routines• Demand-based routines

• Time-based routines

• Miscellaneous routines

Demand Limiting

• Limit energy use

• Monitor demand for energy (in-house meter)

• Adjust quipment operation to limit demand

• Disable certain equipment, or...

• Let temperatures drift from setpoint.

Demand Response

• New concept, similar to Demand Limiting

• Takes adantage of the electrical market

• Automatically adjust electrical usage

• In response to an energy cost signal

• Enabled via the Internet


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Demand Controlled Ventilation

• Control the volume of outside air...

• Based on zone-level demand (CO2 sensing)

• CO2 is a human by-product

• Works well when there’s occupancy diversity

• Schools and theaters

Night Setback / Setup

• Unoccupied mode routine

• Change the setpoints based on time-of-day

• Heating goes to 60 degrees (setback)

• Cooling goes to 80 degrees (setup)

• Fan is set to cycle on calls for htg or clg

Morning Warmup

• Strategy reserved for VAV AHU (why???)

• Transition from unoccupied-occupied mode

• Bring on AHU in a full-heat capacity

• Revert to occ mode when RAT reaches 70

• For single zone equipment, no special MWUcycle is required. Unit will do it’s thing upontransition to the occupied mode.


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Optimal Start

• Built-in routine, employs a learning strategy

• Goal – reach setpoint precisely at occ mode

• Trends data over time (OAT, space temp...)

• Learns from this data, adjusts starting time

Timed Override

• Allows occupants to override unocc mode

• A ‘pushbutton’ at the zone level

• Turns on the VAV AHU, terminal unit

• Typically 1-3 hours, adjustable

• Useful in conjunction with Tenant Billingstrategies.

Economizer

• Utilize outdoor air for ‘free cooling’

• Cool & dry (accounts for OAT & OAH)

• Modulate OA/RA dampers to maintain 55

• When OAT<55, then economizer alone

• When OAT>55, then mechanical clg. also


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Automatic Alternation

• More than 1 piece of equip serves a system

• Primary/Backup sizing criterion...

• ...vs. Lead/Lag sizing criterion

• Even out the runtime hours of each

• Log runtime hours, switch over at intervals

• Applicable to pumps, boilers, chillers...

Reset

• Adjusting or resetting a setpoint of a process

• Based on the value of a related variable

• HW temp setpoint reset based on OAT

• DA temp setpoint reset based on OAT

• SA press stpt reset based on zone demand

• Better match to the HVAC load, improvedcontrollability, energy savings

4) Systems & Sequences• System Schematic

• Typical Sequence Items

• Some ‘Do’s & Don’t’s’


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Airside Systems

• Packaged Rooftop Units

• Fan Coil Units

• VAV Air Handling Units

• Terminal Units (VAV & Fan-powered Boxes)

Waterside Systems

• Pumping Systems

• Boiler Systems

• Chiller Systems

Packaged Rooftop Units


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Packaged Rooftop Units

Sequence Items

• Single zone control

• Heating/cooling of particular space

• Occupied/unoccupied modes of operation

• Constant fan during occupied modes

• Minimum position on outside air damper

• Intermittent fan during unoccupied modes

• OAD should stay shut during setback cycles

Do’s & Don’t’s (+ / -)

+ specify the type of control:

Thermostat (prog or non-programmable)

Microprocessor-based w/BACnet option

DDC by controls contractor

+ specify smoke detectors if/when required

- belabor the inner-workings of the package

- specify a freezestat (when DX cooling &gas heating, no need for it)


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Fan Coil Units

Sequence Items

• Single zone control

• Heating/cooling of particular space


• Modulating control of HW, CHW valves

• For 2-pipe units, changeover via ‘aquastat’

• Face/bypass dampers (OA + HW)

+ specify the type of control

+ specify freezestat, if bringing in outside air

- ‘halfway’ the responsibilities of who provides(stats/sensors, controllers, valves)

Self-contained

‘DDC ready’

No controls

Do’s & Don’t’s (+ / -)


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VAV Air Handling Units

VAV Air Handling Units

Sequence Items

• Maintain supply air temperature setpoint

• Maintain supply air pressure setpoint

• Maintain general space pressure setpoint


• Specialized sequences (Min OA Damper,Economizer, DCV, Reset, Humidification,Dehumidification...)


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+ specify failure modes

+ describe return fan operation

+ specify smoke evac modes (if applicable)

+ describe specialized sequences

- underestimate freeze protection!

Do’s & Don’t’s (+ / -)

Terminal Units

Terminal Units


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Sequence Items

• Reset of CFM setpoint based on space temp

• Primary air damper is modulated

• Minimum position, then heat & htg. position

• FP Parallel–intermittent fan, variable volume

• FP Series–constant fan, volume, var. temp

• Sequence perimeter baseboard withterminal unit heat

+ specify factory/field installation of controllers

+ specify DA sensors if there’s heat

+ use accepted terms (pressure independent)

+ describe unoccupied mode operation

- Belabor industry-accepted modes of op

- Specify more than one sensor per unit(opposite is ok, with a caviat or two)

Do’s & Don’t’s (+ / -)

Pumping Systems


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Sequence Items

• Two pumps, each sized at full capacity

• Primary/backup mode of operation

• Constant or variable volume

• If variable, controlled to maintain dp setpoint

• Typically ‘seasonal’ changeover

+ specify automatic alternation

- Specify a common flow or dp switch forstatus

Do’s & Don’t’s (+ / -)

Boiler Systems


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Sequence Items

• Maintain HW temperature setpoint

• Boilers are sequenced to meet demand

• Boilers operate from their own controls...

• ...or staged/modulated via external control(manufactured boiler sequencer panel, orDDC control)

+ specify reset control (condensing boilers)

+ specify communication if possible

+ prove CA before allowing boiler operation

- specify unoccupied shutdown for boilers

- turn off pumps till boilers cool down (delay)

- specify boiler sequencer panel...if BAS(unless the panel has comm. option)

Do’s & Don’t’s (+ / -)

Chiller Systems


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Sequence Items

• Maintain CHW temperature setpoint

• Chillers are sequenced to meet demand

• Chillers operate from their own controls

+ specify setpoint signal from BAS

+ specify communication if possible

- Vary the flow through the chillers, belowmanufacturer’s recommended minimum

Do’s & Don’t’s (+ / -)

5) Control Devices• Sensors & transmitters

• Switches & two-state devices

• Safeties & limits

• End devices


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OAT Sensors

• Northern exposure

• Out of the direct sunlight

• Away from rainwater accumulation...

• Temperature+humidity variety (good idea!)

Duct & Pipe Temp Sensors

• Probes – general purpose duct sensors

• Probes - 8-inch is good for terminal units

• Averaging – ducts over 4 feet wide

• Averaging – 1 foot of sensor per sq. ft. duct

• Averaging – MA sections, stratification

• Immersion – 2-inch & 4-inch

• Strap-on – specialty situations

Space Temperature Sensors

• Variety of styles – vertical mounting typical

• Decorative or low-profile?

• Visual indication of temperature/setpoint?

• Occupant adjustability?

• Override pushbutton?

• Tamper-proof plastic guard or cage guard?


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Humidity & CO2 Transmitters

• OAH sensor needed for enthalpy calculation

• OA CO2 sensor, as reference for DCV

• RA humidity, good indicator of average

• RA CO2, good indicator of average, however

• ...Individual space CO2 sensors may berequired for ‘true’ DCV

Duct Pressure Transmitters

• Conventional Wisdom: ‘2/3 down trunkline’

• Reality: depends on the geometry

• Fairly true for straight duct runouts

• For looped systems, location is more arbitrary

• For larger systems, multiple sensing points

• Range: 0-2 inches W.C. (or greater)

Pipe Pressure Transmitters

• Convensional Wisdom: ‘end of furthest run’

• Reality: other factors come into play

• Strike a balance between practicality and...

• ...what’s ‘theoretically correct’

• Range: 0-20 psi (or greater)


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Airflow Measuring Stations

• ASHRAE 62.1, LEED credits

• From the simple to the sophisticated!

• Pitot tube and differential pressure xmitter

• Multiple probe approach (larger ductwork)

• Fan inlet probes (pair of probes per inlet)

• Thermal dispersion technology

Flow & BTU Meters

• Flow meter + pair of temp sensors for BTU

• Perform the calculation within the BAS...

• ...or purchase a complete BTU meter

• Receive BTU/flow data via discreet outputs...

• ...or receive this data via BACnet connection

• Minimum upstream/downstream distancerequirements for flow meters (important)

Status for Fans & Pumps

• Fan status–historically a dp or ‘sail’ switch

• Pump status–historically a dp or ‘flow’ switch

• Nowadays - take status of the motor

• Use a current sensing switch

• Reliable, easy to install, easy to set up

• Traditional methods still have their place


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Filter Status

• Monitor pressure drop across a filter bank

• Two-state dp switch – closes when dirty

• Inexpensive, tricky to functionally implement

• Dp transmitter – provides a variable reading

• More costly, yet more intuitive in practice

Safeties & Limits (AHUs)

• Low limit temperature controller

• High limit discharge static pressure switch

• Low limit suction static pressure switch

• Duct smoke detector(s)

• Setpoint or ‘trip’ point adjustment

• Manual reset

Safeties & Limits


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Safeties & Limits

• Typically single-pole devices

• Each to control a DPDT relay

• One contact for hardwired shutdown

• The other contact to report status to the BAS

Safeties & Limits – Failsafe Modes

• OAD/EAD spring closed, RAD springs open

• CHWV springs closed, HWV springs open

• Supply and return fans shut down

• Coil circ pump on (configuration permitting)

• For VFDs – limits need to be wired intoboth ‘drive’ & ‘bypass’ modes

• Limits wired into both ‘Hand’ & ‘Auto’ modes

• Face closed, bypass open, steam valve open

End Devices

• Motorized dampers (damper + actuator)

• One damper for every xx square foot

• Globe valves – traditional valve of choice

• Ball valves – much more popular

• Butterfly valves – for large pipe (>6 inches)

• VFDs – control by sending analog speedreference signal, start/stop command


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Summarizing

• History & background of control systems

• Control system specifications

• Sequences of operation

• Points & points lists

• Common control routines

• Common air & water systems & sequences

• Controls devices

Questions???

[email protected]

Sequences & Control Schemes for Air & Water Systems

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Transcript of Sequences & Control Schemes for Air & Water Systems