Sequences & Control Schemes for Air & Water Systems

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Sequences & Control Schemes for Air & Water Systems

  1. 1. Sequences and Control Schemes for Air and Water Systems Presented to the Illinois Chapter ASHRAE Tuesday, March 9, 2010 By Steve Calabrese, BSEE Control Engineering Corp.
  2. 2. Sequences & Control Schemes for Air & Water Systems March 9, 2010 • Item 1... • Mechanical Contractors...? • Consultants...? • Controls Contractors...? • Electrical Contractors...! Background & Experience ‘How The Heck Did I Get Here?’ 1
  3. 3. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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) 2
  4. 4. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 to change 3
  5. 5. Sequences & Control Schemes for Air & Water Systems March 9, 2010 Direct Digital Control • Microprocessor-based • Expandable I/O • Configurable / Programmable • Extremely flexible • De-centralized (distributed logic) • Networkable And So It Was Written... ΔΔΧ Χλοσεδ λοοπ χοντρολ οφ α χονδιτιον αππλιεδ διρεχτλψ ατ αππλιχατιον υσινγ α διγιταλ λοοπ ωιτη of DDC ASHRAE Definition φεεδβαχκ το σενσε χηανγεσ ιν χονδιτιον. Components of DDC • Closed Loop Control – controlling a process, and at the same time sensing the process. • Digital Loop – microprocessor-based control loop. • Feedback – from above, the sensing of the process, in order to determine subsequent actions to be taken on the process. 4
  6. 6. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 among de-centralized controllers. Analogy of a Control Process 5
  7. 7. Sequences & Control Schemes for Air & Water Systems March 9, 2010 Block Diagram of a Controller Block Diagram-Digital Controller Equipment Level Digital Controller 6
  8. 8. Sequences & Control Schemes for Air & Water Systems March 9, 2010 Wiring Diagram Points List Logic Diagram 7
  9. 9. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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??? 8
  10. 10. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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... 9
  11. 11. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 10
  12. 12. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 to bid the project. • Provide the technical information required to design the project. • Provide the functional information required to operate the installed systems. 11
  13. 13. Sequences & Control Schemes for Air & Water Systems March 9, 2010 Taken from the Pages of Div. 23 ...συβµιτ α χοµπλετε δεσχριπτιον οφ οπερατιον οφ τηε χοντρολ σψστεµ, ινχλυδινγ σεθυενχεσ οφ οπερατιον... Taken from the Pages of Div. 23 ...τηε ωορδινγ οφ τηε χοντρολ σεθυενχεσ ιν τηε συβµιτταλ σηαλλ µατχη ϖερβατιµ τηατ ινχλυδεδ ιν τηε χονστρυχτιον δοχυµεντσ... 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 consultant and controls contractor (at least it should) 12
  14. 14. Sequences & Control Schemes for Air & Water Systems March 9, 2010 So What’s the Problem??? • Best of all worlds, but not always the case. • More often (to some extent), the task is left up to the controls contractor (for good or bad) • + Forces the contractor to understand the design (mech. plans, details, schedules) • + Allows the specifying engineer to evaluate the contractor’s comprehension of the mechanical design intents. Attributes of a Well-written Sequence • Together with the Points List, very powerful tool for specifying a DDC control system. • Outlines the CE’s basic intentions on how the mechanical systems should operate... • How they were designed to operate... • And how they should be controlled to meet that end. Attributes of a Well-written Sequence (cont’d.) • Direct reflection of the mechanical design. • Clear, concise, complete, & unambiguous. • Serves as the final record of system operation, to be included in the as-builts. • Writing good sequences is an art; in addition to knowing the in’s & out’s of the systems design, need to be a good writer. 13
  15. 15. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 • Archaic spec items (pneumatic references) • Subject to substantial change throughout the course of the project. Sequence Styles - Outline 1a. Programmed time-of-day schedule signifies Occupied Mode. 1b. Rooftop unit supply fan starts and runs continuously during Occupied Mode. 1c. Rooftop unit heating and cooling are cycled as required to maintain occupied space temperature setpoints. 14
  16. 16. Sequences & Control Schemes for Air & Water Systems March 9, 2010 Sequence Styles - Paragraph When the programmed time-of-day schedule signifies Occupied Mode, the rooftop unit’s supply fan starts and runs continuously, and the heating & cooling are cycled as required to maintain the occupied space temperature setpoints. Sequence Styles – Short Form Generator room exhaust fan to be controlled by a space-mounted thermostat. Sequence Styles – Long Form Space-mounted thermostat controls the operation of the generator room exhaust fan. Upon a rise in temperature above the setpoint of the thermostat (80 degrees F. adj.), the exhaust fan is energized. Once the space temperature falls back below setpoint and through the differential of 2 degrees f. (adj.), then the exhaust fan de- energizes. 15
  17. 17. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 specify certain descriptions of control • Strike a balance between what’s fundamentally important, and operational details that will ‘come out in the wash’ • Pay attention to detail, so as not to leave out important items, or leave in inapplicable content What a Sequence Should Not Do • Belabor basic control fundamentals, as already accepted by the industry • Detail the operational characteristics of packaged equipment • Use exact setpoints (baselines are acceptable, e.g., 55 deg. F adj.) • Include verbiage for equipment that is not a part of the project (diminishes credibility) 16
  18. 18. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 sequence with 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 the same or similar in all applications. • General descriptions that can be used over and over again. • Beware of copy-paste errors! 17
  19. 19. Sequences & Control Schemes for Air & Water Systems March 9, 2010 Typicals, Customs, From Scratch • Larger and/or more complex systems and equipment. • Common enough to where ‘base’ descriptions can be created. • Customized to fit the application at hand (boiler plant). Typicals, Customs, From Scratch • No explanation needed! Points & Points Lists • A shift in importance from specifying sequences, to specifying what components are required (pre-procurement). • The old days (pre-DDC) vs. Nowadays... • Sequences vs. Points Lists 18
  20. 20. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 a given project. Points List for an AHU Points Lists - Summary • Together with the sequence, points lists are an effective means of specifying a DDC job. • Will give upwards of 90% of what’s required for a project, control-wise. • Still need to read the sequence, for miscellaneous items that probably should be in the points list, but sometimes aren’t. 19
  21. 21. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 20
  22. 22. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 MWU cycle is required. Unit will do it’s thing upon transition to the occupied mode. 21
  23. 23. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 Billing strategies. 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 22
  24. 24. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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, improved controllability, energy savings 4) Systems & Sequences • System Schematic • Typical Sequence Items • Some ‘Do’s & Don’t’s’ 23
  25. 25. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 24
  26. 26. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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) 25
  27. 27. Sequences & Control Schemes for Air & Water Systems March 9, 2010 Fan Coil Units Sequence Items • Single zone control • Heating/cooling of particular space • Occupied/unoccupied modes of operation • Modulating control of HW, CHW valves • For 2-pipe units, changeover via ‘aquastat’ • Face/bypass dampers (OA + HW) Do’s & Don’t’s (+ / -) + specify the type of control Self-contained ‘DDC ready’ No controls + specify freezestat, if bringing in outside air - ‘halfway’ the responsibilities of who provides (stats/sensors, controllers, valves) 26
  28. 28. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 • Occupied/unoccupied modes of operation • Specialized sequences (Min OA Damper, Economizer, DCV, Reset, Humidification, Dehumidification...) 27
  29. 29. Sequences & Control Schemes for Air & Water Systems March 9, 2010 Do’s & Don’t’s (+ / -) + specify failure modes + describe return fan operation + specify smoke evac modes (if applicable) + describe specialized sequences - underestimate freeze protection! Terminal Units Terminal Units 28
  30. 30. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 with terminal unit heat Do’s & Don’t’s (+ / -) + 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) Pumping Systems 29
  31. 31. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 Do’s & Don’t’s (+ / -) + specify automatic alternation - Specify a common flow or dp switch for status Boiler Systems 30
  32. 32. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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, or DDC control) Do’s & Don’t’s (+ / -) + 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) Chiller Systems 31
  33. 33. Sequences & Control Schemes for Air & Water Systems March 9, 2010 Sequence Items • Maintain CHW temperature setpoint • Chillers are sequenced to meet demand • Chillers operate from their own controls Do’s & Don’t’s (+ / -) + specify setpoint signal from BAS + specify communication if possible - Vary the flow through the chillers, below manufacturer’s recommended minimum 5) Control Devices • Sensors & transmitters • Switches & two-state devices • Safeties & limits • End devices 32
  34. 34. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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? 33
  35. 35. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 be required 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) 34
  36. 36. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 distance requirements 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 35
  37. 37. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 36
  38. 38. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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 • Face closed, bypass open, steam valve open • Coil circ pump on (configuration permitting) • Supply and return fans shut down • Limits wired into both ‘Hand’ & ‘Auto’ modes • For VFDs – limits need to be wired into both ‘drive’ & ‘bypass’ modes 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 speed reference signal, start/stop command 37
  39. 39. Sequences & Control Schemes for Air & Water Systems March 9, 2010 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??? scalabrese@controlengineering.net 38

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