High Performance Chilled Water VAV Systems, An Unconventional Look at System Design

High Performance Chilled Water VAV Systems, An Unconventional Look at System Design Brian Fiegen Systems Engineering Manager Trane La Crosse, Wisconsi...
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High Performance Chilled Water VAV Systems, An Unconventional Look at System Design Brian Fiegen Systems Engineering Manager Trane La Crosse, Wisconsin Shane Labuzan Account Manager Trane Central Indiana District Indianapolis, Indiana March 2010

ASHRAE 90.1 Moves Toward Net-Zero Building Stock Median

Building EQ™ (EUIbuilding/EUImedian)

100

ASHRAE 90.1-1999 80

60

ASHRAE 90.1-2004 ASHRAE 90.1-2007 ASHRAE 90.1-2010?

40

20

Net Zero 2 Indiana Building Green Symposium

LEED 2009

LEED 2.2

LEED 2.1

“Golden Rule” of Reducing HVAC Energy Use First, reduce the load. •  Glazing: Avoid glazing which faces east or west, shade exterior glazing, use insulating low-e glass, and make all glazing as small as possible (consistent with use of daylighting) •  Daylighting/Lighting: Design envelope and glazing so the sun provides interior lighting at perimeter, and design efficient supplemental interior lighting that modulates when not needed •  Envelope: Design and construct exterior enclosure to be as airtight as possible 3 Indiana Building Green Symposium

high performance chilled water VAV systems

Agenda

•  Cold air systems –  Benefits –  Common concerns

•  Optimized VAV system controls •  Energy performance comparison

Chilled Water VAV systems

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Lower Supply-Air Temperature Benefits •  Reduces supply airflow –  Less supply fan energy and less fan heat gain –  Smaller fans, air handlers, VAV terminals, and ductwork

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SA Temperature vs. Airflow space sensible supply cooling = 1.085 × airflow × (Tspace – Tsupply) load same

100% cfm

(75°F – 55°F)

same

80% cfm

(75°F – 50°F)

same

67% cfm

(75°F – 45°F)

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Lower Supply-Air Temperature Benefits •  Reduces supply airflow –  Less supply fan energy and less fan heat gain –  Smaller fans, air handlers, VAV terminals, and ductwork •  Can reduce HVAC installed cost •  Can reduce building construction cost

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lower supply-air temperature

Can Reduce HVAC Installed Cost •  Lowering supply-air temperature from 55°F to 48°F reduces supply airflow (cfm) by 26% –  Ducts can be smaller –  VAV terminal units can be smaller –  Diffusers can be smaller –  Air-handling units can be smaller (plus smaller electrical service and VFD’s)

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example

HVAC Installed Cost Savings •  Twelve-story office building in Atlanta, GA (30,000 ft2 per floor) •  One VAV air-handling unit per floor –  Base design: –  Alternate design:

55°F supply-air temperature 48°F supply-air temperature

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example

Air-Handling Unit Selections cfm

size

ESP

TSP

bhp

motor HP

MBh (total)

Base

25,600

50

3.5 in.

4.21 in.

28.4

30

919

Alternate

20,000

40

3.5 in.

4.97 in.

22.2

25

961

•  AHU equipment costs (12 units, including VFDs) –  Base = $204,962 –  Alternate = $167,345

($38,000 savings, or $0.11/ft2)

•  If ductwork and VAV boxes are downsized also: –  Less sheet metal, insulation, and labor = $50,370 ($0.14/ft2) –  Smaller VAV terminals (300 units) = $7,800 ($0.02/ft2) –  Total HVAC cost savings = $96,170 ($0.27/ft2) 10 Indiana Building Green Symposium

lower supply-air temperature

Can Reduce Building Cost •  Smaller indoor air-handling units can allow for smaller equipment rooms and more usable floor space •  Smaller ductwork can allow for a shorter floor-to-floor height, reducing the cost of building materials and labor

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potential reduction in duct size… 55°F supply air (10000 cfm) vs. 48°F supply air (7400 cfm)

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concrete slab floor

55°F supply air

48°F supply air 5 in.

ceiling

What if you could save 5 in. per floor, in a 30-story building? What if you could save 5 in. per floor, in a 3-story building?

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Lower Supply-Air Temperature Benefits •  Reduces supply airflow –  Less supply fan energy and less fan heat gain –  Smaller fans, air handlers, VAV terminals, and ductwork •  Can reduce HVAC installed cost •  Can reduce building construction cost •  Improves occupant comfort –  Lowers indoor humidity levels –  Lowers indoor sound levels

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48°F SA

OA

84°F DB 76°F DP

84°F DB 76°F DP

RA

75°F DB 57% RH

75°F DB 49% RH

MA

79°F DB

81°F DB

SA

55°F DB

180 160

80

OA

48°F DB

(900 cfm)

(670 cfm)

140 120

70

MA 50 30

30

40

40

SA

100 80

60

RA

60 40

SA 50

20 60 70 80 dry-bulb temperature, °F

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90

100

110

humidity ratio, grains/lb of dry air

55°F SA

Lower Indoor Humidity Levels •  Conventional system (55°F supply air)

•  Low-temperature system (45°F to 50°F supply air)

•  Indoor humidity levels of 55% to 60%

•  Indoor humidity levels of 45% to 50%

Lower humidity improves occupant comfort, which can increase employee productivity and student alertness.

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Lower Supply-Air Temperature Common concerns •  Increases reheat energy, reduced economizer savings •  Minimize comfort problems due to cold air “dumping” •  Avoid condensation on air distribution system components

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lower supply-air temperature

Maximize Energy Savings •  Use supply-air-temperature reset (ex: from 48°F to 55°F) during mild weather –  Reduces reheat energy use –  Recovers lost economizer savings

•  Raise space setpoint by 1°F or 2°F –  Lower indoor humidity often allows zone dry-bulb temperature to be slightly warmer –  Further reduces supply airflow and fan energy use

•  Keep same size ductwork –  Further reduces fan energy use –  Allows SAT reset in systems that serve zones with near-constant cooling loads –  Capable of delivering more airflow, if loads increase in future 18 Indiana Building Green Symposium

Supply-Air-Temperature Reset •  Benefits –  Decreases mechanical cooling –  Increases economizing –  Decreases reheat energy

•  Drawbacks –  Increases fan energy –  Raises indoor humidity levels

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SA temperature reset

SA temperature setpoint, °F

Example #1: OA Temperature 60 58 56 54 52 50 48 45

50

55

60

65

70

outdoor dry-bulb temperature, °F

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75

lower supply-air temperature

Minimizing Comfort Problems •  Use linear slot diffusers…

“dumping”

linear slot diffuser

conventional concentric diffuser

…and supply-air-temperature reset (example: from 48°F back up to 55°F) 21 Indiana Building Green Symposium

lower supply-air temperature

Avoiding Condensation •  Properly insulate and vapor-seal ductwork, VAV terminals, and supply-air diffusers

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surface temperatures on duct insulation (wrapped metal duct) • 44°F supply air (Trane district office in Dallas, TX) • fully-ducted return air path (85°F dry bulb above ceiling)

trunk duct (2 in. insulation) outer surface temp = 82°F branch duct (1 in. insulation) outer surface temp = 77°F

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lower supply-air temperature

Avoiding Condensation •  Properly insulate and vapor-seal •  •  •  • 

ductwork, VAV terminals, and supply-air diffusers Maintain positive building pressure to minimize infiltration of humid outdoor air Use linear slot diffusers to increase air motion Monitor indoor humidity during unoccupied periods and prevent it from rising too high During startup, slowly ramp down the supply-air temperature to gradually lower indoor humidity

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examples

Humidity Pull-Down Sequences •  SAT ramp-down schedule supply airflow limit

supply-air temperature

2 hours before occupancy

40% of design

55°F

1 hour before occupancy

65% of design

51°F

no limit

48°F

Scheduled occupancy

or

•  SAT ramp-down based on indoor dew point –  SAT = current indoor dew point – 3°F

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Source: ASHRAE Cold Air Distribution System Design Guide (pp 138-140)

summary

Lower Supply-Air Temperature Benefits •  Reduces supply airflow –  Less supply fan energy and less fan heat gain –  Smaller fans, air handlers, VAV terminals, and ductwork •  Can reduce HVAC installed cost •  Can reduce building construction cost •  Improves occupant comfort –  Lowers indoor humidity levels –  Lowers indoor sound levels

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Optimized VAV System Controls •  •  •  • 

Supply-air-temperature reset Optimal start/stop Fan-pressure optimization Ventilation optimization –  Demand-controlled ventilation at zone level –  Ventilation reset at system level

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Traditional VAV Fan Control

supply fan

VFD

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P

VAV boxes static pressure sensor

Fan-Pressure Optimization static pressure sensor supply fan

P

VAV boxes

VFD

with DDC controllers

BAS

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fan-pressure optimization

Part-Load Energy Savings

static pressure

surge

duct static pressure control

fan-pressure optimization airflow

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fan-pressure optimization

Benefits •  •  •  •  • 

Part-load energy savings Lower sound levels Reduced risk of fan surge Less duct leakage Factory-installation and -commissioning of duct pressure sensor •  Operator feedback to "tune the system"

•  Typical applications: any VAV system!

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Required by ASHRAE 90.1 Since 1999

6.5.3.2.3 Setpoint Reset. For systems with DDC of individual zone boxes reporting to the central control panel, static pressure setpoint shall be reset based on the zone requiring the most pressure; i.e., the setpoint is reset lower until one zone damper is nearly wide open.

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demand-controlled ventilation

CO2 Sensor in Every Zone??

lounge

BAS

rest room

storage

office

CO2

CO2

corridor

CO2 reception area

elevators

vestibule

CO2 CO2 office

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CO2 conference rm

computer room

ventilation optimization

Zone Level: DCV BAS

lounge

rest room

storage

office

CO2

OCC

corridor

TOD reception area

elevators

vestibule

TOD OCC office

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CO2 conference rm

computer room

ventilation optimization

System Level: Ventilation Reset air-handling unit with flow-measuring dampers •  Reset outdoor airflow

SA

RA

CO2

TOD

BAS •  New OA setpoint …per ASHRAE 62

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CO2

OCC

TOD

OCC

DDC/VAV controllers •  Required ventilation (TOD, OCC, CO2) •  Actual primary airflow (flow ring) •  Calculate Vent Ratio

ventilation optimization

Benefits

•  Saves energy during partial occupancy •  Lower installed cost, less maintenance, and more reliable than installing a CO2 sensor in every zone –  Use zone-level DCV approaches where they best fit (CO2 sensor, occupancy sensor, time-of-day schedule) –  Combine with ventilation reset at the system level

•  Earn LEED EQc1: Outdoor Air Delivery Monitoring •  Typical applications: any VAV system!

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Example TRACE® 700 Analysis High Performance VAV system •  48 F supply air •  Optimal start •  Fan-pressure optimization •  SA temperature reset •  Ventilation optimization –  DCV at zone level –  Ventilation reset at system level

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Annual Building Energy Use, kBtu/yr

12,000,000

Houston

Los Angeles

10,000,000

Philadelphia

St. Louis

Pumps Fans Heating

8,000,000

Cooling Plug Loads Lighting

6,000,000

4,000,000

2,000,000

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High Performance VAV System

•  •  •  • 

Reduced energy Reduced materials of construction and first cost Improved comfort Lower sound

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Questions

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