High Performance Chilled Water VAV Systems, An ... Performance Chilled Water VAV Systems.pdf · 3...

March 2010

Brian Fiegen Systems Engineering Manager Trane La Crosse, Wisconsin

Shane Labuzan Account Manager Trane Central Indiana District Indianapolis, Indiana

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

Indiana Building Green Symposium 2

ASHRAE 90.1 Moves Toward Net-Zero

100

80

60

40

20

Net Zero

Bui

ldin

g EQ

™ (E

UI bu

ildin

g/EU

I med

ian)

LEED 2.1

LEED 2.2 LEED

2009

Building Stock Median

ASHRAE 90.1-2007

ASHRAE 90.1-2010?

ASHRAE 90.1-2004

ASHRAE 90.1-1999


“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


high performance chilled water VAV systems Agenda

• Cold air systems – Benefits – Common concerns

• Optimized VAV system controls • Energy performance comparison

Chilled Water VAV systems


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


SA Temperature vs. Airflow

= 1.085 ×

space sensible cooling

load

supply airflow × (Tspace – Tsupply)

same (75°F – 55°F) 100% cfm

(75°F – 50°F) 80% cfm

same

(75°F – 45°F) 67% cfm

same






• Can reduce HVAC installed cost • Can reduce building construction cost


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)


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: 55°F supply-air temperature – Alternate design: 48°F supply-air temperature


example Air-Handling Unit Selections

• 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)

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


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


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


55°F supply air 48°F supply air

concrete slab floor

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?

5 in. ceiling






• Can reduce HVAC installed cost • Can reduce building construction cost • Improves occupant comfort

– Lowers indoor humidity levels – Lowers indoor sound levels


180

160

140

120

100

80

60

40

20

humidity ratio, grains/lb of dry air

110 30 40 50 60 70 80 100 90 dry-bulb temperature, °F

80

70

50

40 30

60 SA

MA

OA

OA 48°F SA 84°F DB 76°F DP

RA 75°F DB 49% RH

MA 81°F DB

SA 48°F DB (670 cfm)

SA

RA

84°F DB 76°F DP

75°F DB 57% RH

79°F DB

55°F DB (900 cfm)

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.



Common concerns • Increases reheat energy, reduced

economizer savings • Minimize comfort problems

due to cold air “dumping” • Avoid condensation on air

distribution system components


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


Supply-Air-Temperature Reset

• Benefits – Decreases mechanical cooling – Increases economizing – Decreases reheat energy

• Drawbacks – Increases fan energy – Raises indoor humidity levels


SA temperature reset Example #1: OA Temperature

50 55 60 45 75 70 65

outdoor dry-bulb temperature, °F

SA

tem

pera

ture

setp

oin

t, °

F

60

58

56

54

52

50

48


lower supply-air temperature Minimizing Comfort Problems

• Use linear slot diffusers…

…and supply-air-temperature reset (example: from 48°F back up to 55°F)

linear slot diffuser

“dumping”

conventional concentric diffuser


lower supply-air temperature Avoiding Condensation

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


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


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


examples Humidity Pull-Down Sequences

• SAT ramp-down schedule

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

supply airflow supply-air limit temperature

2 hours before occupancy 40% of design 55°F

1 hour before occupancy 65% of design 51°F

Scheduled occupancy no limit 48°F

or

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


summary Lower Supply-Air Temperature




• Can reduce HVAC installed cost • Can reduce building construction cost • Improves occupant comfort

– Lowers indoor humidity levels – Lowers indoor sound levels


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


VAV boxes static

pressure sensor

Traditional VAV Fan Control

P supply

fan

VFD


VAV boxes

static pressure sensor

supply fan

Fan-Pressure Optimization

P

BAS

with DDC controllers

VFD


surge

fan-pressure optimization Part-Load Energy Savings

airflow

stati

c p

ress

ure

fan-pressure optimization

duct static pressure control


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!


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.

Required by ASHRAE 90.1 Since 1999


lounge rest room

storage office

office conference rm computer room reception area elev

ator

s vestibule corridor

demand-controlled ventilation CO2 Sensor in Every Zone??

CO2

CO2

CO2

CO2

CO2 CO2

BAS


lounge rest room

storage office

office conference rm computer room reception area elev

ator

s vestibule corridor

ventilation optimization Zone Level: DCV

CO2

CO2

OCC

OCC

TOD TOD

BAS


ventilation optimization System Level: Ventilation Reset

CO2 OCC

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

DDC/VAV controllers

SA RA

air-handling unit with flow-measuring dampers • Reset outdoor airflow

CO2 TOD TOD OCC

BAS • New OA setpoint

…per ASHRAE 62


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!


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


2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

An

nu

al B

uild

ing

En

erg

y U

se,

kB

tu/

yr

Pumps

Fans

Heating

Cooling

Plug Loads

Lighting

Houston Los Angeles Philadelphia St. Louis


High Performance VAV System

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


Questions

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