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
4 Indiana Building Green Symposium
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
5 Indiana Building Green Symposium
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)
6 Indiana Building Green Symposium
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
7 Indiana Building Green Symposium
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)
8 Indiana Building Green Symposium
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
9 Indiana Building Green Symposium
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
11 Indiana Building Green Symposium
potential reduction in duct size… 55°F supply air (10000 cfm) vs. 48°F supply air (7400 cfm)
12 Indiana Building Green Symposium
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?
13 Indiana Building Green Symposium
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
14 Indiana Building Green Symposium
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
15 Indiana Building Green Symposium
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.
16 Indiana Building Green Symposium
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
17 Indiana Building Green Symposium
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
19 Indiana Building Green Symposium
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
20 Indiana Building Green Symposium
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
22 Indiana Building Green Symposium
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
23 Indiana Building Green Symposium
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
24 Indiana Building Green Symposium
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
25 Indiana Building Green Symposium
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
26 Indiana Building Green Symposium
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
27 Indiana Building Green Symposium
Traditional VAV Fan Control
supply fan
VFD
28 Indiana Building Green Symposium
P
VAV boxes static pressure sensor
Fan-Pressure Optimization static pressure sensor supply fan
P
VAV boxes
VFD
with DDC controllers
BAS
29 Indiana Building Green Symposium
fan-pressure optimization
Part-Load Energy Savings
static pressure
surge
duct static pressure control
fan-pressure optimization airflow
30 Indiana Building Green Symposium
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!
31 Indiana Building Green Symposium
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.
32 Indiana Building Green Symposium
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
33 Indiana Building Green Symposium
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
34 Indiana Building Green Symposium
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
35 Indiana Building Green Symposium
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!
36 Indiana Building Green Symposium
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
37 Indiana Building Green Symposium
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
38 Indiana Building Green Symposium
High Performance VAV System
• • • •
Reduced energy Reduced materials of construction and first cost Improved comfort Lower sound
39 Indiana Building Green Symposium
Questions
40 Indiana Building Green Symposium