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 100 Building Stock Median Building EQ (EUI building /EUI median ) 80 60 40 20 ASHRAE 90.1-1999 ASHRAE 90.1-2004 ASHRAE 90.1-2007 ASHRAE 90.1-2010? LEED 2009 LEED 2.2 LEED 2.1 Net Zero 2
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
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
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
SA Temperature vs. Airflow space sensible cooling load = 1.085 supply airflow (T space T supply ) same same same 100% cfm 80% cfm 67% cfm (75 F 55 F) (75 F 50 F) (75 F 45 F) 6
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
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
example HVAC Installed Cost Savings Twelve-story office building in Atlanta, GA (30,000 ft 2 per floor) One VAV air-handling unit per floor Base design: 55 F supply-air temperature Alternate design: 48 F supply-air temperature 9
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/ft 2 ) If ductwork and VAV boxes are downsized also: Less sheet metal, insulation, and labor = $50,370 ($0.14/ft 2 ) Smaller VAV terminals (300 units) = $7,800 ($0.02/ft 2 ) Total HVAC cost savings = $96,170 ($0.27/ft 2 ) 10
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
potential reduction in duct size 55 F supply air (10000 cfm) vs. 48 F supply air (7400 cfm) 12
concrete slab floor 55 F supply air 48 F supply air ceiling 5 in. 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
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
55 F SA 48 F SA OA 84 F DB 76 F DP 84 F DB 76 F DP RA MA SA 75 F DB 57% RH 79 F DB 55 F DB (900 cfm) 30 40 75 F DB 49% RH 81 F DB 48 F DB (670 cfm) 50 SA SA 60 70 80 OA MA RA 180 160 140 120 100 80 60 40 20 humidity ratio, grains/lb of dry air 30 40 50 60 70 80 90 100 dry-bulb temperature, F 110 15
Lower Indoor Humidity Levels Conventional system (55 F supply air) Indoor humidity levels of 55% to 60% Low-temperature system (45 F to 50 F supply air) Indoor humidity levels of 45% to 50% Lower humidity improves occupant comfort, which can increase employee productivity and student alertness. 16
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
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
Supply-Air-Temperature Reset Benefits Decreases mechanical cooling Increases economizing Decreases reheat energy Drawbacks Increases fan energy Raises indoor humidity levels 19
SA temperature reset Example #1: OA Temperature SA temperature setpoint, F 60 58 56 54 52 50 48 45 50 55 60 65 70 75 outdoor dry-bulb temperature, F 20
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
lower supply-air temperature Avoiding Condensation Properly insulate and vapor-seal ductwork, VAV terminals, and supply-air diffusers 22
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
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
examples Humidity Pull-Down Sequences SAT ramp-down schedule 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 SAT ramp-down based on indoor dew point SAT = current indoor dew point 3 F 25 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
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
Traditional VAV Fan Control supply fan P VFD static pressure sensor VAV boxes 28
Fan-Pressure Optimization supply fan static pressure sensor P VFD VAV boxes with DDC controllers BAS 29
fan-pressure optimization Part-Load Energy Savings surge static pressure duct static pressure control fan-pressure optimization airflow 30
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
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
demand-controlled ventilation CO 2 Sensor in Every Zone?? lounge rest room BAS storage office CO 2 CO 2 vestibule corridor reception area CO 2 CO 2 elevators CO 2 CO 2 office conference rm computer room 33
ventilation optimization Zone Level: DCV BAS lounge rest room storage office CO 2 OCC vestibule corridor TOD reception area elevators TOD OCC CO 2 office conference rm computer room 34
ventilation optimization System Level: Ventilation Reset air-handling unit with flow-measuring dampers Reset outdoor airflow SA RA CO 2 TOD CO 2 OCC TOD OCC BAS New OA setpoint per ASHRAE 62 DDC/VAV controllers Required ventilation (TOD, OCC, CO 2 ) Actual primary airflow (flow ring) Calculate Vent Ratio 35
ventilation optimization Benefits Saves energy during partial occupancy Lower installed cost, less maintenance, and more reliable than installing a CO 2 sensor in every zone Use zone-level DCV approaches where they best fit (CO 2 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
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
Annual Building Energy Use, kbtu/yr 12,000,000 10,000,000 8,000,000 6,000,000 4,000,000 2,000,000 Houston Los Angeles Philadelphia St. Louis Pumps Fans Heating Cooling Plug Loads Lighting 38
High Performance VAV System Reduced energy Reduced materials of construction and first cost Improved comfort Lower sound 39
Questions 40