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© 2006 American Standard Inc. Tracer Summit User’s Group 5/22/2012 VSDs and their effect on HVAC system components.

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Presentation on theme: "© 2006 American Standard Inc. Tracer Summit User’s Group 5/22/2012 VSDs and their effect on HVAC system components."— Presentation transcript:

1 © 2006 American Standard Inc. Tracer Summit User’s Group 5/22/2012 VSDs and their effect on HVAC system components

2 e = mc² © 2006 American Standard Inc. speed of light299,792,458 m/s water2 m/s air10–20 m/s

3 500,000 lbs/hr air 600,000 lbs/hr water © 2006 American Standard Inc.

4 work = mass × resistance

5 © 2006 American Standard Inc. velocity path diameter path length fluid density friction factor gravitational constant Darcy-Weisbach Equation p = f L  V² D 2 g c

6 © 2006 American Standard Inc. Darcy-Weisbach Equation resistance  velocity²

7 © 2006 American Standard Inc. System Resistance return duct diffusers and grilles coil dampers supply duct

8 © 2006 American Standard Inc. Fan Laws cfm rpm p rpm²  cfm² hp rpm³

9 © 2006 American Standard Inc. Chiller Laws? resistance  velocity² resistance  “lift”

10 © 2006 American Standard Inc. Practical Application: Free Discharge Fans VSDs and their effect on system components

11 © 2006 American Standard Inc. Free Discharge System n Draw-through cooling tower n Propeller fan sump outdoor air louvers fill propeller fan

12 © 2006 American Standard Inc. cfm rpm p rpm² hp rpm³ General Fan Performance

13 © 2006 American Standard Inc. friction pressure fixed pressure airflow, % static pressure, % system performance Static Pressure system resistance

14 © 2006 American Standard Inc. system performance Free Discharge System airflow, % system resistance static pressure, %

15 © 2006 American Standard Inc. airflow, % fan performance curves Fan Speed (N) N1N1 N1N1 N2N2 N2N2 As fan speed varies … so does airflow volume static pressure, %

16 © 2006 American Standard Inc. fan performance curves Speed N vs. Efficiency  airflow, % N1N1 N1N1 11 22  1'  2' N2N2 N2N static pressure, %

17 © 2006 American Standard Inc. performance curves Fan and System airflow, % N1N1 N1N1  1' N2N2 N2N2 1 static pressure, %

18 © 2006 American Standard Inc. fan performance curves Cooling Tower airflow, % fan power, %

19 © 2006 American Standard Inc. fan performance curves Cooling Tower airflow, % fan and motor power, % potential energy savings 1-speed motor

20 © 2006 American Standard Inc. fan performance curves Cooling Tower airflow, % fan and motor power, % 1-speed motor 2- speed motor

21 © 2006 American Standard Inc. fan performance curves Cooling Tower airflow, % fan and motor power, % VSD

22 © 2006 American Standard Inc. cooling tower application Fan Energy Comparison source: Marley Technical Report H-001A Control strategyEnergy use factor 1-speed fan cycling (base) 100% kWh 2-speed fan cycling39% kWh variable-speed control19% kWh

23 © 2006 American Standard Inc. fan/tower performance curves Free Cooling at Low Load airflow, % fan and motor power, % tower capacity, % tower capacity “free” cooling 0

24 © 2006 American Standard Inc. free discharge fans Summary n Performance approximates the “cube of the speed” n Variable-speed drives (VSDs) are a great option for modulating capacity n When considering VSDs for chilled water plants, start at the cooling tower

25 © 2006 American Standard Inc. Practical Application: Ducted Indoor Fans VSDs and their effect on system components

26 © 2006 American Standard Inc. System Resistance airflow static pressure 3,500 cfm 2.0 in. wg

27 © 2006 American Standard Inc. System Resistance p = f L  V² D 2 g c

28 © 2006 American Standard Inc. 3,500 cfm 2.0 in. wg airflow static pressure System Resistance 2,000 cfm 0.65 in. wg system resistance curve

29 © 2006 American Standard Inc. some devices don’t obey the rules for System Resistance

30 © 2006 American Standard Inc. airflow static pressure system resistance curve valves closed valves open System Resistance

31 © 2006 American Standard Inc. airflow static pressure surge region design modulation range system resistance VAV System actual

32 © 2006 American Standard Inc. 1,100 rpm Fan Performance airflow static pressure wide-open airflow blocked-tight static pressure 1100 rpm

33 © 2006 American Standard Inc. airflow static pressure Fan Speed 1100 rpm 900 rpm 700 rpm 500 rpm

34 © 2006 American Standard Inc. airflow static pressure 1100 rpm 900 rpm 700 rpm 500 rpm VAV System system resistance fan speed

35 © 2006 American Standard Inc. T VAV System 900 cfm500 cfm T air valves System resistance changes as valves modulate …

36 © 2006 American Standard Inc. design airflow, % design power, % comparison of methods Fan Modulation BI fan with discharge dampers2 AF fan with inlet vanes 3 FC fan with discharge dampers 4 FC fan with inlet vanes 5 fan-speed control vaneaxial fan with variable-pitch blades61

37 © 2006 American Standard Inc. fan modulation Objectives n Produce adequate static pressure n Eliminate excess static pressure n Exploit diversity n Maximize energy savings at fan n Provide stable control n Keep everyone comfortable

38 © 2006 American Standard Inc. Static Pressure Control Insufficient static pressure? VAV box delivers too little airflow

39 © 2006 American Standard Inc. Static Pressure Control Excessive static pressure? Wasted energy Poor comfort control Poor acoustics

40 © 2006 American Standard Inc. Fan Control Loop supply fan static pressure sensor S controller

41 © 2006 American Standard Inc. VAV System Modulation airflow static pressure design system resistance actual VAV modulation curve 1100 rpm 800 rpm

42 © 2006 American Standard Inc. static pressure control Sensor at Fan Outlet S supply fan controller VAV boxes

43 © 2006 American Standard Inc. supply fan controller static pressure control Sensor Down 2/3 of Duct S VAV boxes

44 © 2006 American Standard Inc. static pressure control Sensor Down 3/4 of Duct supply fan controller S VAV boxes

45 © 2006 American Standard Inc. static pressure control Sensor Down 3/4 of Duct supply fan controller S VAV boxes

46 © 2006 American Standard Inc. optimized static pressure control Sensor at Fan Outlet supply fan speed or inlet vane position communicating BAS VAV damper positions static pressure S static pressure setpoint

47 © 2006 American Standard Inc. static pressure control methods Performance Comparison Control method Fan outlet Supply duct Optimized Airflow 24,000 cfm (full load) 18,000 cfm Full-load power 60% 100% 55% 43% Fan input power 13 hp 22 hp 12 hp 9.5 hp Fan static pressure 2.7 in. wg 2.1 in. wg 1.9 in. wg 1.5 in. wg

48 © 2006 American Standard Inc. System Demonstration VSDs and their effect on system components

49 © 2006 American Standard Inc. Practical Application: Pumping Water VSDs and their effect on system components

50 © 2006 American Standard Inc. why care about Pump Energy According to the DOE... n Pumps represent 5% of industrial energy consumption n Total cost of owning a pump is 90% energy consumption n Pump energy consumption generally can be reduced by as much as 20%

51 © 2006 American Standard Inc. pumping chilled water ASHRAE Requires variable chilled water flow if: n Total pump power exceeds 75 hp AND n System includes > 3 control valves

52 © 2006 American Standard Inc. pumping chilled water ASHRAE Requires 30% design wattage at 50% flow if: n Any variable-flow pump motor > 50 hp AND n Design head pressure > 100 ft Typical solution: Variable-speed drive

53 © 2006 American Standard Inc. chilled water system Variable Primary Flow gpm

54 1 2 chilled water system Full-Load Pressure Drop © 2006 American Standard Inc. tP control valve PP

55 chilled water system Full-Load Pressure Drop © 2006 American Standard Inc. triple-duty valve gpm

56 © 2006 American Standard Inc. water flow, % pump head, % pumping system Pump & System Curves pump system design point

57 © 2006 American Standard Inc. water flow, % pump head, % pump system design point pump characteristics Power pump power power

58 © 2006 American Standard Inc. variable-flow water system How Does It Unload? Depends on: n Chilled-water system curve n Pump curve n Pump control method u Ride the curve u Different pump sizes u Vary the speed

59 © 2006 American Standard Inc. water flow, % pump head, % pump system design point pump power pump characteristics Ride the Curve part-load point power

60 chilled water system Part Load: Ride the Curve © 2006 American Standard Inc. tP control valve coil flow gpm

61 © 2006 American Standard Inc. variable-flow water system How Does It Unload? Depends on: n Chilled water system curve n Pump curve n Pump control method u Ride the curve u Different pump sizes u Vary the speed safe zone

62 © 2006 American Standard Inc. Pump Characteristics water flow, % pump head, % pump system design point power pump power

63 © 2006 American Standard Inc. water flow, % system pump head, % pump design point power pump characteristics Different Pump Sizes part-load point pump power

64 © 2006 American Standard Inc. water flow, % system pump head, % pump power pump power pump characteristics Different Pump Sizes

65 © 2006 American Standard Inc. water flow, % system pump head, % power pump characteristics VFD = Different Pumps 1750 rpm 1488 rpm 1225 rpm

66 © 2006 American Standard Inc. variable-speed pump Control Methods Pressure control (P) at pump Pressure control (P) at end of system n Critical-valve pressure reset

67 gpm PP chilled water system Part Load: P at Pump © 2006 American Standard Inc. tP

68 © 2006 American Standard Inc. variable-speed pump Control Methods Pressure control (P) at pump Pressure control (P) at end of system n Critical-valve pressure reset

69 gpm chilled water system Part Load: P at End of System © 2006 American Standard Inc. PP tP PP

70 © 2006 American Standard Inc. variable-speed pump Control Methods n Pressure control at pump n Pressure control at end of system n Critical valve pressure reset

71 chilled water system Part Load: Critical Valve Reset © 2006 American Standard Inc gpm valve position tP P

72 water flow, % pump head excess pump pressure 120 dynamic pressure fixed pressure full load part load pump characteristics Part Load: Ride the Pump Curve © 2006 American Standard Inc.

73 pump characteristics Part Load: P Control at Pump water flow, % pump head excess pump pressure dynamic pressure fixed pressure all loads What the pump needs to produce the required system pressure determines minimum pump pressure, motor speed © 2006 American Standard Inc.

74 pump characteristics Part Load: P Control at End of System water flow, % pump head dynamic pressure fixed pressure © 2006 American Standard Inc. PP Pump P “slides” down the system curve …

75 water flow, % pump head pump pressure © 2006 American Standard Inc. pump characteristics Part Load: Critical Valve Reset Pump P constantly reset to lowest possible value … almost all pressure drop is dynamic previous system curve

76 © 2006 American Standard Inc. variable-flow pumping Summary n Energy savings depends on: u Pump selections u Fixed vs. frictional pressure components u Control strategy n Energy savings can approach “cube of speed” n Great application for variable-speed drives

77 © 2006 American Standard Inc. variable-flow condenser water Pump Speed n Determining minimum speed n How variable flow affects: u Pump u Cooling tower u Chiller n Controlling flow to improve system performance

78 © 2006 American Standard Inc. condenser water pump Minimum Speed Determinants: n Minimum condenser flow n Tower static lift n Minimum tower flow u Nozzle selection u Performance n Compare curve with cubic

79 © 2006 American Standard Inc. cooling tower Static Lift static lift

80 © 2006 American Standard Inc. cooling tower Water Distribution

81 © 2006 American Standard Inc. Example 1500 gpm system, 1770 rpm u Minimum flows: Chiller658 gpm Tower750 gpm u Tower static lift12.2 ft u Pump: Speed974 rpm Pump flow875 gpm

82 © 2006 American Standard Inc. variable condenser-water flow Effect on Pump capacity, gpm 0 20 pump head, ft rpm 1505 rpm 1239 rpm 974 rpm 81% 75% 83% 75% 60%

83 © 2006 American Standard Inc. operating dependencies Full Flow Tower design Condenser water temperature & flow Heat rejection Wet bulb Chiller design Condenser water temperature & flow Load

84 © 2006 American Standard Inc. variable condenser water flow Effect on Tower 70 tower entering water, °F condenser water flow, % % fan

85 © 2006 American Standard Inc. 100 chiller power, kW condenser water flow, % °F LCWT variable condenser water flow Effect on Chiller 84°F LCWT Conditions: 70% load 70°F WB Full-speed tower fan

86 © 2006 American Standard Inc. 100 component/system power, kW condenser water flow, % chiller 100 variable condenser water flow Effect on System 0 pump tower system

87 © 2006 American Standard Inc. reducing flow & fan speed Effect on Tower 70 tower entering water, °F condenser water flow, % % 80% 60% 50% fan speed conditions: 70% load 70°F WB

88 © 2006 American Standard Inc. reducing flow & fan speed Effect on System 100 system power, kW condenser water flow, % % 0 80%60%50% fan speed conditions: 70% load 70°F WB

89 © 2006 American Standard Inc. reducing flow & fan speed Effect on System 100 system power, kW condenser water flow, % % 80% conditions: 70% load 50°F WB 60% 50% fan speed

90 © 2006 American Standard Inc. reducing flow & fan speed Effect on System 100 system power, kW condenser water flow, % % conditions: 30% load 50°F WB fan speed 80% 60% 50%

91 © 2006 American Standard Inc. variable condenser water flow Summary Determine what savings, if any, are possible u Are pumps already low power? u Can reducing tower-fan speed achieve most of the savings?

92 © 2006 American Standard Inc. variable condenser water flow Summary If you decide to reduce flow: u Find minimum condenser- water flow rate u Examine system at various loads and wet-bulbs … keep chiller out of surge u Document the sequence of operation u Help commission the system

93 © 2006 American Standard Inc. variable condenser water flow Guidance n Can provide savings … u Finding proper operating points requires more time, more fine-tuning n Two-step process: 1Reduce design pump power 2Is variable condenser-water flow still warranted?

94 © 2006 American Standard Inc. Practical Application: How VSDs Affect Chillers VSDs and their effect on system components

95 © 2006 American Standard Inc. resistance  velocity² VSDs and Chiller Laws Variable-speed drives benefit centrifugal compressors in water chillers u Review “chiller laws” u Explore scientific cause-and-effect relationships u Maximize benefits resistance  “lift”

96 © 2006 American Standard Inc. VSD and centrifugal chillers A Simple Analogy accelerator (speed control of chiller motor) brake (inlet guide vanes for unloading)

97 © 2006 American Standard Inc. Motor runs at constant speed, regardless of load a simple analogy Constant-Speed Chiller Inlet guide vanes restrict refrigerant at off-design conditions

98 © 2006 American Standard Inc. a simple analogy Variable-Speed Chiller Based on load, motor speeds up or slows down

99 © 2006 American Standard Inc. VSDs and centrifugal chillers An Analogy In each case: n Energy is wasted n Mechanical wear-and-tear is increased

100 © 2006 American Standard Inc. Typical Centrifugal Chiller evaporator condenser compressor controller not shown: pressure-reducing device capacity- modulating device

101 © 2006 American Standard Inc. Impeller blade cold refrigerant vapor hot refrigerant vapor

102 © 2006 American Standard Inc. Multistage Compressor impellersinlet vanes

103 © 2006 American Standard Inc. Centrifugal Compressor impeller passage diffuser passage volute

104 © 2006 American Standard Inc. Lift versus Load lift lvg evaporator water lvg condenser water lift  P cnd – P evp lift  T lvg cnd – T lvg evp load  gpm × (T ent evp – T lvg evp ) 56°F 41°F 85°F 99°F 800 gpm load = 500 tons 2 gpm/ton 58°F (T)

105 compressor work © 2006 American Standard Inc. Compressor Work and Chiller Efficiency cooling capacity/“load” head/“lift” lvg evap water lvg cond water 500 tons 58°F

106 refrigerant flow rate © 2006 American Standard Inc. Impeller Dynamics diameter V t  rpm × diameter V r  refrig flow rate rotational speed VrVr R VtVt compressor work resultant velocity tangential velocity radial velocity LIFT LOAD

107 © 2006 American Standard Inc. Compressor Response to Load full loadpart load VrVr VtVt R VrVr VtVt R

108 © 2006 American Standard Inc. Compressor Response to Lift full loadpart load VrVr VtVt R VrVr VtVt R

109 © 2006 American Standard Inc. Lessons Learned n To reduce lift: u Decrease condenser pressure by reducing leaving-tower water temperature u Increase evaporator pressure by raising chilled water setpoint n VSDs optimize chiller lift efficiency 41°F lvg evap water lvg cond water 45°F 75°F 99°F compressor work

110 © 2006 American Standard Inc. various system components Energy Use 20 energy use, % load, % static lift chiller w/low lift chilled water pump (P at end of loop) condenser water pump (stopped at 875 gpm) free discharge fan

111 © 2006 American Standard Inc. VSDs and centrifugal chillers A Simple Analogy accelerator (speed control of chiller motor) brake (inlet guide vanes for unloading) But misleading and technically incorrect

112 © 2006 American Standard Inc. 75% 75°F ECWT chiller efficiency at part load IPLV and NPLV Conditions 50% 65°F ECWT load lift 25% 100% 85°F ECWT

113 © 2006 American Standard Inc. VSDs and centrifugal chillers A Closer Look at IPLV VSDs improve part-lift performance, so running two chillers with VSDs at part load seems more efficient than one chiller at double the same load, but … LoadECWTWeightingkW/Ton 100%0.0185°F %0.4275°F %0.4565°F %0.4565°F0.393

114 © 2006 American Standard Inc. VSDs and centrifugal chillers Performance at 90% Load *Load equally divided ECWT 85°F 80°F 75°F 70°F 65°F Difference –38.4 –30.0 –20.2 – Chillers* Chiller Note: Data shows only chiller power.

115 © 2006 American Standard Inc. VSDs and centrifugal chillers Performance at 90% Load Conclusion: 1 chiller uses less power than 2 chillers chiller power, kW entering condenser water, °F 2 chillers: 45% load each 1 chiller: 90% load

116 © 2006 American Standard Inc. Analyze the System n Model: u Building use u Local weather u Economizers u Utility rates u System design n Use programs like TRACE™, DOE 2.x, Chiller Plant Analyzer, HAP

117 © 2006 American Standard Inc. VSDs and centrifugal chillers Summary n VSDs improve chiller part-lift performance u Lots of operational hours u Reduced condenser water temperatures u Higher costs of electricity n IPLV is not an economic tool

118 © 2006 American Standard Inc. Answers to Your Questions VSDs and their effect on system components

119 © 2006 American Standard Inc. wrap-up VSD Effect Differs n Cubic relationship to speed only occurs in “free discharge” systems n Control parameters affect savings n In chillers, external parameters define lift (pressure difference)

120 © 2006 American Standard Inc. wrap-up VSD Effect Differs n Cooling towers: Nearly cubic n HVAC fans: Not cubic u Depends on control strategy u Fan pressure optimization is best n Chilled water pumps: Not cubic u Affected by valves and control method u Consider pump pressure optimization based on critical valve

121 © 2006 American Standard Inc. wrap-up VSD Effect Differs n Condenser water pumps: Not cubic u Must meet minimum flow or pressure l Tower static lift l Minimum condenser water flow l Minimum tower flow u Reduced flow affects chiller and tower performance u Before applying a VSD, reduce pump design power (CW flow rate)

122 © 2006 American Standard Inc. wrap-up VSD Effect Differs n Power for any chiller is reduced at part load and lift n Chiller savings? Not even close to cubic u VSD helps more at part-lift conditions u MUST reduce lift for VSD to slow down and give benefit u Use same condenser water temperature to compare constant- and variable-speed chillers

123 © 2006 American Standard Inc. VFDs and Gensets Trane Engineers Newsletter volume 35-1 “How VFDs Affect Genset Sizing” by Court Nebuda location.aspx?item=5

124 © 2006 American Standard Inc. references for this broadcast Where to Learn More n 2005 ENL “Cooling Towers and Condenser Water Systems” n Bibliography

125 © 2006 American Standard Inc. mark your calendar 2006 ENL Broadcasts n May 3HVAC systems and airside economizers n Sep 13HVAC design for places of assembly n Nov 8Energy-saving designs for rooftop systems


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