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

© 2006 American Standard Inc.

Tracer Summit User’s Group

5/22/2012

VSDs and their effect on HVAC system components

e = mc²

© 2

00

6 A

merica

n S

tan

dard

Inc.

speed of light 299,792,458 m/s

water 2 m/s

air 10–20 m/s

500,000 lbs/hrair

500,000 lbs/hrair

600,000 lbs/hrwater

600,000 lbs/hrwater

© 2

00

6 A

merica

n S

tan

dard

Inc.

© 2

00

6 A

merica

n S

tan

dard

Inc.

work = mass × resistance

© 2

00

6 A

merica

n S

tan

dard

Inc.

velocity

pathdiameter

pathlength

fluiddensity

frictionfactor

gravitationalconstant

Darcy-Weisbach EquationDarcy-Weisbach Equation

p = f L V²D 2 gc

© 2

00

6 A

merica

n S

tan

dard

Inc.

Darcy-Weisbach EquationDarcy-Weisbach Equation

resistance velocity²

© 2

00

6 A

merica

n S

tan

dard

Inc.

System ResistanceSystem Resistance

return duct

diffusersand grillescoil

dampers

supply duct

© 2

00

6 A

merica

n S

tan

dard

Inc.

Fan LawsFan Laws

cfm rpm

p rpm² cfm²

hp rpm³

© 2

00

6 A

merica

n S

tan

dard

Inc.

Chiller Laws?Chiller Laws?


resistance “lift”


Practical Application:Free Discharge Fans

VSDs and their effect on system components

© 2

00

6 A

merica

n S

tan

dard

Inc.

Free Discharge SystemFree Discharge System Draw-through cooling tower

Propeller fan

sumpsump

outdoor air

louvers

fill

propeller fan

© 2

00

6 A

merica

n S

tan

dard

Inc.

cfm rpm

p rpm²

hp rpm³

General Fan PerformanceGeneral Fan Performance

© 2

00

6 A

merica

n S

tan

dard

Inc.

frictionpressure

fixed pressure

airflow, %

100

20 40 60 80 1000

sta

tic p

ressu

re,

%system performance

Static Pressuresystem performance

Static Pressure

systemresistance

80

60

40

20

0

© 2

00

6 A

merica

n S

tan

dard

Inc.

system performance

Free Discharge Systemsystem performance

Free Discharge System

airflow, %

100

20 40 60 80 1000

systemresistance

80

60

40

20

0

sta

tic p

ressu

re,

%

© 2

00

6 A

merica

n S

tan

dard

Inc.

airflow, %

100

20 40 60 80 1000

fan performance curves

Fan Speed (N)fan performance curves

Fan Speed (N)

N1N1

N2N2

As fan speed varies …so does airflow volume

80

60

40

20

0

sta

tic p

ressu

re,

%

© 2

00

6 A

merica

n S

tan

dard

Inc.


Speed N vs. Efficiency fan performance curves

Speed N vs. Efficiency

airflow, %

100

20 40 60 80 1000

N1N1

1

2

1'2'

N2N2

80

60

40

20

0

sta

tic p

ressu

re,

%

© 2

00

6 A

merica

n S

tan

dard

Inc.

performance curves

Fan and Systemperformance curves

Fan and System

airflow, %

100

20 40 60 80 1000

N1N11'

N2N2

180

60

40

20

0

sta

tic p

ressu

re,

%

© 2

00

6 A

merica

n S

tan

dard

Inc.


Cooling Towerfan performance curves

Cooling Tower

airflow, %

100

20 40 60 80 1000

fan

pow

er,

%

80

60

40

20

0

© 2

00

6 A

merica

n S

tan

dard

Inc.



Cooling Tower

airflow, %

100

20 40 60 80 1000

fan

an

d m

oto

r p

ow

er,

%

poten

tial e

nergy

savi

ngs

poten

tial e

nergy

savi

ngs1-speed

motor

80

60

40

20

0

© 2

00

6 A

merica

n S

tan

dard

Inc.



Cooling Tower

airflow, %

100

20 40 60 80 1000

fan

an

d m

oto

r p

ow

er,

%

1-speedmotor

2-speedmotor

80

60

40

20

0

© 2

00

6 A

merica

n S

tan

dard

Inc.



Cooling Tower

airflow, %

100

20 40 60 80 1000

80

60

40

20

0

fan

an

d m

oto

r p

ow

er,

%

VSD

© 2

00

6 A

merica

n S

tan

dard

Inc.

cooling tower application

Fan Energy Comparisoncooling tower application

Fan Energy Comparison

source: Marley Technical Report H-001A

Control strategy Energy use factor

1-speed fan cycling(base)

100% kWh

2-speed fan cycling 39% kWh

variable-speed control 19% kWh

© 2

00

6 A

merica

n S

tan

dard

Inc.

fan/tower performance curves

Free Cooling at Low Loadfan/tower performance curves

Free Cooling at Low Load

airflow, %

100

20 40 60 80 1000

fan

an

d m

oto

r p

ow

er,

%

100

80

60

40

20

tow

er c

ap

acity

, %80

60

40

20

0

towercapacity

“free” cooling

0

© 2

00

6 A

merica

n S

tan

dard

Inc.

free discharge fans

Summaryfree discharge fans

Summary Performance approximates the

“cube of the speed”

Variable-speed drives (VSDs) are a great option for modulating capacity

When considering VSDs for chilled water plants, start at the cooling tower


Practical Application:Ducted Indoor Fans


© 2

00

6 A

merica

n S

tan

dard

Inc.


airflow

sta

tic p

ressu

re 3,500 cfm2.0 in. wg

© 2

00

6 A

merica

n S

tan

dard

Inc.


p = f L V²D 2 gc

© 2

00

6 A

merica

n S

tan

dard

Inc.

3,500 cfm2.0 in. wg

airflow

sta

tic p

ressu

reSystem ResistanceSystem Resistance

2,000 cfm0.65 in. wg

systemresistance

curve

systemresistance

curve

© 2

00

6 A

merica

n S

tan

dard

Inc.

some devices don’t obey the rules for

System Resistancesome devices don’t obey the rules for

System Resistance

© 2

00

6 A

merica

n S

tan

dard

Inc.

airflow

sta

tic p

ressu

re

systemresistance

curve

systemresistance

curve

valvesclosed

valvesopen


© 2

00

6 A

merica

n S

tan

dard

Inc.

airflow

sta

tic p

ressu

re

surgeregion

design

modulationrange

system resistanceVAV SystemVAV Systemactual

© 2

00

6 A

merica

n S

tan

dard

Inc.

1,100 rpm

1,100 rpm

Fan PerformanceFan Performance

airflow

sta

tic p

ressu

re

wide-openairflow

blocked-tightstatic pressure 1100 rpm

© 2

00

6 A

merica

n S

tan

dard

Inc.

airflow

sta

tic p

ressu

reFan SpeedFan Speed

1100 rpm900 rpm700 rpm500 rpm

© 2

00

6 A

merica

n S

tan

dard

Inc.

airflow

sta

tic p

ressu

re

1100 rpm900 rpm700 rpm500 rpm

VAV SystemVAV System

system resistance

fan speed

© 2

00

6 A

merica

n S

tan

dard

Inc.

TT

VAV SystemVAV System

900 cfm 500 cfm

TT

air valves

System resistance changesas valves modulate …

© 2

00

6 A

merica

n S

tan

dard

Inc.

fan modulation

Objectivesfan modulation

Objectives Produce adequate static pressure

Eliminate excess static pressure

Exploit diversity

Maximize energy savings at fan

Provide stable control

Keep everyone comfortable

© 2

00

6 A

merica

n S

tan

dard

Inc.

Static Pressure ControlStatic Pressure ControlInsufficient static pressure?

VAV box deliverstoo little airflow

© 2

00

6 A

merica

n S

tan

dard

Inc.

Static Pressure ControlStatic Pressure ControlExcessive static pressure?

• Wasted energy

• Poor comfortcontrol

• Poor acoustics

© 2

00

6 A

merica

n S

tan

dard

Inc.

Fan Control LoopFan Control Loop

supply fanstatic pressuresensor

SS

controller

© 2

00

6 A

merica

n S

tan

dard

Inc.

VAV System ModulationVAV System Modulation

airflow

sta

tic p

ressu

re

design

system resistance

actual

VAVmodulation

curve

1100 rpm800 rpm

© 2

00

6 A

merica

n S

tan

dard

Inc.

static pressure control

Sensor at Fan Outletstatic pressure control

Sensor at Fan Outlet

SS

supply fan

controller

VAV boxes

© 2

00

6 A

merica

n S

tan

dard

Inc.

supply fan

controller


Sensor Down 2/3 of Ductstatic pressure control

Sensor Down 2/3 of Duct

SS

VAV boxes

© 2

00

6 A

merica

n S

tan

dard

Inc.


Sensor Down 3/4 of Ductstatic pressure control

Sensor Down 3/4 of Duct

supply fan

controller

SS

VAV boxes

© 2

00

6 A

merica

n S

tan

dard

Inc.

optimized static pressure control

Sensor at Fan Outletoptimized static pressure control

Sensor at Fan Outlet

supply fan speed orinlet vane position

communicating BAS

VAV damperpositions

staticpressure

SS

static pressuresetpoint

© 2

00

6 A

merica

n S

tan

dard

Inc.

static pressure control methods

Performance Comparisonstatic pressure control methods

Performance Comparison

Controlmethod

Fanoutlet

Supplyduct

Optimized

Airflow

24,000 cfm(full load)

18,000 cfm

18,000 cfm

18,000 cfm

Full-loadpower

60%

100%

55%

43%

Fan inputpower

13 hp

22 hp

12 hp

9.5 hp

Fan staticpressure

2.7 in. wg

2.1 in. wg

1.9 in. wg

1.5 in. wg


System Demonstration


© 2

00

6 A

merica

n S

tan

dard

Inc.

why care about

Pump Energywhy care about

Pump Energy

According to the DOE ...

Pumps represent 5% of industrial energy consumption

Total cost of owning a pump is 90% energy consumption

Pump energy consumption generally can be reduced by as much as 20%

© 2

00

6 A

merica

n S

tan

dard

Inc.

pumping chilled water

ASHRAE 90.1-2001pumping chilled water

ASHRAE 90.1-2001

Requires variable chilled water flow if:

Total pump power exceeds 75 hp

AND

System includes > 3 control valves

© 2

00

6 A

merica

n S

tan

dard

Inc.

pumping chilled water

ASHRAE 90.1-2001pumping chilled water

ASHRAE 90.1-2001

Requires 30% design wattage at 50% flow if:

Any variable-flow pump motor > 50 hp

AND

Design head pressure > 100 ft

Typical solution: Variable-speed drive

© 2

00

6 A

merica

n S

tan

dard

Inc.

chilled water system

Variable Primary Flowchilled water system

Variable Primary Flow

1

2

750 gpm

750 gpm

1

2

750 gpm

750 gpm


Full-Load Pressure Drop

© 2

00

6 A

merica

n S

tan

dard

Inc.

tP

control valve

P


Full-Load Pressure Drop

© 2

00

6 A

merica

n S

tan

dard

Inc.

triple-duty valve

1

2

750 gpm

750 gpm

© 2

00

6 A

merica

n S

tan

dard

Inc.

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

pu

mp

head

, %

pumping system

Pump & System Curvespumping system

Pump & System Curves

pump

system

design point

© 2

00

6 A

merica

n S

tan

dard

Inc.

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

pu

mp

head

, %

pump

system

design point

pump characteristics

Powerpump characteristics

Power

100

50

0

pump power

pow

er

© 2

00

6 A

merica

n S

tan

dard

Inc.

variable-flow water system

How Does It Unload?variable-flow water system

How Does It Unload?

Depends on:

Chilled-water system curve

Pump curve

Pump control method Ride the curve

Different pump sizes

Vary the speed

© 2

00

6 A

merica

n S

tan

dard

Inc.

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

pu

mp

head

, %

pump

system

design point

100

50

0

pump power


Ride the Curvepump characteristics

Ride the Curve

part-loadpoint

pow

er


Part Load: Ride the Curve

© 2

00

6 A

merica

n S

tan

dard

Inc.

tP

controlvalve

coilflow

1

2

750 gpm

750 gpm

© 2

00

6 A

merica

n S

tan

dard

Inc.

variable-flow water system

How Does It Unload?variable-flow water system

How Does It Unload?

Depends on:

Chilled water system curve

Pump curve

Pump control method Ride the curve

Different pump sizes

Vary the speed

safe

zone

© 2

00

6 A

merica

n S

tan

dard

Inc.

Pump CharacteristicsPump Characteristics

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

pu

mp

head

, %

pump

system

design point

100

50

0

pow

er

pumppower

© 2

00

6 A

merica

n S

tan

dard

Inc.

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

system

pu

mp

head

, %

pump

design point

pow

er

100

50

0


Different Pump Sizespump characteristics

Different Pump Sizes

part-loadpoint

pumppower

© 2

00

6 A

merica

n S

tan

dard

Inc.

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

system

pu

mp

head

, %

pump

pow

er

100

50

0

pumppower


Different Pump Sizespump characteristics

Different Pump Sizes

© 2

00

6 A

merica

n S

tan

dard

Inc.

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

system

pu

mp

head

, %

pow

er

100

50

0


VFD = Different Pumpspump characteristics

VFD = Different Pumps

1750 rpm1488 rpm

1225 rpm

© 2

00

6 A

merica

n S

tan

dard

Inc.

variable-speed pump

Control Methodsvariable-speed pump

Control Methods Pressure control (P) at pump

Pressure control (P) at end of system

Critical-valve pressure reset

1

2

750 gpm

750 gpm

P


Part Load: P at Pump

© 2

00

6 A

merica

n S

tan

dard

Inc.

tP

© 2

00

6 A

merica

n S

tan

dard

Inc.

variable-speed pump


Control Methods Pressure control (P) at pump

Pressure control (P) at end of system

Critical-valve pressure reset

1

2

750 gpm

750 gpm


Part Load: P at End of System

© 2

00

6 A

merica

n S

tan

dard

Inc.

P

tP P

© 2

00

6 A

merica

n S

tan

dard

Inc.

variable-speed pump


Control Methods Pressure control at pump

Pressure control at end of system

Critical valve pressure reset


Part Load: Critical Valve Reset

© 2

00

6 A

merica

n S

tan

dard

Inc.

1

2

750 gpm

750 gpm

valveposition

tP

P

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

pu

mp

head

excesspump

pressure

120

dynamicpressure

fixed pressure

fullload

partload


Part Load: Ride the Pump Curve

© 2

00

6 A

merica

n S

tan

dard

Inc.


Part Load: P Control at Pump

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

pu

mp

head

excesspump

pressure

dynamicpressure

fixed pressure

allloads

What the pump needs to produce the required systempressure determines minimum pump pressure, motor speed

© 2

00

6 A

merica

n S

tan

dard

Inc.


Part Load: P Control at End of System

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

pu

mp

head

dynamicpressure

fixed pressure

© 2

00

6 A

merica

n S

tan

dard

Inc.

P

Pump P “slides” down thesystem curve …

water flow, %

100

20 40 60 80 1000

80

60

40

20

0

pu

mp

head

pumppressur

e

© 2

00

6 A

merica

n S

tan

dard

Inc.


Part Load: Critical Valve ResetPump P constantly reset to lowest possible value… almost all pressure drop is dynamic

previoussystem curve

© 2

00

6 A

merica

n S

tan

dard

Inc.

variable-flow pumping

Summaryvariable-flow pumping

Summary Energy savings depends on:

Pump selections

Fixed vs. frictional pressure components

Control strategy

Energy savings can approach “cube of speed”

Great application for variable-speed drives

© 2

00

6 A

merica

n S

tan

dard

Inc.

variable-flow condenser water

Pump Speedvariable-flow condenser water

Pump Speed Determining minimum speed

How variable flow affects: Pump

Cooling tower

Chiller

Controlling flow to improvesystem performance

© 2

00

6 A

merica

n S

tan

dard

Inc.

condenser water pump

Minimum Speedcondenser water pump

Minimum Speed

Determinants:

Minimum condenser flow

Tower static lift

Minimum tower flow Nozzle selection

Performance

Compare curve with cubic

© 2

00

6 A

merica

n S

tan

dard

Inc.

cooling tower

Static Liftcooling tower

Static Lift

static lift

© 2

00

6 A

merica

n S

tan

dard

Inc.

cooling tower

Water Distributioncooling tower

Water Distribution

© 2

00

6 A

merica

n S

tan

dard

Inc.

ExampleExample

1500 gpm system, 1770 rpm

Minimum flows:Chiller 658 gpmTower 750 gpm

Tower static lift 12.2 ft

Pump:Speed 974 rpmPump flow 875 gpm

© 2

00

6 A

merica

n S

tan

dard

Inc.

variable condenser-water flow

Effect on Pump

capacity, gpm

0

20

pu

mp

head

, ft

400 800 1200 1600 2000 24000

40

60

80

100

1770 rpm

1505 rpm

1239 rpm

974 rpm

81%

75%

83%

75%60%

© 2

00

6 A

merica

n S

tan

dard

Inc.

operating dependencies

Full Flowoperating dependencies

Full Flow

Tower design

Condenser watertemperature & flow

Heat rejection Wet bulb

Chiller design

Condenser watertemperature & flow

Load

© 2

00

6 A

merica

n S

tan

dard

Inc.

variable condenser water flow

Effect on Towervariable condenser water flow

Effect on Tower

70

tow

er

en

teri

ng

wate

r, °

F

60

80

90

100

110

condenser water flow, %

50 60 70 80 90 100

100% fan

© 2

00

6 A

merica

n S

tan

dard

Inc.

100

ch

ille

r p

ow

er,

kW

50

150

200

250

300


50 60 70 80 90

96°FLCWT

1000


Effect on Chillervariable condenser water flow

Effect on Chiller

84°FLCWT

Conditions:

• 70% load

• 70°F WB

• Full-speedtower fan

© 2

00

6 A

merica

n S

tan

dard

Inc.

100

com

pon

en

t/syste

m p

ow

er,

kW

50

150

200

250

300


50 60 70 80 90

chiller

100


Effect on Systemvariable condenser water flow

Effect on System

0pump

tower

system

© 2

00

6 A

merica

n S

tan

dard

Inc.

reducing flow & fan speed

Effect on Towerreducing flow & fan speed

Effect on Tower

70

tow

er

en

teri

ng

wate

r, °

F

60

80

90

100

110


50 60 70 80 90 100

100%

80%

60%

50%

fan speed

conditions:

•70% load

•70°F WB

© 2

00

6 A

merica

n S

tan

dard

Inc.


Effect on Systemreducing flow & fan speed

Effect on System

100

syste

m p

ow

er,

kW

50

150

200

250

300


50 60 70 80 90 100

100%

0

80% 60% 50%fan speed

conditions:

•70% load

•70°F WB

© 2

00

6 A

merica

n S

tan

dard

Inc.



Effect on System

100

syste

m p

ow

er,

kW

50

150

200

250

300


50 60 70 80 90 1000

100%

80%

conditions:

•70% load

•50°F WB

60%

50%

fan speed

© 2

00

6 A

merica

n S

tan

dard

Inc.



Effect on System

100

syste

m p

ow

er,

kW

50

150

200

250

300


50 60 70 80 90 1000

100%

conditions:

•30% load

•50°F WB

fan speed

80%

60%50%

© 2

00

6 A

merica

n S

tan

dard

Inc.


Summaryvariable condenser water flow

Summary

Determine what savings,if any, are possible

Are pumps alreadylow power?

Can reducing tower-fanspeed achieve most ofthe savings?

© 2

00

6 A

merica

n S

tan

dard

Inc.


Summaryvariable condenser water flow

Summary

If you decide to reduce flow: Find minimum condenser-

water flow rate

Examine system at variousloads and wet-bulbs …keep chiller out of surge

Document the sequence ofoperation

Help commission the system

© 2

00

6 A

merica

n S

tan

dard

Inc.


Guidancevariable condenser water flow

Guidance Can provide savings …

Finding proper operatingpoints requires more time,more fine-tuning

Two-step process:

1 Reduce design pump power

2 Is variable condenser-waterflow still warranted?

© 2

00

6 A

merica

n S

tan

dard

Inc.


VSDs and Chiller LawsVSDs and Chiller Laws

Variable-speed drives benefit centrifugal compressors in water chillers

Review “chiller laws”

Explore scientific cause-and-effect relationships

Maximize benefits

resistance “lift”

© 2

00

6 A

merica

n S

tan

dard

Inc.

VSD and centrifugal chillers

A Simple AnalogyVSD and centrifugal chillers

A Simple Analogy

accelerator(speed controlof chiller motor)

brake(inlet guide vanesfor unloading)

© 2

00

6 A

merica

n S

tan

dard

Inc.

Motor runs atconstant speed,regardless of load

a simple analogy

Constant-Speed Chillera simple analogy

Constant-Speed Chiller

Inlet guide vanesrestrict refrigerant atoff-design conditions

© 2

00

6 A

merica

n S

tan

dard

Inc.

a simple analogy

Variable-Speed Chillera simple analogy

Variable-Speed Chiller

Based on load,motor speeds upor slows down

© 2

00

6 A

merica

n S

tan

dard

Inc.

VSDs and centrifugal chillers

An AnalogyVSDs and centrifugal chillers

An Analogy

In each case:

Energy is wasted

Mechanical wear-and-tearis increased

© 2

00

6 A

merica

n S

tan

dard

Inc.

Typical Centrifugal Chiller

evaporator

condenser

compressor

controller

not shown:pressure-reducingdevice

capacity-modulating device

© 2

00

6 A

merica

n S

tan

dard

Inc.

Impeller

blade

coldrefrigerant

vapor

coldrefrigerant

vapor

hotrefrigerant

vapor

© 2

00

6 A

merica

n S

tan

dard

Inc.

Lift versus Load

lift

lvg evaporator water

lvg condenser water

lift Pcnd – Pevp

lift Tlvg cnd – Tlvg evp

load gpm × (Tent evp – Tlvg evp)

56°F

41°F

85°F

99°F

800 gpm

load = 500 tons

2 gpm/ton

58°F(T)

compressorworkcompressorwork

© 2

00

6 A

merica

n S

tan

dard

Inc.

Compressor Work and Chiller Efficiency

cooling capacity/“load”h

ead

/“lift

”

lvg evapwater

lvg condwater

500 tons

58°F

refrigerantflow rate

refrigerantflow rate

© 2

00

6 A

merica

n S

tan

dard

Inc.

Impeller Dynamics

diameter

Vt rpm × diameter

Vr refrig flow rate

rotationalspeed

Vr

RVt

compressorwork

resultantvelocity

tangentialvelocity

radialvelocity

LIF

T

LOAD

© 2

00

6 A

merica

n S

tan

dard

Inc.

Lessons LearnedLessons Learned To reduce lift:

Decrease condenser pressure by reducing leaving-tower water temperature

Increase evaporator pressure by raising chilled water setpoint

VSDs optimize chiller lift efficiency

41°Flvg evapwater

lvg condwater

45°F

75°F

99°F

compressorworkcompressorwork

© 2

00

6 A

merica

n S

tan

dard

Inc.

various system components

Energy Usevarious system components

Energy Use

20

en

erg

y u

se,

%

40

60

80

100

load, %

40 60 80 1000

20

staticlift

chiller w/low lift

chilled water pump(P at end of loop)

condenser waterpump (stopped at875 gpm)

free discharge fan

© 2

00

6 A

merica

n S

tan

dard

Inc.


A Simple AnalogyVSDs and centrifugal chillers

A Simple Analogy

accelerator(speed controlof chiller motor)

brake(inlet guide vanesfor unloading)But misleading

and technicallyincorrect

© 2

00

6 A

merica

n S

tan

dard

Inc.

75%75%

75°FECWT

chiller efficiency at part load

IPLV and NPLV Conditions chiller efficiency at part load

IPLV and NPLV Conditions

50%50%

65°FECWT

load

lift

25%25%100%100%

85°FECWT

© 2

00

6 A

merica

n S

tan

dard

Inc.


A Closer Look at IPLVVSDs 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 …

Load ECWTWeighting kW/Ton

100% 0.01 85°F 0.572

75% 0.42 75°F 0.429

50% 0.45 65°F 0.324

50% 0.45 65°F 0.393

© 2

00

6 A

merica

n S

tan

dard

Inc.


Performance at 90% LoadVSDs 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

–9.5

+4.3

2 Chillers*

306.4

268.0

230.8

195.2

160.3

1 Chiller

268.0

238.0

210.6

185.7

164.3

Note: Data shows only chiller power.

© 2

00

6 A

merica

n S

tan

dard

Inc.


Performance at 90% LoadVSDs and centrifugal chillers

Performance at 90% Load

0

50

100

150

200

250

300

350

65 70 75 80 85

Conclusion: 1 chiller uses less power than 2 chillers

ch

ille

r p

ow

er,

kW

entering condenser water, °F

2 chillers:45% load each

1 chiller:90% load

© 2

00

6 A

merica

n S

tan

dard

Inc.

Analyze the SystemAnalyze the System Model:

Building use

Local weather

Economizers

Utility rates

System design

Use programs like TRACE™, DOE 2.x, Chiller Plant Analyzer, HAP

© 2

00

6 A

merica

n S

tan

dard

Inc.


SummaryVSDs and centrifugal chillers

Summary VSDs improve chiller part-lift

performance Lots of operational hours

Reduced condenser water temperatures

Higher costs of electricity

IPLV is not an economic tool

© 2

00

6 A

merica

n S

tan

dard

Inc.

wrap-up

VSD Effect Differswrap-up

VSD Effect Differs Cubic relationship to speed only

occurs in “free discharge” systems

Control parameters affect savings

In chillers, external parameters define lift (pressure difference)

© 2

00

6 A

merica

n S

tan

dard

Inc.

wrap-up


VSD Effect Differs Cooling towers: Nearly cubic

HVAC fans: Not cubic Depends on control strategy

Fan pressure optimization is best

Chilled water pumps: Not cubic Affected by valves and control method

Consider pump pressure optimization based on critical valve

© 2

00

6 A

merica

n S

tan

dard

Inc.

wrap-up


VSD Effect Differs Condenser water pumps: Not cubic

Must meet minimum flow or pressure Tower static lift Minimum condenser water flow Minimum tower flow

Reduced flow affects chiller and tower performance

Before applying a VSD, reduce pump design power (CW flow rate)

© 2

00

6 A

merica

n S

tan

dard

Inc.

wrap-up


VSD Effect Differs Power for any chiller is reduced at

part load and lift

Chiller savings? Not even close to cubic VSD helps more at part-lift conditions

MUST reduce lift for VSD to slow down and give benefit

Use same condenser water temperatureto compare constant- and variable-speed chillers

© 2

00

6 A

merica

n S

tan

dard

Inc.

VFDs and GensetsVFDs and Gensets

Trane Engineers Newslettervolume 35-1

“How VFDs Affect Genset Sizing” by Court Nebuda

www.trane.com/commercial/location.aspx?item=5

© 2

00

6 A

merica

n S

tan

dard

Inc.

references for this broadcast

Where to Learn Morereferences for this broadcast

Where to Learn More 2005 ENL “Cooling Towers and Condenser

Water Systems”http://www.trane.com/bookstore/catalog.asp?sectionid=14

Bibliography

© 2

00

6 A

merica

n S

tan

dard

Inc.

mark your calendar

2006 ENL Broadcastsmark your calendar

2006 ENL Broadcasts May 3 HVAC systems and airside

economizers

Sep 13 HVAC design for placesof assembly

Nov 8 Energy-saving designs for rooftop systems

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

Documents

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