Improving Rotary Screw Compressor Performance using Variable Speed Drives

John Cosner, JCI/Frick

A Global Energy Situation….

Efficiency In The Engine Room

Optimum

VFD @ 30% Cap.

S.V. Loaded

Constant Speed

@ 100% Cap.

Constant Speed

@ 100% Cap.

Constant Speed

@ 100% Cap.

Constant Speed

@ 100% Cap.

Constant Speed

@ 30% Cap.

Better

Inefficient

Constant Speed

@ 70% Cap.

Constant Speed

@ 100% Cap.

Constant Speed

@ 60% Cap.

Compressor Facts

• Often the most concentrated and

largest consumers

• Most new installations use rotary

screw compressors.

• At least one compressor will go

to a part load condition.

Constant Torque / Variable Horsepower

Screw Compressors

0%

20%

40%

60%

80%

100%

0% 20% 40% 60% 80% 100%

Speed

To

rqu

e a

nd

Po

wer

HorsepowerTorque

Constant Torque

Horsepower = % Speed

460V / 60Hz = 7.67V : 1Hz

Full Load Power = 500HP / 375KWFull Load Power = 500HP / 375KW

FLA = 580FLA = 580

Running @ 35% Capacity (by speed)Running @ 35% Capacity (by speed)

Voltage = 460vacVoltage = 460vac

HP = 175HP = 175

Amps = 203Amps = 203

Voltage = 161VpwmVoltage = 161Vpwm

HP = 175HP = 175

Amps = 580Amps = 580

MotorMotor

VSDVSD

35% Out35% Out

1260 RPM1260 RPM

21 Hertz21 Hertz

580 Amps580 Amps

132 KW132 KW

0%

20%

40%

60%

80%

100%

0% 20% 40% 60% 80% 100%

Speed

To

rqu

e a

nd

Po

wer

HorsepowerTorque

Variable Torque / Variable HorsepowerPumps & Fans

Torque = % Speed²

Horsepower = % Speed³

Part Load Efficiencies @ Approximately 6:1 CRThrottling, Step Loading (Poppet), Slide Valve w/Var. Vi, VFD w/Var. Vi and Ideal

0.0010.0020.0030.0040.0050.0060.0070.0080.0090.00

100.00110.00

0% 20% 40% 60% 80% 100%

% Capacity

% F

ull L

oad

Po

wer

Slide Valve

VFD

Ideal Eff.

Poppet

Throttling

Improved Part Load Efficiency Solution

Compression Ratio

Minimum Speed

Volume Control

Capacity 20% -100% (720 RPM 3600 RPM )

100% Slide Valve

Base Loaded, Capacity 100% 3600 RPM

100% Slide Valve

The improvement in part load efficiency will vary based on…

Compression Ratio (CR)

194.8psiaD / 48.7psiaS 4:1 CR

194.8psiaD / 19.48psiaS 10:1 CR

Example: Improvement at 40% capacity by speed

over 40% capacity by slide valve

4:1 CR = 19% Improvement

10:1 CR = 33% Improvement

Minimum Speed

• Minimum speed established by the limitations of the:

•compressor

•motor

•oil type

•refrigerant.

NH³

•Sleeve bearings ≥ 1800 rpm.

• Rolling element bearings ≥ 720 rpm

• Viscosity required at 1800rpm is ≥11cSt.

• Viscosity required at 720rpm is ≥18cSt.

Speeds below design limits may cause a mixed film or boundary

lubrication condition.

Compressor Minimum Speed

Oil & Refrigerant

R-22 @ 10˚F (-12˚C)SST1250 RPM Minimum

R-717@ 20˚F (-6.5˚C)SST720 RPM Minimum

R-22 @ 20˚F (-6.5˚C)SST 1500 RPM Minimum

Motor Minimum Speed

• An issue of bearings and heat rejection.

• Air-flows at speeds below

50% speed may not be

adequate.

• Check with manufacturer for

minimum speed.

• Motor stator temperature

protection is now required (NEC 2005).

Fractional HP Squirrel Cage

Fan/s Ensure Adequate Air

Flow At All Speeds

Variable Volume Control

100806040200

5.0

4.0

3.0

2.0

1.0

5.0

2.2

Vi at part load

by slide valve

% Capacity

Vi

15

With VSD at part load

Vi stays ideal

% Capacity to % Capacity to

% Slide Valve Position% Slide Valve Position

Slide Valve w/Variable Vi

VFD w/Variable Vi

HP to $$$HP to $$$ -- 10HP may cost over $5,200 annually (.08 KWH).10HP may cost over $5,200 annually (.08 KWH).

Energy Savings @ Part Load

RWFII-270 @ 20˚F (-6.7˚C) SST / 95˚F (35˚C) SDT 4.06:1 CR on Ammonia

Slide Valve, VFD and Ideal Part Load Efficiencies

0.0050.00

100.00150.00200.00250.00300.00350.00400.00450.00500.00550.00600.00650.00700.00

0% 20% 40% 60% 80% 100%

% Capacity

Ho

rse P

ow

er

S.V. Control

VFD Control

650HP

19

HP

52

HP

107

HP

RWFII-270 @ 20˚F (-6.7˚C) SST / 75˚F (24˚C) SDT / 2.9:1 CR on Ammonia

Slide Valve, VFD and Ideal Part Load Efficiencies

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

400.00

450.00

500.00

0% 20% 40% 60% 80% 100%

% Capacity

Ho

rse P

ow

er

S.V. Control

VFD Control

9

HP

31

HP

62

HP

475HP

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

400.00

450.00

0% 20% 40% 60% 80% 100%

% Capacity

Ho

rse P

ow

er

RWFII-270 @ -40˚F (-40˚C) SST / 95˚F (35˚C) SDT / 18.75:1 CR on Ammonia

Slide Valve, VFD and Ideal Part Load Efficiencies

26

HP

137

HP

82

HP

420HPS.V. Control

VFD Control

RWFII-270 @ -40˚F (-40˚C) SST / 75˚F (24˚C) SDT / 13.45:1 CR on Ammonia

Slide Valve, VFD and Ideal Part Load Efficiencies

335HPS.V. Control

VFD Control

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

0% 20% 40% 60% 80% 100%

% Capacity

Ho

rse P

ow

er

18

HP

60

HP

102

HP

Capacity Control Strategy

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100%

Percent Speed

Perc

ent S

lide V

alv

e

Capacity Control Strategy

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100%

Percent Speed

Perc

ent S

lide V

alv

e

Off Run

SV

Speed

Speed

Typical VSD OperationSlide Valve Unloaded – NH3

Typical VSD OperationSlide Valve Loaded – NH3

Capacity Control Strategy

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100%

Percent Speed

Perc

ent S

lide V

alv

e

Off Run SpeedSVSpeed

Typical VSD Operationwith Standby Mode (Vyper™)

Up To

2 Hrs.

Capacity Control Strategy

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100%

Percent Speed

Perc

en

t S

lide V

alv

e

Stand-byat 100%SVRunSpeed Speed Speed

Up To

2 Hrs

Economizing

Econo. Improvement of HP/TR -40F/95F

0

2

4

6

8

10

12

14

0 50 100 150

Tons

HP

/TR

Non E SV

Econ VSD

Non E VSD

Econ SV

Controlling The VSDControlling The VSD

Manufacturers Controller To Be The Supervisory Manufacturers Controller To Be The Supervisory

ControlControl

��Low Oil FlowLow Oil Flow

��High Motor AmpsHigh Motor Amps

��High discharge PressureHigh discharge Pressure

��Low Mass FlowLow Mass Flow

��The PLC Controls The Speed But Is Routed Through The PLC Controls The Speed But Is Routed Through

The Package Mounted ControllerThe Package Mounted Controller

VSD Control with Quantum LX

Capacity Control Setup

Motor & Drive Setup

Skip Frequencies

Speed trend line

Can I Retrofit My Existing Equipment???

VSD?

• Ensure the VSD is constant

torque.

• Motor is inverter duty rated.

• Minimum speed is identified.

• Ensure environment is

adequate.

• Total cable distance between

drive and motor is within spec.

• Controller is able to protect

the compressor as well as control the drive.

Retrofitting Existing Equipment For Energy Savings

To VSD or Not To VSD?

Qualifying Expense Through Savings (ROI)

Total Load Profile

1. Hours of Operation?

2. Compression Ratio?

3. Cost per KWH?

4. Part Load Profile?

How Do I Accurately Get This Information?

Items 1-3 Are Easy

What is My

Part Load

Profile?

Quantum™ LX Load Profile Screen

The Load Profile Data is Entered

The VSD Energy Savings Calculator

Payback in 1.8 yrs.

What About Step Loading Machines?

Vyper™ Retrofit At JCI-Frick in Waynesboro, PA

In ConclusionIn Conclusion………………....

When Properly Applied, VSDs On Rotary Screw When Properly Applied, VSDs On Rotary Screw

Compressors Can:Compressors Can:

�� Lower Energy Usage & CostLower Energy Usage & Cost

�� Enhance Economizer BoostEnhance Economizer Boost

�� Remove Setpoint OverRemove Setpoint Over--ShootShoot

�� Provide Quicker Response To Load FluctuationProvide Quicker Response To Load Fluctuation

�� Keep Capacity Slides Fully LoadedKeep Capacity Slides Fully Loaded

�� Reduce Maintenance CostsReduce Maintenance Costs

�� Reduce Noise LevelsReduce Noise Levels

�� Properly Applied Is The Properly Applied Is The Key!Key!

Thank You!Thank You!