Energy Efficient Compressed Air Systems. Compressed Air Fundamentals V = volume flow rate P = pressure rise Eff = efficiencies of compressor,

Energy EfficientCompressed Air Systems

Compressed Air Fundamentals

V = volume flow rate

DP = pressure rise Eff = efficiencies of

compressor, motor, and control

dWelec = ∫V dP / [Effcompressor Effmotor Effcontrol]

Internal cooling decreases electrical power

Compressed Air Savings Opportunities

Reduce volume flow rate Reduce pressure rise Increase cooling during compression Increase compressor efficiency Increase motor efficiency Increase control efficiency

dWelec = ∫V dP / [Effcompressor Effmotor Effcontrol]

Compressed Air System

Screw Compressor Operation

Compressed Air System Savings Opportunities• End use

– Eliminate inefficient uses of compressed air• Eliminate air pumps, agitation, cooling and suction

– Use blower for low-pressure applications– Install solenoid valves to shut off air– Install air saver nozzles– Install differential pressure switches on bag houses

• Distribution– Fix leaks– Decrease pressure drop in distribution system

• Compressor System– Compress outside air– Use refrigerated dryer– Direct warm air into building during winter– Use load/unload control with auto shutoff or VSD for lag compressor– Stage compressors with pressure settings or controller– Add compressed air storage to increase auto shutoff

Plant and Compressor Data For Calculating Example Savings

• Plant operates 6,000 hours per year• Electricity cost including demand = $0.10 /kWh• Air compressor produces 4.2 scfm/hp• Air compressor motor is 90% efficient• Air compressor runs in load/unload control with

FP0 = 0.50

Eliminate Inefficient Uses of Compressed Air

Replace Air Pump with Electric Pump

Air motors use 7x more electricity than electrical motors

Example Replace 10 x 1-hp air pumps

with electric pumps Cost Savings = 10 hp / 0.90 x

6/7 x 0.75 kW/hp x 6,000 hr/yr x $0.10 /kWh = $4,300 /yr

Replace Compressed Air with Mechanical Agitation

Compressed air agitation uses ~10x more electricity than mechanical agitation

Example Replace air agitation from

0.5-in pipe at 50 psig with mechanical agitator.

Flow Savings = 11.6 (scfm/lbf) x [1 (in)]2 x 55 psia = 150 scfm

Power Savings = 150 scfm / (4.2 scfm/hp x 0.90) x 0.75 kW/hp x (1-0.50) = 15 kW

Cost Savings = 15 kW x 6,000 hr/yr x $0.10 /kWh = $9,000 /yr

Replace Compressed Air Cooling with Chiller

Compressed Air In

Cold Air Out Warm Air Out

‘Vortex’ coolers use 4x more electricity than electric chillers Example

Replace 10,200 Btu/hr cooler using 150 scfm with chilled water Cost Savings = 150 scfm / (4.2 scfm/hp x 90%) x 75% x 0.75 kW/hp

x (1-0.50) x 6,000 hr/yr x $0.10 /kWh = $6,700 /year

Replace Air With Heat Pipe Cabinet Coolers

Air coolers use 8x more electricity than (heat pipe + small fan) coolers

Example Replace 2,400 Btu/hr compressed

air cooler with heat pipe cooler Cost comp air = 2,400 Btu/hr / 80

(Btu/hr)/cfm / (4.2 cfm/hp x 90%) x 0.75 kW/hp x (1-0.50) x 8,760 hr/yr x $0.10 /kWh = $2,607/year

Cost heat pipe = 0.6 A x 120 V / 1,000 VA/kW x 8,760 hr/yr x $0.10 /kWh = $ 63 /year

Implementation Cost = $1,000

Source: www.airtxinternational.com and www.system-directions.com

Replace Pneumatic Suction Cups with Magnets

Suction cups use 6.0 scfm while holding while magnets use average of 0.3 scfm

Example Replace cups with magnets if holding

3,000 hours per year. Power Savings = 5.7 scfm / (4.2 scfm/hp x

0.90) x 0.75 kW/hp x (1-0.50) = 0.57 kW Cost Savings = 0.57 kW x 3,000 hr/yr x

$0.10 /kWh = $167 /yr

Use Blowers for Low-Pressure Applications

Blowers generate 7.2 scfm /hp at 20 psig while compressors generate 4.2 scfm/hp at 100 psig

Example Install low-pressure blower for

application needing 140 scfm of 20 psig air

Power Savings = 140 scfm x (1/4.2 – 1/7.2) hp/scfm / 0.90 x .75 kW/hp x (1-0.50) = 5.8 kW

Cost Savings = 5.8 kW x 6,000 hr/yr x $0.10 /kWh = $3,472 /yr

Reduce End Use Compressed Air Demand

Reduce Blow-off with Solenoid Valves

Flow from open tube (scfm) = 11.6 (scfm/lbf) x [Diameter (in)] 2 x Pressure (psia)

Example Install solenoid to shut-off blowoff from

3/8-in pipe at 100 psig 80% of time Flow Savings = 11.6 (scfm/lbf) x [3/8

(in)]2 x 115 psia x 80% = 150 scfm Power Savings = 150 scfm / (4.2 scfm/hp

x 0.90) x 0.75 kW/hp x (1-0.50) = 15 kW Cost Savings = 15 kW x 6,000 hr/yr x

$0.10 /kWh = $8,933 /yr Cost of 3/8-inch solenoid valve = $100

Reduce Blow off with Air-Saver Nozzles

Nozzles maximize entrained air and generate same flow and force with ~50% less compressed air

Example Add nozzle to 1/8-in tube at 100 psig Flow Savings = 11.6 (scfm/lbf) x [1/8 (in)]2 x

115 psia x 50% = 10.4 scfm Power Savings = 10.4 scfm / (4.2 scfm/hp x


$0.10 /kWh = $620 /yr Nozzles cost about $10 each

Activate Bag House Air Pulses Using Pressure Differential Instead of Timer

Timers are designed for peak conditions, where demand-based control matches actual conditions

Example Install differential pressure control to

reduce timed pulse from 34 cfm by 60% Flow Savings = 34 cfm x 60% = 20.4 cfm Power Savings = 20.4 cfm / (4.2 scfm/hp x


$0.10 /kWh = $1,214 /yr

Fix Leaks

Fix Leaks

Leaks continuously drain power Leakage rate increases

exponentially with leak diameter Example

Fix one 1/64” leak Power Savings = .25 cfm /

(4.2 scfm/hp x 0.90) x 0.75 kW/hp x (1-0.50) = 0.025 kW

Cost Savings = 0.025 kW x 6,000 hr/yr x $0.10 /kWh = $15 /yr

Leak Diameter (inches)

Leakage Rate (cfm)

Cost ($/year)

1/64 0.25 $151/32 1 $601/16 4 $2381/8 16 $9531/4 63 $3,750

Fix Leaks

Many plants lose ~20% of compressed air to leaks

Example Fix leaks in plant with fully loaded

100-hp compressor and 20% leakage

Power Savings = 100 hp / 0.90 x 0.20 x 0.75 kW/hp x (1-0.50) = 8.3 kW


Leak Fraction (%)

100 hp Annual Cost

($/year)

200 hp Annual Cost

($/year)

400 hp Annual Cost

($/year)0 0 $0 $05 $1,250 $2,500 $5,00010 $2,500 $5,000 $10,00015 $3,750 $7,500 $15,00020 $5,000 $10,000 $20,000

Identify Leaks Using Ultrasonic Sensor

Quantify Leakage By Logging Flow or Compressor Power

Fix Leaks Frequently

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Leak Repair Interval (months)

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Leak Loss = Rate x Time Repairing leaks frequently cuts leak load

at same implementation cost Example

Fix leaks every 2 weeks instead of every 4 weeks if one new 1-cfm leak per week

Reduces leakage by 40%

Fix Leaks Every Four Weeks Fix Leaks Every Two Weekscfm wks cfm-wks cfm wks cfm-wks

1 4 4 1 2 21 3 3 1 1 11 2 2 1 2 21 1 1 1 1 1

Total 10 6

Starve Leaks by Shutting off Branch Headers

Valve on branch header can starve all downstream leaks when area is not in use

Example Install valve to shut off header with

200 cfm leak load for 4,000 hr/yr Power Savings = 200 scfm x 50% /

(4.2 scfm/hp x 0.90) x 0.75 kW/hp x (1-0.50) = 9.9 kW


Use Rubber In Place of Braded Hose

Braided hoses dry-rot and develop leaks that can’t be detected with ultrasonic sensors.

Reduce Distribution System Pressure Drop

Use Looped Piping System

Air Compressor with Linear Distribution Piping

End Use End Use End Use

Air Compressor with Looped Distribution Piping

End Use End Use End Use

If DP < 10 psi at farthest end use, use looped rather than linear design

Design Guidelines Main line: size from

average cfm to get DP = 3 psi

Branch line: size from cfm peak to get DP = 3 psi

Feed lines: size from peak cfm to get DP = 1 psi

Select hose with DP < 1 psi

Avoid ‘Collision’ Connections

Maintain Filters

• Place filter upstream of dryer to protect dryer

• DP filter < 1 psi

Size Dryer for DP< 5 psi

Low Flow: DP = 1 psi High Flow: DP = 6 psi

Then, Reduce Compressed Air Pressure

Work = V DP, thus compressor requires less work to produce air at lower outlet pressure

Fraction savings from reducing pressure =

Fraction savings from reducing pressure = 1% per 2 psi pressure reduction Example

Reduce pressure setting of fully-loaded 100-hp compressor from 110 to 100 psig

(P2high/P1)0.286 = [(110 psig +14.7 psia) / 14.7 psia]0.286 = 1.84 (P2low/P1)0.286 = [(100 psig +14.7 psia) / 14.7 psia]0.286 = 1.80 Fraction savings = (1.84 – 1.80) / (1.84 – 1) = 5.2 % Cost Savings = 100 hp x 5.2% / 90% x 0.75 kW/hp x 6,000 hr/yr x

$0.10 /kWh = $2,582 /yr

1)/P(P

)/P(P)/P(P0.286

12high

0.28612low

0.28612high

Reduce Pressure To Maximum End Use Plus Friction Loss

Dry Air Efficiently

Refrigerated and Desiccant Dryers Refrigerated dryer:

Dries air by cooling Cools to Tdew-point = 35 F Uses 6 W/scfm

Desiccant dryer: Dries air by passing through desiccant, then

purging desiccant of water Cools air to Tdew-point = -40 F to -100 F Uses 16 to 30 W/scfm

Desiccant dryers “should one be applied to portions of compressed air systems that require dew points below 35 F. Because desiccant dryers require a higher initial investment and higher operating costs, Kaeser strongly recommends using refrigerated dryers whenever practical.” Kaeser Regenerative Desiccan Dryers

Desiccant Dryer Purging

Three types of purge Compressed air purge

Uses 15% of compressed air for purging Total is about 30 W/scfm

Heated compressed air purge Uses 7% of compressed air for purging Plus 7 W/scfm for heating Total is about 22 W/scfm

Heater blower air purge Uses 3 W/scfm for blower Plus 13 W/scfm for heating Total is about 16 W/scfm

Purge cycle can be timed or demand-controlled

Source: www.aircompressors.com

Use Refrigerated Rather than Desiccant Dryer

Example: Replace desiccant dryer using compressed

air purge with refrigerated dryer for 200 hp (840 scfm) compressor

Desiccant Power = 840 scfm x 15% / (4.2 scfm/hp x 90%) x 0.75 kW/hp x (1-0.50) = 12.5 kW

Refrigerated Power = (840 scfm x 85% x 0.006 kW/scfm x (1-0.50) = 2.1 kW

Cost Savings = (12.5 kW – 2.1 kW) x 6,000 hr/yr x $0.10 /kWh = $6,215 /yr

Use Demand-Control Rather than Timed Purge

Summer air 4x wetter than winter air. Timed purge set for peak (summer)

conditions Example

Switch from timed to demand-control purge and reduce purge by 50% on dryer for 200 hp (840 scfm) compressor

Timed-Purge Power = 840 scfm x 15% / (4.2 scfm/hp x 90%) x 0.75 kW/hp x (1-0.50) = 12.5 kW

Cost Saving = 12.5 kW x 50% x 6,000 hr/yr x $0.10 /kWh = $3,250 /yr

.016 lbw/lba

.004 lbw/lba

Replace Timed-solenoid with No-loss Drains Winter air holds 50% less water than

summer air Timers designed for peak conditions,

where demand-based control matches actual conditions

Example Replace 3/8-inch timed-solenoid drain that

opens 3 seconds every 30 seconds with no-loss drain that eliminates <90% of air losses.

Flow Savings = 11.6 (scfm/lbf) x [3/8 (in)]2 x 115 (psia) x 10% x 90% = 16.9 scfm

Power Savings = 16.9 scfm / (4.2 scfm/hp x 0.90) x 0.75 kW/hp x (1-0.50) = 1.68 kW


3/8-inch no-loss drains costs $600

Optimize Compressor Cooling

Compress Outdoor Air

Compressing cool dense air reduces compressor work:

Fraction Savings = (Thi - Tlow) / Thi

Fraction Savings = ~ 2% per 10 F Example

Install PVC piping to duct outside air at 50 F to compressor rather than inside air at 80 F.

Fraction Savings = [(80 + 460) - (50 + 460)] / (80 + 460) = 5.9%

Cost Savings = 20 kW x 5.9% x 6,000 hr/yr x $0.10 /kWh = $706 /yr

Direct Warm Air Into Plant During Winter

75% of compressor input power lost as heat Example

Add duct work to direct warm air into plant during winter for compressors drawing 105 kW if heating system operates 2,000 hours per year and is 80% efficient

Heat Load Savings = 105 kW x 75% x 3,413 Btu/kWh x 2,000 hours/year = 537 mmBtu/yr

Cost Savings = 537 mmBtu/year / 80% x $10 /mmBtu = $6,719/year

Summer

Plant

Winter

Air Compressor

Cooling Air

Compress Outdoor Air Compressed

Air To Plant

Employ Efficient Compressor Control

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Blow Off

Modulation

Load/Unload

Variable Speed

On/Off

FP = FP0 + (1-FP0) FC and P = Prated FP

Power Signatures fromModulation and Load/unload Control

180-hp Rotary Screw Air Compressor - Load/Unload Control

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150-hp Rotary-Screw Air Compressor - Inlet Modulation Control

0 20 40 60 80

100 120 140 160 180 200

13:40:00 13:45:00 13:50:00 13:55:00 14:00:00 14:05:00 14:10:00 14:15:00 14:20:00

Current Draw (Amps)

Savings From Switching To Efficient Control

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Modulation

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Variable Speed

On/Off

FP = FP0 + (1-FP0) FC Typical Intercepts

FP0 bypass = 1.00 FP0 modulation = 0.70 FP0 load/unload = 0.50 FP0 variable-speed = 0.10 FP0 on/off = 0.00

Example Switch 100-hp compressor at 50% capacity from modulation

to variable-speed control. FP (modulation) = 0.70 + (1 - 0.70) .50 = .85 FP (variable-speed) = 0.10 + (1 - 0.10) .50 = .55 Savings = 100 hp x (.85 - .55) / .90 x 0.75 kW/hp x 6,000

hr/yr x $0.10 /kWh = $15,000 /yr

Savings Penalty for Inefficient Control

Consider savings from reducing fraction capacity (FC) P1 = Prated x FP1 = Prated x [FP0 + (1-FP0) FC1] P2 = Prated x FP2 = Prated x [FP0 + (1-FP0) FC2] Psave = P1 – P2 Psave = Prated x [FP0 + (1-FP0) FC1] - Prated x [FP0 + (1-FP0) FC2] Psave = Prated x (FC1 - FC2) x (1-FP0) Psave = Unadjusted savings x (1-FP0)

Actual Savings = Unadjusted Savings x (1 – FP0) Example

Calculate actual savings for reducing leaks by 100 scfm if compressor operates in modulation control.

Unadjusted savings = 100 scfm / (4.2 scfm/hp x 0.90) x 0.75 kW/hp x 6,000 hr/yr x $0.10 /kWh = $11,905

Actual savings = $11,905 /yr x (1 – 0.70) = $3,571 /yr

Modulation to Load/Unload with Auto-shutoff

Reduced power 35% and saved $17,000 /yr

Centrifugal Compressor Control

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Throttling w ith Bypass Throttling w ith Unload

Energy-Efficient Centrifugal Compressor Control

• Minimize/eliminate by-pass using effective multi-compressor control

• Operate compressor with throttling/unload control if available

• Adjust throttling/surge pressure points to widen throttling range

Throttling/Surge Pressure Set Points Narrow or Widen Throttling Band

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Poor Throttling Control Throttling w ith Bypass

Poor Throttling/Surge Pressure Set Points

Inlet butterfly valve throttles to 95%, then bypass

Excellent Throttling/Surge Pressure Set Points

AC-3

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644/101=64% throttle

1079=1250 * .746*1.1/.95

Inlet guide vane throttles to 65%, then bypass

Multi-compressor Control

• Cascading pressure set-point control• Single-pressure network control• Smart PLC control

Cascading Pressure Set-Point Control “Stage” compressors into lead and

lag compressor(s) by sequentially reducing load/unload pressures.

Designate compressor with best control efficiency as final lag compressor

Staging allows Lead to run fully loaded and the Lag compressor(s) to turn off or run with smaller part-load penalty if they have better control efficiency.

Simple, effective, and inexpensive However:

Increases pressure Only applicable for compressors

in same location

80

90

100

110

120

Comp 1 Comp 2 Comp 3

P (

psi

g) Lead

Lag 1

Lag 2

Single-Pressure Network Control• When compressors in different

locations, staging compressors using cascading pressure set points doesn’t work, since the compressors see different pressures.

• When more than three compressors are staged using cascading pressure set points, pressure increased and pressure range is large.

• In these cases, use a sequencer with common pressure sensor to stage compressors.

Smart PLC Control

• Monitors pressure, flow and power

• Most flexible (controls different compressor types, locations, manufacturers)

• Trends data

• Accommodates changes

Source: Taming Multiple Compressors, Niff Ambrosino and Paul Shaw, www.plantservices.com

Two Compressors Properly Staged

Savings From Staging Compressors Example: Stage load/unload 100-hp and 50-hp compressors producing 400 scfm

Cr,100 = 100 hp x 4.2 scfm/hp = 420 scfm Cr,50 = 50 hp x 4.2 scfm/hp = 210 scfm

Unstaged FC = 400 scfm / (420 scfm + 210 scfm) = 0.63 P100 = Pr,100 x FP100 = 100 hp x [0.50 + (1 - 0.50) .63] = 81.7 hp P50 = Pr,50 x FP50 = 50 hp x [0.50 + (1 - 0.50) .63] = 40.9 hp Ptotal = P100 + P50 = 81.7 hp + 40.9 hp = 122.6 hp

Staged with auto shutoff FC 100 = 400 scfm / 420 scfm = 0.95 P100 = Pr,100 x FP100 = 100 hp x [0.50 + (1 - 0.50) .95] = 97.6 hp P50 = 0 hp (with auto shutoff) Ptotal = P100 + P50 = 97.6 hp + 0 hp = 97.6 hp

Savings Savings = (122.5 hp – 97.6 hp) / .90 x 0.75 kW/hp x 6,000 hr/yr x $0.10 /kWh =

$12,500 /yr

Savings From Staging Compressors

Example: Stage unstaged load/unload 100-hp and 50-hp compressors producing 400 scfm

Before Staging After Staging

Compressed Air Storage• Necessary for load/unload

compressors• Lengthens load/unload cycle time:

– Reduces compressor wear– Allows sump to completely

blowdown, which reduces unloaded power

– Increases auto-shutoff, which reduces unloaded power

• Acts as additional compressed air capacity to meet demand spikes– Reduces pressure variation to

process– Allows average pressure to be

lowered, which reduces compressor energy use

Locating Compressed Air Storage

• Locating storage after dryer protects dryer from compressed air demand spikes that can prevent dryer from effectively removing moisture.

Sizing Compressed Air Storage

Relationship between volume, flow and pressure from mass balance. Mflowin – Mflowout = dM/dt Vflowin r – Vflowout r = (RT/PV) / dt

2.7 gal/rated scfm (0.36 ft3/rated scfm) or 11.3 gal/rated hp (1.5 ft3/rated hp) of trim compressor guarantees load/unload cycle time > 1 minute with 10 psi pressure band

Example Calculate storage for 100 hp

compressor so load/unload cycle time > 1 minute with 10 psi pressure band.

100 hp x 4.2 scfm/hp x 2.7 gal/scfm =1,134 gal

Primary StorageAir Compressor Dryer

To Plant

Power During Load and Sump Blowdown180-hp Rotary Screw Air Compressor - Load/Unload Control

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Initial rapid power increase when compressor loads

Subsequent slow power increase as pressure builds from load to unload pressure set points.

Initial rapid power decrease as compressor unloads

Subsequent slow power decrease as pressure in sump is bled down to near atmospheric pressure to reduce back pressure.

Blowdown time ~ 30 seconds

Add Storage to Lengthen Load/Unload Cycle and Enable Full Blowdown

Example• Adding storage reduces average power by 2.9% • If 100-hp compressor at 50% load, savings are:

50 hp / 0.90 x 2.9% x 0.75 kW/hp x 6,000 hr/yr x $0.10 /kWh = $725 /yr

Power Savings From Adding Storage (to Achieve Full Blowdown)

For flooded compressor in load/unload mode at FC = 50% and blowdown = 30 sec

0.865

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Volume Storage (gal/rated trim scfm)

Add Storage to Enable Auto Shutoff

Example• Adding storage enables auto shutoff and reduces average power by 14.7%• If 100-hp compressor at 50% load, savings are:

50 hp / 0.90 x 14.7% x 0.75 kW/hp x 6,000 hr/yr x $0.10 /kWh = $3,675 /yr

Add Primary Storage and Reduce Pressure

Small Tank

Big Tank

P = 110

P = 100

P = 100

P = 107

To Plant

To Plant

P = 97

Air Compressor

Air Compressor

Plant requires 100 psig

Small storage doesn’t dampen L/UL swing

Pset = 100-107

Large storage dampens L/UL swing

Pset = 97-107

Add Secondary Storage and Reduce Pressure

To the Plant

Main Receiver

Air Compressor

Process Receiver

P = 115 psig

Process with Intermittent

Compressed Air Demand

Preq = 90 psig

To the Plant

Main Receiver

Air Compressor

Process Receiver

Needle Valve

Process with Intermittent

Compressed Air Demand

Preq = 90 psigP = 100 psig

Example• Adding secondary storage allows reducing pressure by 6 psi which reduces

compressor energy by 3%• If 100-hp at 50% load, compressor savings are:

50 hp / 90% x 3% x 0.75 kW/hp x 6,000 hr/yr x $0.10 /kWh = $750 /yr

U.D. AirSim Software

• AirSim simulates compressed air systems to understand the dynamics between compressed air control, capacity, storage and demand.

• The software is available free of charge at:

http://academic.udayton.edu/kissock/http/RESEARCH/EnergySoftware.htm

D.O.E. AirMaster+ Software

• AIRMaster+ provides a systematic approach for assessing the supply-side performance of compressed air systems.

• AIRMaster+ evaluates the energy savings potential of any or all of the following eight energy efficiency actions:

• Reduce air leaks• Improve end-use efficiency• Reduce system air pressure• Use unloading controls• Adjust cascading set points• Use automatic sequencer• Reduce run time• Add primary receiver volume

• http://www1.eere.energy.gov/industry/bestpractices/

http://www1.eere.energy.gov/industry/bestpractices/

http://www1.eere.energy.gov/industry/bestpractices/

Summary of Key Equations and Relations• Input power (kW) = Voltage (V) x Current (A) x 1.73 x Power factor (kW/kVA) / 1,000 VA/kVA• Annual energy use (kWh/yr) = Input power (kW) x Operating hours (hr/yr)• Annual electricity cost ($/yr) = Annual energy use (kWh/yr) x Unit electricity cost ($/kWh)• Flow from open tube (scfm) = 11.6 (scfm/lbf) x Pressure (psig) x [Diameter (in)] 2

• Input power from flow (kW) = Flow (scfm) x 0.75 kW/hp / (Specific output (scfm/hp) x Motor efficiency) x (1-FP0 ) • Typical compressor/blower specific output: 4.5 scfm/hp at 100 psig 7.2 scfm at 20 psig• Savings from reducing operating pressure ~ 0.5% per psi• Savings from reducing intake air temperature ~ 2% per 10 F• Refrigerated dryer electricity use ~ 4-6 W/scfm Unheated desiccant dryer air use ~ 15% of flow• Recoverable heat from air compressors ~ 75% of electrical power (kW) x 3,412 (Btu/kWh)• Fraction Power = [(Fraction Capacity x (1 – Fraction Power at No Load)] + Fraction Power at No Load

– Typical Fraction Power at No Load (Modulation Control) = 0.70– Typical Fraction Power at No Load (Load/unload Control) = 0.30 - 0.60– Typical Fraction Power at No Load (Variable Speed Drive) = 0.10– Typical Fraction Power at No Load (On/Off) = 0.0

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Compressed Air Storage

• Lengthens load/unload cycle time:– Reduces compressor wear– Allows sump to completely blowdown,

which reduces unloaded power– Increases auto-shutoff, which reduces

unloaded power

• Reduces pressure variation to process– Allows average pressure to be

lowered, which reduces compressor energy use

• 2.7 gal/rated scfm of trim compressor (1.5 ft3/hp) guarantees load/unload cycle time > 1 minute

Dryer Pressure Drop

Low Flow High Flow DPDRYER = 1 psi DPCOMP = 8 psi DPDRYER = 6 psi DPCOMP = 6 psi

Dryer Pressure Drop Causes Short Cycling

• 10 psi DP at compressor reduced to 5 psi “effective DP”

95

100

105

110

115

0:00:00 0:00:30 0:01:00 0:01:30 0:02:00

Pres

sure

(psi

g)

Time (mm:ss)

95

100

105

110

115

0:00:00 0:00:30 0:01:00 0:01:30 0:02:00Pr

essu

re (p

sig)

Time (mm:ss)

Locating Primary Storage

• If DP across dryer is small, locate primary storage downstream of dryer to reduce flow variation through dryer and thoroughly dry air

• If DP across dryer is large, locate primary storage upsteam of dryer to enable compressor to realize full load/unload pressure range

Air Compressor

Main RecieverDryer

To Plant

Main Receiver

Air Compressor

Dryer

To Plant

Inlet Guide Vane vs Inlet Butterfly Valve Throttling

Source: Ingersol Rand Centac Compressor Manual

Stage Compressors for Efficient Control• “Stage” compressors into lead and lag compressor(s) by sequentially

reducing load/unload pressures of the lag compressors.• Designate compressor with best control efficiency as Lag compressor

• Staging allows Lead compressor to run fully loaded and the Lag compressor(s) to turn off or run with smaller part-load penalty if they have better control efficiency.

80

90

100

110

120


P (

psi

g) Lead

Lag 1

Lag 2

Use Efficient Compressed Air Pumps

• Some pumps use ~30% less air than others

• DOE 16% of our industrial motor system energy use. Seventy percent of our manufacturing facilities use compressed air in their production process.

• Compressed Air Audit• “Studies indicate that as much as 35% of the compressed air produced in the market today is wasted to leaks, and everyone has leaks.”

Wayne Perry, technical director, Kaeser Compressors

“It has been our experience that plants which have no formal, monitored, disciplined, compressed air leak-management program will have a cumulative leak level equal to 30% to 50% of the total air demand,” Henry van Ormer, Air Power USA

• “The compressed air system had to run at 98 psi because the grinding area. The header pressure was lowered to 85 psi. Results after 18 months showed that tool repair went down for the grinders, production increased by 30% and total air demand fell from 1,600 to 1,400 cfm.“ Henry van Ormer, Air Power USA

• “The easy answer to many system problems is to jack up the pressure. Unfortunately, the leaks will leak more, and unregulated users will waste more air and more energy.” Norm Fischer, Centrifugal Equipment Service

• “More important, the back pressure sends a false unload signal to the controls, causing premature unloading or extra compressors to be on line,” van Ormer says. “Using a 30 degree to 45 degree directional angle entry instead of a tee will eliminate this pressure loss. The extra cost of the directional entry is usually negligible.”

• Remember, pressure costs money in two ways — power to produce increased pressure costs one half of one percent per psi, and excess pressure produces excess flow that must be compressed. Van Ormer

Source: The top 10 targets of a compressed air audit, Rich Merritt, www.plantservices.com

• “More important, the back pressure sends a false unload signal to the controls, causing premature unloading or extra compressors to be on line,” van Ormer says. “Using a 30 degree to 45 degree directional angle entry instead of a tee will eliminate this pressure loss. The extra cost of the directional entry is usually negligible.”

• “Upgrading to copper or aluminum piping provides excellent value for money and ideal delivery characteristics,” Perry says. “When upgrading, ensure that the physical piping diameter is sized to deliver the required air flow with minimum pressure drop.”

• “Open blow, refrigeration and vortex cooling may all be replaceable with heat tube cabinet coolers with a potential savings of 3.5 kW to 4 kW each on a 30- by 24- by 12-inch average cabinet,” van Ormer says. “The initial cost is usually in the $700 to $750 range with a potential resultant power savings of $1,000 to $2,000 per year each.”

• 14.2 at inlet



• Use booster compressor for high-pressure applications

Single Pressure Network Control

• Narrower pressure band

• However:– Compressors

from same manufacture

– Compressors in same location


Cascading Pressure Set-Point Control

• Simple, effective, and inexpensive

• However:– Increases

pressure

– Only applicable for compressors in same location


Cascading Pressure Set-Point Control• “Stage” compressors into lead and lag compressor(s) by sequentially

reducing load/unload pressures of the lag compressors.• Designate compressor with best control efficiency as Lag compressor

• Staging allows Lead compressor to run fully loaded and the Lag compressor(s) to turn off or run with smaller part-load penalty if they have better control efficiency.

80

90

100

110

120


P (

psi

g) Lead

Lag 1

Lag 2

Sizing Primary Storage

Required Storage Per Blow Down Time (gal)

Compressor Size Blowdown Time (sec)

(hp) 30 45 60 90

10 124 186 247 371

50 619 928 1,237 1,856

100 1,237 1,856 2,474 3,711

150 1,856 2,783 3,711 5,567

200 2,474 3,711 4,948 7,422

250 3,093 4,639 6,185 9,278

300 3,711 5,567 7,422 11,133

350 4,330 6,494 8,659 12,989

400 4,948 7,422 9,896 14,844

Volume Storage Required per Rated Scfm (gal/rated-scfm)

Pressure Band Blowdown Time (sec)

(psi) 30 45 60 90

5 5.5 8.2 11.0 16.5

10 2.7 4.1 5.5 8.2

15 1.8 2.7 3.7 5.5

20 1.4 2.1 2.7 4.1

Documents

Energy Efficient Compressed Air Systems. Compressed Air Fundamentals V = volume flow rate P = pressure rise Eff = efficiencies of compressor,