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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
0.90) x 0.75 kW/hp x (1-0.50) = 1.0 kW Cost Savings = 1.0 kW x 6,000 hr/yr 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
0.90) x 0.75 kW/hp x (1-0.50) = 2.0 kW Cost Savings = 2.0 kW x 6,000 hr/yr 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
Cost Savings = 8.3 kW x 6,000 hr/yr x $0.10 /kWh = $5,000 /yr
Leak Fraction (%)
100 hp Annual Cost
($/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
0
5
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0 2 4 6 8 10 12
Leak Repair Interval (months)
Ave
rage
Lea
k Lo
ad (%
)
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
Cost Savings = 9.9 kW x 4,000 hr/yr x $0.10 /kWh = $3,969 /yr
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
Cost Savings = 1.68 kW x 6,000 hr/yr x $0.10 /kWh = $1,005 /yr
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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Fraction Capacity (FC)
Fra
ctio
n P
ow
er (
FP
)
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
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Savings From Switching To Efficient Control
0.00
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Fraction Capacity (FC)
Fra
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FP
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Blow Off
Modulation
Load/Unload
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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Fraction Capacity
Fra
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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
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KW
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
0.870
0.875
0.880
0.885
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0.900
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0.910
0.0 5.0 10.0 15.0 20.0 25.0
Frac
tion
Pow
er
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 = $750 /yr
U.D. AirSim Software
• AirSim simulates compressed air systems to understand the dynamics between compressed air control, capacity, storage and demand.
• The software is available free of charge at:
http://academic.udayton.edu/kissock/http/RESEARCH/EnergySoftware.htm
D.O.E. AirMaster+ Software
• AIRMaster+ provides a systematic approach for assessing the supply-side performance of compressed air systems.
• AIRMaster+ evaluates the energy savings potential of any or all of the following eight energy efficiency actions:
• Reduce air leaks• Improve end-use efficiency• Reduce system air pressure• Use unloading controls• Adjust cascading set points• Use automatic sequencer• Reduce run time• Add primary receiver volume
• http://www1.eere.energy.gov/industry/bestpractices/
Summary of Key Equations and Relations• Input power (kW) = Voltage (V) x Current (A) x 1.73 x Power factor (kW/kVA) / 1,000 VA/kVA• Annual energy use (kWh/yr) = Input power (kW) x Operating hours (hr/yr)• Annual electricity cost ($/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
Thank you!
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
Comp 1 Comp 2 Comp 3
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
• “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.”
• “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.”
• Use booster compressor for high-pressure applications
Single Pressure Network Control
• Narrower pressure band
• However:– Compressors
from same manufacture
– Compressors in same location
Source: Taming Multiple Compressors, Niff Ambrosino and Paul Shaw, www.plantservices.com
Cascading Pressure Set-Point Control
• Simple, effective, and inexpensive
• However:– Increases
pressure
– Only applicable for compressors in same location
Source: Taming Multiple Compressors, Niff Ambrosino and Paul Shaw, www.plantservices.com
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
Comp 1 Comp 2 Comp 3
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
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