Energy Efficient
Compressed Air Systems
Compressed Air Fundamentals
 dWelec = ∫V dP / [Effcompressor Effmotor Effcontrol]
 V = volume flow
rate
 DP = pressure rise
 Eff = efficiencies of
compressor, motor,
and control
Internal cooling decreases
electrical power
Compressed Air Savings Opportunities
 dWelec = ∫V dP / [Effcompressor Effmotor Effcontrol]






Reduce volume flow rate
Reduce pressure rise
Increase cooling during compression
Increase compressor efficiency
Increase motor efficiency
Increase control efficiency
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 =
$15 /yr
Leak Diameter
(inches)
1/64
1/32
1/16
1/8
1/4
Leakage Rate
(cfm)
0.25
1
4
16
63
Cost
($/year)
$15
$60
$238
$953
$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 = $5,000 /yr
100 hp
200 hp
400 hp
Leak Fraction
Annual Cost Annual Cost Annual Cost
(%)
($/year)
($/year)
($/year)
0
0
$0
$0
5
$1,250
$2,500
$5,000
10
$2,500
$5,000
$10,000
15
$3,750
$7,500
$15,000
20
$5,000
$10,000
$20,000
Identify Leaks Using Ultrasonic Sensor
Quantify Leakage By Logging
Flow or Compressor Power
Fix Leaks Frequently
Fix Leaks Every Four Weeks
cfm wks
cfm-wks
1
4
4
1
3
3
1
2
2
1
1
1
Total
10
Fix Leaks Every Two Weeks
cfm
wks
cfm-wks
1
2
2
1
1
1
1
2
2
1
1
1
6
 Reduces leakage by 40%
20
Average Leak Load (%)
 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
15
10
5
0
0
2
4
6
8
Leak Repair Interval (months)
10
12
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
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
Air Compressor with Linear Distribution Piping
End Use
End Use
End Use
Air Compressor with Looped Distribution Piping
End Use
End Use
End Use
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 =
(P2high /P1 )
0.286
 (P2low /P1 )
(P2high /P1 )
0.286
0.286
1
 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
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 demandcontrol 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 noloss 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
Summer
Plant
Winter
Cooling
Air
Compress
Outdoor Air
Air
Compressor
Compressed
Air To Plant
 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
Employ Efficient Compressor Control
1.00
Fraction Power (FP)
0.75
Blow Off
Modulation
0.50
Load/Unload
Variable Speed
On/Off
0.25
0.00
0.00
0.25
0.50
0.75
1.00
Fraction Capacity (FC)
FP = FP0 + (1-FP0) FC
and
P = Prated FP
Power Signatures from
Modulation and Load/unload Control
180-hp Rotary Screw Air Compressor - Load/Unload Control
150-h p R o tary -S crew A ir C o m p resso r - In let M o d u latio n C o n tro l
250
200
180
200
160
14 0
Current Draw (Amps)
Cu
rre
nt
Dr
aw
(A
m
ps
)
120
100
80
150
100
60
50
40
20
0
0
13:40:00
13:45:00
13:50:00
13:55:00
14:00:00
14:05:00
14:10:00
14:15:00
14:40:00
14:20:00
14:42:00 14:44:00 14:46:00 14:48:00 14:50:00 14:52:00 14:54:00 14:56:00 14:58:00 15:00:00
Savings From Switching To Efficient Control
 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
1.00
Fraction Power (FP)
0.75
0.50
0.25
0.00
0.00
0.25
0.50
Fraction Capacity (FC)
0.75
1.00
Blow Off
Modulation
Load/Unload
Variable Speed
On/Off
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
1.00
Fraction Power
0.80
0.60
0.40
0.20
0.00
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
Fraction Capacity
Throttling w ith Bypass
Throttling w ith Unload
0.90
1.00
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
1.00
Fraction Power
0.80
0.60
0.40
0.20
0.00
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
Fraction Capacity
Poor Throttling Control
Throttling w ith Bypass
0.90
1.00
Poor Throttling/Surge Pressure Set Points
Inlet butterfly valve throttles to 95%, then bypass
4/1/08 12:14 AM
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1079=1250 *
.746*1.1/.95
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4/6/08 12:14 AM
4/5/08 12:14 AM
1,200
4/4/08 12:14 AM
4/3/08 12:14 AM
4/2/08 12:14 AM
KW
Excellent Throttling/Surge Pressure Set Points
AC-3
644/101=64%
throttle
1,000
800
600
400
200
0
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
120
110
P (psig)
 “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 partload penalty if they have better
control efficiency.
 Simple, effective, and inexpensive
 However:
 Increases pressure
 Only applicable for compressors
in same location
Lead
Lag 1
100
Lag 2
90
80
Comp 1
Comp 2
Comp 3
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
Before Staging
After Staging
 Example: Stage unstaged load/unload 100-hp and 50-hp
compressors producing 400 scfm
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
To Plant
Air Compressor
Primary
Storage
Dryer
Power During Load and Sump Blowdown
180-hp Rotary Screw Air Compressor - Load/Unload Control
250
200
Current Draw (Amps)
 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
150
100
50
0
14:40:00 14:42:00 14:44:00 14:46:00 14:48:00 14:50:00 14:52:00 14:54:00 14:56:00 14:58:00 15:00:00
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)
0.910
0.905
Fraction Power
0.900
0.895
0.890
0.885
0.880
0.875
0.870
0.865
0.0
5.0
10.0
15.0
20.0
25.0
Volume Storage (gal/rated trim scfm)
For flooded compressor in load/unload mode at FC = 50% and blowdown = 30 sec
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
P = 110
To Plant
Air Compressor
Small Tank
Plant requires
100 psig
Small storage
doesn’t dampen
L/UL swing
Pset = 100-107
P = 100
P = 107
To Plant
Air Compressor
P = 100
P = 97
Big Tank
Large storage
dampens L/UL
swing
Pset = 97-107
Add Secondary Storage and Reduce Pressure
P = 115 psig
Preq = 90 psig
To the
Plant
Air Compressor
Process
Receiver
Main Receiver
P = 100 psig
Process with
Intermittent
Compressed
Air Demand
Preq = 90 psig
To the
Plant
Air Compressor
Needle
Valve
Main Receiver
Process
Receiver
Process with
Intermittent
Compressed
Air Demand
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
DPDRYER = 1 psi DPCOMP = 8 psi
High Flow
DPDRYER = 6 psi DPCOMP = 6 psi
115
115
110
110
Pressure (psig)
Pressure (psig)
Dryer Pressure Drop Causes Short Cycling
105
105
100
100
95
95
0:00:00
0:00:30
0:01:00
Time (mm:ss)
0:01:30
0:02:00
0:00:00
0:00:30
0:01:00
0:01:30
0:02:00
Time (mm:ss)
• 10 psi DP at compressor reduced to 5 psi “effective DP”
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
To Plant
Dryer
Main Reciever
Air Compressor
To Plant
Main Receiver
Dryer
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
120
P (psig)
110
Lead
Lag 1
100
Lag 2
90
80
Comp 1
Comp 2
Comp 3
• 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.
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
boost
er
comp
ressor
for
highpress
ure
applic
ations
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
120
P (psig)
110
Lead
Lag 1
100
Lag 2
90
80
Comp 1
Comp 2
Comp 3
• 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.
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
Pressure Band
50
619
928
1,237
1,856
(psi)
30
45
60
90
100
1,237
1,856
2,474
3,711
5
5.5
8.2
11.0
16.5
150
1,856
2,783
3,711
5,567
10
2.7
4.1
5.5
8.2
200
2,474
3,711
4,948
7,422
15
1.8
2.7
3.7
5.5
250
3,093
4,639
6,185
9,278
20
1.4
2.1
2.7
4.1
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)
Blowdown Time (sec)