Energy Saving Opportunities for Compressed Air Systems

1
Energy Saving Opportunities for Compressed Air
Systems

• Kelly Kissock, Ph.D., P.E.
• Department of Mechanical and Aerospace
  Engineering
• University of Dayton Industrial Assessment Center

2
Compressed Air Systems
3
Compressed Air System Savings Opportunities

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

4
Eliminate Continuous Blow-off with Solenoid
Valves

• Known
• Open tube air loss V (scfm) 11.6 x ID (in)2
  x P (psia)
• Solenoid valves cost between 30 - 350
• Operate up to 600 cycles per minute from 50-150
  psig
• Controlled by process machines, photo sensors,
  etc.
• Action
• Replace continuous blowoff from 100 psig, 3/8-in
  pipe with solenoid valve open 20 of time
• Savings
• V tube 11.6 x 3/8 (in)2 x 115 (psia) 188
  scfm
• 188 scfm x 80 x 0.75 kW/hp / (4.2 scfm/hp x
  0.90) 30 kW
• 30 kW x 6,000 hr/yr x 0.10 /kWh 18,000 /yr
• If load/unload at 60 6,200 /yr x 40 7,200
  /yr

5
Reduce Compressed Air with Air-Saver Nozzles

• Known
• Open tube air loss V (scfm) 11.6 x ID (in)2
  x P (psia)
• 3/8-in tube, 100 psig, 2000 hr/yr, 4.2 scfm/hp
  comp
• 3/8-in vortex nozzle consumes 31 scfm
• Action
• Add nozzle to tube
• Savings
• V tube 11.6 x 3/8 (in)2 x 115 (psia) 188
  scfm
• V nozzel 31 scfm
• (188 31) scfm x 0.75 kW/hp / (4.2 scfm/hp x
  0.90) 31 kW
• 31 kW x 2,000 hr/yr x 0.10 /kWh 6,200 /yr
• If load/unload at 60 6,200 /yr x 40 2,480
  /yr

6
Activate Bag House Air Pulses Using Pressure
Differential Instead of Timer

• Known
• Timed pulse uses 34 scfm at peak production
• Production is currently 60 of peak
• Action
• Install differential pressure control
• Savings
• 34 scfm x 40 x 0.75 kW/hp / (4.2 scfm/hp x
  0.90) 2.7 kW
• 2.7 kW x 6,000 hr/yr x 0.10 /kWh 1,620 /yr
• If load/unload at 60 1,620 /yr x 40 648
  /yr

7
Use Blower For Low-pressure Applications

• Known
• Air compressor at 100 psig 4.2 scfm/hp
• Low-pressure blowers at 20 psig 7.2 scfm /hp
• Tank currently uses 140 scfm of comp air
• Action
• Install low-pressure blower
• Savings
• 140 scfm x 0.75 kW/hp x (1/4.2 1/7.2) hp/scfm
  / 0.90 11.6 kW
• 11.6 kW x 6,000 hr/yr x 0.10 /kWh 6,960 /yr
• If load/unload at 60 6,960 /yr x 40
  2,780 /yr

8
Purchase Ultrasonic Sensor and Fix Leaks

• Known
• Most compressed air systems lose between 5 and
  20 of compressed air to leaks.
• To find leaks
• listen with the unaided ear or ultrasonic
  sensor.
• monitor compressor power when all production
  machinery is off.
• Inspect system for leaks once a week
• Action
• Fix single 1/32-inch leak
• Savings
• 1 scfm x 0.75 kW/hp / (4.2 scfm/hp x 0.90) 0.2
  kW
• 0.2 kW x 6,000 hr/yr x 0.10 /kWh 120 /yr
• If load/unload at 60 120 /yr x 40 48 /yr

9
Starve Leaks in Unoccupied Areas by Shutting off
Branch Headers

• Known
• Total plant leak load 200 cfm
• Half of plant operates 6,000 hr/yr and half of
  plant operates 2,000 hr/yr
• Action
• Install solenoid valve to shut off air to unused
  area
• Savings
• 200 scfm x 50 x 0.75 kW/hp / (4.2 scfm/hp x
  0.90) 20 kW
• 20 kW x 4,000 hr/yr x 0.10 /kWh 8,000 /yr
• If load/unload at 60 8,000 /yr x 40 3,200
  /yr

10
Install No-loss Drains Instead of Timed Solenoids

• Known
• 3/8-inch drain with timed solenoid opens 3
  seconds every 30 seconds to discharge condensate.
• 3/8-inch no-loss float-type drains eliminates
  90 of air losses.
• 3/8-inch no-loss float-type drains costs about
  600
• Action
• Replace timed solenoid drain with no-loss drain
• Savings
• V drain 11.6 x 3/8 (in)2 x 115 (psia) 188
  scfm
• Fraction time open (3 sec / 30 sec) 10
• 188 scfm x 10 x 0.75 kW/hp / (4.2 scfm/hp x
  0.90) 3.7 kW
• 3.7 kW x 90 x 6,000 hr/yr x 0.10 /kWh 2,000
  /yr
• If load/unload at 60 2,000 /yr x 40 800
  /yr

11
Use Looped Piping to Decrease System Pressure
Drop

• Use looped rather than linear design
• Main line size from average cfm to get DP 3
  psi
• Branch line size from cfm peak to get DP 3psi
• Feed lines size from peak cfm to get DP - 1 psi
• Hose can generate DP 4 to 5 psi (proper
  selection of hoses is important!)
• Total DP lt 10 psig

12
Avoid Deadhead Connectionsto Decrease System
Pressure Drop
13
Adequately Size Supply Piping/Hoseto Decrease
System Pressure Drop
14
Properly Size and Maintain Filters and Dryers to
Decrease System Pressure Drop

• Place filter upstream of dryer to protect dryer
• DP filter lt 1 psid
• DP refrigerated dryer lt 5 psid ( 90 F inlet and
  40 F outlet)

15
Reduce Compressed Air Pressure to Benefit from
Decreased System Pressure Drop
Fractional Savings
0.5 per psi

• Known
• Compressor draws 20 kW while producing 120 psig
  air
• Action
• Reduce pressure setting to from 110 to 100 psig
• Savings
• (P2high/P1)0.286 (110 psig 14.7 psia) / 14.7
  psia0.286 1.84
• (P2low/P1)0.286 (100 psig 14.7 psia) / 14.7
  psia0.286 1.80
• Frac savings (1.84 1.80) / (1.84 1) 4.8
  
• 20 kW x 4.8 x 6,000 hr/yr x 0.10 /kWh 580
  /yr

16
Compress Outdoor Air

• Known
• Compressors work harder to compress hot,
  expanded air.
• Compressing cooler outside air reduces
  compressor work
• Fractional Savings (Thi - Tlow) / Thi 2 per
  10 F
• Compressor draws 20 kW, Tin 80 F, Tout 50 F
• Action
• Install PVC piping to duct outside air to
  compressor
• Savings
• Frac Savings ((80 460) - (50 460)) / (80
  460) 5.5
• 20 kW x 5.5 x 6,000 hr/yr x 0.10 /kWh 660
  /yr
• If load/unload at 60 660 /yr x 40 264
  /yr

17
Replace Unheated Purge-Type Desiccant Dryer with
Refrigerated Dryer

• Known
• Refrigerated dryer cools to Tdew-point 35 F,
  and uses 6 W/scfm
• Desiccant dryer cools air to Tdew-point -40 F,
  but use 15 of compressed air for purging
• Current use 840 scfm from compressor at 4.2
  scfm/hp
• Action
• Install refrigerated dryer
• Savings
• Purge power
• 840 scfm x 15 x 0.75 kW/hp / (4.2 scfm/hp x
  90) 25 kW
• Refrigerated dryer power
• (840 scfm x 85 x 0.006 kW/scfm 4.3 kW
• (25 kW 4.3 kW) x 6,000 hr/yr x 0.10 /kWh
  12,420 /yr

18
Consider Pre-cooling Compressed Air to Dryer

• Reduces load on dryer
• Saves 1-2 W /scfm

19
Direct Warm Air Into Plant During Winter
Winter

• Known
• gt75 of compressor input power lost as heat
• Compressors draws 105 kW, heating system 80
  efficient, operates 2,000 hours per year
• Action
• Change ventilation or add duct work to direct
  warm air into plant during winter
• Savings
• 105 kW x 75 x 3,413 Btu/kWh x 2,000 hours/year
  540 mmBtu/yr
• 540 mmBtu/year / 80 x 10 /mmBtu 6,750 /year

20
Stage Compressors

• If the same load/unload pressures for two
  compressors are the same, both compressors will
  operate at part-load.

• Stage compressors into a lead and lag
  compressor by setting the load/unload pressures
  of the lag compressor 5 psi less than the lead
  compressor.

• Staging allows the Lead compressor to run fully
  loaded and the Lag compressor to turn off or run
  at minimal load, increasing efficiency.

21
Stage Compressors

• Known
• Two 100-hp, compressors operating between 95
  105 psig at 70 capacity
• FP (FC x (1 FPNL) FPNL
• Action
• Set base between 95 105 psig and lag between
  90-100 psig
• Savings
• Current
• FP (.7 x (1 .6) .6 .88
• Power 2 x 100 hp x .88 / .90 x .75 kW/hp 147
  kW
• Proposed
• Base 100 hp x 1.00 / .90 x .75 kW/hp 83 kW
• Lag FP (.2 x (1 .6) .6 .68 P 100 hp
  x .68 / .90 x .75 kW/hp 57 kW
• Base Lag 83 kW 57 kW 140 kW
• Savings (147 140) kW x 6,000 hr/yr x 0.10
  /kWh 4,200 /yr
• Savings if auto shutoff (147 100) kW x 6,000
  hr/yr x 0.10 /kWh 28,200 /yr

22
Run Trim Compressor in Load/Unload with
Auto-shutoff or use Variable Speed Compressor
FP (FC x (1 FPNL) FPNL
23
Power Characteristics of Load/unload and
Modulation Control
24
Modulation to Load/Unload with Auto-shutoff
Reduced Power 35 and Saved 17,000 /yr
25
Switching from Modulation to Load/Unload with
Auto Shutoff

• Known
• 100-hp compressor at 60 capacity with FPNLmod
  .7 and FPNLl/ul .5
• Action
• Switch compressor control from modulate to load
  unload
• Savings
• Modulate
• FP (FC x (1 FPNL) FPNL (.6 x (1
  .7) .7 .88
• Power 100 hp x .88 / .90 x .75 kW/hp 73 kW
• Load/unload
• FP (FC x (1 FPNL) FPNL (.6 x (1
  .5) .5 .80
• Power 100 hp x .80 / .90 x .75 kW/hp 67 kW
• Savings
• (73 67) kW x 6,000 hr/yr x 0.10 /kWh 3,600
  /yr

26
Add Compressed Air Storage to Lengthen Cycle
Time and Increase Auto Shutoff
Original AirSim Pressure/Power Trends
AirSim Pressure/Power Trends after adding 500
gallons storage

• Savings
• Average power draw from 17 kW to 14.5 kW after
  adding storage
• (17 14.5) kW x 6,000 hr/yr x 0.10 /kWh
  1,500 /yr

27
Add Local Storage w/ Valve and Reduce Compressed
Air Pressure
28
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
• Peak input power (kW) Rated motor power (hp) x
  Service factor x 0.75 kW/hp / Motor efficiency
• 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)
• 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) 0.50 - 0.60
• Typical Fraction Power at No Load (Variable Speed
  Drive) 0.10
• Typical Fraction Power at No Load (On/Off) 0.0

29
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
  /