Reducing the Cost of Compressed Air http:web'umr'eduiacclassroomme219Reducing the Cost of Compressed

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Reducing the Cost of Compressed
Airhttp//web.umr.edu/iac/classroom/me219/Reduci
ng20the20Cost20of20Compressed20Air.ppt

• Chapter 6, pp.360-369
• Fall 2003

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Background

• Compressed air is commonly used in industrial
  facilities to perform a wide variety of tasks
  such as cleaning, operating pneumatic equipment,
  and even refrigeration.
• However, there is a widespread waste of energy
  associated with compressed-air systems and a
  general lack of awareness about the opportunities
  to conserve energy.

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Energy Costs Associated With Compressed Air

• Example, p361
• Compressor Power 125 hp (1.341 hp 1 kW)
• 93.21 kW
• Operating Hours 6000 h/yr
• Electricity Cost 0.085/kWh
• Motor Efficiency 0.90
• Annual Energy Usage (Power Operating Hours) /
  Efficiency
• (93.21 kW)(6000 h/yr) / (0.90)
• 621,417 kWh/yr
• Annual Electricity Cost Annual Energy Usage
  Unit Cost of Energy
• (621,417 kWh/yr)(0.085/kWh)
• 52,820/yr

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Cost Reduction Procedures

• Repairing Air Leaks on Compressed-Air Lines
• (pp.361-364) Example 6-22
• Installing High-Efficiency Motors
• (pp.365-366)
• Using Outside Air for Compressor Intake
• (p.367)
• Reducing the Air Pressure Setting
• (pp.367-368) Example 6-23

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1. Repairing Air Leaks

• Air leaks are the greatest single cause of energy
  loss in manufacturing facilities associated with
  compressed-air systems.
• Leaks occur at joints, flange connections,
  elbows, reducing bushes, sudden expansions, valve
  systems, filters, hoses, check valves relief
  valves, extensions, and the equipment connected
  at the air lines.

Compressed Air System image from http//www.pge.c
om/003_save_energy/003b_bus/003b1d8a10_compair_gui
de.shtml
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1. Repairing Air Leaks

• Equation 6-89 Mechanical Energy Wasted
• Equation 6-90 Leak mass flow rate
• Equation 6-91 Power Savings
• Power Saved Power wasted m air w
  comp,in

w comp,in w reversible comp, in nRT1
(P2/P1) (n-1)/n -1
?comp ?comp (n-1)
m air C discharge 2/(k1) 1/(k-1) P line A
kR2/(k1) Tline
RTline
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1. Repairing Air Leaks

• Example 6-22 Energy and Cost Savings by Fixing
  Air Leaks
• Determine the energy and money saved per year by
  sealing a leak equivalent to a 3-mm-diameter hole
  in the compressed air line
• P1 (Atm) 101 kPa Operation 4200 h/yr
• P2 700 P1 Electricity 0.078/kWh
• 801 kPa
• T inlet 20C ? comp 0.8
• 293.15 K ?motor 0.92
• T line 24C Cdischarge 0.65
• 297.15 K n (ideal gas) 1.4

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1. Repairing Air Leaks

• Example 6-22 Solution Method
• Equation 6-89, w comp,in 296.9 kJ/kg
• Hole cross-sectional area 7.069 x 10-6 m2
• Equation 6-90, mair 0.008632 kg/s
• Equation 6-91, Power Wasted 2.563 kW
• Energy Savings (Power saved) (Operating hours)
  / ?motor
• (2.563 kW) (4200 h/yr) / (0.92)
• 11,700 kWh/yr
• Cost Savings (Energy savings) (Unit cost of
  energy)
• (11,700 kWh/yr) (0.0078/kWh)
• 913/year

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2. High-Efficiency Motors

• Energy efficient motors are constructed with
  better bearings and windings to reduce frictional
  and electrical resistance losses.
• Depending on the horsepower rating of a given
  high-efficiency motor, operating efficiencies may
  be from 1 to 10 higher than the operating
  efficiencies of the existing motors.
• The electrical energy consumed by a motor is
  inversely proportional to its efficiency.
  Therefore, high-efficiency motors cost less to
  operate, but they also usually cost more to
  purchase. However, the energy savings usually
  make up for the price differential during the
  first few years.
• Efficiencies of motors used to power compressors
  usually range from about 70 to over 96.

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2. High-Efficiency Motors

• Equation 6-92 Motor Efficiency
• Welectric Wcomp / ?motor
• Equation 6-93 Electric Power Saved
• Equation 6-94 Energy Savings
• Energy savings W electric, saved Annual
  operating hours

W electric, saved W electric, standard W
electric, efficient W comp (1/? standard 1/?
efficient) (Rated Power) (Load Factor) (1/?
standard 1/? efficient)
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3. Outside Air for Intake

• Compressor work is directly proportional to the
  inlet temperature of air. Therefore, the lower
  the inlet temperature of the air, the smaller the
  compressor work.
• Outside air is generally cooler, and thus denser
  than the air in the compressor room. Therefore,
  it is advisable to install an intake duct to the
  compressor inlet so that the air is supplied
  directly from the outside.
• Eq 6-95 Power Reduction Factor

f reduction W comp,inside W
comp,outside W comp,inside
T inside T outside 1 T outside
T inside T
inside
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4. Reducing Pressure Setting

• Another source of energy waste is compressing the
  air to a higher pressure than required by the
  air-driven equipment.
• Energy savings can be realized by determining the
  minimum required pressure and reducing the air
  pressure control setting accordingly.
• Eq 6-96 Power reduction factor

f reduction (Wcomp,current Wcomp,reduced)/
Wcomp,current 1 - (P2,reduced / P1)(n-1)/n
-1 (P2/P1)(n-1)/n -1
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4. Reducing Pressure Setting

• Example 6-23 Reducing the Pressure Setting to
  Reduce Cost
• Determine how much money will be saved as a
  result of reducing the pressure of the compressed
  air.
• P1 (Atm) 85.6 kPa
• P2 (original) 900 P1
• 985.6 kPa
• P2,reduced 800 P1
• 885.6 kPa
• n (ideal gas) 1.4
• Current Cost 12,000/yr

f reduction 1 - (885.6/85.6) (0.4/1.4) -1
(985.6/85.6) (0.4/1.4) -1
0.060 Cost Savings Current Cost f
reduction (12,000/yr)(0.06) 720 / year