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AIR COMPRESSORS

    Compressed air, a valuable resource in industry, can be one of the most expensive processes in manufacturing facilities.  Annual operating costs of air compressors, dryers and supporting equipment can account for up to 70 percent of the total electric bill.  In manufacturing plants, compressed air uses range from small hand tools such as drills, grinders and staplers to agitation of liquid storage and process tanks and pneumatic transfer of materials.  The majority of compressed air usage is at a line pressure of 90 to 100 psig.
    Because the greatest single cost of manufacturing may be attributed to compressed-air, it follows that the greatest potential for energy conservation may exist with compressed air equipment.   The following recommendations in this module illustrate the energy savings derived from the optimization of a compressed air system.

   General Rules of Thumb:

  • The average cost of electricity is $0.05/kWh ($15/MMBtu)
  • There are 2000 hours per year per shift (based on the assumption that one shift is 8 hours per day, 5 days per week, 50 weeks per year)
  • Cost of compressed air leaks (100 psig):  $30 to $90/leak/shift/year
Notes:                    Before choosing the following targeted recommendations READ THE FOLLOWING:     Pay back estimates for the following recommendations will use the equation below.  They will vary depending on the, application, type of installation, and purchase quantity of material and labor associated with each recommendation.  It will be up to the person doing the analyses to use the URL references below each equation to help estimate an implementation cost.

    The data correlating to the variables below each equation will be prompted for in order to execute a calculation.  Frequently the fuel cost (FC) associated with the specific recommendation will be prompted for in order to calculate the annual cost savings (ACS). Unless otherwise specific to a particular recommendation the ACS will be calculated as follows:
 

Air data tables: ref: National Bureau of Standards Handbook # 115
 
 
Temperature of intake air, F
Intake Volume Required to Deliver 1000 cu ft of free air at 70 F 
Percent HP savings or increase relative to 70 F intake.
30
925
7.5 % savings
40
943
5,7 % savings
50
962
3.8 % savings
60
981
1.9 % savings
70
1000
0
80
1020
1.9 % savings
90
1040
3.8 % savings
100
1060
5.7 % savings
110
1080
7.6 % savings
120
1100
9.5 % savings

 
 
Hole Diameter, in
*Free Air Wasted, cu ft per year, by a leak of air at:
**Energy loss, Btu/hr
-----------------------------------------
----------------100 psig-------------
-----------------------------------------
3/8
79,900,000
1667
1/4
35,500,000
740
1/8
8,880,000
185
1/16
2,220,000
46
1/32
553,000
11.5
1/64
138,250
2.9
-----------------------------------------
---------------70psig---------------
-----------------------------------------
3/8
59,100,000
1004
1/4
26,200,000
447
1/8
6,560,000
340
1/16
1,640,000
28
1/32
410,000
7
1/64
102,500
1.75.

    *  Based on nozzle coefficient of 0.65
    ** Based on 100,000 Btu of fuel/kWh

  1. Reduce pressure of compressed air system
  2. Reduce compressed air use
  3. Use smaller compressor
  4. Use cool intake air
  5. Eliminate leaks
  6. Use engineered nozzles

1.  Reduce pressure of compressed air system.
        Increases in air pressure set-points may have been made to overcome excessive air leaks or because new compressed air equipment was installed using undersized distribution piping.  Considering that most compressed-air tools and equipment can be operated satisfactorily with a 90 psig pressure set point and that the average well-designed compressed-air distribution system will experience a pressure drop of approximately 10 psig at the farthest distribution point, the set-point for the compressors should be 100 psig.  It is recommended that all adjustments to compressor set-points be done incrementally and that the cause of an excessive pressure set-point be identified.  The following equation is left to describe the potential savings that can be obtained when a set-point is optimized.



2.  Reduce compressed air use
         Reducing absolute volumetric flow can reduce air compressor costs.  Considering this the following suggestions are made.  Reduce or eliminate compressed air cooling when outside air can cool process streams/equipment; reduce the use of compressed air to a minimum for cooling and/or agitating; Eliminate or reduce the use of compressed air for process operations; replace compressed air cooling with water or air cooling; reduce the use of compressed air pressure systems for safety systems in favor of direct acting units.  The following equation illustrates the potential savings that can be achieved.
     

    Power Engineering Books
    ASHRAE
    SullAir
    Thomas Register

    HP = compressor horsepower
    HY = operating hours per year
    ARF = air reduction factor (estimation of amount of air reduction that can accommodate process and equipment needs)
    CON = conversion factor, 0.7465 KW/hp
    LF = compressor load factor
    h = efficiency of compressor motor

    Press the calculate button to execute an estimation

    HES = hourly energy lost from a continuous air leak (Air data table scroll to second table)
    HY = operating hours per year
    FRAC = fraction of time that compressor air can be turned off (obtain from plant personnel)

    Data Conversions?
    RPN Calculator?

    Press the calculate button to execute an estimation

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3. Use smaller compressor
        The use of a smaller compressor can reduce plant operating costs when an oversized compressor is used in a process application that is unnecessary.  This can include situations where the process demands have been reduced due to a change in manufacturing, and/or the compressor cost was not weighed against an optimal sizing to a particular process.  The following equation shows the savings that can occur when a smaller compressor is used.

4. Use cool intake air and check filters

        Install, clean, or replace compressor air filters regularly, to maintain required suction pressure.
    Each 1% decrease in suction pressure costs 1% in compressor mass flow output and efficiency (ref.- "Compressed Air Systems : A Guidebook On Energy Costs And Savings").
    Install compressor air intakes in coolest locations; use heat exchangers to cool air intake to compressor.  The following
    equation illustrates the savings that can be achieved when cooler intake air is used.


     


    Power Engineering Books
    ASHRAE
    SullAir
    Thomas Register

    HP = compressor horsepower
    PS = percent savings (see Air data table concerning cool intake air)
    LF = load factor
    HY = operating hours per year
    CON = conversion factor, 0.7465 KW/hp
    h  = efficiency of compressor motor

    Data Conversions?
    RPN Calculator?

    Press the calculate button to execute an estimation

    BACK TO FRONT PAGE


5.  Eliminate leaks
         Eliminate leaks in lines and valves carrying compressed air or other gases; remove or close off unneeded compressed air lines to eliminate potential leaks.  Leaks typically occur in fittings, valves, regulators and in hand-tools.  Large compresed-air leaks can be identified with normal hearing, while small leaks can only be identified with ultrasonic detection equipment.  The energy and savings, possible through a reduction in compressed air leaks is calculated as follows.

6. Engineered Nozzles
         Open end nozzles should be replaced by engineered nozzles which are able to induce a large airflow entrainment while still using a smaller volume of air than the open jets.  The velocity of the resulting airflow is reduced, but the mass flow of the air is increased, thus increasing the cooling effect.  Energy savings result due to a decrease in compressor work that is required to provide the nozzles with compressed air, and are calculated as follows.
 
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