The presentation gives an idea as to how the compressed air system is designed and the performance of the compressed air system. The losses, conservation of energy, the cost of leakages etc are discussed in the presentation

1. COMPRESSED AIR SYSTEM . ENERGY
CONSERVATION OPPORTUNITIES
Manohar Tatwawadi
total output power solutions
A2-806, PALLADION, Balewadi, Pune, 411045
+91 9372167165, mtatwawadi@gmail.com

2. INTRODUCTION
Compressed air systems are one of the energy intensive load
of industrial sector and account for 3.0-4.0 % of total electrical
power utilised by Indian industrial sector
Compressed air is preferred in many industrial applications
due to its convenience and operational safety. The common
applications are
• Spray painting
• Pneumatic operation of instruments
• Pneumatic conveyor for materials
• Pressure testing of various products
• Pneumatic operation of grinders, drills, chipping, hammer
press, etc..
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3. Typical Life Cycle Cost of Air
Compressor
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4. Life cycle cost of air compressor
• The initial cost is low compared to the energy cost in lifetime.
• This indicates the importance of replacing the inefficient ones
and installing energy efficient compressors.
• The overall efficiency of most of the compressed air systems is
10 % or so, because of poor efficiency of compressors,
pressure drop in pipeline and conditioning equipment; leakage
and low efficiency of end use itself.
• It is always beneficial to switch over to other source of energy
if possible (For example, pneumatic conveyors, drills, chipping,
grinding, hammer press, etc. can be easily replaced by
electrical power operated equipment) and energy saving
achieved will be in the order of 60-90 %.
• Hence, compressed air requirement has to be critically studied
/ analysed before installation.
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5. COMPRESSOR TYPES AND CAPACITY
CONTROL
• Compressors are classified into three types based on
their operating principle –
• Reciprocating type,
• Screw type and
• Centrifugal type.
• Air compressors are selected based on air pressure &
flow required, air quality required (permitted moisture
/ oil content), plant air demand profile over time and
permissible down time.
• The range of capacity and operating pressure of various
types of compressors are presented in next slide.
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6. Capacity and Operating Pressure of
Compressors
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7. COMPRESSOR TYPES
• Screw compressors are of high speed and high capacity ones and the
output is continuous. This high speed needs noise suppressors. Dry type
screw compressors are available upto 15 bar and lubricated type from 3.5-
10.0 bar.
• Reciprocating types are highly energy efficient and have better capacity
control and the output is pulsating one. These may be single acting for
small capacities and double acting for larger capacities. Reciprocating type
compressors are most commonly used since its part load and full load
efficiency are better than screw type. Reciprocating compressor needs
more maintenance whereas screw type needs minimum maintenance.
Hence, screw type is used where the production down time is more
expensive.
• Centrifugal type is employed only for higher capacity air requirements
with single or multi-stage. Root blowers (do not compress air internally
but against system back pressure as in aeration of liquids) are chosen
when the pressure requirement is more than centrifugal blowers and less
than reciprocating compressors.
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8. Capacity and Control
• Air delivered from compressors may contain traces of moisture / oil.
• lf the applications demand oil-free air, non-lubricated compressor is
to be used.
• For moisture control, driers have to be used. Since driers consume
significant amount of power, only the air for special applications
shall be dried.
• In screw type, capacity control is achieved by throttling the suction
and opening discharge to the atm. Putting off the motor (through
pressure switch sensing) of the compressor is the capacity control
for reciprocating type upto a capacity of 2.8 M3/min. and for higher
capacities; it is cylinder unloading with suction valve open. Suction
side inlet guide vane control is used in centrifugal type for capacity
control.
• For low range of capacity and pressure, the compressor system is
air-cooled type and for higher range, it is water cooled type.
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9. Typical Compressed Air System
Charging
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10. CONDITIONING EQUIPMENT
• Air filter, inter and after cooler, filter, moisture separator and drier system
form the conditioning equipment of compressed air system.
• Dust in ambient air causes excessive wear of moving parts and leads to
malfunctioning of various valves. Air filter will filter the dust in intake air.
• Moisture separator will remove moisture in air in condensed form only.
Moisture drain is installed at inter and after cooler, receiver and
distribution line at various points to remove the condensed water from
the compressed air.
• Filter is installed in the line to separate the oil, dust and corroded particle
from air which otherwise may damage the end use equipment.
• The safety valve will open when the pressure inside the receiver exceeds
a preset limit.
• Air driers are used to maintain the moisture content in air at required
level for the process.
• The pressure of compressed air will drop across all conditioning
equipment.
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11. CONDITIONING EQUIPMENT
After coolers do not affect the power consumption of compressors.
• However, poor heat removal across after coolers will lead to carry
over of moisture to distribution system or to drier (if drier is used)
resulting in increased power consumption of driers.
• Even over sizing of after cooler is desirable. About 65 % moisture is
removed at the outlet of after cooler and 96 % of moisture after the
drier. Every 10O C rise in discharge temperature doubles the
moisture content.
The importance of inter and after cooler is……….
• lf discharge air temperature is high, volume increases significantly,
air carries more moisture and the cooling increases the mass flow
rate of air in the piping. .
• Cooling saves upto 7 % of energy for compression. .
• lf condensed water flows to end use, it will damage instruments
and make instrument sluggish, corrosion of receiver and air lines.
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12. Air Driers Requirement
• lf inlet ambient air is at 32OC, 50 % relative humidity
(RH) & 1 atm (a), after compression to 8 atm (a), the
volume of air reduces by 8 times, moisture becomes
400% and the air temperature is very high.
• This air is passed through water-cooled inter and after
coolers to bring down air temperature to near to
ambient temperature.
• At this temperature, air cannot hold more than 100 %
RH, hence the excess 300 % RH moisture is condensed.
The air after the after cooler is 100% wet saturated.
• This saturated air will condense its water vapour
wherever cooling due to expansion takes place.
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13. Air Driers Requirement
The need for an air drier can be understood from the following facts. .
• 100 CFM of air at 7 kg/cm2 contains 110 litres/day of water vapour.
• Instruments operating with wet air have 1/25th of life compared to
the one operating with dry air.
• Operator wastes about 10-15 % of air for draining water at various
points whenever starting the operation.
• When oil is used as lubricant in compressor, oil combines with
water and forms an emulsion, which rapidly degrades the control
system by corroding the jets and orifices of pneumatic instruments.
• Drier saves maintenance cost. Hence, air driers are installed to
maintain the moisture content in air at required level for the
process.
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14. Selection Criteria For Air Dryers
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15. Heat Less Type Drier
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16. Refrigeration type drier
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17. External air heated type drier
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18. Heat of compression type drier
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19. Split Flow - No Purge Loss Type Drier
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20. AIR CHARGING AND LEAKAGE TEST
• lf a compressor is rated as 30m3/min. at 7 bar, it means that it can draw
30m3 of air from ambient and compress it to 7 bar gauge pressure.
• The actual capacity can be estimated by conducting the charging test and
the test procedure is given below.
1. Drain the receiver to zero pressure and close valve V1 and moisture drain
valve (Figure).
2. Note down the volume of receiver tank and pipe line between
compressor and condensate receiver.
3. Energise the compressor and charge the receiver tank from 0 to rated
pressure and note down the time taken.
4. Present capacity of the compressor = ((P2- P1)/Pa)*(V/t)*(Tr/Ta) m3/min.
Where ;
• P1 & P2 - Initial and final receiver pressure (absolute), bar .
• Pa - Atmospheric pressure (absolute), bar,
• V - Volume of receiver & pipe line, m3 .
• t- Time taken for charging receiver from P1 to P2 , min.
• Tr & Ta - Air temperature at receiver & intake, 0K.
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21. Leakage Test
• An air leak of 20-50 % is very common in most of the plants.
Air leakage is difficult to detect and can go unnoticed for
long time. The major leakage point may be identified by the
hissing sound of air where as the minor leakage can be
located only by applying soap solution on the safety and
check valves, valve stems, threaded joints, flanges, filter,
hose, connectors, moisture trap & drain.
The procedure to estimate the leakage rate is as below.
• Open valve V1 and close V2, V3 & V4. (Figure).
• Energise the compressor and note down the loading and
unloading cycle time of the compressor.
• Air leak = Load time x present capacity/(Load + Unload) time
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22. Compressor Performance
Capacity of a Compressor
• Capacity of a compressor is the full rated volume of flow of
gas compressed and delivered at conditions of total
temperature, total pressure, and composition prevailing at the
compressor inlet.
• It sometimes means actual flow rate, rather than rated
volume of flow.
• This also termed as Free Air Delivery (FAD) i.e. air at
atmospheric conditions at any specific location. Because the
altitude, barometer, and temperature may vary at different
localities and at different times, it follows that this term does
not mean air under identical or standard conditions.
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23. Compressor Efficiency Definitions
• Several different measures of compressor
efficiency are commonly used: volumetric
efficiency, adiabatic efficiency, isothermal
efficiency and mechanical efficiency.
• Adiabatic and isothermal efficiencies are
computed as the isothermal or adiabatic power
divided by the actual power consumption.
• The figure obtained indicates the overall
efficiency of compressor and drive motor.
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24. Isothermal Efficiency
• The calculation of isothermal power does not include power needed to
overcome friction and generally gives an efficiency that is lower than
adiabatic efficiency.
• The reported value of efficiency is normally the isothermal efficiency. This
is an important consideration when selecting compressors based on
reported values of efficiency.
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25. Volumetric Efficiency
• For practical purposes, the most effective guide in
comparing compressor efficiencies is the specific power
consumption ie kW/volume flow rate , for different
compressors that would provide identical duty.
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26. Effect of Intake Air Temperature
• As a thumb rule, "Every 4°C rise in inlet air temperature results in a higher
energy consumption by 1 % to achieve equivalent output". Hence, cool air
intake leads to a more efficient compression.
• It is preferable to draw cool ambient air from outside, as the temperature
of air inside the compressor room will be a few degrees higher than the
ambient temperature. While extending air intake to the outside of
building, care should be taken to minimize excess pressure drop in the
suction line, by selecting a bigger diameter duct with minimum number of
bends.
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27. Effect of Air Intake Temperature
• The effect of intake air temperature on the power
consumption of the compressor is related as given below.
• Y = -4.66745+0.326074 X - 0.001 91 X2
• Where X: Intake air temperature (range 10-500 C)
• Y = Increase in power consumption (range 0-8 %)
• It may he noted that every 40 C rise in intake air
temperature leads to 1 % increase in power consumption
for same mass flow rate.
• This is applicable to the compressed air after the
intercooler also (inlet to second stage). lf the temperature
is higher, the density is low and volume is higher for same
mass flow requirement. Consequently higher power is
required for compression.
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28. Dust Free Air Intake
• Dust in the suction air causes excessive wear of moving parts
and results in malfunctioning of the valves due to abrasion.
• Suitable air filters should be provided at the suction side. Air
filters should have high dust separation capacity, low-pressure
drops and robust design to avoid frequent cleaning and
replacement.
• See Effect of pressure drop across air filter on power
consumption.
• Air filters should be selected based on the compressor type
and installed as close to the compressor as possible.
• As a thumb rule "For every 250 mm WC pressure drop
increase across at the suction path due to choked filters etc,
the compressor power consumption increases by about 2%
for the same output.
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29. Effect of pressure drop across air filter
on power consumption
Hence, it is advisable to clean inlet air filters at regular intervals to minimize pressure
drops. Manometers or differential pressure gauges across filters may be provided for
monitoring pressure drops so as to plan filter-cleaning schedules.
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30. Dry Air Intake
• Atmospheric air always contains some amount of water vapour,
depending on the relative humidity, being high in wet weather. The
moisture level will also be high if air is drawn from a damp area - for
example locating compressor close to cooling tower, or dryer exhaust
is to be avoided.
• The moisture-carrying capacity of air increases with a rise in
temperature and decreases with increase in pressure.
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31. Elevation
• The altitude of a place has a direct impact on the volumetric efficiency of
the compressor. The effect of altitude on volumetric efficiency is given in
the table below.
• It is evident that compressors located at higher altitudes consume more
power to achieve a particular delivery pressure than those at sea level, as
the compression ratio is higher.
•
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32. Cooling Water Circuit
• Most of the industrial compressors are water-cooled, wherein the
heat of compression is removed by circulating cold water to
cylinder heads, inter-coolers and after-coolers. The resulting warm
water is cooled in a cooling tower and circulated back to
compressors. The compressed air system performance depends
upon the effectiveness of inter-coolers, after coolers, which in turn
are dependent on cooling water flow and temperature.
• Further, inadequate cooling water treatment can lead to increase,
for example, in total dissolved solids (TDS), which in turn can lead to
scale formation in heat exchangers. The scales, not only act as
insulators reducing the heat transfer, but also increases the
pressure drop in the cooling water pumping system.
• Use of treated water or purging a portion of cooling water (blow
down) periodically can maintain TDS levels within acceptable limits.
It is better to maintain the water pH by addition of chemicals, and
avoid microbial growth by addition of fungicides and algaecides.
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33. Efficacy of Inter and After Coolers
• Efficacy is an indicator of heat exchange performance- how well
intercoolers and after coolers are performing.
• Inter-coolers are provided between successive stages of a multi-
stage compressor to reduce the work of compression (power
requirements) - by reducing the specific volume through cooling the
air - apart from moisture separation.
• Ideally, the temperature of the inlet air at each stage of a multi-
stage machine should be the same as it was at the first stage. This is
referred to as "perfect cooling" or isothermal compression. The
cooling may be imperfect due to reasons described in earlier
sections. Hence in actual practice, the inlet air temperatures at
subsequent stages are higher than the normal levels resulting in
higher power consumption, as a larger volume is handled for the
same duty (See Table in the next slide).
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34. EFFECT OF INTER-STAGE COOLING ON
SPECIFIC POWER CONSUMPTION OF A
RECIPROCATING COMPRESSOR
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35. EFFECT OF INTER-STAGE COOLING ON
SPECIFIC POWER CONSUMPTION OF A
RECIPROCATING COMPRESSOR
• It can be seen from the Table that an increase of 5.5°C in the inlet air
temperature to the second stage results in a 2 % increase in the specific
energy consumption.
• Use of water at lower temperature reduces specific power consumption.
However, very low cooling water temperature could result in condensation
of moisture in the air, which if not removed would lead to cylinder
damage.
• Similarly, inadequate cooling in after-coolers (due to fouling, scaling etc.),
allow warm, humid air into the receiver, which causes more condensation
in air receivers and distribution lines, which in consequence, leads to
increased corrosion, pressure drops and leakages in piping and end-use
equipment.
• Periodic cleaning and ensuring adequate flow at proper temperature of
both inter coolers and after coolers are therefore necessary for sustaining
desired performance. Typical cooling water requirement is given in Table
in next slide.
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36. Cooling Water Requirement
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37. Pressure Settings
• Compressor operates between pressure ranges called as loading
(cut-in) and unloading (cut-out) pressures. For example, a
compressor operating between pressure setting of 6 – 7 kg/cm2
means that the compressor unloads at 7 kg/cm2 and loads at 6
kg/cm2 . Loading and unloading is done using a pressure switch.
• For the same capacity, a compressor consumes more power at
higher pressures. They should not be operated above their
optimum operating pressures as this not only wastes energy, but
also leads to excessive wear, leading to further energy wastage The
volumetric efficiency of a compressor is also less at higher delivery
pressures.
• The possibility of lowering (optimising) the delivery pressure
settings should be explored by careful study of pressure
requirements of various equipment, and the pressure drop in the
line between the compressed air generation and utilization points.
Typical power savings through pressure reduction is shown in Table.
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38. Pressure Settings
• Pressure Reduction Power Savings
• The pressure switches must be adjusted such that the compressor
cuts-in and cuts-out at optimum levels.
• A reduction in the delivery pressure by 1 bar in a compressor would
reduce the power consumption by 6 – 10 %.
• If the low-pressure air requirement is considerable, it is advisable to
generate low pressure and high-pressure air separately, and feed to
the respective sections instead of reducing the pressure through
pressure reducing valves, which invariably waste energy.
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39. Pressure Settings
Compressor modulation by Optimum Pressure Settings
Very often in an industry, different types, capacities and makes of
compressors are connected to a common distribution network. In such
situations, proper selection of a right combination of compressors and
optimal modulation of different compressors can conserve energy.
Where more than one compressor feeds a common header, compressors
have to be operated in such a way that the cost of compressed air
generation is minimal.
• If all compressors are similar, the pressure setting can be adjusted such
that only one compressor handles the load variation, whereas the
others operate more or less at full load.
• If compressors are of different sizes, the pressure switch should be set
such that only the smallest compressor is allowed to modulate (vary in
flow rate).
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40. Modulation of Compressors
• If different types of compressors are
operated together, unload power
consumptions are significant. The
compressor with lowest “no load
power” must be modulated.
• In general, the compressor with
lower part load power consumption
should be modulated.
• Compressors can be graded
according to their specific energy
consumption, at different pressures
and energy efficient ones must be
made to meet most of the demand
(See Table).
Pressure
Bar
No. of
Stages
Specific Power
kW/170 m3/hr
(kW/100cfm)
1 1 6.29
2 1 9.64
3 1 13.04
4 2 14.57
7 2 18.34
8 2 19.16
10 2 21.74
15 2 26.22
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41. Air Lines, Filter & Moisture
• lf air filter is installed near the compressor itself, it will draw
hot air since the compressor generates lot of heat during its
operation.
• Hence, it is preferable to extend the suction line (larger
diameter is required to minimise pressure drop) outside
the compressor room so that fresh, dust and moisture free
(far away from cooling towers) air is drawn for
compression.
• lf more moisture is there, energy is consumed for
compressing it and to condense the same at inter and after
coolers.
• Moisture carrying capacity of air increases with increasing
temperature and decreases with increased pressures.
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42. Pressure Drop in Air Lines
Minimum pressure drop in air lines
• Excess pressure drop due to
inadequate pipe sizing, choked
filter elements, improperly sized
couplings and hoses represent
energy wastage.
• The Table illustrates the energy
wastage, if the pipes are of
smaller diameter.
• Typical acceptable pressure
drop in industrial practice is 0.3
bar in mains header at the
farthest point and 0.5 bar in
distribution system.
Pipe
Nominal
Bore(mm)
Pr. Drop
(bar)/100M
Equi. Power
loss (kW)
40 1.80 9.5
50 0.65 3.4
65 0.22 1.2
80 0.04 0.2
100 0.02 0.1
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43. Air Lines Pressure Drops
• Equivalent lengths of fittings Not only piping, but also fitting are a
source of pressure losses. Typical pressure losses for various fitting
are given in Table.
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44. Energy Saving by Use of Blowers
• Blowers in place of Compressed Air System
Since the compressed air system is already
available, plant engineer may be tempted to
use compressed air to provide air for low-
pressure applications such as agitation,
pneumatic conveying or combustion air.
• Using a blower that is designed for lower
pressure operation will cost only a fraction of
compressed air generation energy and cost.
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45. Air leakages through Different Size
Orifices
• The amount of free air wasted for different nozzles sizes
and pressure.
• TABLE FOR DISCHARGE OF AIR (m3/MINUTE) THROUGH
ORIFICE (ORIFICE CONSTANT Cd – 1.0)
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46. Cost of Compressed Air Leakage
It may be seen from Table that any expenditure on stopping leaks
would be paid back through energy saving.
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