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The KAESER Compressed Air Seminar Part 9 1
9. The Air Main
9.1 Air Losses
9.2 Pressure Drop
9.3 Mains Sizing and Selection of Pipework
9.4 Air Mains with or without Air Dryers
9.5 Outdoor Air Main
9.6 Selection of Pipe Materials
9.7 Identification of Pipelines
9.8 Save Energy and Lower Costs
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The KAESER Compressed Air Seminar Part 9 2
9.1 Air Losses
Air losses increase operating costs !!!
Example: a 3 mm diameter holeAir loss: 0,5 m/min (6 bar)0,5
m/min x 60 min/h = 30 m/h30 m/h x 8,000 h/year = 240,000
m/year240,000 m/year x 0.02 P = 4,800 \HDULeakagesCompressed air
must be routed with the minimum of air volume reduction (through
leaks)And thus at minimum cost from the compressed air installation
to the consumers.
Correspondingdiametre of hole
Size mm
Air lossat 6 barm/min
LosskW
1 0.065 0.3 240,--
2 0.240 1.7 1,360,--
4 0. 980 6.5 5,200.--
6 2.120 12.0 9,600,--
Example:0.1 kWh is needed to compress 1 m of air at 7,5 bar. At
4,000 service hours per year and power costs of0.10
N:KWKHIROORZLQJVXPIRUDWRWDOOHDNDJHLQDQDLUPDLQRIP/min is
calculated:5 m/min x (4,000 h x 60 min/h) x 0,1 kWh/m x 0,10 N:K -
\HDULeakage losses raise the costs of producing compressed air or
lower the performance of pneumatictool or machine.
9. The Air Main
* Electricy price: 0.10 N:Kworking period: 8,000 sh/year
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The KAESER Compressed Air Seminar Part 9 3
Measuring leakages by emptying the air receiver
9. The Air Main
Supply line shut off
Amount of leakage
VR x (pI pF)VL =
t
VL = volume of leakVR = receiver volumepl = initial receiver
pressurepF = final receiver pressuret = measuring time
This method of measurement is suited to airsystems in which the
pipework volume is lessthan 10 % of the air receiver volume. If
not,the accuracy of the measurement cannot beguaranteed.
Beispiel:VB = 500 lpl = 9 bar (g)pE = 7 bar (g)t = 3 minVL = 500
l x (9 bar 7 bar)/3 min
= 333 l/min
Leakage losses in the air main:333 l/min
Tools not in use
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The KAESER Compressed Air Seminar Part 9 4
Measuring leakages by measuring the cut-in periodof the
compressor with pneumatic equipment out of use
VL = volume of leak in m/minVC = volume flow of compressor in
m/mint = sum of time units during which the compressor ran under
loadT = total time for measuring procedure
Example:Volume flow VC of the compressor = 3 m/minLoad time t of
the compressor t = t1 + t2 + t3 + t4 + t5 = 120 sTotal time T for
the measuring procedure = 600 s
9. The Air Main
1
2
3
4
5
6
7
8
Workingpressure
bar(
g)
time
VK x tVL =
T
3 x 120VL = = 0,6 m/min = 20%
600
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The KAESER Compressed Air Seminar Part 9 5
Measuring consumer leakages
9.2 Pressure Drop
Air tools only work at their best if sufficient airvolume at the
correct pressure is available.The following illustration shows
howinsufficient pressure affects the performance ofa tool:
Pressure drop is caused by:
Internal friction (molekules)
Friction on the pipe walls
Turbulence
high flow velocities (small pipe bore sizes)
9. The Air Main
In plants in which a lot of air tools, machinesand equipment are
used, hose coupling andvalves cause significant air leakages.
Using the previously described methods, twomeasurements will be
carried out:
a.) tools, machines and equipment areconnected for normal use
(total leakage)
b.) the shut-off valves upstream of thecouplings for the air
consumers are closed (airmain leakage)
The difference between a) and b) is the sum ofthe losses at the
tools and their fittings.
%
kW
Pressure in bar (g)
Perform
ance
The normal pressure required by an air tool is 6 bar (g).
Increasing the pressure in thecompressor system to compensate for
the pressure drop (pressure loss) costs money.Example: V = 30 m/min
requires 160 kW of power at 7 bar (g). At 8 bar (g), 6% more
poweris required, i.e. approximately 9.4 kW more.Costs: 9.4 kW x
0.10 N:K[K\HDU -- \HDU
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The KAESER Compressed Air Seminar Part 9 6
The right fittings are also a pressure loss factor
A = valve (ball valve recommended)B = filter (to separate water
and rust)C = pressure regulator (for constant
working pressure)D = lubricator (mostly oil mist lubricators)E =
quick release coupling (flexibility)F = hose (3-5 m in length)G =
spring balancer (working help)
Pressure drop in the air main
On a well designed air main a pressure drop of 0.1 bar is to be
expected.
9. The Air Main
A
B
C E
F
G
1. Main supply line 0.03 bar2. Distribution line (ring)0.03
bar3. Branch line 0.04 bar4. Refrigeration dryer 0.2 bar5. Filter,
regulator,
lubricator and hose 0.5 barmax. 0.8 bar
Switching difference 0.2 bar1.0 bar
Max. pressure at the compressor 7.0 bar (g)Consumer pressure 6.0
bar (g)Pressure difference 1.0 bar
0.1 bar
1
23
4
5
Pressure drop 0.8 bar
3
5
At low working pressures, e.g. 3 bar (g), apressure drop of 0.1
bar means, however, arelatively higher loss of energy than in 7 bar
(g)air main.
The max. pressure drop in the air main 1.5 %
of the working pressure
Air supply using a ring main Air supply using a dead end
split-flow main
D
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The KAESER Compressed Air Seminar Part 9 7
9.3 Mains Sizing and Selection of Pipework
The following points must be observed when sizing and installing
an air main:
Flow impedance of fittings, converted into equivalent pipe
length
Pipe flow calculations
Equivalent length of pipe in mFittings
25 40 50 80 100 125 150 200 250 300 400
shut-off valve opened half closed
0,35
0,58
0,610
1,016
1,320
1,625
1,930
2,640
3,250
3,960
5,280
diaphragm valve 1,5 2,5 3,0 4,5 6 8 10 - - - -
elbow valve 4 6 7 12 15 18 22 30 36 - -
seat valve 3-6 5-10 7-15 10-25 15-30 20-50 25-60 30-75 - - -
check plate 2,0 3,2 4,0 6,4 8,0 10 12 16 20 24 32
bendR = 2d
0,3 0,5 0,6 1,0 1,2 1,5 1,8 2,4 3,0 3,6 4,8
bendR = d
0,4 0,6 0,8 1,3 1,6 2,0 2,4 3,2 4,0 4,8 6,4
knee bend 1,5 2,4 3,0 4,8 6,0 7,5 9 12 15 18 24
T-piece in direction of flow 0,5 0,8 1,0 1,6 2,0 2,5 3 4 5 6
8
T-piece in direction ofbranche flow
1,5 2,4 3,0 4,8 6,0 7,5 9 12 15 18 24
adaptor piece 0,5 0,7 1,0 2,0 2,5 3,1 3,6 4,8 6,0 7,2 9,6
9. The Air Main
Bore size of the pipeline:- air consumption- length of the air
line- working pressure- pressure drop- flow impedance
Fittings and connectors:- type of outlets- shut-off valves-
condensate separators- tool lubricators- dust filters- oil filters-
pressure regulating valves- hoses- couplings
Selection of pipe materials:- ambient conditions (humidity,
temperature, chemical pollution of theair)
- quality of the air (humidity content, oilcarry-over,
temperature)
- costs- expected service life
Installation:- ring mains- interconnecting lines- dead end
lines- pipe connections- fittings
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The KAESER Compressed Air Seminar Part 9 8
Example:- pipe length (air main) 100m- bore size of pipe 100 mm-
required fittings
Fittings No. off Equivalent pipe length in mmper fittimg
total
seat valve 4 30 120
bend r = d 12 1,6 19
T-piece 2 6 12
adapter piece 4 2,5 10
Total 161
Total pipe length (technical flow length):
Ltotal = Lstraight + Lcomparable
or, estimated
Ltotal = 1,6 x Lstraight
Minimum bore size of pipelines
Air deliverym/min
Working pressure 7,5 bartotal length:
up to 50 m to 100 m to 200 m over 200 m
Working pressure 10 bartotal length:
to 50 m to 100 m to 200 m over 200 m
Working pressure 13 bartotal length:
to 50 m to 100 m to 200 m over 200 m
up to 0,5 1 1 1 1 1 1 up to 1,0 1 1 1 1 1 1 1 1 1 up to 1,5 1 1
1 1 1 1 1 1 1 up to 2,0 1 1 2 1 1 2 1 1 2up to 3,0 1 1 2 1 1 2 1 1
2up to 5,0 1 2 2 1 2 2 1 1 2up to 7,5 2 2 2 2 2 2 1 2 2up to 10 2 2
2 2 2 2 2 2 2
up to 12,5 2 2 3 2 2 3 2 2 2 up to 15,0 2 2 3 2 2 3 2 2 3up to
17,5 2 3 DN100 2 3 DN100 2 3 DN100up to 20,0 3 3 DN100 3 3 DN100 2
3 DN100up to 25,0 3 DN100 DN100 3 DN100 DN100 3 DN100 DN100
up to 30,0 3 DN100 DN100 3 DN100 DN100 3 DN100 DN100
up to 40,0 DN100 DN100 DN125 DN100 DN100 DN125 DN100 DN100
DN125
Nominal bore size of pipelines - comparison
mm (DN: Diameter Nominal) Nominal bore size (inches)
DN 6 R 1/8DN 8 R 1/4DN 10 R 3/8DN 15 R 1/2DN 20 R 3/4DN 25 R 1DN
32 R 1 1/4DN 40 R 1 1/2DN 50 R 2DN 65 R 2 1/2DN 80 R 3DN 100 R 4DN
125 R 5DN 150 R 6
9. The Air Main
seestraightlinegraphforairlines
seestraightlinegraphforairlines
seestraightlinegraphforairlines
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The KAESER Compressed Air Seminar Part 9 9
Determination of pipeline bore size with astraight line
diagram
How to use the straight line graph:Mark axes A and B according
to the total pipe length and the air delivery. Connect both points
with a straight linethat cuts axis C.Now mark axes E and G with the
system pressure and the required pressure loss. Connect both points
with astraight line that cuts axis F.Now connect the two points of
intersection on axes C and F with a straight line and read off the
required pipelinebore size on axis D.
9. The Air Main
pipe lengthl[m]
FAD[m
3/h] [m
3/min]
bore sized[mm]
pressure loss
'p[bar]
gaugepressurep[bar]
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The KAESER Compressed Air Seminar Part 9 10
No falling pipelinevertical branch lines possible
9.4 Air Mains with or without Air Dryers
Air main without air dryer (pipe-laying principles in wet
areas)
Air main with air dryer
9.5 Outdoor Air Main
9. The Air Main
Compressor Air receiver withcondensate drain
Filter, waterseparator,regulator,lubricator
Filter,regulator Condensate
(water) drain
a) Installation in ducting or shaftsAdvantage: no spatial
restrictionDisadvantage: complicated, expensive andpoor
accessibility; separators must be locatedin frost-free space
b) Underground installationAdvantage: low costsDisadvantage:
repair and maintenanceexceptionally difficult, pipes in stainless
steelbecause of corrosionc) Overground installation with
supportedor suspended pipesAdvantage: relatively low
costDisadvantage: spatial restriction, risk offreezing, not
aesthetic
Winter operation is possible with shortoutdoor pipelines, and
with a pressure dewpoint of + 3 C (refrigeration dryer), if:
Larger bore sizes (no freezing) are used
The compressed air is subsequently re-warmed and a condensate
drain/filter isinstalled at the point of re-entry into aheated
area
The pipeline is vented during non-operational periods
The corresponding section of the pipelineis heated
CompressorAir receiver withcondensate drain
Dryer, incl.condensate drain
Regulator,lubricator
Regulator
Falling pipelineapprox. 2
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The KAESER Compressed Air Seminar Part 9 11
9.6 Selection of Pipe Materials
9. The Air Main
Concrete duct
Condensate line
water separator
Inspection pit
Cover
Drain valve
Nominalpressure(bar())
Seamless steelpipe
DIN 2448
Max. permissible working pressure in (bar())
up to 120 C up to 200 C
2,5
6
10
16
25
40
64
100
St 35
St 35
St 35
St 35
St 35
St 35.8
St 35.8
St 35.8
2,5
6
10
16
25
40
64
100
2
5
8
13
20
36
50
80
Influence of temperature on themaximum permissible presssureIf
the temperature increases, the yield pointof all materials reduces.
DIN sheet 2401shows the interrelationship betweennominal pressure
and the max. permissibleworking pressure for various materials.
Example: An air main that is designed for 6bar (g) and is
subjected to a test pressure of6 bar (g) may only be operated at
apressure of 5 bar (g) at temperatures above120 C.
Air line
Example of an outdoorseparator not subject tofreezing
temperature
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The KAESER Compressed Air Seminar Part 9 12
Steel pipelines
Seamlesssteel pipe
Threaded pipe Stainless steel pipe Copper pipe
Type
Black or galvanized to DIN2448
Medium weight to DIN 2440Heavyweight to DIN 2441
black or galvanized
Seamless to DIN 2462Welded to DIN 2463
Soft, in coils to DIN 1786Hard, in straight lengths
to DIN 1754/1786
Material e.g. St 35to DIN 1629
Seamless St 00 to DIN 1629Welded St 33 to DIN 17100
e.g. material No. 4301,4541, 4571
Copper
Dimensions 10,2 - 558,8 mm 1/8 - 6 6 - 273 mm6 - 22 mm soft6 -
54 mm hard
54 - 131 mm hard
Permissibleworkingpressure
12,5 - 25 bar 10 bar Up to 80 bar and, in part,higher
16 - 140 baraccording to type
Pipe ends Plain Tapered, plain or threaded Plain Plain
Pipe connection Welded Screw, welded Welded(inert gas arc
welding)
Srew, brazed (fittings),welded
Advantages Air light pipe connection Many fittings(for srew
connections)
Airtight connection, nocorrosion
No corrosion smooth innerwalls
Disadvantages
Corrosion on black steelpipe
Installation by experiencedtechnicians only
Corrosion (sometimes ongalvanized piping too)High flow and
friction
Impedance can leak afterlong service life
Time consuming installationbecause of thread cutting or
weldingInstallation by experienced
technicians only
Installation by experiencedtechnicians only
Limited number of fittings
Special installation skillsrequired
Possibility of coppersulphate formation
Plastic pipelines
Material GIRAIR (PVC) Polyamide Polyethylene netted
Polyethylene
Dimensions 16 - 110 mm 2 - 40 mm 10 - 160 mm 10 - 160 mm
DIN DN 8061/62 DN 16982 DN 8074 DN 16893
Permissibleworking pressure
at 20 C
12,5 bar Up to 100 bar 10 bar Up to 20 bar
Pipe ends Plain Plain Plain Plain
Pipe connection Cold solvent welding Fittings Welding
Fittings/press fit
Advantages No leaksFittings of the same
materialHardly inflammable
Higher pressuresHigh chemical resistance
No leaksFittings of the same
materialNormal inflammability
High temperatureresistance
High chemical resistanceHardly inflammableMax. resistance to
aggressive compressed air
Simple to lay, low weight, no corrosion
Disadvantages Limited size range in partHigher coefficient of
expansionOnly normal inflammability in part check in each
individual casePossibility of static charge in partLegislative
warranty only in partSome fittings of metal
9. The Air Main
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The KAESER Compressed Air Seminar Part 9 13
Material comparison
The following selective criteria should be taken into account
when planning an air main.The costs of material and installation
are disregarded in this table.
Steel pipe to DIN 2440, 2441, 2448Criteria
Individualrequirements
Black,threaded
Black,welded
Galvanized,threaded
Galvanized,welded
Copper toDIN 1786,
17545
StainlessSteel toDIN 2462,
2463
Plastics
Size rangeup to 50 mmup to 100 mmover 100 mm
*XX(X)
XXX
XX(X)
XXX
XX(X)
XXX
XXX
Pressure rangeup to 10 barup to 12,5 barover 12,5 bar
*X (DIN2440/41)
X1)
X1)
X (DIN2440/41)X
1)
X1)
X (DIN2440/41)X
1)
X1)
X (DIN2440/41)X
1)
X1)
XXX
XXX
X(X)(X)
CorrosionAir quality
* 3 3 2 2 2 1 1
Temperature rangeup to 20 Cup to 50 Cup to 80 Cover 80 C
*XXXX
XXXX
XXXX
XXXX
XXXX
XXXX
X(X) up to 8 bar
--
Flow characteristik * 2 2 2 2 1 1 1
Toxicologicalcharacteristic
3 3 3 3 3 1 1
Anti-static 1 1 1 1 1 1 3
Installation effort,specialists and
mates
3X-
2X-
3X-
2X-
2X-
2X-
1-X
Weight 3 3 3 3 3 3 1
Maintenance 3 2 3 2 1 1 1
Air tightness * 3 1 3 1 1 1 1
SUM of* criteria
8 6 7 5 4 3 3Technically advantageous
Example:High demands on the air quality (no corrosion), minimum
energy loss(airtight and hydraulically smooth inner walls), simple
installation,normal working pressure, e.g. 7 bar (g).Used in the
following areas: air and space travel
precisionmechanics/optics/watch and clock making wood processing
electrics textiles printing food industries office machines/DP
machine tools chemicals.
1 = very good, 2 = acceptable, 3 = with limitations1) DIN 2448
according to quality specification to DIN 1629
9. The Air Main
* = identification of thecriteria relevant to theindividual
selection
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The KAESER Compressed Air Seminar Part 9 14
9.7 Identification of pipelines
Medium Group Colour and number
Air 3 grey RAL 7001
Water 1 green RAL 6018
Inflammable liquids 8 brown RAL 8001
Gas 4/5 yellow RAL 1012
Steam 2 red RAL 3003
Acid 6 orange RAL 2000
Solvent 7 violet RAL 4001
Oxygen 0 blue RAL 5015
9. The Air Main
DIN 2403 and BS 1710 states that in theinterests of safety,
correct maintenance andeffective fire-fighting methods, it is
imperativethat pipelines are clearly identified according tothe
flow medium.This identification must indicate dangers in orderto
prevent accidents and personal injury.The colour code
identification to DIN 2403provides immediate information on the
mediumflowing at the point of identification
The colour code must be clearly marked:
at the start of the pipeline
at the end of the pipeline
at junctions
at wall breakthroughs
at fittings
using coloured rings along thecomplete length
Compressd Air8 bar
SignsUsing either text or codenumbers
3.1 8 bar
No. of the sub-group
Group number
Grey = Colour of Group 3 Air RAL 7001
Classification for compressed air
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The KAESER Compressed Air Seminar Part 9 15
9.8 Save Energy and lower your CostsPlanning, Installation,
Redevelopment of an Air Mainby Erwin Ruppelt
9. The Air Main
Compressed air may be a versatilecarrier of energy, but contrary
toopinions that are still very widespread, itis a very expensive
one. It only becomesa really economical option when all
thecomponents of the air system airproduction, treatment and
distributionare optimally matched to each other.Correct sizing of
the system, correctmaintenance and, when necessary,appropriate
refurbishing of the air mainare all requirements maintaining
systemefficiency. The following articledescribes points that should
beobserved in the trade.
If one takes all expenditure for energy,cooling, maintenance and
depreciation ofthe compressor into account then a cubicmetre of
compressed air, depending on thesize, loading, state of repair and
type ofcompressor, costs between DQG0.03. In many plants, great
value is placedon really efficient production of compressedair:
older compressors are being replacedby efficient fluid-cooled screw
compressors.Because of the more effective cooling andlower
maintenance requirements of thesecompressors, up to 20% of former
costs forthe production of compressed air can besaved. Naturally,
it is precisely in the tradewhere many uses for
reciprocatingcompressors are still to be found. Theseinclude
transportable and mobile workshopcompressors that are often used to
powerair tools on construction sites as well. Inthese cases, where
easy transport andhandling are paramount, oil
lubricated,single-stage reciprocating compressors givethe craftsman
the possibility of usingcompressed air economically and
flexibly.The lower delivery limit of mobilecompressors lies at
around 20 l/min, theupper limit at around 450 l/min; their
workingpressure is normally between 4 and 10 bar.Pressure up to 20
bar are possible forspecial applications such as in glassworking
(fig. 1).
Also, single-stage reciprocatingcompressors with gauge pressure
up to 10bar are well suited as reasonably pricedstationary supplies
in smaller workshopshaving a low air demand.
In the displacement range between 120and 2000 l/min they are the
ideal solutionif there is no continuous compressed airrequirement.
In contrast, in workshopsthat need working pressures up to 15
bar,
Fig. 1: Air at a maximum gauge pressure of 10 or20 bar, needed
for inserting putty in windows forexample, is generated by
transportablereciprocating compressors of this series. Thesesmall,
powerful compressors can be takenpractically anywhere and can be
used for manyof the different craftsmans tasks
a two-stage reciprocating compressor isrecommended they are
ideal ifcompressed air is not required constantly.In contrast,
workshops needing a workingpressure up to 15 bar, a
two-stagereciprocating compressor isrecommended. The duty of both
thesingle and the two-stage compressorshould not exceed 70%
however,otherwise under higher load conditions,the maintenance
requirement would bedisproportional. The same applies if theair
delivery exceeds 2000 l/min orcompressed air is
demandedcontinuously. In such situations, it is farbetter to use a
suitably sized screwcompressor because a reciprocatingcompressor is
distinctly inferior to a screwcompressor when maintenance costs
andefficiency are compared.
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The KAESER Compressed Air Seminar Part 9 16
9. The Air Main
In most workshops, much attention is paidto reasonably priced
air production.Correct treatment of the application isusually
ignored. This is to be regretted,because only properly treated air
reducesmaintenance costs for consumer equipmentand the air
distribution system.In 80% of all applications refrigerationdryers
are more than sufficient for such airtreatment.
Fig. 2: A compact, ready to use systemcomprising a fluid-cooled
screw compressor,refrigeration dryer and 300 l air receiver
provideseconomical compressed air production, dryingand storage for
workshops with an air demand ofaround 1100 l/min
If one accepts average production costs of0.02 SHUP3 of
compressed air then theannual loss amounts to 5,256.- 7KHOHDN-rate
in a well maintained compressed airnetwork should not be more than
10% of totalair consumption. The average value through,is between
20 and 25%. This means that aloss equating to that given in the
example isnot the exception but rather the rule. Theleak is not
confined to one isolated point butnormally comprises countless
small leaks inthe hundredth or tenth of millimetre rangeand amount
to quite large losses whenadded up together.High losses of energy
are also caused if thecompressor installation is enlarged to
combatincreasing air demand, but air main bores arenot increased to
match the new conditions. Ina modern compressed air system
thepressure loss in a fixed air main should notexceed 0.1 bar. In
fact, the average value inmost plants is much higher at around 0.7
to0.8 bar. In other words: if a pressure loss of0.8 bar in a fixed
air main was reduced to 0.1bar through refurbishing then 4.2% of
thepower costs could be saved. If a powerrequirement of 25 kW is
taken as a basis,this saving corresponds to 1.05 kW everyservice
hour. If the total annual service hoursadd up to 8,000 then the
annual powerrequirement would reduce by 8,400 kWh, orfor a kWh
price of 0.11 FRVWVDYLQJVRI924.- ZRXOGEHPDGHThey often save on
filters (with theirpressure losses) and consume only 3% of
the overall power used by the compressorto produce the
corresponding volume ofcompressed air. Added to this are thecost
savings brought about by lowermaintenance and repair effort on
pipelinesand pneumatic equipment that are up toten times the
investment needed fordrying. In workshops with an air demandup to
1100 l/min there are reallyeconomical, compact systems
availablethat are ready to plug in and use,comprising a
fluid-cooled screwcompressor, a refrigeration dryer and anair
receiver (fig. 2).
A defective air main is direct cause ofwasted energy
Leakages in the air main causeconsiderable losses of energy
andincrease power costs. At 6 bar air mainpressure, a leak of 3 mm
diameter causesa pressure loss of 0.5 m
3/min! In an hour,
that is 30 m3. On the assumption that an
air main is pressurized the whole yearthrough, the loss amounts
to 262,800 m
3.
AS 31
Fig. 3: The pressure drop between thecompressor and the
pneumatic equipment ismaximum 0,8 bar in a well designed air
main
Planning a new air main
The first decision in the design of a newair main is whether air
should be suppliedon central or decentral basis. Inworkshops in the
trade a central system isgenerally installed. In such
installations,none of the problems arise that occur incentral
systems supplying large plants,namely high installation costs,
freezing ofinsufficiently insulated outdoor pipelines inwinter or
increased pressure dropscaused by very long pipelines.
Pressure drop: 0,8 bar
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The KAESER Compressed Air Seminar Part 9 17
9. The Air Main
Right sizing
Pipe size should always be calculatedfirst. The basis of this
calculation is amaximum pressure drop of 1 bar betweenthe air
consumers, including normaltreatment of the air (refrigeration
drying),(see fig. 3).
The following pressure losses can beassumed:
main supply line 0.03 bardistribution line 0.03 barconnecting
line 0.04 bardryer 0.3 barfilter, regulator,lubricator and hose 0.6
bar
total (maximum) 1.0 bar
Looking at this list you can see howimportant it is that the
pressure losses inthe individual pipeline sections arecalculated
together with all fittings andshut-off valves, etc. Just inserting
thestraight length of pipe into the formula ortables is not
sufficient. More important,the equivalent pipe length of the
pipelinemust be calculated. Generally, at the startof planning, it
is impossible to know theoverall composition of the air main with
allits fittings and shut-off valves, etc. Tocompensate for this,
the equivalent pipelength is calculated by multiplying thestraight
length of pipe in metres by thefactor 1.6. The bore size of the
pipelinecan then be evaluated using a straightline graph as shown
on page 10.
Laying pipelines with energy saving inmind
Pipelines should be laid as straight aspossible in order to save
energy. Bendscircumventing pillars, etc. can be avoidedby laying
the pipeline in a straight linenext to the hindrance. High
pressurelosses caused by 90corners can bereduced with 90large
radius bends.Instead of water traps, which are stilloften
encountered, ball valves or butterflyvalves with full bores should
be used. Inthe wet pipe section, i.e. in moderninstallations in the
compressor space, theinlets and outlets to the main pipe mustemerge
in the form of an upward loop orat least to the side. The main
pipelinemust be installed with a fall of 2% and a
condensate drain must be fitted at the lowestpoint of this line.
In the dry area the pipelinescan be installed horizontally and the
outletscan point directly downwards.
Steel or plastic pipe materials?
In view of material characteristics, no generalrecommendations
can be made with regardto the material selection. Prices are no
guideeither to making a positive decision.Galvanized, copper and
polymer pipelinescost about the same when installation andmaterial
costs are considered. Theinstallation of stainless steel pipelines
cancause a cost increase of about 20%,although, in the meantime,
better installationmethods have brought about lower prices.Many
manufacturers these days providetables from which optimum
conditions forevery pipeline material can be taken. Beforethe final
decision is made however, thesetables should be studied in detail,
the in-service loading taken into account and aspecification
catalogue for the pipelineswritten. This is the right way to make a
reallygood choice of materials. The individualpipelines should be
connected using eitherwelding or solvent welding or screw
andadhesive techniques. Even though removal ismade more difficult,
these types ofconnections are very secure and leakagesare kept to a
minimum.
Refurbishing an existing air main?
Although many faults and subsequentproblems can be avoided when
a new airmain is designed, refurbishing of an existingpipework is
often thwart with difficulty. Thiscan become a really helpless task
if the airmain is subsequently fed with moist air again.Before any
refurbishing is attempted, acentral drying unit must be
installed.If, after installation of an air treatmentsystem, the
pressure drop within the air mainis still quite large even though
sizing issufficient, then deposits within the pipescaused by
contaminants drawn along by theair are the problem. These
contaminantshave reduced the available cross-section offlow to a
minimum. If these deposits havealready hardened then in most cases
onlyreplacement of the section can solve theproblem. Often though,
if reduction is not toograve, the flow cross-section can
beincreased by blowing through and drying outthe pipeline.
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The KAESER Compressed Air Seminar Part 9 18
9. The Air Main
The amount of leakage is then calculated onthis basis using the
following formula:
VC x tVL =
T
VL = leakage volume in m3/min
VC= compressor volume flowin m
3/min
t = time the compressor ran on loadin min
T = total time in min
To determine the leakage caused by theconsumers, all tools,
machines and the sumtotal of all leaks are measured (see page
5,fig. a). Then the shut-off valves immediatelyupstream of the
consumers are closed andair main leakage is measured (see
formulaand page 5, fig. b). The difference betweenthe sum total of
all leaks and air mainleakage itself equals the leakage of
theconsumers and their fittings. To locate theleaks more precisely,
the measurementshould be carried out several times.
Experience shows that about 70% of theleaks are to be found in
the last few metresof the network, i.e. at the final take-off
points.These leaks can be located with the help ofsoapy water or
special sprays. An air mainoften suffers from numerous large leaks
if itwas originally a wet main fitted with old jointssealed with
sisal and then supplied with dryair. These joints dry out after a
while, causingleaks. Ultrasonic equipment is recommendedfor precise
location of air main leaks.
When all these leaks have been located andremoved and the
cross-section of thepipelines matched to the current
airconsumption, the resulting air main is (again)an efficient
compressed air distributionsystem.
Narrowed branch lines can be improvedby laying a new line in
parallel with theold one (fig. 4a, overleaf). In the sameway, a
second ring solves the problemscaused by a narrow ring main (fig.
4b,overleaf).
If such a dual branch or ring main iscorrectly designed, then
increasedreliability is gained as well as anoticeable reduction in
pressure drop.Another method of reducing pressuredrop is the
integration of so-calledintermediate networks (fig. 4c,
overleaf).
All these refurbishing measures only leadto success if the leaks
in the existing airmain are reduced to a minimum.However, before
searching for leaks acheck of the overall leakage volume mustbe
made.
With the help of a compressor relativelysimple methods are used
to measure thelosses involved. All consumers are shutdown and then
the cut-in times of thecompressor over a defined period aremeasured
(see page 4).
Fig. 4: Refurbishing or enlargement of an existing airmain using
a parallel pipeline (4 a), by installing asecond ring (4 b) or
installation of so-calledintermediate lines (4 c); 1: screw
compressor; 2:hose; 3: ball valve; 4: pressure switch
Old air mainExtended air main
Old air mainExtended air main
Old air mainExtended air main