Feature Report
Design and Specification of
A Compressed Compressed Air Air System System
Prasanna Kenkre
Jacobs Engineering India Pvt. Ltd.
A
n industrial compressed-air sys-tem is expected to supply air of defined quality, required pres-sure and desired quantity to all
the plant air and instrument air con-sumers. With air being one of the most critical utilities of a chemical plant, a compressed air system should func-tion efficiently and cost effectively. effectively. Therefore, designers should consider parameters such as air quality, air consumption and supply, storage and distribution and control management in their desi gns.
Most equipment manufacturers supply air compression and dr ying systems as packages comprised of many units put together. Hoever, Hoever, the purchaser of the system has the option of buying this complete pack-age system or requesting only a por-tion of it. !t is commonly observed observed that most compressed-air users de-sign and and install the air storage and distribution system themselves. "or
A practical overview overview of what to look out for when specifying a compressor
and its associated components Table 1 Typical process conditions and !uality re!uirements of instrument and plant air
Prasanna Kenkre
Jacobs Engineering India Pvt. Ltd.
A
n industrial compressed-air sys-tem is expected to supply air of defined quality, required pres-sure and desired quantity to all
the plant air and instrument air con-sumers. With air being one of the most critical utilities of a chemical plant, a compressed air system should func-tion efficiently and cost effectively. effectively. Therefore, designers should consider parameters such as air quality, air consumption and supply, storage and distribution and control management in their desi gns.
Most equipment manufacturers supply air compression and dr ying systems as packages comprised of many units put together. Hoever, Hoever, the purchaser of the system has the option of buying this complete pack-age system or requesting only a por-tion of it. !t is commonly observed observed that most compressed-air users de-sign and and install the air storage and distribution system themselves. "or
A practical overview overview of what to look out for when specifying a compressor
and its associated components Table 1 Typical process conditions and !uality re!uirements of instrument and plant air
Fluid Compressed Air
Service Instrument and plant air
Reuirements!
Process and design conditions
"perating pressure# barg $ % $.&
'esign pressure# barg ().&
"perating temperature# *C Ambient
'esign temperature# *C +, do-n to lo-est ambient site tem
2
perature
/ualit0
'e- point at operating pressure at At least (,*C belo- t1e lo-est ambi
1
air dr0er outlet # *C ent site temperature
2a3imum solid particle si4e# 5m 67
2a3imum ualit0 o8 contaminants 7
,.( mg9m or ,.,$ ppm :-9-;
:oil# liuid and gas;
#otes$ %lant air does not need to be dried. Typically &'()* at operating pressure or &+)* at atmospheric conditions in cold climate.
instance, in most " /front-end engineering and design0 and basic engineering 1obs the process licen-sors or the engineering contractors clearly demark the scope of ork on a piping and instrumentation dia-gram /%2!0. Thus on a %2! of a compressed air system, the compres-sor and dryer along ith associated instrumentation and piping ill be shon simply as a dotted block indi-cating the equipment manufacturers3 scope. The donstream piping, stor-age receiver, distribution and instru-mentation ill be shon in much more detail indicating that the engi-neering responsibility lies ith the oner or his or her detail-engineer-ing contractor. ue to this predeter-mined
ork-scope split, process and mechanical engineers are entrusted ith preparation-of-enquiry specifications of compressed air systems
that ill serve as input for the sup-plier of the compressor and dryer.
This article is intended for readers ho ant to gain a basic understand-ing of the components of a compressed air system. !t also presents best prac-tices that ill prove helpful to a pro-cess engineer riting specifications for such a system.
4equirements
The main components of a conventional air-compression and drying system are shon in "igure 5. 6ir supplied by the compressor is split after the primary air receiver into to streams. 6 ma1or part of the air stream is dried and utili7ed in the plant as instrument air. The other stream is not treated further and serves as plant air. !f pressure in the instrument air header falls belo a certain pre-set value, then a lo-pressure sitch
What is the trend of air demand < intermittent or continuous; /%890 ill close the shutoff valve and temporarily shut don the supply of plant air.
What is the expected quality of the compressed air in the plant; Consumers or end users. 6s a first
:efore initiating the specification, the folloing points need to be con-sidered for installing a proper air-system configuration$
Ho much total compressed air is required in the plant;
6t hat pressure is the air to be sup plied to the consumers of compressed air in the plant; Who are the end-users or air consumers of compressed air in the plant;
"#
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step, it is necessary to identify all equipment, machinery, instruments and tools that require compressed air in order to function. 6 list should be
Conventional air compression and drying system
$nstrument air
$nst air low
Air intake filter and silencer
%ackage boundary
press switch %&F controller
%S'
%ackage
Flow orifice
Dryer
boundary
Secondary air
Drive 1( Compressor Compressor )oisture
*1st stage+ *,nd stage+
reciever
Steam turbine
separator
ST
%re-filter
After-filter
%lant air
Air intake filter and silencer
After-cooler
$nter-cooler
Flow orifice
Shutoff valve
Drive ,( electric
$nter-cooler After-cooler
motor
%rimary air
)
reciever
Automatic drain
Automatic drain
trap
Stand-by Stand-by
)oisture
trap
compressor compressor separator
*1st stage+ *,nd stage+
Condensate
Cooling Cooling
Steam in out water supply water return
Air compression and supply
Air storage and drying
Air distribution
Figure 1 Shown here are the main components of a compressed air system
prepared that contains data, includ-ing number and type of consumers, minimum and maximum air-pressure requirements of each user, air flo required by each user, utili7ation factor and so on.
*ompressed air has a number of industrial uses based on its service. 6 ma1or application of compressed air, hen used as instrument air, is valve actuator control. =ther common applications of instrument air include use in laboratories, rotating equipment seals, paint spraying and poder coating, climate control and so on. !ndustrial orkshops have consumer tools, such as pneumatic hammers, drills, grinders
and such. >tility stations are often installed in a plant for general purposes and require plant air. :reathing air stations are provided in most chemi-cal plants. "ood, pharmaceutical and electronic industries require mostly process air. 6ll of these users must be carefully identified and listed.
Quality. The air quality depends on the levels of contaminants that the end users can tolerate ithout affect-ing the smooth function of process /Table 50. Typical contaminants com-monly encountered in compressed air systems include solids /dirt, dust, pipe scales, and particles from com-pressor ear0, liquids /ater and oil0 and gases /ater vapor, oil, chemical vapors0. :ased on the services ca-tered to, the quality of compressed air ranges from plant air /least critical0, process air and instrument air /criti-
cal0 to breathing air /most critical0. The cost of producing compressed
air goes up ith each quality level. ach increased quality level requires installing additional purification equipment and leads to a higher initial capital investment. The future operating cost ill also rise due to increased energy consumption and maintenance. Therefore, the air quality level should be determined as the first step.
The quality class of compressed air can be assigned as listed in detail in the international standard !8= ?(@A-5, hich bases the classes on particle si7e,
moisture and oil content in the air. "or example, the air quality specifica-tion for instrument air is ritten as !8= ?(@A-5 *lass '.'.5, hich means 5 micron particulate filtration, &+)" /& +)*0 de point and .? ppm B
the air quality, e can still try to r educe the load and thus the quality level expected from them by eliminating or minimi7ing sources of contamination. This can be done in a number of ays.
A
/.5 mgBm 0 oil filtration. The air class may also change from client-to-client based on the purity requirement of air for the particular service. The most stringent quality class in this regard is *lass . !t does not mean that the contaminant level ill be 7ero, but rather that the levels of particulate matter, de point and oil content of the air supplied ill be as per any values /typically loest0 speci-fied by the user. :ased on its equip-ment capabilities, the manufacturer must agree in riting that it can pro-vide air of such a class.
8ome points to be considered hen talking about air quality are given belo$
Minimizing or eliminating sources of contamination. *ontaminants can enter the system at the compressor in-take or could be introduced in the air stream by the system itself. Though equipment, such as separators, fil-ters, dryers and condensate drains are used to improve
C1emical Engineering ---.c1e.com Januar0 ),(7 "1
"or example, locate the compres-sor3s air-intake filters in a safe nonha7ardous area in open air outside the plant building aay from sources of dirtC dustC moistureC toxic, corrosive and flammable gasesC and also at sufficient height /about A to ( m0 from ground level to avoid dust, debris, insects and so on. 6s the air intake is sub1ect to extreme conditions ith various contaminants causing foul-ing, corrosion and other problems, the material of intake filters should be selected ith great care. Typically, the air intake filter and piping is made of stainless steel.
6lso, one should avoid using lubricated air compressors in applications here high quality is desired. Grouping of consumers. *onsum-ers ith similar air quality and pres-sure level can be grouped along ith airtreatment equipment in close proximity. !f different air quality re-quirements exist in the same plant then the plant can be divided into dif-
Feature Report econo mic and opera ferent tional units. analy The air sis0, treatm highent qualit equipy air ment may can be be kept supdedicat plied ed to ith a the end dedic users ated, ith lubric highantquality free require comp ments. ressor . Hoe ver, if "or there exampl is a e, if sufonly ficien one tly consu high mer requir require ement s of lubrica highe nt-free r air air, qualit only y /say air @D being or supplie more0 d to it , then needs the to be entire treated plant , can thereb be y suppli reduci ed ng ith costs. this 6ltern qualit atively y /based level. on
Quanti ty — Estima ting system ca pacity and margin s. :efore installing a compr essor, the quantit y of air flo require d by the plant should be knon . The require d compr essedair capacit y is the sum of air require -ments of instru ments, tools and process operati ons assumi ng normal plant operati on at
full load /taking into accoun t the operati onal load factor of each piece of equip ment0. 6 study is typical ly carried out to understand the various applica tions requiring compr essed air and the duratio n of their operati on.
each tool, but rather the sum of the avera ge air consu mptio n of each. "or exam ple, in most plants the capac ity of a comp ressor is the capac -ity requir ed for opera ting both instru -ment and plant air. Typic ally, the tool Hoev air er, the syste total ms air are require kept ment is separ not ate simply from the the sum of instru maxim ment um reand quirem plant ents air for syste
m. uring plant shutdo n, the tool air require ments are especia lly large and can be met by hired portabl e compr essors. !n this ay, oversi7 ing the instrume nt and plant air compr essor to cover this tempor ary large deman d of air can be avoide d.
!n case it is planne d to supply tool air from the same compr essor,
then care should be taken to ensure that$ there is no interco nnectio n betee n piping of the to air system s donstream of the dryerC the receive r si7e is adequa te enough to supply instrument air at all timesC and that a lo pressur e sitch is installe d that can cut-off the tool air supply in case the instru ment air
press ure drops.
The tool air requir ement can be cal-
can be used to si7e the tool air compr essor.
When designi ng a compr essed air system , the approa culate ch d as should the be to sum minimi of the 7e the numb deman er of d and tools properl times y si7e the the air compr consu essorC mptio oversi7 n per ing tool should times be the avoide load d. factor Eariati . The on in load air facdeman tor d over takes time is care a of the ma1or time a consid partic eration ular . %lants tool is ith a being ide utili7 variati ed. on in This detotal mand toolneed a air compr requir essor ement
operati ith ng the efficie help ntly of under flo partial meter load. s Thoug install h the ed on air main compr heade essor rs and efficie at ncy vario ill in- us crease points ith in the si7e, syste oversi7 m. ed The compr electr es-sors onic are data extrem logge ely rs that ineffici track ent comp becaus ressor e they activi use ty more over energy time per also unit help volum monit e of air or the produc dema ed nd. hen The operati data ng at thus par-tial meas load. ured can be used !n to existin si7e a g ne installa plant. tions, "or the air ne deinstall mand ations is the monito comred press
or capacit y may be calcula ted as the exampl e shon in Table '.
Sizing for future deman d. 6lay s keep in mind that a plant may need a ne proces s unit someti me in the near future. 6s an exampl e, say that this unit ill have a require ment of approx imatel y ( A
#m Bh and the ap plicati on lies in the
same the ations pressur comp or e and ressor supply quality has tool air range beco as this as that me may of oversi lead to Table 7ed oversi7 '. ue for ing the to the curre compr availab nt essor. ility of use. When these !n such data such deman ell in a ds are advanc case, encoun e the tered, during logica the l si7ing appro stage, ach ( ill they A be to #m Bh install can be are met a added any small to the time er existin recipr by g flo ocatin future of g unit compr A,+ essor of A installa #m B ( tions A h and a #m B or ne h at a tempor capacit later ary y is stage rented estimat hen installa ed as actutions. A,F ally A #m B neede d. h. Pressu 6lthou Thus re gh in care level. this shoul %roces case it d be s may taken engine seem to ers that avoid specify the fuaddin airture g pressur require extra e ments margi require are ns to ments taken cover for the care future proces of, in applic s in reality
their basis hile the valve and pneum atic tool manuf acturers rate their valve and tools for specifi c purpos es as given in their literatu re. ach air consu mer has a certain operati ng pressur e require -ment to functio n correct ly. The highest orkin g pressur e require ment of a consu mer is used to determ ine the
correct install ation press ure /or the com press or disch arge press ure0. !n the same syste m for the consu mers here such high press ures are not requir ed a selfregul ating valve /or a press ure contr ol valve /%*E 00 can be install ed upstre am to reduc e the press ure at the consu mer3s inlet.
To decide the installa tion pressur e, the pressur e at the compr essor discharge flange needs to be estimat ed. To estimat e this pressur e, the losses encoun tered in the circuit due to equip ment /filters, dryers, flo elements, heat exchan gers, piping and so on0 must be added to the maximum pressur e value require d at the consu mer end. The
exampl e given in Table A clarifie s this point.
press ure drop in the filters are lo initial ly but inTable A creas shos e over that time. the "or orkin exam g pres- ple, a sure is desicdeterm cant ined dryer by afteradding filter system may pressur accu e mulat losses e to the desic maxim cant um fines presover sure time, value hich require can d at the cause consu an mer increa end. sed The press equip ure ment drop pressur and e drops increa are sed depend poe ant on r vendor consu design mptio and the n. values used in the exampl The e are flo typical regul values ation encoun of a tered. comp The res-
sor may bring about flo variati ons in the system . 6s pressur e drop throug ha given pipe is directl y proport ional to the square of florat e /G P 2
Q 0 throug h the pipe, the pressur e drop ill increas e in case of a higher flo deman d. To compe nsate for this varying pressur e drop due to compr essor regulat ion, a margin is consid ered.
6s a rule of thumb
",
for comp resse d air syste
ms in the range of 5 psig
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Table , .stimating air compressor capacity
S0mbols!
Assumptions!
7
C < Compressor capacit0# =m 91
Air consumption 9 instrument 7.) 7
=m 91
7
I < Instrument air reuirement# =m 91
Air consumption 9 utilit0 station ),, 7
=m 91
7
P < Plant air reuirement# =m 91
> o8 utilit0 stations -orking (, >
N < =umber o8 instruments in plant
simultaneousl0
u < =umber o8 utilit0 stations in plant
Flo- margins to account 8or !
U < =umber o8 utilit0 stations -orking
a; Leaks and 8uture e3pansion ), >
Formulae!
b; Air d0er regeneration ), >
C = I + P
c; Compressor -ear and e88icienc0 :onl0
I = N :Air consumption 9 instrument;
8or reciprocating t0pe# in addition to a ? b; ), >
U = u :> o8 utilit0 stations -orking simultaneousl0;
P = U :Air consumption 9 utilit0 station;
Calculations!
For an e3ample -e consider t1e 8ollo-ing 8igures!
N< @+& :sa0;
u< , :sa0;
U< , (,>
I< @+& 7.) (#&),
7
=m 91
7
=m 91 :Considering centri8ugal t0pe and appl0ing 8lo- margins a ? b to
I< (#&), (.) 3 (.) )#($$.$
above 8lo-;
P< ),, (#),,
7
=m 91
C< )#($B (#),,
7#7$$.$
7
=m 91
D 7#@,,
7
=m 91
7
ence t1e estimated compressor si4e is 7#@,, =m 91
/approximately @&? barg0, for every ' psi /.5+ bar0 increase in compressor discharge pressure, the folloing to changes occur$
should be con-sulted for obtaining exact values of pressure drops across the equipment at maximum florates. These realistic values should then be used for calcu-lating the compressor discharge pressure. 6lso, an attempt should be made to select equipment and instruments ith minimum pressure drop.
:uild a basis of hat is expected from the compressed air system.
=perating ith a loer pressure than needed ill lead to erratic func-tion of instruments and endanger the process. 6 higher pressure, on the other hand, ill cause more energy consumption and may lead to system leaks and thus increases of the plant operating costs in future.
!dentify the scope of supply.
%resent sufficient and precise tech-nical data to the equipment manufac-turer to design this system.
nergy consumption increases by approximately 5D at full output flo.
nergy consumption increases by another .&5D due to unregulated usage /unregulated usage is typically considered to be about A&(D of air demand0.
The combined effect is a net ris e of about 5.&'D IJ.
With this information in mind, one should be careful in finali7ing the sys-tem pressure. The calculated value of the compressor3s discharge pressure should not be simply rounded to the nearest hole number. !nstead, equip-ment manufacturers
6 package enquiry specification
The hole idea of riting an enquiry specification for the package system is to do the folloing$
8ome important points for the ven-
dor and the buyer hich should be put in the specification in a clear and con-cise ay are given belo.
. E!uipment" system and site details. quipment details should contain some data given by the de-sign engineer and some information left for the equipment manufacturer to confirm. ata, such as number of compressors and dryersC
capacities requiredC operating and design condi-tionsC fluid propertiesC alloed noise levelC expected air quality at the dryer outletC maximum pressure drop across dryers and filtersC and dryer outlet temperature are to be given by the designer. =n the other hand, data such as equipment-rated capacity confirma-tionC number of compressor stages re-quiredC absorbed poer and efficiency at shaftC suction- and discharge-flange si7e and ratingC consumption of utili-ties like cooling ater and instrument airC design temperature based on com-pressor discharge temperatureC dryer
cycle timeC drying periodC regeneration periodC cooling periodC tie-in point listC instrumentation and control schemesC and so on, are given by the vendor.
C1emical Engineering ---.c1e.com Januar0 ),(7 "/
8ystem details should cover the operational and control philosophy, number of orking and spare equip-ment, quality, quantity, pressure re-quirements of air, schematic sketch and so on.
:attery limit conditions, utility availability, meteorological and cli-matic conditions, site location, geo-technical data, and any limitations on plant dimensions details constitute the site details.
Scope of supply. 6 vendor must understand hat exactly he has to fur-nish to the buyer. *ommonly, vendors supplying compressors also supply receivers, filters and dryers
together to form hat is called as the air com-pression-and-drying package. Typical details listed in the scope of supply are equipmentC interconnecting pipingC control panelC instrumentationC plat-forms and laddersC boltsC lugsC skidsC fabricationC surface preparation and paintingC inspection and testingC first fill /desiccant, oil0 supplyC installationC documentationC site shipmentC author-ity approval and certification.
#eference and procedure. !ndus-
trial equipment manufacturers have their on set of internal manufactur-ing quality standards. Hoever, most
Table / .stimating the working pressure
Pressure reuired at consumer end
P
barg
Feature Report
Element
0pical pressure
drop
Final 8ilter
P 1 ,.7
bar
chemical process industries /hence-
Air distribution piping
P 2 ,.(
bar
'ust 8ilter :dr0er a8ter 8ilter;
P 3 ,.(
bar
forth referred in this article as KclientL0
'r0er
P 4
,.(&
bar
require the vendors to adhere to global
Coalescing 8ilter :dr0er pre 8ilter;
P 5 ,.(
bar
manufacturing codes, standards, guide-
Flo- element
P 6 ,.)&
bar
lines, good recommended practices,
Compressor a8tercooler
P 7 7 ,.(
bar
directives /for instance, 68M, 6%!,
Compressor intercooler
P 8 8 ,.(
bar
6#8!0. !nternational clients operating
Compressor regulation range
P 9
,.&
bar
multiple industrial units at times have
otal pressure drop
P
(.+
bar
their on set of technical standards
and guidelines that the vendor has to
Pressure reuired at t1e
P
comply ith. Typically, a list of such
compressor disc1arge 8lange
P +.+
barg
codes and standards to be fo lloed is
to the vendor3s proprietary design.
The folloing variables, if analy7ed
available in the design basis of a pro1-
ect and needs to be conveyed to the
There may be certain technicalities in
correctly, ill provide a fair idea of
vendor through specification.
terms of fabrication or state-of-art de-
the compressor type to be selected be-
This section should also contain
velopment that only the vendor may
fore consulting a compressor vendor
administrative, procedural and other
be better aare of. "or example, air
for details$
temporary requirements to be fol-
intake filters or moisture separators
5. Hours of operation per month
loed, including submission of expe-
are entirely a vendor-supplied propri-
'. #ature of demand /continuous or
rience record proforma, complying to
etary item. This ill be designed by
intermittent0
equipment qualification criteria, in-
the vendor based on the particle-si7e
A. %ressure and flo requirements
structions for delivery to site, schedul-
retention and moisture data given by
+. nvironment /clean or dirty0
ing, arranties, and spare and main-
the design engineer.
6 preliminary selection of the type of
tenance agreements.
uring the technical bid analysis
air compressor can be made from the
8pecifying equipment data
/T:60 stage based on the s pecifica-
typical graph of inlet flo versus dis-
tion given by the designer, different
charge pressure, as given in the %86
6 compressed air and drying package
vendors offer their proposals that have
handbook I2J. "or example, suppose
contains many types of equipment,
to be evaluated technically for energy
e ant to select an air compressor for
such as air-intake filters, compres-
efficiency and lifetime operating cost.
5, acfm and a discharge pressure of
sors, inter-coolers, after-coolers, mois-
The data furnished by the vendor need
5'' psig. :y using such a graph, e ill
ture separators, receivers and so on.
to be thoroughly checked by the engi-
observe that for our application e ill
The engineer ho rites specifica-
neer to see that all of his or her techni-
end up selecting the folloing types of
tions for the package does not neces-
cal and operational requirements are
compressors$ reciprocating /single and
sarily si7e all this equipment. :ased
in line ith that given in the specifi-
multiple stage0, rotary scre and cen-
on rules of thumb, good engineering
cation. 6ny other additional data fur-
trifugal /single and multiple stage0.
practices and sound technical as-
nished as a result of proprietary design
6ll three types of compressors can suit
sumptions, he or she can fairly esti-
should also be checked at least for cor-
the application. 8o ho do e decide
mate the capacities and si7es of these
rectness and compliance to standards.
hich type of compressor is the best;
pieces of equipment. This may help
6ir compressor selection
The anser is that e must not select
different engineers from disciplines
any compressor that simply fulfills the
like piping, static equipment, electri-
uring compressor and drive selec-
flo and pressure requirements, but
cal and so on to get at least prelimi-
tion, it must be kept in mind that in
the one that is best suited to the ap-
nary data to proceed ith their ork.
most industries it is the compressor
plication /see Table +0.
"or example, due to availability of
that utili7es more electricity than any
8uppose for the same application
equipment si7es, the layout engineer
other equipment. 4ecords sho that in
given above e further kno that
can assign preliminary locations for
many instances during the first year
the nature of load ill be continuous,
this equipment /hich ill be sup-
of operation, the operating cost as al-
heavy /high florate0 and the system
plied as packages or skids0 on the al-
most tice that of the initial purchase
has to be lubricant free. "or high flo-
lotted plot plan and fix the area for
price of the equipment.
rates and oil-free conditions centrifu-
the air unit in the basic stage.
When selecting ne compressors,
gal compressors are a common choice.
The si7es of some of the equipment
industries ith existing compressed-
6lso
centrifugal compressors ork
estimated by the engineer may not
air installations have an advantage.
ell under continuous load rather
necessarily match that given by the
They monitor their current air de-
than variable load. ue to these rea-
vendor. Though seeming correct on
mand and supply trends and also the
sons a centrifugal compressor ill be-
paper, such equipment may or may
reliability and suitability of existing
come a first choice for our application.
not give the desired result. This may
air compressors. The data thus ob-
*orrect florate units
be either due to the capabilities and
tained ill prove useful to them in
limitations of the selected vendor3s
selecting and si7ing any future com-
6s air is compressible it ill occupy
manufacturing and machinery or due
pressed air installations.
different volumes
at different tem-
""
C1emical Engineering ---.c1e.com Januar0 ),(7
Table " Compressor Selection
Compressor t0pe Reciprocating
ScreCentri8ugal
Gest suited 8or!
Flo-rates Lo-
2edium ig1
=ature o8 air demand Fluctuating or
Continuous or Continuous or
var0ing
stead0 stead0
strument air cannot be compromised
=ature o8 operation Intermittent
Continuous Continuous
at any cost. The capacity of the spare
compressor is kept the same as the
"perating e88icienc0 at 2ost e88icient
Hood Poor# suscepti
lo-er 9 part loads
bilit0 to surge
largest duty compressor.
ven to cater to the normal opera-
Reliabilit0 and ig1 -ear
Hood 2edium main
tion, sometimes multiple compres-
maintenance
tenance but
8reuent
sors are installed in a plant. "or ex-
ample, for a certain knon capacity
Comple3 and
Eas0 and loC1eck 8or un
e have a compressor installation of
8reuent main
maintenance balance and
tenance
vibration
' 5D. This actually means that
e have to installed compressors,
peratures and pressures. There is
pressure / P 0 and final pressure / P '0
out of hich one is orking and the
no global standard for specifying air
in absolute units. 6 gage value is only
other a standby. 8electing this instal-
compressor florates. *are should be
a representation of pressure. !t does
lation may mean that e get a single
taken to avoid confusion due to usage
not include the atmospheric pressure
compressor hose orking capacity
of different units like cubic feet per
and hence is not the true pressure
is very large. !nstead e can opt for
minute /*"M0, standard air capacity
of the gas. Typically in instrument
a combination of a number of smaller
/8*"M0, actual air-compressor capac-
air systems, the overall compression
compressors, hich may prove an at-
ity /6*"M0, inlet air capacity /!*"M0,
ratio is about nine. ue to this high
tractive economic and operating alter-
free air delivery /"60, normal cubic
compression ratio e may need mul-
native than having one large compres-
A
meters per hour /#m Bh0 and so on.
tiple stages.
sor. 9ikeise, a A (D combination
*ompressor vendors rate their com-
The compression ratio per stage is
here e have three installed com-
pressors in terms of volume. The ven-
limited by the discharge temperature
pressors, out of hich to are orking
dor catalogs typically state compres-
and usually does not exceed four. Ho-
and the third a standby is an another
sor flos in *"M. !t also seems logical
ever, sometimes for small si7ed air
option. "or critical services, the option
and easy to visuali7e equipment si7e
units ith intermittent duty, a higher
of keeping a spare rotor handy is also
in terms of volume rather than mass.
compression ratio may be used by the
considered at times.
8ometimes mass florate /kgBh0 of
vendor. Table ( can be used for choos-
enerally, a combination of differ-
gas is given by a design engineer ith
ing number of stages.
ent drives is used to run compressors.
the understanding that mass of a gas
Though an engineer can state the
6 petroleum refinery may have units,
remains constant. !n such cases, the
value of the number of stages in the
for instance a hydrogen generation
moisture content in the gas /if any0
specification, this value is sub1ect
unit /H>0 or a sulfur-recovery unit
should be subtracted from the given
to the manufacturing capabilities of
/84>0, here excess high-pressure
florate. The vendor should be told if
the vendor.
/H%0 steam is generated in the pro-
the florate is et or dry.
!n general, variable speed control is
cess. !f the generated excess steam is
When dealing ith process gas ap-
achieved by using a steam turbine, gas
not being used or exported elsehere
plications, the unit 8*"M is commonly
turbine or diesel or gasoline engines.
and is sufficient to drive a turbine,
used hile "6 finds a more common
*onstant speed control is achieved
then a steam-driven turbine can be
usage in compressed air applications.
by electric motors. Eariable speed can
selected as the main drive hile an
#umber of stages and drives
also be obtained from electric motors
electric motor may be used to operate
ith variable speed drives. rive se-
the other compressors.
Multiple stages are used in compres-
lection can be done based on the chart
"or example, a compressor ith a
sors to achieve higher pressures. 6s
given in the !nstrument ngineers
steam-turbine drive may supply (D
high-pressure compression is carried
Handbook I$J.
of the total flo requirement hile the
out in multiple stages, intercoolers
=perational philosophyBspares
compressor ith electric motor and
provided beteen the stages remove
variable speed drive /E80 may sup-
heat of compression and bring don
Most plants install at least to com-
ply A(D of the total flo requirement.
the temperature to approximately
pressors, one orking and the other a
The spare ill also be E8 driven and
that at the compressor inlet. 6s a r e-
spare or standby. 6 spare air compres-
si7ed to supply (D of the total flo
sult of this cooling, the density of air
sor is required in the system to ensure
requirement in case of emergency.
increases and volumetric florate of
maximum reliability and availability
This leads to electric poer saving,
the gas going to the next stage reduces.
of compressed air during emergency
increased reliability due to usage of
ue to this volumetric reduction the
scenarios, such as equipment failure.
a reliable source of utility /steam in
ork of compression and hence the
Mechanical failure of a compressor
this case0 and also extraction of useful
poer need reduces.
ill directly affect instrument air sup-
ork from excess steam. 6ll the three
The number of stages required is
ply in the plant after the stored air ca-
compressors ill be si7ed for (D of
determined by the overall compres-
pacity of the air receiver is completely
the air demand. The actual operating
sion ratio. The compression ratio is
exhausted. 6 spare compressor is in-
schemes are decided and approved by
calculated considering both the initial
stalled here process criticality of in-
the chemical plant personnel along
C1emical Engineering ---.c1e.com Januar0 ),(7 "0
Table Calculating air receiver si2e
Feature Report
Ambient air temperature T *C @,
Capacit0 reuired C 7
=m 91 7#@,,
Capacit0 correction
< 7#@,, :)+7 @,; 9
:8ree air deliver0;
:, )+7; < @.B
oldup time reuired t min (,
Table 0 Choosing number of stages
based on compression ratio
Ambient pressure P
a
bar (.,(
Compression =umber o8 stages
Initial9storage pressure P 1
barg $
Ratio :P 29 P 1;
bara B.,(
(%@ ( stage# sometimes ) stages
Final9destination P
2
barg @.&
@%), ) stage# sometimes 7 stages
pressure
bara &.&(
),
7 stages
olume 7
m
($$
ith the design engineer based on their previous experience, cost effec-tiveness and 46M studies.
4unning a smaller compressor at full load proves more energy efficient than running a larger compressor at lo load. 6lso if there is a large varia-tion in air demand /like lo demands during eekends0 then e can sitch off one of the to orking compressors. There may be a combination of
operating compressors based on se-quential controls to avoid running the larger compressor at such times.
6ir receiver si7ing
P a N 6tmospheric pressure, bara The air receiver is used to store a cer-tain volume of compressed air and sup-ply it for use as needed. !n the event of a failure or a shutdon of the operat-ing compressor, the receiver provides the necessary air supply for the time needed to s tart /manually or automat-ically0 the standby air compressor.
The receiver volume is at ambient temperature. P N !nitial pressure or compressor discharge pressure, bara #o air is being supplied to the re-ceiver by the compressor. P 2 N "inal pressure or minimum pres-sure required at the air consumer end, bara
6n air receiver located on the dis-charge side of a reciprocating com-pressor also helps to dampen pressure pulsations. ue to availability of a large vapor space, the receiver pro-vides radiant cooling and also collects any condensed liquid.
The air receiver is si7ed such that it supplies a compressed air demand for an amount of time required for the air pressure to drop from compressor discharge pressure to the minimum pressure required at the air consumer end. The si7e of an air receiver can be calculated by the formula /based on :oyle3s la, P% & a constant 0$
The time t , also knon as the resi-dence time for receiver si7ing, is a function of criticality of the system, operator intervention for mainte-nance and piping diameter. This time typically varies from ( to 5( min. !n plants here provision of auto start of spare compressor is given, the residence time may be reduced to 5.( to ' min considering reliability of auto start and sub1ect to client3s approval and operating experience. 6n example is provided in Table .
The initial pressure / P 0 is usually taken as V
the pressure at the compres-sor discharge flange considering line losses to be negligibleC and the final pressure is taken as the pressure re-quired at the instrument for proper operation. 8ometimes the receiver volume calculated from the given for-mula may turn out to be too large to be economical. To reduce the receiver volume
=
t C Pa ⋅
⋅
/50
9ocation of air receiver
The air receiver is typically installed at to different locations in the com-
pressed air system. The receiver lo-cated immediately donstream of compressor but before the dryer is knon as the et receiver or pri-mary receiver. The receiver located donstream of the dryer is knon as the dry receiver or secondary re-ceiver.
The main function of the et receiver is to act as a pulsation dampner / typi-cally for piston reciprocating compres-sor0 and bring about a stabili7ation in pressure. !t provides additional radiant cooling to help condense some moisture and reduce load on the dryer. =n the other hand, the dry receiver meets the high short-term air demand from consumers by the air stored in it, thus avoiding cycling of the compressor.
/% 0, the value of the term / P ( P1 − P2 )
' P 20 should be increased. To achieve this, the value P should be increased. 8toring Where, A
% N 4eceiver volume, m
t N Time alloed for pressure drop / P ' P 20 to occur, min
air at a higher pressure by installing a smaller reciprocating ma-chine ill reduce receiver si7e and prove economical compared to install-ing a receiver ith high storage vol-ume. 8ometimes for a critical system, an additional receiver operating in parallel can be installed for additional reliability, if required.
C N "ree air delivered at compressor A
discharge, #m Bh
The assumptions for this exercise are the folloing$
Most rotary scre compressors /lu-bricant in1ected0 are equipped ith capacity control by inlet valve mod-ulation and are designed to match the output from the compressor ith the demand from consumers. Thus it seems that an air receiver can be avoided in this case.
Hoever, absence of an air receiver ill not shield the compressor from pressure fluctuations from the de-mand side donstream of the receiver. 6lso the ability to keep the compres-sor unloaded for longer time during periods of light loads ill not be avail-able. Thus the requirement for an air receiver is a must.
The folloing mountings are essen-tial for an air receiver$
6utomatic drain trap and manual drain tapping
%ressure gage
"usible plugs
8afety valve
9evel transmitter
"
C1emical Engineering ---.c1e.com Januar0 ),(7
Manhole
The receiver inlet no77le should be located in the loer portion of the vessel and the outlet no77le should be located at the top to assist settling of liquid droplets
Table 3 Air Dryer Selection
'r0er C1emical deliues Re8rigerant dr0er 'esiccant eat o8 compression 2embrane
cent dr0er
dr0er dr0er dr0er
Gasic Single to-er -it1 a Combination o8 air -in to- Single or t-in to-ers 2embrane
con8iguration saltpacked bed toair 1eat e3c1anger ers -it1 -it1 desiccant packed unit
8ollo-ed b0 re8rigerant
desiccant beds
toair 1eat e3c1anger. packed
ariation! C0clic dr0 beds
ers indirect cooling
t1roug1 t1ermal stor
age medium
'r0ing action 2oisture is ab Cooling air 8rom com 2oisture 2oisture adsorption in Selective ad
sorbed b0 salt bed. pressor disc1arge adsorp desiccant bed sorption. 2oist
Salt dissolves in in airtoair 1eat e3 tion in
air enters t1e
-ater and is lost to c1anger to reduce desiccant
dr0er. ater
drain during peri load on t1e dr0er 8ol bed
permeates
odic draining lo-ed b0 direct cool
t1e mem
ing in re8rigeranttoair
brane -alls
1eat e3c1anger.
-1ile dries air
Indirect cooling in
continues to
t1ermal storage media
travel 8urt1er
'r0ing Salt beds o8 sodium# Re8rigerant 9 t1ermal 'esiccant Single to-er! Rotat 2embrane
medium potassium# calcium mass media like ing desiccant drum in
and t1ose -it1 a
Silica gel# single pressure vessel.
urea base
alumina It uses 1ot air taken
and mo directl0 at a point a8ter
lecular compressor disc1arge
sieves 8or regeneration purge.
-in to-er! 'esiccant
bed :1eat regeneration
b0 1ot air taken directl0
a8ter compressor dis
c1arge;
'r0ing =ot possible# salt =ot applicable. Possible Possible =ot possible#
medium is used up and
membrane
regeneration makeup o8 salt is
1as to be re
reuired
placed
'e- point (&%&,*F belo- inlet 7&%7B*F %@, to %@, to %(,,*F @, to %@,*F
attained air temperature
%(,,*F
Appro3imate ,.) ,.+B ) to 7 ,.$ 7 to @
po-er
reuirement#
k9(,, c8m
%ipelines are typically carbon steel, except lines ith smaller diameter in The air dryer is selected based on
Materials of construction
The most common material of construction /M=*0 used for a plant- and instrument-air system is carbon steel. The compressor and dryer package parts in contact ith moist air shall be selected ith care. *orrosion alloance ill be included as per pro1ect standard or design basis. The equipment mate-rial is specified by the design engineer and is sub1ect to confirmation and 1us-tification by the vendor.
The compressed air receiver is made of carbon steel. 6s the compressed air receiver also serves the purpose of condensate collection and most liquid is knocked off and collected at the receiver bottom, it is susceptible to corrosion. To avoid this, the receiver is typically provided ith an internal protective resin coating /for example, example, heat-cured phenolic resin0.
the range .( to ' in. are galvani7ed carbon steel. This is done typically because lines that are smaller in diameter can get clogged by any rust, corrosion or other solids caused by carbon-steel corrosion or eroding, and may create problems for the instruments donstream to hich they supply air.
6ir dryer selection
Water in compressed air, either in the Water liquid or vapor phase, can cause a variety of operational problems for consumers of compressed air. %rob-lems encountered may include free7-ing of outdoor air lines, corrosion in piping and equipment, malfunction-ing of pneumatic process-control in-struments, fouling of processes and products and so on. Hence, using an air dr yer becomes necessary to re-move the ater vapor from the com-pressed air.
the required pressure de point. To select the correct dryer, first it is im portant to understand the concept concept of de point. 6tmospheric 6tmospheric air contains moisture. !f e keep on cooling air e ill attain a temperature here the moisture contained in air ill begin to condense and drop out. This tem perature at hich condensation condensation first occurs is the de point of air at atmospheric pressure. !f e compress atmospheric air, it ill occupy a smaller volume. ue to compression the ater molecules ill come closer, coalesce and condense out. This temperature at hich ater vapor ill begin to condense at the applied higher pres-sure is the de point at the applied pressure, or pressure pressure de point. Thus the pressure de point /de point at higher pressure0 ill be different than the de point of air at atmo-spheric pressure.
!n general, air at a temperature
C1emical Engineering ---.c1e.com ---.c1e.com Januar0 ),(7 "3
Feature Report some of the moist ure to higher be than remo atmosp ved heric off ill hile hold the more highe moistu r re, and tempe air at a rature presill sure enabl higher e the than air to atmosp hold heric on to ill some hold moist less ure. moistu The re. The press air ure leaving de the point compr is essor more is both meani at a ng-ful higher as it pressur indica e and tes temper the ature de than point atat the mosph opera eric. ting Thus press at the ure. compr essor outlet The a vendo pheno r menon must occurs be here higher provi pressur ded ith e ill maxi cause
mum florat e, require d de point, maxim um and minim um inletair pressur es, maxim um and minimum inletair temper atures, maximum coolin gater temper atures, maxim um pressur e drop for dryer design. Table @ provid es guideli nes for dryer selecti on.
%refilters are installe d
upstrea m
of the air dryer to protect the drying mediu m /exam ple, desicca nt0 from getting contam inated. 6fterfilters are installe d donst ream of the air dryer to preven t desicca nt fines from enterin g the system donst ream. 6fterfilters also help in remov al of vapor, harmfu l chemic als,
micro -organis ms and so on. :oth the filters also serve to coale sce oil and moisture dropl ets, hich can then be drain ed. =ver time, the filters may get clogg ed and cause increa sed system resist ance and energ y consu mption. Henc e, timel y filter maint enanc e is very
import ant in compr essed air system s. iffere ntial pressur e gages should be installe d across filters to keep a check on the pressur e drop throug h them.
:eside s these filters, small filters may also be installe d at the pointof-use end. Their functio n is to filter particl es generat ed in the distrib u-tion piping.
be as desig closedned loop or prope ring istrib rly. main very header ution s. !n piping possi ble the attem ring pt header The shoul the air compr d be flo is essedmade split air to into distrib mini to ution mi7e directi piping press ons ill be ure from a si7ed drop. point based "or and on the exam can 6*"M ple, flo to for a locate an end minim air -user um suppl in to pressur y, differe e drop storag nt of .5 e and directi barB5 dryin ons. m of g Thus piping. syste for a 6ir ms particu velocit closer lar air ies of to the consu the consu mer order mer the air of ( to end, flo is 5 mBs and availab are mini le from quite mi7in both comm g pipe directi only bends ons of mainta . the ined. header. !ncorre 6s the ct air 6ir si7ing flo is distri may halved, lead to butio the n excess velocit pressur syste y e drop, ms reduce are hence s and mainl piping also y system the desig pressur s ned should e drop.
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%iping in air systems should not contain loops or be installed undergro
un d. !n ad di ti on to in str u m en t air , if ot he r co m pr es se d ai r se rv ic es li ke pl an t ai r or to ol ai r ar e su pp lie d fr o m th e sa m
e compres sor then no cross connecti ons should be kept beteen these three air services donstre am of the dryer. 5
Edited (y Gerald )ndrey
4eferen ces
K!mproving *ompressed 6ir 8ystem %erformance- a sourcebook for industryL, >.8. e partment of nergy, nergy fficiency and 4eneable nergy 2 *ompressed 6ir *hallenge.
K%86 ngineering ata bookL, 5'th ed. < 8ection 5A, *ompressor s and xpanders, "igure 5A& A, *ompressor
*o ver age *h art, a s %ro ces sor s 8u ppl ier s 6s sn., Tul sa, =k la.
:el a . 9ip tak , K!n str um ent n gin eer s Ha nd bo ok%ro ces s *o ntr olL , Ard ed. < *h apt er ?, 8e cti on ?.F , *o mp res sor *o ntr ols an d =p tim i7a tio n, "ig
