All About Compressor

7/28/2019 All About Compressor

1/15

INTRODUCTION

Compressed air also called as the fourth utility plays a vital rolein the manufacturing process of any industry.

The generation of compressed air is one of the major energy consumingactivity taking place in any industrial operation. Often 'Compressors' arereferred to as 'Power Guzzlers' . An approximate estimate indicates thataround 1500 MW, is consumed nationwide, just to compress air in industry.Moreover, the use of compressed air is increasing linearly with our industrialgrowth. This presents a worthy target for the application of energyconservation (Encon) technologies. Many energy conscious engineersexposed to factory environments believe that, atleast 10% of this can besaved. Therefore, it is in the national interest that compressed air begenerated and used with efficiency & economy, wherever possible. This notonly results in energy savings, but also a saving in rupees.

Types of Air Compressors :-

# Positive displacement type air compressors :-

Positive displacement compressors mechanically displace a fixed volume of air into a reduced volume. They will deliver a nearly constant volume, whenoperated at a fixed speed; while the discharge pressure is determined by thesystem load conditions. The different types of positive displacementcompressors are as follows:-

Reciprocating air compressors :-

Compressed air is generated by the to & fro movement of the piston in thecompression chamber (Just like an IC Engine). Each movement compresses afixed quantity of free air at a specific pressure. According to the type of construction, reciprocating compressors can be further classified as singlestage / double stage and single acting / double acting compressors. Thevertical type air compressors are suitable for applications ranging between50 - 150 CFM, and the horizontal balance type is most suited for applications

ranging from 200 - 5000 CFM. The schematic representation of a doublestage reciprocation is shown in figure.

Double Stage Reciprocating Compressor:

Rotary type air compressors :-Air is compressed by two rotating, intermeshing rotors (in some cases onerotor is kept stationery and the other rotates). The action of the rotary


2/15


3/15

of compressed air required for his plant. To estimate this quantity, thefollowing should be determined.

Maximum compressed air consumption

Maximum compressed air consumption is the total quantity of compressedair required by all the pneumatic equipments connected in the plant,operating in full load condition. To estimate this compressed airconsumption, the air consumption per unit depending on the equipmentconnected should be considered. Wherever process air is used dueconsideration has to be given.

Utilisation factor (Use factor or Load factor)

Use factor is the ratio of total output over a period, to total output whencompressor operates at full load continuously, over that period. In any

industry, where a large number of pneumatic tools / application is involved,all may not be operating simultaneously. In these cases, the use factor is of immense help to the factory manager, to determine the approximateaverage compressed air consumption. This use factor can be determinedwith the help of work-study people. In cases where use factor for each classof machines cannot be estimated, it is a common practice to use an overallfactor of 50%, of the total consumption of all machines in the plant.

Average consumption of air

Average consumption of air can be estimated, considering both the

maximum air consumption and the utilisation factor. A provision of 5% of thetotal average capacity requirement should be made to cover leakagelosses. Also a provison of 10% of the total average capacity requirementshould be given for unaccounted usage and future expansion. Theeffect of altitude should also be given due consideration during theestimation (eg. For compressors installed at 60m above sea level and at atemperature of 50 0 Deg C, the efficiency drop will be around 15%). Theestimated air consumption should be 115% of the calculatedaverage air consumption.

Selection of compressors

Having calculated the total average capacity requirement, the factorymanager can select the type of compressors required.

Rotary SlidingVane

---------------------


4/15

-------

Rotary Screw -x-x-x-x-x-x-x--xx-xx-xx-xx-Piston Liquid -o-o-o-o-o-o-

Rotary Lobe -$-$-$-$-$-$-$-Centrifugal -c-c-c-c-c-c-c-Reciprocating(Single or TwoStage)

-R-R-R-R-R-R-

Reciprocating(More than twostage)

-RR-RR-RR-RR-

Chart to guide selection of air compressors(Source Indian Standards IS 6206 - 1985)

The chart has the capacity (calculated average air consumption) across the Xaxis and pressure across the Y axis. Having known the pressure requirementand the average air consumption, the type of compressor best suited can beselected from the given curves. The graph is self-explanatory.

While selecting the compressors, the following points should be kept in view

a) As a first step, identify the base load and fluctuating load.b) For base (Steady) loads Select centrifugal compressors (best for very highcapacity) or screw compressors.

c) For fluctuating loads Select screw compressor with built-in VFD (the bestoption) or reciprocating compressors.

Selection of compressed air layout

Compressed air system layout, is the process of geometrically locating theair compressor, air piping and work-stations for maximum plant efficiencyand economy. Other factors like production variables, maintenance, location,capacity of utilities and energy consumption should also be considered.Compressor locations should be selected, so as to minimise the length of piping between the air compressor and the largest user of compressed air

user. In systems with a large distribution network, it is preferable to havecompressor centrally located, to minimise the length of piping between thecompressor and the farthest end in the plant.

Some common types used are

a) Loopb) Unit Loop


5/15

c) Gridd) Unit Grid

Different compressed air layouts

Out of these four headers, loop header is the most desired and energyefficient. In some cases, where there is a minimal usage of compressed air atfarthest ends, separate compressed air systems can be installed. For this,the requirement of compressed air has to be estimated, considering loadfluctuations. When multiple pressures are required for different plantoperations, consideration should be given to separate distribution systemsand compressors.

The other important considerations are

a) Adequate space between compressors for regular inspection &

maintenanceb) Proper ventilationc) Optimised cooling arrangements.

Compressed air piping

Having seen the compressed air layout, the next important thing to beconsidered is the compressed air piping system. The following points shouldbe kept in view, while designing the piping, for maximum economy,reliability and high efficiency. This subdivision intends to address primarilythe Encon aspects to be considered, while designing the compressed air

piping system.Minimum pressure drop - 0.3 bar between the compressor plant and thefarthest end of compressed air consumption.

Depending on the length of the pipeline, the airflow speed should be keptbetween 6 - 10 m/s to achieve moderate pressure drop. Pressure drop can bereadily obtained from the figure.

Pressure drop in pipeline

(Source Indian Standards IS 6206 - 1985)

High degree of condensate separation throughout the systemWhile laying the pipelines, adequate water separators (like traps etc.,)should be provided at suitable points, to drain condensate. The waterseparators should be placed in between long straight pipes, with a slightslope to facilitate easy water flow towards the separators.

Minimum number of joints, bends and fittings in pipelineFurther to minimising the joints, it should be ensured that joints are welded,


6/15

instead of flexible or screwed joints wherever possible. This facilitates inminimising the leakages and pressure drop.

Compressed air conditioning

Compressed air needs to be treated to various levels depending on theapplication. The treatment involves removal of dust, oil and moisture. Theremoval of dust, oil & moisture is achievedby filters and dryers.

Filters

Filters are used to remove contaminants, especially dust (Solids), oil &moisture.

Solids

Solid dust is found in compressed air because of Suction air qualityDessicant, activated carbon provided in dryersPipeline rust and scaleBurnt compressor lubricating oil

It is preferable to remove dust to the extent required at the point of use.Filters should have a low pressure drop, as the energy loss across the filter isin the form of pressure drop. A surface filter like pleated paper, ceramic orsintered bronze filter can be used for dust removal. Pleated paper / celluloseare best suited, as they offer very low pressure drop and can handle highdust loads without significant pressure drop. A pressure drop of 0.3 to 1.0 isallowable.Hence, it is recommended to use a pleated paper, ceramic or sintered bronzefilter for dust removal and maintain a pressure drop between 0.3 to 1.0 psi.(14.22 psi = 1 kg/cm 2 )

Oil

Oil from the compressor comes as vapour and liquid aerosol. The vapourcools and becomes liquid in the line. For most industrial applications only theliquid oil need to be removed and is done by a coalescing filter. Coalescersare depth filters and the most energy efficient. Eg., a wet 0.01 [email protected]% efficiency has a pressure drop of 6 PSI, while a 0.01 micron @99.97% efficiency has a pressure drop of 3 PSI only. Hence it isrecommended to install the 99.97 % efficiency, 0.01 micron coalescingfilter for all industrial applications at design stage itself.


7/15

Oil vapour removal

This is required for breathing air and applications where the air is in touchwith the product. Oil vapour produces small stain products. Oil vapour isremoved by activated carbon filter. The dryer media will have a pressure

drop of 0.1 kilogram per square centimetre for a fully saturated wet media.

Energy cost in filters

It costs 7% of the total energy for 1 kilogram per square centimetre pressuredrop at 7-10 ksc operating pressure. This can be higher for higher pressure.Hence,

The filters have to be sized properly

The filters have to be replaced when the pressure drop is high. It is advisable

to change the filters, when the pressure drop reaches 0.5 kilogram persquare centimetre. A stand-by filter can also be arranged to facilitate thechanging of filters. However, the economics of replacement of a particularfilter based on pressure drop can be accurately found from the figure.

Filter replacement chart

Moisture removal

Moisture is found in liquid as well as in suspended form. Moisture in liquidform is called as condensate and the moisture in suspended form is called as

aerosol.

Aerosol is normally removed using water separators. The common types of water separators are

a)Ceramic cartridgeb)Baffle platec)Demister pad type

The demister pad type is the most desired and energy efficient waterseparator, as it is good in removing water droplets and has a lower pressure

drop also.

Drain valves are required to automatically remove condensates. Watercondensates, where there is cooling and settles down in low velocity areas.

The areas of removal are inter and after coolers, moisture separators, airreceivers, drop legs, dryers and filters. The condensate comes with oil anddust, which is a difficult liquid to drain. Float valves do not work well, whenthe condensate is dirty.


8/15

Timer based drain valve assures positive draining. These valves are reliable.However, they waste energy, as lot of air is purged during drain cycle.

The latest technology is to sense condensate electronically. The controller

thus discharges the condensate only, thereby saving air. The capacitancesensing works for pure water and oily condensate. These valves haveenormous energy saving potential and it is the best solution for condensatedraining. Further, the operation is noise free, as no air is vented.

It is therefore recommended to install electronic sensing technology formoisture removal at design stage.

Dryers:-

Moisture in vapour phase needs to be removed. If it is not removed, it will

condense when it is cooled, i.e, at the point of use. There are two methods of drying

Adsorption drying (Dessicant Dryers)Refrigeration Drying

The summary of types of dryers and their advantages areType of

DryerCapital

CostRunning

CostDew

Point o CPressure

DropBest

Suitable for

Dessicant

heatlessLow High -40 oC Medium 150 CFM

Dessicantheated High Medium

-20 o to-40 oC High

100-750CFM or

decided bydew point

DessicantHOC High Very Low

-20 o to-40 oC High

> 1000 CFMfor

lubricatedRefrigeration

dryer Medium Low +3oC Low Compressors100 CFM

Compressor utilities:-

The compressors can also be classified according to the type of cooling. Theyare

a) Air cooled compressors:-


9/15

These compressors use the fan for forced cooling of the compressors. Due tothe low cooling efficiency, this type of cooling is mostly used for low capacitycompressors having intermittent usage.

b) Water cooled compressors:-

For heavy duty or continuous applications water cooling system is adopted,as the efficiency of cooling is high.

The cooling system along with the auxilaries, such as, Cooling towers,Cooling tower fans etc., consume considerable energy. The energyconsumption for such utilities, should also be considered at design stageitself, so as to reduce the overall operating cost.

The following points are to be kept in view, while designing the compressedair system.

Cooling system:-

Cooling water pumps:

The cooling water pumps are designed such that, only one pump caters to aseries of compressors. This is a common feature in almost all the industries.

The pumps are designed keeping in view that all compressors are operatedcontinuously. But in actual practice, only 50 to 60 % of the compressors areoperated. This results in unnecessary pumping of water, leading to a higherpower consumption. A VFD for the cooling water pumps can also be

considered, wherever dedicated systems are not possible. This will result inconsiderable amount of power savings, as well as good process control.Alternatively, dedicated cooling water pumps i.e., a separate pump for eachcompressor can be installed at the design stage itself. This not only savespower, but also increases the flexibility of operation.

Moreover, the pump capacities have to match with the heat removal fromthe compressor. The suggested norms below can be considered foroptimising the water & power consumption vis-a-vis the CFM generated.

Water consumption/CFM = 350 LPM/1000 CFM (Typical 7.0 kilogram per

square centimetre Compressor)

Power consumption/CFM = 2.0 kW/1000 CFM (Typical 20 m head pump)

Cooling tower

Cooling tower is one area where not much importance is given. This alsoconsumes considerable amount of power by way of cooling tower fans. The


10/15

cooling tower fans operate continuously throughout the year. The load is lessduring nights and high in days. This fluctuation has to be kept in view and atemperature indicator controller (TIC) has to be incorporated at design stageitself. This temperature control should automatically cut off cooling tower fanat 30 0 Deg C and start the cooling tower fan at 32 0 Deg C (Suggested

temperatures are typical desired temperatures).

Energy Consumption (Encon) aspects to be considered at design

The following Encon aspects have to be considered, while designing thecompressed air system.

Suction air intake to air compressor

The compressors generate heat, due to their continuous operation. This heat

gets dissipated inside the compressor room / chamber / leading to hot airfeed to the compressor intake. This results in lower volumetric efficiency andhigher power consumption. For an approximate 4 0 Deg C rise in temperature,the power consumption increases by 1% for the same output. Hence, it isrecommended to provide a separate suction duct from outside (atmosphere)to the compressor directly.

Segregation of HP & LP Compressed air system :-

Pressure Vs Power Consumption:-

Higher the pressure, higher is the power consumption. In any industry, thecompressed air system is designed for the maximum pressure requirement,even if the requirement is very minimum.

While calculating the average compressed air consumption of the plant, thetotal requirement of Low Pressure (2.5 to 3.5 bar) and High Pressure (above3.5 bar) compressed air has to be estimated. If any, say LP or HP airconstitutes more than 30 % of the average compressed air consumption,then separate compressed air system has to be installed. The advantage of segregating HP & LP compressed air user has many advantages. They are:-

Reduces the leakages proportionally, as the leakage levels are high athigher pressures.

Reduces the overall operating cost. Say a 20 % reduction in pressureresults in 20% reduction in power consumption of the compressors.Moreover, the wear & tear of the compressors are less at low pressures.

Increases the life of instrument valves, as higher pressure tends to


11/15

damage the joints, packings etc., frequently

Reduces the investment on pressure reducing valves at design stage itself.It is therefore recommended to segregate the low pressure and highpressure compressed air system at the design stage itself.

Install screw compressors with built-in variable frequency drives (VFD) for fluctuating loads.

Variable speed drives eg. (variable frequency drives) can be installed for alltypes of air compressors. However, it is best suited for screw aircompressors. The advantages of installing VFD for screw air compressors are:

All the compressors connected to a common system operate at a constantpressure. The operating pressure will be lesser than the average operating

pressure of loading / unloading system. Hence, energy saving is achieveddue to pressure reduction.

The compressors need not operate in load / unload condition. This savesthe unload power consumption.

An air leakage in the compressed air system also comes down since theaverage operating pressure is less.

Generally, high capacity air compressors are operated with loading /unloading control, as in the case of screw & reciprocating compressors andwith inlet vane control for centrifugal compressors.

In loading / unloading type of control, receiver pressure is sensed and thecompressor load / unloads depending on the pressure. Hence a compressoroperates within a band of pressure range. Generally air compressors operatewith 1 kilogram per square centimetre pressure range.

For example, a compressor operates between load pressure of 6 kg/cm 2 &unloads at a pressure of 7 kg/cm 2 ; (the average pressure is 6.5 kg/cm 2 ,bandwidth 1 kg/cm 2). The power consumption of the compressor operatingconstantly at 6 kg/cm 2 with VFD comes down by 5 to 6 %. By installation of aVFD, it is possible to maintain a bandwidth of 6 kg/cm 2 to 6.1 kg/cm 2 . Themajor advantage of variable speed drive is that if 4 or 5 compressors areconnected to a common header, then by installation of VFD in onecompressor, the energy savings achieved due to pressure reduction iscumulative (power consumption comes down in all compressors).

Since the average operating pressure with VFD is less (6 ksc instead of 6.5


12/15

ksc as per earlier example) the air leakages in the system is also minimised.

The installation of VFD facilitates to reduce / increase the speed of thecompressor depending on the requirement. This completely avoids unloadingand saves unload power consumption, which is normally 25 to 35% of the full

load consumption.

Recently, screw compressors with built-in variable frequency drives, havebeen introduced in the Indian market. This system facilitates fine - tuning of the compressor capacity precisely to meet the fluctuating compressed airdemand. This leads to precise pressure regulation. It accurately measuresthe system pressure and adjusts the speed to automatically maintain aconstant pressure.Hence it is recommended to install screw compressors with built-in VFD forfluctuating loads, at design stage itself.

Minimise leakages by design

Quantity of air losses through small holes, cracks, leaky couplings, joints, etc,can add up to a very large value. With proper installation and maintenance,leakage losses should not exceed 3% of the total capacity of thecompressor. Keeping this in view, the compressed air system has to beprovided with good controls, so as to keep the leakage levels to a bareminimum of 5% of the total compressed air generation. The following shouldbe taken care of at design stage, so as to check and maintain the leakagelevels at the optimised levels

Welded joints should be used instead of screwed joints as far as possible.

Install ball valves at the user ends, to facilitate easy opening & closing of valves.

Initiate a system to replace the flexibles, rubber hoses, joints, packings,etc., in regular intervals (Say once in 3 months).

The compressors are to be provided with hour meters for measuring theloading / Unloading periods. The increase in the loading period for the sameproduction levels, indicates the increase in leakage levels.

Normally, most of the engineering plants operate for two shifts only.However, some units (eg., heat treatment shops, etc.,) have to operatecontinuously in these plants. In order to support this activity the wholecompressed air system has to function continuously.

In such large size plants, having many individual work shops and acentralised compressor house, individual shop-wise solenoid control valves


13/15

for compressed air lines have to be installed.

The solenoid valve helps in cutting the compressed air supply to theindividual shop when there is no activity. This minimise the leakage loss andpressure drop to a considerable level, as most of the work shops do not

operate continuously. Hence, it is recommended to install individual shopwise solenoid control valves for the compressed air line at design itself, so asto minimise the compressed air consumption during non-active periods.

Substitution of compressed air

Compressed air is highly energy intensive and costly. So the factory managerhas to think of the possibilities of replacing compressed air, with anequivalent source at design stage. The possible areas for substituting

compressed air are:-

Agitation:-Normally compressed air is utilised for agitation purposes in ETP

tanks, Pretreatment tanks, etc., For agitation purposes, the quantity of airrequired is important than the pressure (required only to push through thewater column through - a max of 10m (1.0 kg/cm 2) height). This can bereplaced with a Roots Blower. A high pressure blower, can generatepressures upto 2.0 kg/cm 2) agitation, thereby saving enormous power andcost on layout.

Cooling Compressed air for cooling is a common practice in engineering,auto component & tyre industry. Cooling also requires large quantity of air atlow pressure. Compressed air for cooling purposes can be replaced with ablower cooling. This not only saves power, but also effects a good cooling.

Curtain:-Compressed air for providing air curtains is also a common practice adoptedin many industries. Air curtains are required, so as to avoid heat loss fromthe ovens, shots from shot blasting machine, etc,. Compressed air for suchair curtains can be replaced with a blower.

An approximate estimate indicates that a net power saving of 50% ispossible by replacing the compressed air with blower air. So it isrecommended to substitute compressed air with a suitable equivalentsource, say a roots blower, blower etc.

Recovery of compressed air:-

Like substitution, recovery of compressed air is also possible. Recovery of


14/15

high pressure compressed air and utilising it for some low pressure non-critical applications has to be thought of at the design stage itself.Incorporating the recovery arrangements at initial stage would reduce thecompressed air consumption, thereby resulting in power savings. Forexample, Heavy Commercial Vehicle like Truck Tyre PCI air (HP 11 kg/cm 2)

of Tyre curing can be recovered and used for Light Commercial Vehicle TyrePCI application (LP 5 kg/cm 2).

Therefore, it is recommended to recover the compressed air and utilise it forsome other purpose.

Install transvector nozzles for cleaning hoses

The use of compressed air for cleaning applications using is one of thecommon activity in any industry. The cleaning application requires large

quantity of air at low pressure (2.5 ksc). Whereas compressed air at 6.0 kscis used for cleaning applications in almost all the industries. This is mainlydue to the usage of centralised compressed air for cleaning applications.

As a first step, one should think of operating a separate on/off package aircompressor at low pressure for cleaning applications. Wherever suchseparate compressed air system for cleaning cannot be justified, transvectornozzles can be installed for the compressed air cleaning hoses, to minimisethe compressed air consumption.

Transvector nozzles are based on the venturi effect - The passing of high-

pressure air through a constricted hole, creates a vacuum This results in theatmospheric air getting sucked through the circumferential holes provided.

The proven results show that almost 30 to 40 % of the atmospheric air isutilised, thereby reducing the compressed air consumption. This offers agood potential to consider installation of transvector nozzles for the cleaninghoses at design stage.

Install electrical tools as much as possible, instead of pneumatic tools The replacement of pneumatic tools with electrical tools will result in a powersavings of 30%. This is mainly due to the high cost and energy intensivenessof compressed air. Moreover, pneumatic tools are highly leak-prone. This

results in unnecessary wastage of compressed air.

Previously, the electrical tools had some design problems like overweight,overheating, frequent armature failures, etc,. Now these have been takencare of and a new generation of high frequency electrical tools are alsoavailable. So it is recommended to consider electrical tools in place of pneumatic tools at design stage itself.


15/15

Install flat belt drives for reciprocating air compressors

The V-belt drives are commonly used for the reciprocating type of aircompressors. The V belt drives consume more power, due to the 'wedge in'and 'pull out' action of the belt. The present trend is to convert the V-belt

drives to flat belt drives, resulting in a power savings of atleast 5%.Nowadays, good quality synthetic flat belts are available to ensure thesmooth running of compressors. Therefore it is recommended to install flatbelts for the reciprocating air compressors at the design stage itself.

Documents

All About Compressor