CAS EEMaking

Enhancement Happen

Part 2: The basics

of industrial

compressed air

systems

CAS EE – Making Enhancement Happen

Part 2.

The basics of industrial compressed air systems

Enersize Ltd

Pasi PeltomaaDecember 2010

ENERSIZE LTD Friitalantie 13, FI-28400 Ulvila T +358 207 980 310 info@enersize.com www.enersize.com

Contents

Purpose of this document 1

Introduction 2

1 Need for compressed air 5

2 End-using of compressed air 7

3 Distribution of compressed air 10

4 Treatment of compressed air 12

5 Production of compressed air 14

Conclusions 16

References 17

Copyright © 2010 Enersize Ltd

Purpose of this document

This document is the second part of Enersize Ltd’s “CAS EE - Making Enhancement Happen”

document. The purpose of this second part is to introduce the reader to the basics of an industrial

compressed air system. In the first part — “CAS EE – MEH” — the reader was introduced to some

basic information concerning industrial energy efficiency. The conclusions stated that

understanding energy efficiency from the bigger picture helps in understanding better certain areas

of industrial energy efficiency. Compressed air systems are such an area of industry, and with the

awareness achieved from part one, it is easier to understand compressed air.

Despite the name — “The basics of an industrial compressed air system” — this document takes a

strong energy efficient perspective concerning industrial compressed air systems. The document

divides compressed air into five areas: (1) the need for compressed air, (2) end-use, (3) distribution,

(4) treatment, and (5) production.

The minimum level for debating energy efficiency in these areas is the Best Available Technique for

a compressed air system. These techniques are well-known improvement possibilities for an

industrial compressed air system. The BAT-level for compressed air systems is from the European

Commission Reference Document on Best Available Techniques for Energy Efficiency. With proper

techniques we also need an appropriate approach for a compressed air system. With such an

approach the need for compressed air is a core question and a basis of the system.

Part two of CAS EE - MEH takes a look into the basics of an industrial compressed air system with

a strong energy efficient point of view.

Copyright © 2010 Enersize Ltd 1

Introduction

Compressed air has been used for centuries. The earliest mechanical compressors were bellows,

which were used for making bronze and iron in ancient times. The industrial use of compressed air

began in the 19th century when mines used compressed air. Nowadays compressed air systems

are widely used throughout industry, and they are often referred to as the “fourth utility”, after

electrical energy, gas, and water. Almost every industrial plant, from a small machine shop to a

pulp and paper mill, has some kind of compressed air system. In many cases, the compressed air

system is so vital many facilities could not operate without it. Compressed air is a comfortable

utility. It is safe and clean to use and it can be stored at the receivers, also there is always

availability for compressed air, because almost every industrial plant has its own compressed air

system [Elliot 2006; Radgen & Blaustein 2001; U.S. DoE 2003; Peltomaa 2008; Qin & McKane

2008; Šešlija et al. 2009].

A compressed air system consists of several parts, where the main parts are: compressed air

production, treatment, distribution, and end-use. Compressed air production happens with

compressors. There are a lot of different kinds of compressors, but the two most basic

compressor types are displacement compressors and turbo-compressors. This is because the

pressure which is needed and the amount of required flow vary a lot, so there is no existing

compressor which can produce air effectively in every kind of pressure and flow area. After

production, compressed air needs after-treatment, because the air consists of water, oil, and

different kinds of contaminants. Typical after-treatment possibilities are different kinds of filtering,

dryings, and water separation. Compressed air storage is also a natural part of compressed air

production and treatment. In Picture 1, the block diagram identifies the basic components of a

compressed air system by separating them into the main areas [Radgen & Blaustein 2001; U.S.

DoE 2003; Peltomaa 2008; Qin & McKane 2008; Šešlija et al. 2009].

Picture 1. Basic components of a compressed air system with the basic areas. Modified and

reproduced from U.S. DoE 2003.

Copyright © 2010 Enersize Ltd 2

Compressed air distribution happens through a network. Many demands, such as pressure, flow,

ambient conditions, future estimations, and plant layout, all depend on the network size, shape,

and material chosen [BOGE 2004; The ERI 2000]. After distribution, there is the end-use area of

the compressed air system. Industry uses compressed air in many ways, but the two main ways

can be separated: first, as an integral component in industrial processes, e.g. pneumatic sorting,

plastics molding, stirring in high temperature processes like steel and glass and so on, blowing

glass fibers and glass containers; secondly, compressed air can be used as an energy medium,

e.g. driving compressed air tools and pneumatic actuators [EC 2009].

An industrial compressed air system is an overall system where each component is fitted to

correspond with every other component. The system’s goal is to produce cost-effective air, the

quality of which meets the demands of end-use. To meet this goal, a systematic approach is

required all the way from production and treatment to distribution and end-use. The focus must be

shifted from individual components to total system performance with good planning,

manufacturing, and maintenance of the system [McKane 2010; EC 2009; U.S. DoE 2003].

Typically, compressed air systems use a lot more energy than is needed to meet demand [Radgen

& Blaustein 2001; U.S. DoE 2003; Šešlija et al. 2009]. A simple way to understand why we need

consideration and a critical approaching to compressed air systems is to observe the life cycle cost

of a compressed air system (Picture 2). It is a fact that the energy cost is a major component of

CAS. If we split the energy cost and take a look at where energy divides, we can see that only a

small fraction of consumed energy gets a point of use.

Life Cycle CostingPower 160kW Operating hours: 4000h Lifetime: 15a

24%

47%

6%

15%

8%- Investment costs:list price- Maintenance costs:5% of inv. costs p.a.

- Energy costs:motor eff. = 90%load factor = 1;el. price 0.06€/kWhrise in prices = 0

-Interest rate = 10%

Delivered compressed air Investment costsmaintenance cost Heat lossesLosses in CA system

Picture 2. Life Cycle Costing of a compressed air system. The main cost factors are investments,

maintenance, and energy. Energy costs are divided with McKane estimations for energy dividing in

CAS. Only 8% of the total cost gets a point of use. Modified and reproduced from Radgen &

Blaustein 2001; McKane 2010.

Copyright © 2010 Enersize Ltd 3

Because the facts are what they are and lot of energy is wasted all the time, we are taking a look

into the basics of industrial compressed air systems from the energy efficient point of view. The

minimum level for approaching industrial CAS energy efficiency in this document is from the

European Commission Reference Document on Best Available Techniques for Energy Efficiency

(more information from [EC 2009]). It is not enough that we know the proper techniques for

achieving an energy efficient compressed air system. The overall approach for compressed air

systems must take an energy efficient point of view. The need for compressed air is a core

question (Picture 3), and through this question the approach to a proper compressed air system

can begin. After realizing the need for compressed air, we start to look at the end-use and

distribution of compressed air. Finally, there is the treatment and production of compressed air.

Picture 3. Steps for an energy efficient approach to a compressed air system. The need for

compressed air is a core question. Reproduced from Hietaniemi & Lappalainen 2005.

Copyright © 2010 Enersize Ltd 4

1 Need for compressed air

Industry uses compressed air in a wide variety of operations; however, there are two main ways.

The first is as an integral component in industrial processes, e.g. pneumatic sorting, plastics

molding, stirring in high temperature processes like steel and glass and so on, blowing glass fibers

and glass containers. There are also the oxidation, fractionation, cryogenics, refrigeration, filtration,

dehydration and aeration uses of compressed air. Secondly, compressed air can be used as an

energy medium for driving compressed air tools, packaging and automation equipment, and

conveyors [EC 2009; U.S. DoE 2003].

Regardless of where or how much compressed air is used, compressed air needs should always

be critically analyzed. As we know, most of a compressed air system’s life cycle costs come from

energy. So compressed air should only be used if it improves safety, increases productivity, and

reduces labor. This means that air use should be at minimum quantity and pressure with the

shortest possible duration [The ERI 2000; U.S. DoE 2003].

The main requirements for air needs are: air quality, air quantity (capacity), and load profile

(pressure and artificial demand). These requirements affect the whole compressed air system

(production, treatment, distribution, and end-use) [The ERI 2000; U.S. DoE 2003]. The question of

air quality gives us good guidelines for system requirements. Compressed air can be divided into

four wide ranges from plant air to breathing air (Table 1).

Plant air Instrument air Process air Breathing air

Air tools, air-actuated

valves, general plant air

Labo ra to r i e s , pa i n t

s p r a y i n g , p o w d e r

coating, climate control

Food and pharma -

ceutical process air,

electronics

Hospital air systems,

diving tank refill stations,

respirators for cleaning

and/or grit blasting

Table. 1. Compressed air ranges from plant air to breathing air. Reproduced from The ERI 2000.

Typically, industrial applications use the first three categories. The higher the quality, the more the

air costs to produce. The quality of air which is needed at end-use is followed by requirements of

dryness and the amount of contaminants. The next question is the required capacity or quantity for

the compressed air system. This is an important factor for compressor capacity, which needs to

produce the proper amount of air. The total air capacity is the average consumption of each

process and energy medium. The third main question is the load profile of the compressed air

system. Variation in demand over a period of time is a very important issue when an energy

efficient compressed air system is under consideration. A plant which has wide variation, needs to

operate efficiently also in part-loads. Artificial demand and pressure are also topics which belong to

load profile. Artificial demand is the excess volume of air which is produced because of

unregulated end-use areas. Unregulated end-use areas need higher pressure than necessary end-

use areas, so this artificial demand should be eliminated. Pressure in end-use areas is a

Copyright © 2010 Enersize Ltd 5

requirement for the system. In these requirements, pressure losses which come from compressed

air treatment and distribution must be taken into account [The ERI 2000].

It is no secret that compressed air is often used incorrectly in industry. Compressed air is, as earlier

mentioned, safe and clean to use and it can be stored by the receiver, and there is always

availability for compressed air because almost every industrial plant has its own compressed air

system. This leads to a situation where compressed air is chosen for applications where other

energy sources are more economical. Inappropriate uses of compressed air include any

applications which can be carried out economically using energy sources other than compressed

air [The ERI 2000; U.S. DoE 2003].

The following presents some tasks where compressed air should not be used. Electrical cabinets

should be cooled down using air conditioners or fans, not compressed air vortex tubes. A vacuum

system should be applied instead of creating a vacuum using the compressed air Venturi method.

Use blowers, fans, mixers or nozzles and not compressed air for cooling, aspirating, agitating,

mixing, or inflating packaging [Šešlija et al. 2009; The ERI 2000]. Open blowing of compressed air

wastes energy. So instead of open blowing, high efficiency nozzles could be applied, or if there is

no a need for high pressure, considering the use of a blower or fan could be correct. In mixing

applications, mechanical methods typically waste less energy than compressed air [U.S. DoE

2004b].

Do not use compressed air for cleaning parts, use brushes, blowers or a vacuum for this. Low-

pressure blowers, electric fans, brooms, and high efficiency nozzles are more efficient for parts

cleaning than using compressed air for these tasks [Šešlija et al. 2009; The ERI 2000; U.S. DoE

2004b]. Do not use a compressed air blast for moving parts, use blowers, electric actuators or

hydraulic machinery. Use low pressure in blow guns and air lances. Use possible high pressure

blowers that are automatically turned off when the cutting object is absent, rather than using

compressed air [Šešlija et al. 2009; The ERI 2000]. Using compressed air for personnel cooling is

absolutely prohibited due its possible dangers. For cooling, use fans or HVAC upgrading [U.S. DoE

2003; U.S. DoE 2004b].

Some other important areas which everyone should notice are, for example, un-regulated end-use

areas. If there are no pressure regulators, the end-user operates using full pressure, and there may

not be any need for full pressure. Also, if there are abandoned areas in industrial factories, where

there is no longer any need for compressed air, compressed air should not flow there any longer,

preventing unnecessary leaks and air loses [Šešlija et al. 2009; The ERI 2000; U.S. DoE 2003].

Copyright © 2010 Enersize Ltd 6

2 End-using of compressed air

Industry uses compressed air in a wide variety of operations; however, there are two main ways.

The first is as an integral component in industrial processes, e.g. pneumatic sorting, plastics

molding, stirring in high temperature processes like steel and glass and so on, blowing glass fibers

and glass containers, and also the oxidation, fractionation, cryogenics, refrigeration, filtration,

dehydration and aeration uses of compressed air. Secondly, compressed air can be used as an

energy medium for driving compressed air tools, packaging and automation equipment and

conveyors [EC 2009; U.S. DoE 2003]. End-use areas where compressed air is vital should also be

recognized for all possible tasks for optimizing compressed air consumption. Optimization is

possible to achieve, for example, by replacing existing components for more energy efficient

components or better use of existing components [Šešlija et al. 2009].

Replacing existing components with more efficient components is a relevant task by means of an

energy efficient compressed air system. For example, Patrikainen has found that compressed air

consumption can be clearly reduced in winder. Compressed air is used in winder by actuators,

blowers, nozzles, and vacuum techniques. Tests have shown that using nozzles in blowing is

preferable for reducing air consumption. For example, there is the possibility to reduce air

consumption by 25% in tail threading by using nozzles. Also, modifications for vacuum technology

could significantly reduce energy versus an old system [Patrikainen 2008].

Also the importance of pressure regulation at certain end-use areas can be easily shown with the

following picture (Picture 4) and Table 2. Picture 4 shows the case where a cylinder is swept of

work piece from a roll case. The cylinder volume is 32.3l, the cycle is 10 times per minute, and

compression ratio is 5.08 at 4.1 bar. It is assumed that the work performed can be done with 4.1

bar.

Picture 4. Cylinder swept of a work piece from a roll case (original BC Hydro) [Booth et al.].

Copyright © 2010 Enersize Ltd 7

Pressure (bar)

Air (m3/min)

Power (kW)

Energy use (kWh/a)

Lost (kWh/a)

Lost (€/a)

4.1 1.64 10.8 40 564 0 0

4.8 1.86 12.3 46 019 5 455 545

5.5 2.08 13.7 51 404 10 840 1 084

6.2 2.30 15.2 56 859 16 295 1 630

6.9 2.52 22.3 62 314 21 750 2 175

Table 2. Comparing operation at different cylinder pressures according to Picture 4. The table is

based on the following assumptions: two 7.5 h shifts per day, 250 work days/year and an

electricity price of 0.1€/kWh. Modified and reproduced from (original BC Hydro) Booth et al..

The single most important action which affects compressed air system energy efficiency is the

reduction of air leaks [EC 2009; Radgen & Blaustein 2001; Saidur et al. 2010; Šešlija et al. 2009;

The ERI 2000; U.S. DoE 2003]. Radgen & Blaustein have argued that reducing leaks can decrease

annual energy consumption by 20% in 80% of compressed air systems where leak prevention is

applicable and cost effective [Radgen & Blaustein 2001]. Typically, the problem areas for leakages

are end-use areas and the distribution system of a compressed air system. Leakage rates are a

function of supply pressure in an uncontrolled system, and also leaks also increased with higher

system pressure. Leakage rates are also proportional to hole diameter [U.S. DoE 2004a]. The

following table (Table 3) gives information on leakage rates with certain holes with different

pressures, and Table 4 gives directional guidance on understanding how much compressed air

leaks cost.

Pressure Hole diameter (approximately equivalent) [inches], [mm]Hole diameter (approximately equivalent) [inches], [mm]Hole diameter (approximately equivalent) [inches], [mm]Hole diameter (approximately equivalent) [inches], [mm]Hole diameter (approximately equivalent) [inches], [mm]Hole diameter (approximately equivalent) [inches], [mm]

[bar] 1/64 , 0.4 1/32 , 0.8 1/16 , 1.6 1/8 , 3.2 1/4 , 6.4 3/8 , 9.5

4.8 0.0082 0.033 0.132 0.527 2.107 4.751

5.5 0.0091 0.036 0.148 0.588 2.353 5.300 Leakage

6.2 0.010 0.041 0.162 0.654 2.605 5.850 ratesa

6.9 0.011 0.044 0.179 0.714 2.857 6.428 [m3/min]

8.6 0.014 0.055 0.217 0.868 3.460 7.801

Table 3. Leakage rates with certain holes at different pressures. a) For well-rounded holes, multiply

the values by 0.97 and sharp-edged holes, multiply the values by 0.61. Modified and reproduced

from U.S. DoE 2004a.

Copyright © 2010 Enersize Ltd 8

Hole diameter

[mm]

Air loss with 6bar

[m3/min]

Power needs

approximately

[kW]

Annual

production

costs [€]

Annual cost of

production, treatment

and distribution [€]

1 0.080 0.4 320 768

3 0.670 4 3 200 6 432

5 1.857 10 8 000 17 827

10 7.850 43 34 400 75 360

Table 4. Costs of compressed air leaks. The table basis for the following estimations: electrical

energy price of 0.1€/kWh (industrial electricity average price in EU is 0.09–0.12 €/kWh), system

operates 8000h/a, and the cost of compressed air treatment is 0.02€/m3. Modified and

reproduced from Šešlija et al. 2009.

The reduction of operating pressure is also an important task due to leak prevention. As we can

see from Table 3, a higher pressure increases leakage rates. Usually, pressure increases are for

compensating a lack of capacity. This is counterproductive. Higher pressure leads to higher leaks

[Šešlija et al. 2009]. When we are thinking about possible savings, such as in Table 4, we must

remember that repairing leaks will not reduce energy costs by the full amount, unless all the fixed

leaks have created an air demand on the full load compressor and, after the leaks have been fixed,

there is no longer any need for the compressor. The most common method for estimating leak

savings is the rule-of-thumb method where the assumption is that all energy would be saved if the

leaks were fixed. So the rule-of-thumb method overestimates possible savings. The actual amount

of possible savings is a function of the type of compressor and the local capacity controls [Schmidt

& Kissock 2004; Ormer]. These savings vary from 0% up to 95% [Ormer]. The point is not to ignore

the importance of leak prevention, but rather to emphasize that the size of possible savings from

leak prevention might be easily overestimated.

Copyright © 2010 Enersize Ltd 9

3 Distribution of compressed air

The compressed air which an end-use area uses needs a proper distribution system. There are

various demands which set challenges to guarantee the continuous, reliable, and efficient

operation of the end-use area. Firstly, the distribution system must allow sufficient volume flow and

the required working pressure so that each end-use device has the proper volume flow and air

pressure at all times. The distribution system must allow the required air quality to each end-use

device at all times, and pressure losses should also be minimized for energy efficient reasons

[BOGE 2004; EC 2009; Radgen & Blaustein 2001; Šešlija et al. 2009; U.S. DoE 2003; The ERI

2000]. Secure operation and safety rules are also important. Maintenance and repair work on lines

must not shut down the entire distribution system, and all safety rules must be followed to prevent

accidents [BOGE 2004].

The main distribution line should be in the form of a ring as it is the most cost-effective method to

distribute compressed air and also it increases the security of the line. The ring line forms a closed

distribution ring. With part of a ring it is possible to isolate without interrupting the supply of

compressed air for other areas, and also the alteration or extension of the distribution line is made

easy with a ring line. In the distribution ring line the compressed air has a shorter route than with a

stub line, thereby reducing the pressure drop over the system [The ERI 2000; BOGE 2004]. The

following picture (Picture 5) gives an example of a ring form distribution system.

Picture 5. Example of a ring form distribution system [The ERI 2000].

Copyright © 2010 Enersize Ltd 10

The main factors which affect the inside diameter of a distribution system are volume flow, the

effective flow length of the pipeline, and operating pressure. There are some possibilities for

determining the inside diameter of the distribution system. For example, calculation with a formula,

determining with a nomogramme or determining with the bar graph [BOGE 2004]. Table 5 gives a

quick utilization for determining the maximum flow at a certain pipe at 7 bar pressure. The future

demands of a system should also be noticed when sizing of the distribution system is done. Table

6 shows the power losses for different pipe sizes with flow 30 m3/min at 7 bar pressure.

Nominal pipe [DN] Max flow [m3/min]

10 0.3

25 1.5

50 6.0

65 10.8

80 14.4

100 24.6

150 54.0

Table 5. Quick utilization for determining the maximum flow at certain pipe at 7 bar pressure.

Modified and reproduced from The ERI 2000.

Nominal pipe [DN] Pressure drop per 100m (bar) Equivalent power lost (kW)

50 2.6 18

65 0.9 5

80 0.2 0.8

100 0.1 0.4

Table 6. Power losses for different pipe sizes with a flow of 30 m3/min at 7 bar pressure. Modified

and reproduced from The ERI 2000.

The compressed air receiver is also a kind of device in the distribution system, thus it is also part of

compressed air treatment. In the distribution system the receiver’s tasks are to provide storing

capacity for compressed air and create more stable pressure conditions to effectively balance

fluctuations in pressure [The ERI 2000; BOGE 2004]. One important issue in the distribution

system is leak prevention. The reduction of air leaks is the single most effective action which

increases compressed air system energy efficiency [EC 2009; Radgen & Blaustein 2001; Saidur et

al. 2010; Šešlija et al. 2009; The ERI 2000; U.S. DoE 2003]. There are many potential areas for

leaks in a distribution system, such as couplings, hoses, tubes, fittings, pipe joints, quick

disconnects, condensate traps, valves, flanges, packings, thread sealants, etc. [U.S. DoE 2004a].

The next section provides more information on leaks.

Copyright © 2010 Enersize Ltd 11

4 Treatment of compressed air

Compressed air needs treatment. In 1 m3 of atmospheric air there are contaminants of over 180

million particles of dirt with sizes ranging from 0.01...100μm. Also, 1 m3 of atmospheric air consists

of water in the form of atmospheric humidity 5–40 g/m3 , oil in the form of mineral oil aerosols, and

unburnt hydrocarbons 0.01...0.03 mg/m3, and traces of heavy metals such as lead, cadmium,

mercury, and iron. When atmospheric air is compressed to 10 bar overpressure, the concentration

of impurities rises over 11 times. After compression, the air content is over 2 billion particles of

impurities [BOGE 2004]. The following example gives information on water in compressed air: a 30

kW compressor with production of 5 m3/min at 7.5 bar produces about 20 liters of water per

working shift. Per annum this is almost 6,000 liters [Kaeser Kompressorit Oy].

As we can see, compressed air is not very clean and it needs treatment to be suitable for different

kinds of end-use areas. The quality of compressed air has been specified in the standard DIN ISO

8573-1 (Table 7). The standard includes six quality classes for the filtering degree, oil content, and

dew point.

Class OILincl. vapours

[mg/m3]

DIRTparticle size

[μm]

DIRTconcentration

[mg/m3]

WATERres. water

[g/m3]

WATERdew point

[℃]

1 0.01 0.1 0.1 0.003 -70

2 0.1 1 1 0.117 -40

3 1 5 5 0.88 -20

4 5 15 8 5.953 +3

5 25 40 10 7.732 +7

6 - - - 9.356 +10

Table 7. Classes of DIN ISO 8573-1 standard. Reproduced from BOGE 2004; The ERI 2000.

For compressed air filtering there is a wide range of filters: cyclone separator, pre-filter, micro filter,

active carbon filter, and sterile filter. A cyclone separator is located just after the compressors and

its task is to remove water from compressed air. The pre-filter is usually before the micro filter, if the

compressed air is very dirty. If the requirements are for high quality air, there is micro filter. After the

micro filtering compressed air is actually oil free, but there are still some hydrocarbons and some

odors and taste substances. With an active carbon filter hydrocarbons can be removed from the

air. If we need totally sterile and bacteria free air, we need sterile filtering [BOGE 2004]. Every filter in

a compressed air system causes a pressure drop and this leads to a situation where we need

higher pressure from the compressor. For example, a 1 bar pressure drop in a 7 bar net pressure

causes energy losses of 6–7% [Hingorani 2009].

Copyright © 2010 Enersize Ltd 12

To prevent unnecessary pressure drops a filter process should be optimized with the following

tasks. Firstly, identify the possible filter locations and then define the following parameters: flows,

pressures, temperatures, allowed pressure drops, compatibility and possible needs for validation in

critical places. Then defining the types and concentrations of contaminants in specific locations

and also defining the needs for retention of efficiency and the number of filtration stages for each

specific location. Finally, choosing the right filter element and housing for each specific location

[Šešlija et al. 2009]. The following table (Table 8) gives some general guidelines for filter types.

Filter type Application Max Δp with working pressure of 7 bar

Special demands

Regular filters Particle removal 0.14 - 0.5 No

Micro filter Removal of all kinds of particles and fluids 0.17 - 0.7

Installation of an ordinary prefilter

Active carbon filterRemoval of fumes and

odors 0.0017 - 0.13Installation of ordinary

and micro prefilter

Sterile filter Removal of biological load

3.0 - 5.3Installation of ordinary,

micro and active carbon prefilter

Table 8. General guidelines for filter types. Modified and reproduced from Šešlija et al. 2009.

Compressed air can be dried using three main principles: condensation, diffusion, and sorption

[BOGE 2004]. The most common drying methods are refrigeration drying (the condensation

principle) and adsorption drying (the sorption principle). Typically, if there is no need for very dry air

(dew point +3℃...+5℃), a refrigeration dryer is the right choice. A refrigeration dryer uses only 3%

of the energy which a compressor needs for compressed air production, and an adsorption dryer

uses 10–20%. An adsorption dryer should be preferred only when there is a need for very dry air

(dew point -40℃...-60℃) or if the distribution system is in contact with the outside atmosphere

[Enersize Ltd; Kaeser Kompressorit Oy]. A compressed air receiver is a kind of treating device. Air

can cool down and water can also separate in a receiver [BOGE 2004; The ERI 2000].

Copyright © 2010 Enersize Ltd 13

5 Production of compressed air

A compressor is at the heart of a compressed air system. It produces compressed air by

increasing the pressure of air. The need for compressed air in an end-use area creates demands

for the compressor. Typically, the pressure level in industry is 6–10 bar, but higher levels are also

possible, all the way to 200 bar. Air flow also needs to vary, from a couple of liters per minute up to

the hundreds of cubic meters per minute. So there are no “one size fits all” compressors. Two

basic compressor types are displacement compressors and turbo-compressors. In displacement

compressors, the air flows into a chamber where the reduction of chamber volume causes the air

pressure to increase prior to discharge. At a constant speed, the airflow keeps constant with some

variation in discharge pressure. Most common displacement compressors are screw and

reciprocating compressors. In turbo-compressors, the air flows through running wheels which are

equipped with blades. The running wheel accelerates the air to a high speed. Impellers and

discharge volutes or diffusers change the velocity energy into pressure energy [Radgen & Blaustein

2001; U.S. DoE 2003; Peltomaa 2008; Qin & McKane 2008; Šešlija et al. 2009].

Consideration in making the right choice between compressors is important. One way to compare

the efficiencies of compressors is to compare their specific energy consumptions. This indicator

shows how much energy is used to produce a certain amount of flow. The following table (Table 9)

gives examples of how efficiencies change between compressors in different capacities. All of the

compressors are working at 7 bar pressure.

Description Capacity

(m3/min)

Specific Energy Consumption (kWh/

m3)

Part Load efficiency

Lubricated piston

0.12 - 1.5

1.5 - 15.015.0 - 60.0

0.141

0.1180.100

Good

GoodExcellent

Non-lubricated piston

0.12 - 1.5

1.5 - 15.015.0 - 60.0

0.153

0.1290.112

Good

GoodExcellent

Oil-injected vane/screw

0.12 - 1.5

1.5 - 15.015.0 - 60.0

0.141

0.1240.112

Poor

FairFair

Non-lubricated toothed rotor/screw

1.5 - 15.0

15.0 - 60.060.0 - 120.0

0.119

0.1060.106

Good

GoodGood

Non-lubricated centrifugal

15.0 - 60.0

60.0 - 120.0Over 120.0

0.124

0.1060.100

Good

ExcellentExcellent

Table 9. Efficiencies of compressors with different capacities. Reproduced from The ERI 2000.

Copyright © 2010 Enersize Ltd 14

Choosing the right compressor is the first step toward the energy efficient production of

compressed air. A compressor with a sophisticated control system improves the ability to adjust

the compressor flow to meet the demands of the end-use area. This happens by optimizing the

transition between the non-loading state, the loading state, and the non-operating state of the

compressor. Predictive control uses fuzzy logic and other algorithms to predict the future demands

of end-use by considering the history of system behavior. The type of compressor largely

determines the type of control systems which are going to be selected. If there is only one

compressor and the demand for compressed air is quite steady, then a simple control system

might be appropriate. On the other hand, complex systems with several compressors, varying

demands and many types of end-users raise challenges for proper control of the system [Radgen

& Blaustein 2001; Šešlija et al. 2009; The ERI 2000].

There are also some possibilities to improve compressor technologies, but improvements are

usually customized for different segments of industry. Another point of view for improvement is to

improve production methods to achieve closer clearances to reduce the gap leakage of machines.

We must remember that the laws of thermodynamics set limits on improving the energy efficiency

of compressors. Power drive improvements are also a way of improving compressed air

production. There is the possibility of using high-efficiency power drives for improving the efficiency

of a compressor. We must remember that power drive improvements are most cost-effective with

small systems (under 10 kW) and they provide the largest savings for new systems, because it is

not very likely that users change new drives into existing compressors without changing the

compressor itself [Radgen & Blaustein 2001; Šešlija et al. 2009].

If a compressor is used under variable demands, then integration of a variable speed drive into the

compressor can lead to energy efficiency improvements [EC 2009; Falkner 2009; Radgen &

Blaustein 2001; Šešlija et al. 2009; The ERI 2000]. We must remember that VSD-compressors’ air-

ends have a typically sized corresponding optimum tip speed of 70% load. VSD-compressors

cannot operate only in the optimum area, they operate in the full 0–100% load area. So, VSD-

compressors are typically more efficient than normal on/off compressors by approximately 50% of

their capacity range. If the compressor load frequently reaches a level which is outside of this 50%

area, its efficiency decreases [St Baker 2008; St Baker 2009].

Compressors generate heat while compressing air. Actually, the main principle of thermodynamics

states that the entire electrical power intake is converted into heat. The heat is low grade and

usually wasted but, under certain conditions, this heat can be used for other functions [BOGE

2004; Radgen & Blaustein 2001; U.S. DoE 2003; Šešlija et al. 2009; Saidur et al. 2010; the Energy

Research Institute 2000]. Recommendations for its usage depends on the situation of the

consumers whose demands meet the amount of generated heat and also whose usage is

provided with relevant equipment (heat exchangers, pipe lines, regulators, etc.), and the price of

alternative solutions [Šešlija et al. 2009]. According to heat recovery, compressor station

temperature has a clear effect on compressor efficiency. Whether heat is recovered or not, heat

from compressors must be rejected from a compressor station because of thermodynamics. The

compression of warm air requires more energy than cool air, because cool air is more dense than

warm air. The following effects have been recognized from temperature decreasing effects on

compressor power needs (1% decrease / 3°C decrease - 2% decrease / 5°C decrease) [EC 2009;

Saidur et al. 2010; Tamrotor kompressorit Oy].

Copyright © 2010 Enersize Ltd 15

Conclusions

An industrial compressed air system which should correspond to the Best Available Technique

level and also where needs are properly adjusted requires a wide approach. The main requirements

for air needs are: air quality, air quantity, and load profile. These requirements affect the entire

compressed air system. Realizing the needs of compressed air gives us good guidelines for

looking into the end-use area and the distribution of compressed air. Finally, there comes the

treatment and production of compressed air.

There are many important factors to take into account when we move towards an energy efficient

compressed air system. First, preventing the inappropriate use of compressed air because

compressed air is often used incorrectly in industry. There are many applications which can be

carried out more economically using energy sources other than compressed air. In the end-use

area, there is the important issue of reducing leaks. Leak reduction is the single most important

action towards an energy efficient compressed air system. Another important task in the end-use

area is, for example, preventing the unnecessary use of air and reducing operating pressure. An

efficient distribution system needs actions from a properly sized and planned piping layout to leak

prevention, which is also a relevant task in the distribution area. Compressed air treatment consists

of important tasks such as optimizing the filtering process and reducing unnecessary drying.

Compressed air production consists of several tasks which should be seen all the way from correct

compressor choice with a sophisticated control system to heat recovery and suitable temperature

for the compressor station.

We should remember that an industrial compressed air system is an overall system, where every

component must be fitted to correspond to every other component. The goal of an industrial

compressed air system is to produce cost-effective air, the quality of which meets the demands of

end-use. To meet this target, a systematic approach is required all the way from the need of the

system and end-use to production. The focus must be shifted from individual components to total

system performance with good planning, manufacturing, and maintenance of the system.

Energy is the biggest cost in the life cycle cost of a compressed air system, and the fact is that,

typically, a compressed air system uses a lot more energy than is needed to meet demand. Such

facts are also opportunities and possibilities for increasing the energy efficiency of industrial

compressed air systems. As a result, we always have to look at the basics of industrial

compressed air systems from an energy efficiency point of view.

Copyright © 2010 Enersize Ltd 16

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Copyright © 2010 Enersize Ltd 17

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Copyright © 2010 Enersize Ltd 18

Enersize Ltd.

Head office:Friitalantie 13 FI-28400 ULVILA Finlandtel. +358 207 980 310

www.enersize.com

Project Director

Mr. Pasi PeltomaaResearch and Development Managertel. +358 207 980 310pasi.peltomaa@enersize.com

Enersize wants to lead the process industry into a new culture of energy usage. Together.