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APPLICATION NOTE

COMPRESSED AIR

Jean Timmermans (Laborelec) and Hugh Falkner (Atkins)

June 2015

ECI Publication No Cu0116

Available from www.leonardo-energy.org



Publication No Cu0116

Issue Date: June 2015

Page i

Document Issue Control Sheet

Document Title: Application Note – Compressed Air

Publication No: Cu0116

Issue: 03

Release: June 2015

Author(s): Jean Timmermans (Laborelec) and Hugh Falkner (Atkins)

Reviewer(s): Bruno De Wachter, Hugh Falkner

Document History

Issue Date Purpose

1 February

2007

Initial publication

2 November

2011

Upgrade for adoption into the Good Practice Guide

3 March

2015

Revised version

Disclaimer

While this publication has been prepared with care, European Copper Institute and other contributors provide

no warranty with regards to the content and shall not be liable for any direct, incidental or consequential

damages that may result from the use of the information or the data contained.

Copyright© European Copper Institute.

Reproduction is authorised providing the material is unabridged and the source is acknowledged.





Page ii

CONTENTS

Summary ........................................................................................................................................................ 1

Introduction .................................................................................................................................................... 2

The cost of compressed air ........................................................ ................................................................. ............ 2

Avoiding inappropriate use of compressed air .......................................................... ............................................. 2

Air Pressure .................................................................................................................................................... 3

Air Quality ...................................................................................................................................................... 4

Air quality requirements ............................................................ ................................................................. ............ 4

Particulate removal ................................................................................................................................................ 5

Moisture content ....................................................................................................... ............................................. 5

Oil removal ............................................................................................................................................................. 6

The Distribution network ................................................................................................................................ 7

Network design ............................................................... ................................................................. ....................... 7

Condensate drain traps .......................................................................................................................................... 7

Compressed air leakage ............................................................. ................................................................. ............ 8

The compressor choice .............................................................. ................................................................. ............ 8

Air Compressor control ................................................................................................................................. 10

Dealing with variable demands ........................... ................................................................. ................................ 10

Air receivers .................................................................................................... ...................................................... 10

Load/unload control ............................................................................ ............................................................... .. 10

Several compressors in cascade ................................................................................ ........................................... 11

Example showing the benefit of using an additional small compressor to deliver air during times of

low demand ...................................................... ................................................................. ..................... 11

Variable Speed drives ........................................................................................................................................... 14

Specifying an air compressor ........................................................................................................................ 15

Compressor Inlet Conditions ................................ ................................................................ ................................ 15

Heat recovery from air compressors ............................................................................................................. 16

Opportunities for heat recovery ................................................................................................................. .......... 16

Matching the demand for and availability of waste heat ................................................................ ..................... 16

Costs of implementing heat recovery .............................................................. ..................................................... 17

How much heat can my compressor provide in different applications? .............................................................. 18

Practical notes ...................................................................................................................................................... 18

Conclusions ................................................................................................................................................... 19

Further reading ............................................................................................................................................. 20





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Page 1

SUMMARY

The supply of compressed air accounts for between 10 to 15% of the electricity bill of the average industrial

consumer. Fortunately, compressed air systems frequently offer some easy to identify and rectify energy

saving opportunities. These should be a priority at all sites using this versatile but costly resource.

The flexibility of compressed air means that it can be used for an ever growing number of applications on a

site, with insufficient thought given to the impact on the overall system, or its true cost. Even when poorly

maintained, a compressed air system may appear to function satisfactorily, and so problems will not become

apparent without systematic maintenance procedures being in place.

This application guide gives an overview of technical and organizational solutions that will maximize the

energetic ef ficiency of the system. Implementation of these measures can save up to 25% of the systems’

energy consumption.

Low or no cost actions:

Identify and fix leaks on a regular basis. This can reduce compressed air use by over 10%.

Reduce the system pressure to the minimum needed. A reduction of 1 bar will save 7%.

Fit pressure indicators across air filters to show when they require changing (at 0.5 bar(g) or in line

with manufacturers specification).

If blowdown guns must be used, limit the pressure to 2.0 bar(g), which is also a safety requirement.

If possible, isolate unused equipment or network legs for all or part of the time to reduce leakage.

Replace older unreliable condensate drain traps with modern and more energy efficient and reliable

electronic condensate drain traps.

Higher cost actions

Look at where compressor air is being used. Are there lower cost alternatives?

Move the air compressor inlet to a cooler location. A reduction of 10°C can save 3.5% in energy

consumption.

Air quality requirements should be carefully specified according to need. Excessive air quality

increases energy costs. General purpose treatment for the whole network can be supplemented by

localised higher quality treatment equipment for particularly demanding applications.

Reduce inefficient frequent cycling by installing an air receiver close to the compressors, and also next

to applications with particularly high but periodic demand.

Where using several air compressors, ensure that they are configured to give the lowest overall cost

of compressed air production.

When purchasing air compressors, ensure that the full load and part load energy performance is

specified, and measured in accordance with specific inlet conditions.

Look for opportunities to use the heat losses from an air compressor installation for space or water

heating.

Maintenance of compressors and dryers is important to maintain best efficiency and avoid unplanned

outages.

Finally, maximizing the energy efficiency of a compressed air system is not a one-time action. Continuous

monitoring of key parts, and adjusting them when necessary, is indispensable.





Page 2

INTRODUCTION

THE COST OF COMPRESSED AIR

The versatility, convenience and relative safety of compressed air mean that is widely used throughout

industry, accounting for 10% - 15% of the consumption of electricity at a typical site. It is used for processes as

diverse as paint spraying, blow moulding, vacuum production and transportation applications.

It is though easy to overlook its cost, since with an electricity price of 0.1 euro/kWh, and a typical compressor

efficiency of just 10%, this equates to 1.0 euro/kWh, and that is before the often considerable losses between

the compressor and the end use application. This Application Note describes the many considerations in

designing a reliable and efficient compressed air system, and highlights the energy saving opportunities that

can often be found.

For a typical air compressor, the energy consumption represents 75% of the lifetime cost of operation, and so

energy efficiency should be the priority for reducing the total costs of compressed air.

Figure 1 – Lifetime costs of ownership of an air compressor.

AVOIDING INAPPROPRIATE USE OF COMPRESSED AIR

Since compressed air is clean, readily available around the factory, and simple to use, it can often be chosen

for applications for which it is not the most cost efficient option. Compressed air is a premium energy source,

but its cost means that it should only be used where there is no feasible alternative, and then at the lowest

satisfactory pressure and quality. For many common uses for compressed air there are substitutes with lower

running costs that should be considered, as illustrated by table 1.

Application Possible alternatives

Drying, cleaning, cooling Low pressure blowers, fans, brushes.

NB: Where compressed air is unavoidable, specialist high

efficiency nozzles can be used that enhance the cooling effect.

Air driven mixers or diaphragm pumps Electric motor driven equipment

Compressed air driven hand tools Electric motor driven equipment, but the extra weight of

unsupported equipment will be tiring for operators.

Sparging Low pressure blowers

Table 1 – Alternatives to compressed air.





Page 3

AIR PRESSURE

Not all equipment requires a standard 7 bar(g) supply. Some requires less, some more. Generating at excess

pressure can increase costs by an average of 7% for every additional 1 bar pressure. Understanding the correct

pressure requirements for each application is therefore a good starting point for rationalising systems.

For equipment with particularly high pressure requirements, a dedicated high pressure compressor avoids the

whole network having to be at a higher pressure. Similarly, applications that only require a low pressure could

be supplied by a low pressure compressor, or even a more energy efficiency alternative to compressed air

(table 1).

In all systems, the actual pressure seen by attached machinery will be less than that leaving the compressor

house, as there will be additional pressure drops in the distribution network. If the distribution piping system

causes an excessive pressure drop, this will also result in a higher required outlet pressure of the compressor.

This in turn raises the consumption for every kind of unregulated usage, including leaks.

Equipment connecting hoses often present the largest pressure drop in the system. If the pressure is too low,

then the tool duty will be reduced. This means that it will take longer to do the work. If the tool becomes

inadequate, it will need to be replaced by a new one of higher rating. Checking the air pressure at the entry to

the equipment while at maximum flow will indicate if this is a problem.

Figure 2 – Influence of system pressure on compressed air power demand (from GPG241 “ Energy Savings in the

Selection; Control and Maintenance of Air Compressors” ).

Some sites save energy by progressively reducing the pressure until somebody complains, for example they

will notice that motors or actuators will take longer to operate. Increasing the pressure slightly until users no

longer complain will then give the new actual required air pressure. To avoid possible unexpected problems,

this should only be done by a compressed air operator with a good knowledge of the applications of

compressed air on that network.





Page 4

AIR Q UALITY

AIR QUALITY REQUIREMENTS

Air quality is formally defined in terms of the quantities of contaminants present, which include water,

particulates and oil carry-over from the compressor. The compression process means that for a 7 bar(g)

system, the contaminant level in the intake air will be multiplied by a factor eight.

Requirements can range from basic treatment only for construction, mining and general hand tools, through to

the very high quality air essential for pharmaceuticals or semiconductor processing. Providing the right quality

of air is essential to prevent wear of compressed air driven equipment, or spoilage of material that is in direct

contact with the compressed air. Not only is air treatment equipment costly, it also requires a direct source of

energy to run it and gives an additional system pressure drop to overcome.

It is common to have general purpose air treatment equipment within the compressor house, and then

additional higher quality air treatment equipment dedicated just to the equipment that requires it. This point

of use treatment equipment also ensures the removal of any contaminants picked up in the distributionnetwork. Figure 3 illustrates a typical arrangement for supplying air to different applications on an industrial

site.

Figure 3 – Example of compressed air treatment system for high quality air (from GPG 216 Energy Saving in the

Filtration and Drying of Compressed Air).

The ISO8573 series of air quality standards provides a clear basis for the specification and selection of

treatment equipment, with typical quality classes for a range of applications shown in table 2.

Application classes Typical quality classes

Dirt Water Oil

Construction 5 5 4

Fluidics, power circuits 4 4 4

Food and beverages 3 1 2

Paint Spraying 3 3 3

Sand blasting 3 3 -

Micro-electronics manufacture 1 1 1

Table 2 – Typical air quality classes for selected applications.





Page 5

PARTICULATE REMOVAL

In addition to particulates in the intake air, additional contaminants such as sludge, rust and pipescale can be

picked up in the distribution network. Filters to remove particulates should be fitted to avoid any risk of

contaminating products. The filters should be selected for the flow and size of particulate they are designed to

remove. For critical uses, polished stainless steel, copper or PVC pipes are used to avoid corrosion.

It is good practice to fit a pressure gauge across filters to detect when the filter needs cleaning. A larger filter

will show a lower pressure drop for the same flow, so check filters are not under-sized. In the absence of

detailed data, a quick check is that the filter flanges have a diameter that is not smaller than that of the piping.

Finer filters have a larger pressure drop, and so the filters should not be specified with a higher quality than is

actually needed.

In critical systems where the compressed air supply cannot be switched off, fitting duplex filters enables

replacement without turning the system off.

MOISTURE CONTENT

The compression process means that the compressed air that is leaving the compressor will be warmer than

the ambient air, and fully saturated with moisture. As the air cools down in the pipes, the moisture will

condense. This moisture could lead to pipe corrosion and contamination at the compressed air end use device.

It should normally be reduced by an air dryer. As an example, a 500 l/s compressor would produce over 1,000

litres of water vapour per week1

However, air dryers lead to additional system energy consumption from their own internal power

consumption, from purging air losses, and from the pressure drop that they add to the system. This may be 3

to 5 psi altogether, and even higher if the dryer is undersized. To minimise energy costs, compressed air should

therefore only be dried to the degree that is needed for the duty.

The selection of a compressed air dryer should be based upon the required dew point and the estimated costs

of purchase and operation. Different types of compressed air dryers are available which offer particular

degrees of dew point suppression (see table 3).

Refrigeration dryers are popular general purpose dryers, which chill the air to condense out the moisture for

removal, and then re-warm the air to have a lower dewpoint.

For applications that demand much dryer air, two column desiccant driers are most commonly used. While

one of the columns is absorbing moisture from the air, the other is being regenerated, and then after a while

the duties swapped around. The regeneration of the absorbent is by the use of heated or un-warmed air, with

there being several variants for different applications. This regeneration is energy intensive, and so the

different options for this should be carefully considered when selecting a drier.

In applications where the inlet air only has a low moisture content, and/or the air demand is considerably

below the rated one, the drying capacity will be greatly in excess of what is needed. It is therefore now usual

for desiccant dryers to be fitted with dew-point sensors that reduce this energy loss by stopping regeneration

as soon as the absorbent is adequately dry. This can give an excellent payback in new and retrofit applications.

1 Based on 12.5g or water for each m

3 of free saturated air at 15C and 48 hour per week operation, (GPG216).





Page 6

Waste heat regenerative dryers use the hot air produced by an air compressor to remove the moisture from a

heat wheel, and are available as integrated compressor-dryer packages.

Pressure dewpoint Dryer type Typical levels of

filtration installed

Added energy cost

+3°C Refrigeration General purpose 5%

-20°C Waste heat regenerative Depends on compressor

configuration

3-5%

-40°C to -70°C Desiccant High efficiency before

the dryer, and dust

removal after it.

8-21%

Table 3 – Pressure dewpoints and energy costs of compressed air drying equipment.

OIL REMOVAL

Filters to remove oil carried over from the compressors should be fitted in such a way that the oil cannot

contaminate the products that it comes into contact with.

For critical processes, oil carryover can be eliminated by the use of lubrication free rotary screw air

compressors. Such compressors are more costly, usually less efficient and may require more maintenance. But

these disadvantages should be offset against the avoidance of need for subsequent separate oil removal. For

very large air demands, centrifugal compressors are also inherently oil free, but only the largest sites would

normally consider these machines.

Note that the oil carried over from the compression process will be damaged, and so should not be considered

an alternative to proper equipment lubrication.





Page 7

THE DISTRIBUTION NETWORK

NETWORK DESIGN

The piping network that distributes the produced compressed air to the application also merits attention. Any

type of obstruction, restriction, or roughness in the system will cause resistance to air flow and a pressure

drop, resulting in a loss of energy. Using ring-shaped networks will result in lower pressure drops compared to

antenna shaped networks. To minimise pipe friction and avoid moisture running past drain traps, a maximum

air speed of 6m/s is recommended.

It is not unusual for an initially well designed system to be expanded over time on an ad hoc basis, which can

lead to the development of localised bottlenecks in the system.

Figure 4 – Design of a compressed air network (from GPG385 “ Energy efficient compressed air systems” ).

CONDENSATE DRAIN TRAPS

Condensate drain traps are used to remove moisture from the distribution network. These devices have a

small reservoir, which periodically opens to expel the collected water to pipes that transfer the water to an oil-

water separation unit. This separates and collects any oil, and discharges the water to drain (see figure 4).

During the valve opening time the compressed air in the pipework can also leak direct to atmosphere, and so

the duration of this should be minimised.

Traditional mechanical types are prone to failure in the open position, and simple timed electrical valves can

be very wasteful by venting for too long. Manual vents such as on the bottom of air receivers might also be left

open, which is another large waste of compressed air. A worse case example is that of a half open manual

drain that could vent 43 l/s of compressed, equivalent to 17 kWh/day. To overcome these problems, electronic

condensate drain traps sense the water level in the reservoir, minimising the discharge time and offeringgreater reliability. These should be used in both new and retrofit applications.





Page 8

COMPRESSED AIR LEAKAGE

The average industrial compressed air system running without proper maintenance could have a leak rate of

20% or higher. Proactive leak detection and repair can reduce leaks to less than 10% of the compressor output,

with 5% leakage being regarded as excellent. Managing leakage should be a high priority, as the financial cost

is high, leaks can be easy to fix, and excess leakage can lead to the compressors having inadequate capacityand hence drops in pressure.

Even a small hole of just 3.0 mm diameter on a 7 bar(g) system will consume 2.1 kW of compressor (electrical)

power, and so regular identification and repair is a highly cost effective use of time.

While leakages can come from any part of the system, the most common problem areas are:

Couplings, hoses, tubes, and fittings

Filter regulator lubricators

Open condensate traps and shut-off traps

Non-operational equipment still connected to the network

The starting point for a leakage campaign should be a quantitative assessment of the amount of leakage,

which can then be re-measured after the leaks have been fixed to give feedback on the effectiveness of the

work.

Leakage can only be measured at times of no regular demand, since in this state all the air flow will be

supplying the leaks. Where a flow meter is fitted, this will give a direct reading. Where no meter is fitted, then

the leakage can be estimated by dividing the on-load time by the total cycle time of the compressor. This ratio

and a knowledge of the capacity of the compressor will give a good indication of the steady state leakage.

Air leaks are rarely possible to see and cannot always be heard, even during quieter times of no production. If

the approximate location of a leak is known, bubbles formed from the application of soapy water to thesuspected area will show the precise location. For larger sites, an ultrasonic acoustic detector is often used.

These can recognize the high-frequency hissing sounds associated with air leaks, even where it is not possible

to get close to the system and in locations where there is a lot of background noise.

A time-efficient and systematic way to reduce leakage is the leak-tag method. With this method, leaks that are

identified during the execution of operational tasks or maintenance actions are marked with a two-part tag.

One part stays at the leak and the other part is returned to the maintenance department, identifying the

location, size, and description of the leak to be repaired.

Even on the best systems some leakage is inevitable. Because of this, isolating unused plant and even network

legs while not in use should be considered. Valves can be controlled by, for example, time-switches orequipment interlocks, but care must be taken to ensure that lack of air does not present a danger.

THE COMPRESSOR CHOICE

Three main types of packaged compressors are used for the production of compressed air, capable of

supplying compressed air to the flow and pressures shown in figure 5:

Rotary screw compressors.

Rotary centrifugal compressors.

Reciprocating or piston compressors.

By far the most common type of air compressor is the oil injected rotary screw compressor. It consists of two

rotors inside a casing that compress the air internally. The rotors are oil cooled, and this oil also seals the





Page 9

internal clearances. The oil is, in its turn, cooled by air or by water. These are reliable, requiring little

maintenance, have a long lifetime, and are quiet and compact.

Two stage compressors with inter-cooling offer higher efficiency, but at a higher cost, and so only have a

modest market share. A variant are oil free models which are more efficient, but also considerably more

expensive and so only used where very clean air is essential. Sliding vane compressors are much less common,but have the benefit of low noise operation, in particular in smaller sizes.

For sites with a large and steady compressed air demand, centrifugal compressors are ideal. The centrifugal

compressor is a dynamic compressor, meaning that its working principle is based on a transfer of energy from

a rotating impeller to the air. An impeller rotates inside a volute casing. The air is taken in at the centre and is

pressed against the casing by the angular momentum of the impeller. This produces a high pressure discharge

at the end of the volute. These types of compressors are designed for higher capacity as their flow is

continuous. They give good efficiencies, are inherently oil free and require little maintenance.

It is not unusual to use a mix of centrifugal compressors to supply base load, with rotary screw compressors

topping up the air supply to meet the total demand.

In applications that require very high pressure, piston (reciprocating machines) are required. They increase the

pressure of the air by reducing its volume. More precisely, they take in successive volumes of air, confine them

in a closed cylindrical space, and elevate them to a higher pressure by a compression piston. Piston

compressors are available in a single-stage or multi-stage configuration, depending on the pressure level.

These can achieve high efficiencies, but do require regular maintenance to achieve this, and they are

inherently noisy.

Figure 5 – Compressor coverage chart by flow and pressure (from GPG241 “ Energy Savings in the Selection;

Control and Maintenance of Air Compressors” ).





Page 10

AIR COMPRESSOR CONTROL

DEALING WITH VARIABLE DEMANDS

Compressors are most efficient at full load, with efficiency reducing as flow demand decreases. Selecting the

best way to control both the individual compressor and group of compressors to avoid this area of low

efficiency will have a big impact on overall costs of producing compressed air.

AIR RECEIVERS

The pattern of air demand is important, with systems finding it easier to maintain a steady pressure with slow

moving demands than occasional short high demands. These peaks can be smoothed out by the use of air

receivers that give a reservoir of air, de-coupling the demand from the instantaneous compressor output.

Lowering these demand peaks also enables a smaller compressor to be used to satisfy the demand, saving

capital cost and reducing time spent at inefficient low load.

The air receiver will typically be sized at around 10% of the volume of air produced per minute by the aircompressors. To assist with condensate separation, the receiver should be mounted outside in a cold

environment. It is also good practice to install additional air receivers adjacent to any equipment that has

particularly large and “peaky” air demands.

LOAD/UNLOAD CONTROL

Very small portable compressors of up to 10 l/s will simply switch the motor on/off to maintain the pressure in

the integral air receiver tank. This method of control is inherently efficient, but the frequent switching is not

possible for larger motors.

The most basic concept for standard industrial compressors is the load/unload control. The system pressure is

monitored and unloads the compressor when the discharge pressure is adequate. When the system pressure

reaches a predetermined minimum level, the compressor is loaded again, and the pressure will rise. Since the

motor runs continuously, an unloaded rotary screw compressor will consume 15 to 35% of full load power

while delivering no useful work. When the demand is volatile, there are frequent changes between load and

unload, resulting in a consumption during unload periods of 40% or even higher.





Page 11

Figure 6 – Part load performance of rotary screw air compressors (from GPG241 “ Energy Savings in the

Selection; Control and Maintenance of Air Compressors” ).

Most compressors have a ‘sleep’ or ‘automatic’ mode in which they turn off after running unloaded for 5 or 10

minutes. This delay is required to ensure that the maximum number of motor stop-starts per hour is limited to

avoid over-heating, and should not be bypassed.

SEVERAL COMPRESSORS IN CASCADE

During periods where the air demand is very low, the unload time of the compressor can become very high.

This is an inefficient operating zone, and so it is usual to have several compressors of smaller nominal

capacities instead of one larger compressor. The compressors will then be turned on and off in sequence so

that any time air compressors that are operating will do so at full load, with the exception of the final “top up”

compressor. At a minimum there would be one compressor. The simplest cascade consists of two

compressors, with one about twice the size of the other. The cost of operating an over-sized compressor at

30% rated flow can be 70% higher per cfm than at rated flow, so having a separate compressor for use during

times of low demand can give good savings.

EXAMPLE SHOWING THE BENEFIT OF USING AN ADDITIONAL SMALL COMPRESSOR TO

DELIVER AIR DURING TIMES OF LOW DEMAND

An industrial site operating 24/7 uses a 55 kW rotary screw compressor capable of delivering

10 m3/min (350 cfm) at 7.5 bar. Taking account of the motor losses and auxiliary equipment

such as cooling fan and oil pump, the total electrical input is 63 kW at full load.

However, since the site air demand has fallen since the original design, it now requires just 5

m3/min (175 cfm). From Figure 6, the energy consumption when running at 50% capacity is

~65% of full load power consumption.

[To be continued on the next page]





Page 12

In the simplest systems, a stack of pressure switches with typically 0.5 bar span will be used to progressively

turn on/off compressors as demand varies. Substituting these switches for a combination of electronic

pressure and demand-related signals gives much better control and versatility:

The pressure variation inherent in the cascade control is eliminated, reducing the peak pressure that

is being generated.

The pressure can be easily varied, for example a lower pressure set at times of low demand.

By monitoring the rate of change of pressure, it is possible to predict when each compressor will be

needed, enabling them to be turned off immediately rather than left to “run on”.

Rotation of duty between different compressors, and/or to preferentially use the most efficient

compressors.

The following simulation (Figure 7) illustrates the effect of staging multiple compressors in the case of simple

pressure switch control.

Three compressors with the same nominal capacity (10 Nm3/min), work in a load/unload regime in order to

deliver the required compressed air. A series of pressure switches are used to switch individual air

compressors on/off in response to the system air pressure rising and falling with demand.

Compressor Load pressure (pLow) Unload pressure (pHigh)

Comp. 1 6.5 bar 7 bar

Comp. 2 6.25 bar 6.75 bar

Comp. 3 6 bar 6.5 bar

[Continuation of the example]

The total annual energy consumption = 8,760 hrs x 63kW x 65% = 358,722 kWh per

year. This compressor therefore operates with a Specific Energy Consumption (SEC) of 23.4

kW /100 cfm.

If an additional smaller 30 kW compressor was used that could still meet the actual demand

(175 cfm @ 7,5bar), then the efficiency could be improved. The total electrical input for this

smaller compressor at rated flow is 34.5 kW (and from Figure 6 93% of this at 90% flow).

Hence the new annual energy consumption is:

8,760 hrs x 34.5 kW x 93% = 281,065 kWh

This equates to a reduced SEC of 18.1 kW /100 cfm.

The energy saving equals 77,657 kWh per year, which at an electricity price of electricity of

0.1€ / kWh amounts to €7,766 per year.

If the cost to supply and install the smaller compressor is €20,000, then the simpl e payback

would be 2.6 years. (Note that the larger compressor should still be kept and maintained, as

it will provide back up during maintenance, and provide additional capacity when demand

increases.)





Page 13

These pressure “steps” are needed so that the air pressure controlled cascade of micro -switches holds

sufficient pressure difference between the settings.

The produced flow (bottom toothed line) and resulting air pressure (upper toothed line) are shown in Figure 7:

Figure 7 – Three compressors in cascade: resulting air pressure and produced flow.

This simulation shows that the cascade concept in the pressure set points, which is needed to prioritize the

compressors, leads to a significant variation in the system’s pressure. Moreover, the unload period is still fairly

high, resulting in a specific average consumption (113 Wh/Nm3) that is still fairly high as well.

Adding such a centralized control system to the previous example results in the following system pressure

(upper toothed line) and production flow (bottom toothed line).

The resulting discharge pressure has lower variability than in the previous example, resulting in a more stable

system pressure and a lower specific consumption (111 Wh/Nm3).

Figure 8 – Electronic Control: resulting air pressure and produced flow.

4.25

4.75

5.25

5.75

6.25

6.75

7.25

0 6 0

1 2 0

1 8 0

2 4 0

3 0 0

3 6 0

4 2 0

4 8 0

5 4 0

6 0 0

6 6 0

7 2 0

7 8 0

8 4 0

9 0 0

9 6 0

1 0 2 0

1 0 8 0

1 1 4 0

1 2 0 0

1 2 6 0

1 3 2 0

1 3 8 0

1 4 4 0

1 5 0 0

1 5 6 0

1 6 2 0

1 6 8 0

1 7 4 0

1 8 0 0

seconds

P r e s s u r e ( b a r ( e ) )

0

10

20

30

40

50

60

F l o w

( m ³ / m i n )

4.25

4.75

5.25

5.75

6.25

6.75

7.25

0 6 0

1 2 0

1 8 0

2 4 0

3 0 0

3 6 0

4 2 0

4 8 0

5 4 0

6 0 0

6 6 0

7 2 0

7 8 0

8 4 0

9 0 0

9 6 0

1 0 2 0

1 0 8 0

1 1 4 0

1 2 0 0

1 2 6 0

1 3 2 0

1 3 8 0

1 4 4 0

1 5 0 0

1 5 6 0

1 6 2 0

1 6 8 0

1 7 4 0

1 8 0 0

seconds

P r e s s u r e ( b a r ( e ) )

0

10

20

30

40

50

60

F l o

w

( m ³ / m i n )





Page 14

VARIABLE SPEED DRIVES

Rotary screw compressors can be supplied with integrated variable speed drives (VSDs) to improve part load

performance. Such a VSD will raise the compressor speed when the discharge pressure drops and vice versa.

With this type of control, the compressor discharge pressure can be held within narrow limits, resulting in a

lower average discharge pressure.

Where there is a steady demand, energy savings of 15% can be seen at mid load, with larger savings possible

where the alternative is a compressor with frequent unloaded running. At full load, the additional losses in the

VSD mean that the air compressor will not be as efficient as a fixed speed compressor of the same size. This

means there should never be more than one variable speed compressor on a system, and in multiple

compressor systems the variable speed compressor should always be configured to provide the “top up” air

flow to track the varying demand.

Figure 9 – Relative part load performance of a Variable speed air compressor (from GPG241 “ Energy Savings in

the Selection; Control and Maintenance of Air Compressors” ).

If retrofitting a VSD controller to an existing air compressor, check with the manufacturer that the cooling will

still be adequate, and that the proposed controls will be effective. Retrofitting will also lose out on the benefit

of being able to remove the step up gears. The VSD can control the motor to work at the higher speed that is

required for driving the compressor “air end”, so these gears are no longer necessary

In this case, a variable speed compressor with a nominal capacity of 15 Nm3/min is added to the previous

system. By doing so, the specific consumption is reduced to 103 Wh/Nm3.

To obtain the best results, the capacity of the variable speed compressor should be higher than the flow

variation. If not, so called ‘regulation gaps’ will occur. For these certain ranges of demand, the regulation

system will fall short of an optimal answer and one or more compressors will run unloaded.





Page 15

SPECIFYING AN AIR COMPRESSOR

Air compressors should be tested in accordance with ISO1217, which includes standard temperature, pressure,

humidity and density conditions of inlet air. This will ensure that all quotations are to the same reference

conditions; otherwise it is difficult to compare efficiency. Better still, if you know the typical operating

conditions at your site, ask for manufacturers to specify performance at these conditions.

Care should be taken when interpreting catalogue statements of power consumption, as the specified “kW”

might refer to several different measurements, such as: total package power, rated motor power, motor shaft

power or actual motor power consumption.

Further, in some compressors the motor will be run beyond its rated (100%) duty into the service factor at

110% load or even slightly more. This not only reduces the motor efficiency, but also prolonged operation at

these loads will reduce the motor lifetime.

It is therefore important to check the datasheet to be sure what the actual electrical consumption will be, and

to check the actual load factor of the motor. Although the cost of a less stressed motor will be slightly higher,

this will be re-paid through a longer running life.

The most useful method of comparing efficiency is specific energy consumption, which gives the true cost of

generating air in terms of energy per unit output, for example kW/scfm, Wh/Nm3 or J/l.

COMPRESSOR INLET CONDITIONS

The higher the temperature of the intake air, the greater the energy needed to produce compressed air. This is

because warmer air is less dense, meaning that mass flow reduces, and hence efficiency worsens. This effect is

most noticeable in rotary screw compressors.

In practical terms this means that the intake should be in a sheltered location outside the building. Moving theair intake can be a low cost measure, but take care not to lead to excessive inlet pressure rise of any new

pipework. Well-designed installations use outside air, and have their air intake on the north side of the

building, far away from heat sources like steam conducts, burners, ovens, et cetera. This can result in a

reduction of the inlet temperature of 10 °C, saving about 3.5% of energy.

The inlet filter is important for protecting the air compressor, and should follow the specification of the

manufacturer. Ensuring that there are no excessive contaminants in the air in the intake region will prolong

the life of this filter. A high pressure drop across the filter will mean that the air compressor will have to work

harder, and so fitting a differential pressure gauge is recommended to indicate when the filter needs changing.





Page 16

HEAT RECOVERY FROM AIR COMPRESSORS

OPPORTUNITIES FOR HEAT RECOVERY

Up to 90% of the electrical consumption of an air compressor is lost as heat, much of which can be recovered

in the form of hot air or hot water. Depending on the system, hot water can be produced at a temperature of

up to 90°C, and hot air up to 80°C.

In practice, simple space or water heating applications work best and are found to be the most economic.

Possible applications for this “free” heat are:

Direct heating of neighbouring rooms

Preheating of the combustion air of boilers

Hot air for drying applications

The heating of sanitary or washdown water via a heat exchanger

Direct use of the cooling water as feed water for steam boilers

MATCHING THE DEMAND FOR AND AVAILABILITY OF WASTE HEAT

Using a similar approach to the way which the economics of Combined Heat and Power systems are appraised,

it is important to understand what proportion of the waste heat can actually be utilised. Key considerations

include the match between the available heat and heat demand of the quantities, temperatures and times of

the heat energy, and the distance of the demand from the source.

Figure 10 – Matching of heat su pply and demand (from GPG238 “ Heat recovery from air compressors” ).





Page 17

Best opportunities are found where the total demand is considerably greater than the available heat, as then

most or all of the waste heat can be utilised. For example, hot air from compressors can be vented directly into

a large factory space, so displacing space heating. Where the average demand is only of a similar size to the

available heat, differences in the detailed profiles of each mean that at some times heat will still go to waste,

and at others supplementary heating will be needed. Both of these factors will adversely impact the

economics.

Annual operating hours should be calculated taking account of non-working days such as holidays and

weekends, and the times of day when the compressors are switched off. Some thermal storage of energy is

possible at times of low demand, such as hot water or pre-heating the building with space heating.

Figure 11 – Simplified arrangements of common heat recovery systems (from GPG238 “ Heat recovery from air

compressors” ).

For longer distances, both the cost of ductwork and the heat losses can become high. This means there is a

limit to the distance between the compressor and the point of heat demand in order for heat recovery to be

beneficial.

In case of space heating, summer diversion of hot air can avoid over-heating. For smaller and more

temperature critical spaces, thermostatic control of motorised dampers to divert the heat directly to the

outside should be used.

Even where the temperature is too low for direct use, waste heat can be used for pre-heating, for example in a

domestic water heating system.

COSTS OF IMPLEMENTING HEAT RECOVERY

While the heat is free, the costs of capturing, moving and storing should be included in the project evaluation.

In particular, the following should be accounted for:





Page 18

Pipework or ductwork and insulation

Controls

Dampers or valves (manual or motorised)

Auxiliary fans or pumps

Water storage vessels

The actual financial value of the heat is also less than may first appear, as it is usually offsetting a lower cost

source of heat than that produced by electricity. For example, if a natural gas alternative costs one third the

price of the electricity used to drive the air compressor, then this lower price and assumed gas heating

efficiency should be used for calculating the value of the recovered heat.

Nevertheless, paybacks of less than two years are usually achieved on heat recovery systems. In some cases

the recovered heat will be sufficient to eliminate the need for a new conventional heating system, hence

actually giving a negative payback.

HOW MUCH HEAT CAN MY COMPRESSOR PROVIDE IN DIFFERENT APPLICATIONS?

The following rules of thumb will give a rough estimate of the service that the recovered heat can fulfil. These

will quickly give an idea of the possibilities before undertaking more detailed calculations.

Space heating: As a rough guide, each kW of compressor power can heat 5 – 10 m2 of a typical factory, more

than this for a space with good insulation and pipework with low leakage. This should be sufficient to give an

idea of the feasibility of a potential use before undertaking more detailed calculations. The use of additional

hot air or hot water radiators can work well in most sites, except for where high temperature radiant heaters

are used. Where ductwork is installed, use multiple outlets to promote an even mix of air temperature.

Water heating: As a rough guide, the waste heat from a 60 kW compressor can provide sufficient hot water for

100 people in a typical factory.

PRACTICAL NOTES

It should be remembered that heat recovery is a by-product; the efficient supply of compressed air should not

be compromised by attempts to optimise heat recovery.

It is important that the compressor is still cooled adequately, as running too hot will cause its

efficiency and remaining life to decrease

For long lengths of ductwork, an additional fan might be necessary to ensure sufficient flow

A fire shutter might be needed when a duct passes through a wall or another type of fire barrier

It is best practice to fit an indicator so that it is clear whether a damper is set in the Summer or Winter

position

On oil injected compressors, the oil recovery vessel safety valve should be diverted to avoid hot oil

being expelled into the hot air stream





Page 19

CONCLUSIONS

The easy availability of compressed air on the shop floor, changes in factory production, and the many uses it

can be put to, mean that the compressor installation can over time be running loads that it was never designed

for.

The separation between the compressed air users and the plant engineer responsible for the supply of

compressed air can mean that simple problems can go unnoticed, and so without intervention the energy cost

and performance of the compressed air system will deteriorate over time. With an effective cost of typically

1.0 euro/kWh for compressed air energy leaving the compressor, and considerably more than this by the point

of delivery, it is a valuable but costly commodity that repays regular inspection for energy saving

opportunities.

Fortunately, many of the readily achievable energy savings are low cost actions relating to leakage or excess

pressure, which can be identified and fixed with little specialist knowledge. However, new leaks start to appear

almost as soon as old ones have been fixed, and so a periodic regime is important to keep this under control.

While the efficiency of the compressor is important, for overall optimisation it is best to work backwards from

the point of use, thereby ensuring that the whole system is optimised for the actual rather than estimated

need. Once the system is working well, the opportunities for free hot water or air can be explored, giving a

modest financial rebate on the cost of this versatile but costly “Fourth Utility”.



Publication No Cu0116 Page 20

FURTHER READING

Improving Compressed Air System Performance: A Sourcebook for Industry

US Compressed Air Challenge:

http://www.compressedairchallenge.org/library/#Sourcebook

E. M. Talbott, Compressed Air Systems (second edition), The Fairmont press Inc., Georgia (USA), 1992

J.P. Rollins (Compressed Air and Gas Institute), Compressed Air and Gas Handbook, Prentice Hall PTR,

New Jersey (USA), 1989

The following are available from British Compressed Air Society:

http://www.britishcompressedairsociety.org.uk/pdf/carbontrust

o GPG385 - Energy efficient compressed air systems

o GPG241 - Energy Savings in the Selection; Control and Maintenance of Air Compressors

o GPG216 - Energy Saving in the Filtration and Drying of Compressed Airo GPG238 - Heat Recovery from Air Compressors (Carbon Trust)

Compressed Air v2

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