8

CHAPTER 2

LITERATURE REVIEW

The various research works attempted in the area of energy

conservation and specifically in the area of air compressor and pneumatic

systems have been referred and discussed here. The articles from journals,

magazines, books, specific reports and web sources have been collected,

reviewed and presented in the following sections.

2.1 STUDIES ON AIR COMPRESSORS AND ENERGY

CONSERVATION

As described in the source book for industry (U.S. 2003), though

the production of compressed air is one of the most expensive processes in

manufacturing facilities, compressed air itself is used inappropriately in

several ways such as open blowing, sparging, atomising, cabinet cooling and

vacuum generation. The energy wasted in compressed air systems because of

poor installation and maintenance can account for up to 50% of the energy

consumed by the compressor, and about half of this amount can be saved by

practicing energy conservation measures.

A broad classification of various possibilities for energy

conservation in air compressors and compressed air supply system as given

by several researchers is given in Figure 2.1 and are discussed in detail in the

following sections.

9

Figure 2.1 Energy conservation opportunities in air compressors

Type of compressor

Selection of

size Optimisation

of compressor Selection of

ancillary equipments

Air inlet temperature

Effective inter

cooling Heat recovery Pressure

bandwidth

Selection of apt control method

Intelligence in

control Flexibility in

control

Pressure required

Leakage of

air Improper

usage of air Variation in

consumption

High efficiency motors

Variable

Speed Drives IC engine for

drive

Maintenance Systems

approach Awareness and

training Benchmarking

Compressor Process Parameters Control strategy Usage

parameters Drive systems Management

Energy conservation opportunities in air compressors

10

According to John (1995), the opportunities for cost savings in

compressed air supply system includes but not limited to waste heat recovery,

compressed air leakage reduction, use of outside air for compressor,

compressor control, air pressure control, compressor selection and usage of IC

engine for compressor driving.

The solution for compressed air leaks is to make leak finding and

correcting as a part of the normal maintenance process and repeating leak

survey at least once a year (John Holdsworth 1997). Over-pressurization can

also result from short sighted selection of ancillary equipments. Bill Howe

and Bill Scales (1998) report that the opportunities for improved compressed

air efficiency where air is used internally, but uneconomically are less

understood. Supplying air at required pressure, appropriate use of air,

automated controls are some of the recommendations given by the authors.

Cost effective efficiency opportunities in production and usage of

compressed air are often ignored by the industries due to various reasons and

selection of correct compressor control also plays a major role in the energy

consumption by the air compressors (Robert 1999). Leaks, inappropriate

usage of compressed air, poor selection of compressors and ancillary

equipments, pressure problem and poor attempts to solve these problems are

some other common causes of inefficiency in the compressor system.

According to David (1999), estimates and actual measurements of

compressed air systems show that 10 % to 35% air is lost due to leak or

improper use. Proper maintenance, sound design and appropriate usage of the

compressed air can contribute for energy savings.

Three methods have been proposed for estimation of compressed

air consumption by John (1999), out of which measuring the time for few of

11

the load / unload cycles of the compressor and calculating the consumption

based on that is the one reported as the quickest method which can give useful

information. Providing separate duct to enable cool air intake and using

synthetic lubricating oil for compressors are attempted in few glass

manufacturing industries, with about 5 % energy savings (Gopalakrishnan

et al 2001).

Durmus Kaya et al (2002) attempted energy conservation with

repairing air leaks, installing high efficiency motors, reducing the average air

inlet temperature by using outside air and reducing compressor air pressure.

The pay back periods for the investments made with these measures were

very less. Proper maintenance and appropriate use of compressed air can

contribute to cost effective and energy efficient compressed air system, along

with the control mode (U.S 2003). Different efficiencies are considered for

performance evaluation of compressors (Ueno et al 2003).

Maintenance, monitoring, blocking leakage, minimising air inlet

temperature, minimising allowable pressure dew point at air intake, controls,

properly sized pipes, heat recovery, usage of natural gas engine for driving air

compressor, system improvement and improvement in the motor are the

options described by Christina et al (2003). Asfaw (2005), lists leak and air

supply at higher pressure than required are the major causes besides over

sized compressors, running compressors when not needed, wrong application

of compressed air etc.

Bureau of Energy Efficiency (2005) suggests reducing the air

intake temperature and every 4 degree centigrade rise in inlet air temperature

results in higher energy consumption by 1% to achieve equivalent output.

Chris and Kelly (2004) presented a methodology for modelling air

compressor performance and calculating projected energy savings from easily

12

obtainable performance data such as full-load power, no-load power, rated

capacity and average fraction full-load power or average fraction rated

capacity. Using load/unload type of control and fixing the leak are the major

contributors for the reported savings.

The drivepower of selected industrial processes has been

investigated and found that 42% of the electricity goes to drivepower (pumps,

compressors, etc.). An estimate shows that 6.3 % of drive power can be saved

without major changes in the equipment (Gabriel 1993). Normally in

industries, compressed air consumption is not constant throughout a given

time period and it varies depending on the simultaneous usage of the utilities.

One such varying consumption, as reported by an energy audit report

(EnergAir) is shown in Figure 2.2.

Figure 2.2 Variation in energy consumption (EnergAir)

Tim

e

13

2.1.1 Studies on Compressor Control

Compressed air system control is one of the most important

determinants of overall system energy efficiency. The type of control selected

for any system is mainly determined by the type of compressor used and the

air consumption profile. Importance of the correct selection of compressor

control methodology is insisted by John (1995), Bill Howe and Bill Scales

(1998), Robert (1999) and Christina et al (2003). Even simulation tools are

used as powerful methods for designing digital control systems (Wojciech

1996).

For air supply system with multiple compressors, varying demand,

and many types of end uses require a more sophisticated strategy. In any case,

careful consideration should be given to both compressor and system control

selection because they can be the most important factors affecting system

performance and efficiency (Compressed air system controls fact sheet #6

1998).

Robert (1999) reports that out of several compressor control

strategies load/unload type of control is the most efficient modulation control

for small and medium size. Gary (1999) and Joseph (2002) express that air

compressor control should provide efficient volume and pressure regulation

of compressor air delivery. Energy savings of 15 % to 20 % and long term

maintenance savings of 7 % to 10 % is possible for advanced control systems.

Inadequate compressor control causes over pressurisation of the compressors

which is the most common and most significant energy misuse.

The goal of supplying the air at required quality and pressure with

maximum possible efficiency cannot be accomplished by a simple controller

for a multiple compressor system (Joseph 2002). Compressor control system

14

selection affects the performance considerably (U.S. 2003). Start/stop,

load/unload, modulating control, multi step control and variable frequency

drive are some of the methods used in the industries. Compressor modulation

plays an important role in the energy savings of compressors for residential

cooling and inverter based control is less successful in the U.S. due to higher

initial cost (Kurt et al 2004).

2.1.2 Studies on air receiver size

Compressed air receiver serves to stabilize the compressed air

supply and smoothens pressure fluctuations in the network when air is

consumed. The size of the receiver can be decided based on air consumption,

network, type of regulation and permissible pressure difference in the network

(Meixner and Kobler 1978).

The size of the receiver is governed by the rate at which

compressed air is consumed and the capacity of the compressor if the air

consumption is assumed as at constant rate (Werner and Kurt 1975). Other

factors controlling the size of the air receiver are capacity control provided for

the compressor and the maximum cycle rate. But, the storage function and

assumed relatively constant consumption of air are the principal determinants.

Majumdar (2006) lists that the delivery volume of compressor, air

consumption, pipeline network, type and nature of regulation and permissible

pressure difference are the parameters on which the receiver size depends.

NCDENR (2004) reports that choosing a receiver, or storage tank, to fit the

needs of the system demand and prevent system pressure from dropping

below minimum required pressure during times of peak demand is important.

A drop in pressure causes the end tools to function improperly. The common

response to the tool malfunction is to increase the system pressure. The

15

energy used in increasing system pressure could have been saved through the

use of a properly sized receiver.

2.1.3 Studies on the use of variable speed drives

Some literature support the usage of variable frequency drives

(VFD) to compressors. Most of the compressors run below their capacity and

for about 70% of the compressors, the air demand varies from 40 % to 80 %.

Motors fitted to such compressor systems cannot give savings beyond certain

level because, standard production motors cannot run below certain speed

without heating problems (Elie 2002).

Bob (2002) reports that adjustable frequency drives help to improve

the compressed air system efficiency and that can be further improved by the

automated control system. Variable frequency drives fitted to a boiler air

supply fan for providing optimum air at different operating conditions could

save considerable electrical energy within a month in a fan fitted with

13.5 kW motor (Ibrahim and Engin 2004).

Reimund (2005) found that the use of variable speed drives (VSD)

compressor gives good energy savings but a VSD compressor is not a

standard compressor with frequency converter. The design of the compressor

has to consider the specific demands on the total system. If there is a

combination of VSD compressors and fixed speed compressors, a

sophisticated regulation is required.

At the same time, several literature report that usage of VFD is a

trustable solution for energy conservation only for certain conditions. Vern

(2000) observed that application of VFD for turbo machinery does not

16

produce energy savings always and sometimes fail in achieving the results

particularly in the retrofitted systems due to various reasons.

Mark (2002) observed the power consumed by compressor at

different load conditions with different types of drives or controllers and are

presented in Table 2.1. From the Table 2.1, it can also be noted that the

variable frequency drive is more efficient only if the average consumption is

within certain range and otherwise various other methods are efficient. For

lower percentage of loads, online / offline is more efficient.

Table 2.1 Power requirement at various load conditions (Mark 2002)

% Load

% of power Var. SR

Drive

Var. Freq. Drive

OL/OL*w/ Storage

MOD ACS GEOM

0 20 50 20 60 20 20 10 20 50 28 64 28 28 20 20 50 36 68 36 36 30 30 50 44 72 44 44 40 40 50 52 76 52 64 50 50 50 60 80 60 67 60 60 60 70 84 84 72 70 70 70 78 88 88 78 80 80 80 85 92 92 85 90 90 90 93 97 97 93

100 100 100 100 100 100 100 Avg.0-100

60-100 0-60

52.7 80

34.3

63.6 80

51.4

61 85 44

76 92 66

64 92 46

62 86 47

Where, Var. SR Drive refers to Variable Switched Reluctance Drive, Var. Freq. Drive refers to Variable Frequency Drive, Ol/OL refers to Online/Offline, MOD refers to Modulation control, ACS refers to Auto Control Select, GEOM refers to Geometry control.

17

Michael (2003) reports that VSD compressors are more costly

compared to standard compressors when operate at 100 % capacity as extra

energy is wasted in the addition of VSD itself. It is also reported that

retrofitting standard compressors with VSD is not efficient always and at

certain flow rate, the fixed speed compressor is more efficient than the

variable speed compressors as shown in Figure 2.3.

Figure 2.3 Flow rate and specific power (Michael 2003)

Usage of variable frequency drives is rare for industrial air

compressors because of high initial cost (U.S. 2003).

2.1.4 Managerial approach required

Jay Stein (1996) identified a list of fourteen common mistakes

committed during the implementation of energy efficiency projects.

Inadequate definition of the baseline for energy savings, limited or

inappropriate solutions, neglect of interaction between the building systems,

18

failure to verify the results and inadequate operation and maintenance of

efficiency measures are some of the common mistakes.

Khemiri-Enit and Annabi (1996) report that one of the technical

instruments of energy saving is the energy audit study, that is useful to

determine the actual consumption, reveal the anomalies and suggest

corrective measures. Nair and Sugathakumar (1996) used a linear

programming approach for the installation of energy saving devices with an

aim of optimising the subsidy given with given target of energy savings.

Energy conservation efforts can result in huge amounts of savings if equated

to carbon dioxide emissions and the expenses to provide necessary trainings

on energy conservation can be met easily (John 1998).

Hans-Dieter and Eberhard (1998) expressed that for reaching the

goal of reduction in energy conditioned emissions, an emphasis on rational

use of energy is required in small and medium sized enterprises as the

potentials exist there. These potentials are often not realized due to existing

obstacles in enterprises, which however can be overcome with suitable tools.

Apart from important financial obstacles, missing characteristic energy ratios

to enable a comparison with the benchmarking as well as missing information

in the enterprises about energy saving measures represent the largest obstacles

for an energy management in operation avoiding energy losses.

Felipe and Nicolás (1998) express energy conservation programs

should be based on the practical knowledge of energy experts, who can

diagnose every case and prescribe the relevant recommendations. The

decisions can be made taking into account a combination of scientific theories

and analytic techniques, experimental methods and individual experience and

judgment as well as common sense.

19

Paresh (1998) proposes a ‘systems approach’ to design a new

system and for debottlenecking of existing system, to improve the operation

and to reduce the lifecycle cost of a compressed air system. Grimaldi et al

(2000) reports that achievement of a more rational use of energy can be

achieved with a well-focused energy analysis, based on experimental data to

point out possible relevant energy savings in large and complex industrial

plants.

Concept of total productive energy management (TPEM) is

introduced by Mohammad (2000). This approach requires the involvement of

all members of the facility in the energy conservation efforts. Commitment of

top management, incentives and continuous trainings are required for the

successful implementation of TPEM. In a survey conducted in European

Union, in many cases users were not aware of the compressed air costs and in

some cases, the managers were not sufficiently aware about the availability of

cost effective energy saving measures (Peter and Edgar 2001), even though

cost data is available.

George (2001) reports benchmarking can be used with focus on

energy efficiency improving measures. Some questions need to be answered

before benchmarking and several methods available for benchmarking. Key

issues need to be considered carefully to obtain meaningful metrics and useful

results.

Douglas and Laura (2001) have identified eight key elements

which are important for best energy management practice. The commitment

by the top management, clearly defined goal for energy reduction,

communication of the goals throughout the organization, assignment of

responsibility, formation and tracking of energy metrics, identifying potential

projects, adaptation of the projects considering risks and rewarding for

20

achieving the goals are the elements. The authors also identified that no

company engages all of these practices to the best degree.

Pinch analysis technique can be used to estimate the target energy

requirement for any process and can be used as a tool for energy efficiency

improvement (Jimmi 2002). Industrial energy outsourcing (IEO) offers one of

the most exciting opportunities for achieving the win-win combination of a

reduction in both energy consumption and energy cost while offering a

profitable business opportunity for an energy service provider (Michael and

Simon 2003).

Money is being wasted on inefficient compressed air systems

throughout the world and most of the companies are unaware about the waste

in their current air system (Michael 2003). James (2004) insist on valuing the

energy savings in money value is as important as taking conservation efforts,

because, it creates more interest in the minds of the upper management

towards energy conservation than the conventional energy units. Energy

savings to the extend of 9,00,000 kWh and 23,45,000 kWh are reported in

two industrial cases (Barbara 2004) by the implementation of system level

approach to the air compressors.

Asfaw (2005) expresses that the energy conservation measures for

air compressors are less expensive and have quickest pay back period. At the

same time, the small and medium sized industries have no in-house expertise

for energy management which require an intensive training.

Compressed air system economics depends on several factors.

According to Compressed air systems fact sheet #9 (1998), too many

decisions regarding compressed air systems are made on initial cost basis, or

with an "if it is not broken, don't fix it" attitude. But to achieve optimum

21

compressed air system economics, users should select the compressor based

on life-cycle economics. Also, proper sizing of the components, turning off

unneeded compressors, using appropriate control and storage strategies, and

operating and maintaining the equipment are important for peak performance.

Gopalakrishnan et al (2005) insists that the energy assessment

process, which explores the opportunities for energy conservation, is a

complicated process for a large sized industry containing the more interacting

energy consuming systems. A structured systems approach and plant - wide

energy assessment is needed in such a case. There are number of essential

actions to be taken to optimise compressed air systems which need to be done

systematically (Scot 2005).

2.1.5 Studies on delivery pressure and pressure bandwidth

John (1995) expresses that in general the operating pressure

bandwidth is maintained at much higher level due to various reasons that exist

inside the plant.

The ratio of power reduction if the operating pressure is reduced is

given by Durmus Kaya et al (2002) as

FRi = ((Pdp + Pi)/Pi)(k-1)/(k x N) – 1.0/((Pdc + Pi)/Pi)(k-1)/(k x N) – 1.0 (2.1)

where Pdp is the discharge pressure at proposed operating pressure conditions,

Pdc is the discharge pressure at current pressure conditions Pi is the inlet

pressure, k is the ratio of specific heat for air (k= 1.4), N is the number of

stages. All ‘P’ are in any constant units; ‘k’ and ‘N’ are dimensionless.

22

Increasing the pressure increases the unregulated usages such as

leakages, open blowing and production application without pressure

regulation or with wide open regulation. This artificial demand increases the

energy consumption (Compressed air system fact sheet # 4 1998).

Dan Howett (2003) advocates that utility costs of the compressor

may be lowered by producing and storing air at higher pressure level and

reducing it to end user level. It depends on the efficacy – the standard cubic

feet of air produced per brake horse power - of the compressor system. But

this approach is countermanded by Richard (2004).

2.1.6 Studies on energy conservation in compressors and pneumatic

systems

Several attempts have been made already, to optimise compressor

and its accessories as well as pneumatic systems so as to reduce the energy

consumption. Fujiwara and Osada (1995) used computer simulation for

analysing the performance of the screw compressors. Computer based tools

are also developed for identifying energy saving opportunities in industries,

compressed air system is one of its application areas (Gopalakrishnan et al

1997). Pascal et al (2001) propose a global model for the thermodynamic

analysis of reciprocating air compressor based on five main and four

secondary dimensionless parameters used to predict the performance of a

reciprocating air compressor under various operating conditions.

Exergy analysis as a tool was used for the design, optimization,

and performance evaluation of energy systems (Recep et al 2002). Kagawa et

al also used exergy approach for the energy assessment of pneumatic cylinder

actuation system and reported that the approach is effective on clarifying the

energy distribution in pneumatic cylinders. Attempts had been made to

23

optimise screw compressors during the design stage itself (Stosic et al 2003).

Jiang et al (2003) used an integrated CAD/CAM method for design and

manufacturing of scroll compressors.

Yukio et al (2000) attempted with meter in circuit for energy

saving in pneumatic systems to reduce the air consumption. The reduction of

the air consumption is possible in proportion to the size, irrespective of the

load condition changes. Grzegorz and Johann (2004) suggested the use of

multiple on-off valves to minimise the compressed air consumption in the

actuators used in mobile robots.

2.2 FUZZY LOGIC SYSTEM AND APPLICATIONS

Effective optimisation of the compressed air supply system requires

an intelligent system due to several variations and uncertainties associated

with it. Fuzzy based approach is one such system and the related referred

articles are discussed here. In fuzzy logic models, information is processed in

terms of fuzzy sets, made precise through the definition of an associated

membership function. The specific inference is then processed by the fuzzy

set combined with fuzzy rules. The fuzzy logic model combines one or more

input signals, which are defined by the fuzzy sets, with a collection of fuzzy

rules to produce an output that can be compared with actual values in the real

world (Mamdani and Assilina 1975 and Sugeno 1999).

The advantage of fuzzy logic is that the use of fuzzy logic enables

the heuristic rule based technique commonly applied to discrete variables to

be extended for use in the continuously variable situation, without

significantly increasing the size of the rule base. The fuzzy controller

expresses in natural language the human expertise on the control of the non-

linear system by means of a set of linguistic expressions and fuzzy controller

24

performance is in real time, under a wide range of working conditions

(Lia et al).

Fuzzy logic modelling is a powerful tool for exploring complex

problems (Chen et al 2000). The fuzzy methods give good performance in

controlling non- linear system (Boada et al 2006). Shepherd and Batty (2003)

used fuzzy control strategy to provide energy efficient environmental

conditions, which is a multi variant control problem. Aprea et al (2004) used

fuzzy based algorithm to continuously vary the compressor speed with a help

of an inverter to save the energy in the compressors of refrigeration plant and

energy saving of nearly 13% is reported.

Fuzzy based control system is used for several applications:

integrating the front wheel braking and steering control to improve the vehicle

handling capacity at various driving conditions and manoeuvres (Boada et al

2006), vehicle trajectory controlling (Cho and Yi 2004), chatter suppression

in end milling process (Liang et al 2004), controlling unmanned lawn mover

(Lia et al), X-ray inspection of soldering defects (Wei and Tapio 2002),

evaluate and monitor the gap condition of wire electrical discharge machining

(Yan and Liao 1998).

Fuzzy logic model with genetic algorithm is used to overcome the

uncertainties in the fish stock – recruitment process (Chen et al 2000). Fuzzy

control system is used for ventilation control of naturally ventilated buildings

(Ljiljana and Mahroo 2004), evaluation of welding efficiency and quality of

laser welding (Casalino and Memola 2004), quality assurance in resistance

spot welding, (Lee et al 2001), tool wear monitoring (Srinivasa Rao and

Srikant 2004), for controlling the liquid level in a tank (Heider and

Chukwuma 2000) and many more.

25

2.3 TARGET COSTING

Target costing is not a costing system as such, rather it is an activity

which is aimed at reducing the lifecycle costs while ensuring quality,

reliability and customer requirements (Yutaka 1993). For over a decade,

target costing has been recognised as an important tool for lowering costs and

increasing competitiveness. Target costing is a process whereby an

organization determines the ‘‘estimated selling price’’ for its product or

service, less the ‘‘desired profit,’’ with the remainder equalling the ‘‘target

cost.’’ The target cost amount must cover all of the firm’s expenses for

producing/procuring the product or service (Lisa 2002). In target cost

management system, interactive control and simultaneous engineering

approach are required (Takeyuki 1995).

Gopalakrishnan et al (2004) expressed that target costing is a

structured approach to determine the cost at which a proposed product,

meeting the quality and functionality requirements, must be produced in order

to generate the desired profits. Companies in automotive, electronic and

process industries have reaped the benefits of target costing, still it need to

find applications in other segments of the industry.

Target costing concept is used for several applications other than

identifying the manufacturing cost of a product. The relationship among the

loss function, process capability indices and control charts to establish goal

control limits is determined by extending the target costing concept (Hsin-

Hung Wu 2004), target costing has been used as a means to improve the

management of supply chains (Archie and Wilbur 2000), for quality

management and for depicting the relationship among Taguchi loss function,

process capability indices, and traditional control charts to setup goal control

limits (Hsin-Hung Wu 2003).

26

Jürgen and Richard (1998) integrated design costs and Quality

Function Deployment (QFD) to optimise product development resources. Ugo

and Paulo (2007) suggested a methodology for the product development

process in an automotive company, aiming at the correct systematic approach

of Value Engineering (VE) and target-costing in cost management.

2.4 REENGINEERING

Reengineering is the fundamental rethinking and radical redesign of

business processes to achieve dramatic improvements in critical,

contemporary measures of performance such as cost, quality, service and

speed (Michael and James 1993).

Process reengineering can be considered to be a combination of

industrial engineering techniques, operations research methods, management

theory and information systems analysis that utilise the power of information

technology to radically change processes of organizations to achieve dramatic

performance improvements in order to compete in the markets within which

they operate (Love and Gunasekaran 1997).

Business process reengineering has been touted as necessary for

dramatic improvement in the competitiveness of the organization and it needs

a rigorous and systematic assessment of the factors needed for the

organization. It requires a cross functional effort and makes radical change in

one or more processes of the organization (Tor 1999).

Peter and Amrik (1999) insist that to be a truly world class

organization, the company needs to work as a team and all the functional

areas of the business need to be properly integrated with each understanding

the importance of the cross functional process. Lyu (1996) integrated two

27

different approaches – kaizen and automation are integrated to reengineer a

manufacturing process and it resulted in 50% improvement in the labour

productivity.

Fromme (1996) expressed that restructuring of production can lead

to a 30% reduction in energy demand. In total, energy savings of 47% of

current demand can be achieved. These savings can be achieved by local

means. Many of these measures are achieved at no cost or with low cost. A

general lack of awareness stemming from traditional thinking and structures,

compounded by a lack of financing possibilities constitute some of the most

important obstacles.

According to many investigators, reengineering should focus on

processes and not to be limited to thinking about organizations only

(Subramanian et al 1999). Reengineering is also applied for several areas such

as heat treatment (Zainul 2005), production planning (Erne and Victor 1997),

materials management system (Mohanty and Deshmuk 2001), service

operations (Ram and Jayanth 1998), cultural change in laboratory's design

process (Kane and Robert 1995) and many more.

2.5 SUMMARY, MOTIVATION AND SCOPE OF THIS

RESEARCH WORK

The important points discussed in the area of energy conservation

in compressed air supply system and the area of tools used in literature are

given in the Table 2.2.

28

Table 2.2 Summary of literature review

Sl. No

Points discussed, [Researcher, year]

Recommendations Remarks

1 Delivery Pressure [John (1995), Bill Howe and Bill Scales (1998), Durmus Kaya et al (2002), Dan Howett (2003), Richard (2004), Asfaw (2005)].

Reduction of delivery pressure can reduce the energy consumption.

General methodology to identify the lowest possible delivery pressure and minimum operating pressure bandwidth is hardly available.

2 Air receivers [Werner and Kurt (1975), Meixner and Kobler (1978), NCDENR (2004), Majumdar (2006)]

Air receivers are used to smoothen the fluctuations and its size depends on consumption, compressor output and control strategy.

Focus on the effect of air receiver on energy consumption is found to be rare.

3 Compressor control [John (1995), Wojciech (1996) Bill Howe and Bill Scales (1998), Robert (1999), Gary (1999), Joseph (2002) Chiristina et al (2003)]

Compressor control system affects the performance considerably; maximum possible efficiency cannot be accomplished by a simple controller.

Methods to control the compressors for varying load and uncertain variations in load are found to be rare.

4 Drive systems, VFD [Vern (2000), Elie (2002), Mark (2002), Durmus Kaya et al (2002), Michael (2003), Ibrahim and Engin (2004), Reimund (2005)]

Selection of drive is important. Each drive system is advantageous only under certain conditions.

VFD is advantageous for narrow range of loads.

29

Table 2.1 (Continued)

Sl. No

Points discussed, [Researcher, year]

Recommendations Remarks

5 Managerial approach for energy conservation [Jay Stein (1996), Khemri-Enit and Annabi (1996), John (1998), Hans-Dieter and Eberhard (1998), Felipe and Nicolás (1998), Paresh (1998), Grimaldi et al (2000), George (2001), Peter and Edgar (2001) Asfaw (2005), Gopalakrishnan et al (2005)]

Management approach is required in implementation of energy saving programmes. Benchmarking of energy consumption is needed. Some managers not aware of the energy conservation potentials in compressed air systems.

A comprehensive methodology for implementation of energy conservation efforts in industries hardly exists.

6 Fuzzy logic system [Mamdani and Assilina (1975), Sugeno (1999), Chen et al (2000), Chen et al (2000), Wei and Tapio (2002), Cho and Yi (2004), Liang et al (2004), Srinivasa Rao and Srikant (2004), Ljiljana and Mahroo (2004), Boada et al (2006)]

Fuzzy logic system is suitable for problems with uncertainty. Effectively used in several applications.

Usage of fuzzy logic system for energy conservation in air compressor systems is found to be rare.

7 Target costing (TC) [Yutaka (1993), Takeyuki (1995), Jiigren and Richard (1998),Lisa (2002), Gopalakrishnan et al (2004), Hsin-Hung Wu (2003; 2004), Ugo and Paulo (2007)]

TC is a useful tool to achieve effective cost control. Used for various applications other than cost control of new products.

Application of TC approach for energy conservation in compressed air system is to be attempted

8 Reengineering [Michael and James (1993), Lyu (1996), Love and Gunasekaran (1997), Tor (1999), Peter and Amrik (1999), Mohanty and Deshmuk (2001), Zainul Huda (2005)]

Fundamental rethinking and redesign of existing system. It is applied to several requirements.

Application of RE for energy conservation in compressed air supply system is yet to be applied.

30

From the referred articles the following can be summarised and

based on that the area of this research work is identified.

i) Energy saving is important and good amount of energy can be

saved in air compressors.

ii) Several measures have been suggested by the researchers for

energy conservation in air compressors.

iii) Reduction of operating pressure of air compressors

recommended by the most of the researchers to reduce the

energy consumption. But the methodology to identify the

lowest possible or optimum pressure for a specific industry is

not available in the open literature.

iv) Still in most of the industries, air compressors are run with

high pressure due to various reasons mainly non availability of

methods to estimate the optimum pressure.

v) Variable speed drives fitted to compressor are effective for

certain range of consumption only.

vi) Compressor control also plays a role in energy consumed by

the compressor. Most of the compressed air systems use

load/unload type of regulation for the control of the

compressor.

vii) As the complexity of compressed air system increases, the

solution becomes more complicated.

viii) Energy conservation attempts require a systematic approach

with intelligent tools if the system is complicated.

ix) Energy conservation activities require a change in the

management approach of energy conservation efforts.

31

Keeping the above in consideration, the scope of this research

work has the following objectives:

i) To suggest a suitable method to identify the optimum

operating pressure for air compressors so that it can be

employed by the industries readily to conserve energy

consumed by air compressors. It is done considering different

working conditions, different combinations and variations in

the parameters.

ii) To suitably modify the existing compressor controller so that

it dynamically adjusts the pressure based on the pattern of the

consumption of air.

iii) To study the possibility of optimising pressure bandwidth

using fuzzy based method at various levels of consumption.

iv) To use a target costing and reengineering based approach for

energy conservation in compressors.

Above objectives are attempted individually and the results are

presented in the following chapters.