EDR Design Briefs Compressed air

7/31/2019 EDR Design Briefs Compressed air

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Summary

Compressed aircommonly called the fourth utilityis in high

demand in most industrial facilities. Despite its widespread appli-

cation, however, up to two-thirds of the compressed air systems

in operation have either an obvious problem that affects produc-

tion or a hidden problem that drives compressed air production

costs higher[3]. In some cases, according to the U.S. Department

of Energy, compressed air generation may account for as much as

30% of the total electricity consumed by the facility [2].

Compressed air is one of the most expensive uses of power in an

industrial facility. Opportunities for increasing the overall effi-

ciency of compressed air systems occur on both the supply side

and the demand side of the systems.

On the supply side, typical opportunities include installation of a

variable speed drive (VSD) compressor, a more efficient dryer

matched to the quality and quantity of compressed air demand,

the addition of compressed air storage, and modifications to or

implementation of more effective compressor and system con-

trols. The optimization goal for the supply side is to operate the

compressors at their highest efficiency point. This goal may be

achieved by operating the minimum number of compressors at

full load and at the lowest possible pressure range, and using a

VFD-controlled compressor for trimming.

On the demand side, opportunities include reducing compressed air

usage through selecting appropriate equipment and practices, imple-

design brief

Summary 1

Introduction 2

Supply Side System Components 6

Demand Side SystemComponents 20

Optimization of Existing

Compressed Air Systems 22

Best Practices Design Methods

for a New Compressed Air System 31

Notes 33

For More Information 34

Compressed Air


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menting effective leak repair programs, managing pressure swings without addi-

tional compressor capacity, and lowering system pressure when appropriate.

Optimizing a compressed air system has the potential to generate energy effi-

ciency improvements in the range of 20% to 50%[1]. By incorporating Best

Practices design methods into the front-end development of new construc-

tion or major retrofit projects, end users of compressed air can achieve dra-

matically lower electricity consumption, along with improved reliability and

stability of the compressed air system, monetary savings from reduced elec-

tricity consumption and reduced wear and tear on the equipment, and

reduced emissions of greenhouse gases associated with electricity generation.

Introduction

Compressed Air in Industrial FacilitiesCompressed air systems are an integral part of many industrial processes

up to 90% of industrial facilities utilize a compressed air system.

Compressed air is a valuable source of power for hand-held tools and for

equipment used for pressurizing, atomizing, agitating, and mixing applica-

tions. The convenience and relative safety of using compressed air for

motive force have made it as commonplace as electricity, gas, and water

systems in industrial facilities, which is why industrial facility engineers and

operating staff often refer to compressed air as the fourth utility in the plant.

Compressed air provides end users with a practical method to convert

electrical power into mechanical power, enabling end users to store, trans-

mit, and ultimately deliver mechanical power to the required point of use.

Examples of applications include actuating air cylinders, operating hand

tools (e.g., hammers, rotary guns), blowing molten plastics into molds,

mixing, and powering automatic conveying, sorting, and packaging equip-

ment. Pneumatic hand tools are typically smaller, lighter, and more maneu-

verable than hand tools driven by electric motors; they deliver power

smoothly and are not easily damaged by overloading.

The industrial sector uses compressed air in a wide range of applications,

and plants frequently have multiple compressor systems with extensive

distribution systems to provide compressed air throughout the facility. In

addition to the examples of applications listed in Table 1 [1], some manu-

page 2 Compressed Air


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page 3Compressed Air

facturers utilize compressed air for combustion in boilers firing liquid fuels

and for process operations, such as painting, oxidation, fractionation, cryo-

genics, refrigeration, filtration, dehydration, and aeration.

Compressed air is also important in non-manufacturing sectors, including

transportation, construction, mining, agriculture, recreation, and service

industries. Examples of applications in these sectors are shown in Table 2[1].

Current Efficiency and Cost

The use of compressed air for performing mechanical or process-related

work is inherently inefficient; the overall efficiency of a typical com-

pressed air system can be as low as 10% to 15% [2]. For example, to oper-

ate a 1-horsepower (hp) air motor at 100 pounds per square inch gauge

(psig), approximately 7 to 8 hp of electrical power must be supplied to the

air compressor.

Table 1: Typical Uses of Compressed Air in

Industrial/Manufacturing SectorsIndus try Ex am ple s o f Co m pre s s e d Air Us e s

ApparelConveying, clamping, tool powering, controls and actuators, automated

equipment

Automotive Tool powering, stamping, controls and actuators, conveying

Chemicals Conveying, controls and actuators

FoodDehydration, bottling, controls and actuators, conveying, spraying, coatings,

cleaning, vacuum packing

FurnitureAir piston powering, tool powering, clamping, spraying, controls and actua-

tors

GeneralManufacturing

Clamping, stamping, tool powering and cleaning, controls and actuators

Lumber & Wood Sawing, hoisting, clamping, pressure treatment, controls and actuators

Metals and

Fabrication

Assembly station powering, tool powering, controls and actuators, injection

molding, spraying

Petroleum Process gas compressing, controls and actuators

Primary Metals Vacuum melting, controls and actuators, hoisting

Pulp & Paper Conveying, controls and actuators

Rubber & PlasticsTool powering, clamping, controls and actuators, forming, mold press power-

ing, injection moldingStone, Clay and

Glass

Conveying, blending, mixing, controls and actuators, glass blowing and mold-

ing, cooling

TextilesAgitating liquids, clamping, conveying, automated equipment, controls and

actuators, loom jet weaving, spinning, texturizing


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Generating compressed air is expensive. A survey conducted by the U.S.

Department of Energy (DOE) indicated that for a typical industrial facility,

approximately 10% of all of the electricity used was attributable to generat-

ing compressed air, and that in some cases compressed air generation may

have accounted for as much as 30% of the total electricity consumed by a

facility. As illustrated in Figure 1, the cost of the electricity is approximately

76% of the total cost of owning and operating a compressed air system[2]

.

The total cost of producing compressed air at 100 psig has been estimat-

ed to be in the range of 18 to 32 cents per 1,000 cubic feet [4]. Compressed

air systems consume an estimated 90 billion kWh per year in the United

States, accounting for approximately $1.5 billion in energy costs, and elec-

tricity production for compressed air is responsible for 0.5% of total green-

house gas generation in the United States[3]. However, the operation of

these systems is often overlooked as a contribution to the overall produc-

tion cost. Once installed, a typical compressed air system is largelyneglected as long as it continues to satisfy production needs; occasional

failures in meeting production needs may result in additional inefficiencies

as piecemeal expansion without appropriate analysis and optimization of

the overall system performance takes place.


Table 2: Typical Uses of Compressed Airin Non-Manufacturing Sectors

S e c to r Ex am ple s o f Co m pre s s e d Air Us e s

Agriculture Farm equipment, materials handling, crop spraying, dairy machines

Mining Pneumatic tools, hoists, pumps, controls and actuatorsPower Generation Starting gas turbines, automatic control, emissions controls

Recreation

Amusement parks air brakes

Golf courses seeding, fertilizing, sprinkler systems

Hotels elevators, sewage disposal

Ski resorts snow making

Theaters projector cleaning

Underwater exploration air tanks

Service IndustriesPneumatic tools, hoists, air brake systems, garment pressing

machines, hospital respirator systems, climate control

Transportation Pneumatic tools, hoists, air brake systems

Waste Water Treatment Vacuum filters, conveying


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Energy Savings Potential and Benefits

Despite their widespread application, up to two-thirds of the compressed

air systems in operation have some sort of problems, from an obvious air

leak to an improper sequencing control that keeps all compressors run-

ning continuously without consideration of demand. These problems may

stem from a wide range of causes, including:

I Improper compressor types and/or sizes

I Inappropriate controls

I Improper cleanup equipment (such as filters, dryers, regulators)

I Unsound installation and operation practices

I Poor maintenance practices

Poorly optimized systems waste energy, increase operating costs, and

reduce reliability. According to the DOEs Industrial Technologies

Program, optimizing compressed air systems could improve their energy

efficiency by 20% to 50%[3]. In California, the recently completed California

Energy Efficiency Potential Study estimates that implementing high-effi-

ciency compressed air systems at industrial facilities in the state could gen-erate annual savings of 1,000 MWh and 115 MW[5].

In addition to the energy savings potentials described above, optimizing

compressed air systems can lead to reduced greenhouse gas emissions

from electricity generation and improved reliability.


Source: Energy Tips Compressed Air Tip Sheet #1, August 2004.[2]

Figure 1: Typical Lifetime Compressed Air Costs


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Key Elements of Compressed Air Systems

A compressed air system consists of a supply side and a demand side. The

supply side comprises compressors, air treatment equipment, and primary

compressed air storage. It is responsible for providing an adequate amount

of clean, dry air at the lowest operating pressure to meet the end-userequirements. The demand side consists of the distribution system and the

end-use equipment. The objective of the demand side is to distribute and

utilize the compressed air as efficiently as possible while eliminating, or at

least minimizing, wasted air.

Given the continuous interaction of supply-side and demand-side elements,

it is essential that owners of compressed air systems adopt an integrated

(whole system) approach to design, operation, and maintenance. Figure 2

illustrates the components of a typical industrial compressed air system.

Supply Side System Components

The key supply-side components are the compressors and compressor

controls, air treatment equipment, and primary compressed air storage.

Compressors

Compressors fall under two general types: positive displacement com-

pressors, and dynamic compressors. Positive displacement machines trap

a volume of air in a compression chamber and mechanically compress the

air by reducing the volume of the chamber. Dynamic machines change the

velocity of a continuous flow of gas into pressure by means of impellers

rotating at high speed, and downstream diffusers. The pressure and flow

relationship in a dynamic machine is influenced by the shape of the rotor

and the diffuser volutes. Figure 3 shows the family tree of positive dis-

placement and dynamic compressor types.

Compressor efficiency is expressed as the ratio of the input power to the

production rate of compressed air at a specific pressure. The industry

norm for comparing compressor efficiency is expressed in terms of brake

horsepower per actual cubic feet per minute (bhp/100 ACFM) at a com-

pressor discharge pressure of 100 psig. Rated capacities, pressures, and

package power are generally available in the manufacturer data sheets for



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new compressors; for older compressors, generic ratings may be available

in the AIRMaster+ program[7] and on utility websites[8].

Compressors typically found at industrial facilities include rotary and recip-

rocating compressors (both are positive displacement machines), and cen-

trifugal compressors (dynamic machines). A brief discussion of each of these

three common compressor types follows.

Rotary Screw Compressors

These are by far the most common compressors found at or specified for

industrial facilities. Figure 4 is a drawing of a direct drive rotary screw com-


Source: Improving Compressed Air System Performance, a Sourcebook for Industry, November 2003.[1]

Figure 2: Components of a Typical Compressed Air System


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pressor showing the two intermeshing rotors: a male rotor with helical cut

lobes and a female rotor with helical cut grooves mounted in a stator case.

As the two rotors turn and mesh together, a fixed volume of air is drawn

from the inlet port on the left side of the compressor shown. As the lobeand groove begin to mesh, the trapped gas is compressed in the decreas-

ing volume between the two rotors; the compressed charge of air is forced

along the two rotors until it is discharged at a higher pressure at the right

side port on the drawing. A fixed volume of air is drawn into the two

rotors for a constant rotor speed, so the discharge pressure rises or falls as

compressed air demand decreases or increases.

Rotary screw compressors are available in lubricant-injected and lubricant-

free designs. Lubricant-injected compressors use a specialized fluid that is

injected into the inlet air stream to remove some of the heat of compres-

sion, form a seal between the rotors, and lubricate the rotors as they mesh

along their length. The oil is removed from the compressed air after the dis-

charge port, first by gravity as the air flow slows, and later by coalescing

filters that remove all but a few parts per million of oil that remains



Figure 3: Components of a Typical Compressed Air System


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aerosolized. Typical sizes for these workhorse compressors range from 3 to

900 hp, although the more common sizes are less than 200 hp. Capacitiesfor these compressors are 8 to 5,000 CFM at pressures from 50 psig up to

250 psig, with higher pressures possible for multi-stage compressors.

Specific power ratings for lubricant-injected rotary screw compressors cover

a wide range of values depending on both the operating pressure of the sys-

tem and the size of the compressor. Smaller compressors with load/unload

type capacity controls require as much as 37 kW per 100 ACFM for a 5 hp

compressor discharging at 175 psig and 21 kW per 100 ACFM for a 20 hp

compressor discharging at 100 psig. Larger industrial compressors of 100 to

200 hp sizes have a lower specific power, and are commonly rated at

between 18 to 25 kW per 100 ACFM for similar discharge conditions.

Compressors larger than 200 hp typically have similar specific power ratings.

Some industrial applications require oil-free air. Lubricant-free compres-

sors do not use lubricants in the compressed air; bearings and timing gears

are lubricated, but they are kept isolated from the compression chamber.

In a lubricant-free rotary compressor, external timing gears prevent the

rotors from touching. However, because the heat of compression is not

removed by a lubricant and the sealing between rotors is less efficient,

these compressors are often operating at higher rotor speeds, and often

have two-stage designs with an intercooler between the two stages to

remove the heat of compression. The resulting compressed air is lubricant

free and safe for use in critical applications. These compressor types are


Source: Quincy Compressor

Figure 4: Cutaway View of a Direct DriveRotary Screw Compressor


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available in a wide range of sizes, from 25 hp up to approximately 4,000

hp for capacities of 90 to 20,000 CFM at pressures of 50 to 150 psig.

Reciprocating Compressors

Reciprocating compressors are similar to bicycle pumps, in that a piston

attached to a crankshaft moves up and down in a cylinder, compressing

the air as the volume of the compression chamber is reduced. Single-act-

ing reciprocating compressors provide compression of the gas charge in

one direction only, whereas double-acting compressors provide compres-

sion in both directions of the piston travel. Large multi-stage, water-cooled,

double-acting industrial compressors have the highest efficiency, but are

loud, expensive to maintain and operate, and are becoming less common

in industrial facilities. Increasingly they are being replaced with rotary

screw or centrifugal compressors. Reciprocating compressors are availablein a wide range of sizes, from fractional and single digit horsepower, air-

cooled single-acting hobby compressors up to 25 hp, and double acting,

typically water-cooled compressors in sizes up to approximately 500 hp.

With compressor efficiency ranges as high as 13.2 to15.8 kW per 100

ACFM for 400 hp double-acting, multi-stage compressors at discharge pres-

sures of 80 to 125 psig respectively, they are 5% to 10% more efficient than

rotary screw compressors at similar pressure ratings, but with significant

trade-offs for maintenance and installation costs[8].

Centrifugal Compressors

Centrifugal compressors are typically large capacity, water-cooled

machines designed to satisfy high flow demands at relatively stable flow

rates. Pressure in a centrifugal compressor is achieved partly by the rapid-

ly rotating impeller, spinning to over 50,000 RPM (approximately 50% of

pressure is generated in the impeller), and partly by converting the veloc-

ity of the air flow to pressure after the impeller by means of a diffuser and

volute. These machines are usually multi-stage, with two to four stagesbeing the most common configuration for compressors at 100 to 150 psig

ratings. Between stages, the compressed air is cooled to ambient temper-

atures in water-cooled intercoolers prior to recompression in the next

stage. Centrifugal compressors are available in a wide range of sizes, with

capacities of 300 to more than 100,000 ACFM at pressures up to 125 psig.



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Capacity control of individual compressors is accomplished through one of sev-

eral control strategies. All of the strategies below depend on monitoring the dis-

charge pressure of the compressor to respond to changes in the system demand:

I Start/Stop

I Load/Unload

I Inlet Modulation

I Variable Displacement

I Variable Speed Drive

Each of these strategies is described in more detail below, along with com-

pressor performance graphs for all but the Start/Stop control strategy,

which essentially operates at full load whenever it is active. The perfor-

mance curves show the compressor controls effect on an individual com-

pressor motor load as the compressor capacity changes; the performance

curves are all based on lubricant-injected rotary screw compressors.

I Start/Stop This simple approach turns on the compressor when pressure

drops below a set point, and then turns it off after reaching the higher limit.

Demand is met by storage during periods when the compressor is off. This

strategy is typically used on smaller motors and compressors, as rapid

start/stop cycles on large motors can lead to overheating and failure.

I Load/Unload The motor runs continuously for a load/unload com-

pressor, or until a pre-set time interval of unloaded operation is com-

pleted in order to prevent motor damage in this strategy. Sensing a

drop in system pressure, the load cycle starts by fully opening the inlet

valve, and closing the sump pressure relief valve; compressed air con-

tinues to be produced until the upper pressure set point is reached, at

which point the compressor inlet valve is closed, and pressure in the

sump is slowly bled off for the unloaded phase of operations. During

unloaded periods, the compressor will continue to consume 15% to

35% of full power while delivering no compressed air. The four curves

in Figure 5 correspond to system performance for a load/unload com-

pressor equipped with varying amounts of compressed air storage. A

load/unload compressor essentially has two operating points, either

loaded, or unloaded. A compressor with a large volume of compressed

air storage capacity operates closer to the virtual linear line between



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the load and unload points, while compressors associated with lower

levels of storage are represented by the curves that are increasingly

bowed upward. The upward bowing represents increased average

power demand for a given average flow of compressed air.

I Inlet Modulation Throttling the inlet air flow reduces the output of a com-

pressor to closely match the demand. This strategy cannot be used on lubri-

cant-free rotary screw or reciprocating compressors. For lubricant-injected

rotary screw compressors, the modulating range is limited to about 40% of

rated capacity; below this threshold the compressor is unloaded.

Modulating controls provide good capacity control in the control range, but

at a penalty in efficiency at part load operations. Often, rotary screw com-

pressors with modulation control are also equipped with blowdown torelieve pressure in the sump. By relieving the sump pressure, the input

power is dramatically reduced. However, similar to a load/unload com-

pressor operation in unload mode, the compressor delivers no compressed

air to the system after the sump pressure is relieved while continuing to run

at 15% to 35% of full load (see Figure 6).



Figure 5: Performance Curves for Load/unload Control withVarying Levels of Storage Capacity


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ume using sliding or turn valves; the strategy is used in concert with

inlet modulation to provide accurate pressure control and improved part

load efficiency.

I Variable Speed Drive Especially suitable for lubricant-injected rotary

screw compressors, a VSD slows the rotor speed and air flow through

a compressor while retaining the compression ratio of a constant

speed compressor when demand drops. Similar in closely matching

demand, but superior in performance to inlet modulation control for a

constant speed compressor, a VSD extends the range of efficient part

load operations to low capacity levels with a near linear relationship

between power and capacity, with accurate control of pressure (+/- 1

psig) and matching of capacity to system demand (see Figure 8).

System Controls

Multiple compressor systems benefit from a control system to orchestrate

the sequencing and operations of the compressors. As previously stated,

the primary goal of a multi-compressor control system is to operate all but

one compressor at full load, with one compressor designated to provide



Figure 8: VSD control performance curves for compressorwith and without unload control


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trim capacity. However, system controls can range from relatively simple

to very complex, with compressor control based on real-time flow and

pressure measurements of the system.

A single compressor control system typically consists of a simple pressure

transducer that produces a signal based on the discharge pressure to the com-

pressor. As the discharge pressure decays with increased compressed air flow,

a signal is sent to the compressor capacity control to increase the inlet flow,

begin to load, or start up, depending on compressor control type. For a sin-

gle compressor with modulation control, load/unload, variable volume, VSD

or start/stop compressor control, this type of system may be perfectly ade-

quate. But in a large industrial multi-compressor system with combinations of

dynamic and positive displacement machines, and varying compressor con-

trol types, managing system capacity to meet demand while optimizing forenergy efficiency can be challenging for the best master control systems.

Many older multi-compressor systems are equipped with electro-mechanical

controls that start up compressors based on a series of cascading pressure set

points. An individual compressor is started up whenever the discharge pres-

sure drops below the set point; the next compressor in line starts up when its

discharge pressure sags below a pressure set below that of the previous com-

pressor. Imprecise control and slow response times, characteristic of these sys-

tem controls, can often result in a wide swing of system pressure, and multi-

ple compressors are often forced to run at inefficient part load conditions.

For many systems with two, three, or more screw compressors, the addi-

tion or conversion of a constant speed compressor to an appropriately

sized VSD trim compressor may be a relatively low-cost option that meets

the system demand for variable capacity while maintaining high overall

system efficiency. The VSD compressor is able to modulate over a wide

range of flows, allowing the other compressors to either remain off or

operate at high load conditions.

In other instances, however, a more sophisticated master control system

may be a more appropriate solution. New microprocessor-based control

systems now available are capable of monitoring conditions throughout a

complex compressed air system. These control systems manage compres-



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sor operations based on measured flow and pressure in a real-time fash-

ion; they are capable of communicating with practically all compressor

brands and communications protocols, and they work with centralized

engine rooms and/or remote satellite compressors. A sophisticated master

control system harmonizes the operations of all of the compressors for thehighest possible system efficiency. The limitations of such a master con-

trol system are typically governed by initial cost, practicality, and the cost

of additional control points and functionality.

Air Treatment Equipment

An important part of a compressed air system is the air treatment equip-

ment that protects the tools and process equipment served by the system.

This air treatment equipment includes dryers and filters to remove mois-

ture and contaminants from the compressed air. Ambient air supplied tothe air compressors may contain moisture, oil, and dirt, and can dramati-

cally affect equipment life or operation:

I Moisture - Ambient air contains varying amounts of water vapor,

which can condense in pipes. Sludge, rust, freezing, and tool damage

can result from poor moisture control.

I Oil - Lubricant-injected rotary screw compressors and reciprocating

compressors always have some carryover of oil, but excessive amounts

can lead to problems with tools or processes exposed to compressed

air. In some processes, food processing for example, use of oil-free

compressors and elimination of any other oil contamination is an

important function for cleanup equipment.

I Dirt - Dust and grit that enter the system through the air intake are

concentrated by the compression process by a factor of seven times

for 100 psig air. Combined with oil and/or moisture, dirt can lead to

equipment failure or poor operation of tools.

The international standard that specifies the quality of compressed air is

ISO8573.1. This standard specifies limits for three categories of air quality:

I Maximum allowable dew point temperature

I Maximum particle size for any remaining particles

I Maximum remaining oil content



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Each category is given a rating number between 1 and 6, as shown in

Table 3. In many cases a Class 4 rating is adequate for compressed air

needs for hand tools and other pneumatic equipment. Cleanup of com-

pressed air beyond the needs of the process equipment can lead to addi-

tional pressure drops, excessive use of compressed air for purge air typedryers, and additional costs to install, operate, and maintain the cleanup

equipment.

Dryers

Dryers are designed to remove water vapor carried into the system from

ambient inlet air. Three types of dryers are briefly discussed below:

I Refrigerated dryers are the most common means of removing water,

with cycling and non-cycling refrigerated dryers capable of achieving

dry air at dew points between 35F and 39F.

I Desiccant dryers adsorb moisture from the air in desiccant beds. The

desiccant beds typically require regeneration when the desiccant has

reached its capacity for holding moisture. This can be achieved by

using purge air previously dried and heated, or by using compressor

waste heat. Purge air requirements in the regenerating tower can be a

relatively large efficiency loss to the system.

I In a deliquescent dryer, the medium absorbs water as it dries the air.

These dryers are non-regenerative, and the medium is used up as it

changes from a solid to a liquid.

The most efficient dryer for a compressed air system is one that minimizes

pressure losses, minimizes power consumption, and meets the design flow


*PDP - Pressure Dew Point, F

Table 3: ISO 8573 Air Quality Classifications

Oil Carry-

Ov e r

Dus t Carry-

Ov e r

Moisture

Carry-Over

Cla s s m g /m 3 g m g /m 3 PDP* (de g F) m g /m 3

1 0.01 0.1 0.1 -70 0.003

2 0.1 1 1 -40 0.12

3 1 5 5 -20 0.884 5 15 8 +3 6

5 25 40 10 +7 7.8

6 -- -- -- +10 9.4


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and dew-point requirements for supply of dry air to the demand side.

Refrigerated dryers typically have the lowest capital cost and operating

cost; however, desiccant or deliquescent dryers may be necessary for

meeting lower dew point requirements.

Filters

Filters serve two primary purposescleaning grit and dust from inlet air,

and removing oil or other contaminants from the discharged compressed

air. As with dryers, optimization of filtration systems involves specifying fil-

ters capable of removing the contaminants while minimizing any pressure

drops, either on the inlet side or the discharge side. Maintenance and

replacement of filters are key elements in maintaining the system efficien-

cy and reliability of the system.

Compressed Air Storage

Compressed air storage receivers are simple but effective additions to

compressed air systems to address demand surges (high flow but short

duration events) and reduce pressure swings in a system. By storing a vol-

ume of compressed air, the receiver can meet sudden compressed air

demands much more quickly than a compressor, and thus can prevent or

delay startup of another compressor. Tank capacity ranges from a few gal-

lons to several thousand gallons. The volume of storage should be sized

to meet a flow event by supplying compressed air over a sufficient length

of time that the pressure drop from the demand event will not cause

another compressor to start up to meet the intermittent demand.

In large distribution systems, remote compressed air storage or remote

metered storage may be a good solution to intermittent flow events. Large

intermittent flow events from equipment located far from the central stor-

age or engine room can lead to localized equipment operation problems,

cause compressors to come on line for short load cycles and then contin-ue to run afterward at no-load, or affect other equipment on the system

(e.g., poor operations from flow event pressure sags). With long distribu-

tion lines, the delay between the start of a large remote flow event, with

its localized pressure sag, and the response from either centralized com-

pressed air storage or compressor capacity response, may be too long to

meet the equipment demand in a timely manner. A metered storage sys-



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tem equipped with a check-valve allows localized storage of compressed

air near equipment that creates the demand; the storage tank builds up a

sufficient volume of compressed air between event cycles, with the entire

storage capacity devoted to meeting the intermittent demand. Other equip-

ment and the compressors do not see the event, and the rest of the sys-tem is unaffected by the now isolated pressure swings.

Although compressed air receivers are passive pieces of equipment, they

can be a critical element in the supply side of a compressed air system;

they help the system operate at higher efficiency by minimizing the num-

ber of compressor startups necessary to meet fluctuating compressed air

demands. Careful consideration of total storage needs, location of storage,

choice of wet or dry storage, and integration of storage with the con-

trol system will provide significant benefits in most settings. While notquite a cure-all, additional storage can be one of the single best modifica-

tions for existing system operational and efficiency improvement.

Demand Side System Components

The key demand-side components include the distribution piping, valves,

connectors, filter/regulator/lubricators (FRL), remote storage receivers and

metering devices, and end-use equipment.

The distribution piping transports compressed air to the point of use. The com-

pressed air may be treated by the FRL to achieve the proper pressure and air

quality before being used. Successful system operation is dependent on main-

taining an adequate flow of compressed air at a pressure sufficient to meet the

requirements of the end-use equipment. Failure to meet flow or pressure require-

ments can result in production problems or equipment damage.

Preventing unnecessary pressure losses is thus a key consideration in con-

figuring a distribution system. Any devices in the distribution system that

increase friction also reduce the pressure and flow of compressed air

delivered to the end usepotentially causing improper operation of the

end-use equipment. Figure 9 is a pressure profile diagram that illustrates

how pressure losses add up in a system between the compressor discharge

and the final point of use.



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Ideally, system capacity should match demand at all operating conditions.

Thus distribution systems should be designed with maximum expected

flows and velocities in mind. As shown in Figure 9, large pressure drops

between the cleanup equipment (e.g. dryer and filter pressure drop) and

the end use are possible; often the control range of a system is set higherthan necessary to prevent or mitigate problems associated with system

pressure losses. Specifying low (pressure) loss filters and FRLs, and ade-

quately sizing the disconnects and drop hoses can help minimize these

point-of-use pressure drops and lead to substantial savings opportunities

by allowing lower discharge pressure for the compressors.

Careful consideration in choosing and setting up end-use equipment can

also make a significant difference in overall system performance. For

example, remote end-use equipment that imposes large but intermittentloads on a system may benefit from additional local metered storage tanks

to isolate the load from the rest of the system; another common situation



Figure 9: Pressure Profile for a Compressed Air System at a Point in Time


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is use of engineered nozzles instead of open blowing. Understanding the

system pressure profile is very useful in specifying end-use equipment that

can operate on actual available pressure within an existing system.

Optimization of Existing Compressed Air Systems

The energy required to operate a compressed air system can be reduced

in many ways, some of which are low-to-no cost, while others require cap-

ital expenditures. Because a compressed air system operates as an inte-

grated set of components, changes to one component may cascade

through the system, affecting the operations of other parts. Sometimes,

however, operating improvements in one area will only have a beneficial

impact if other actions are taken, such as reducing system pressure before

implementing a leak repair program.

Implementing the basic principles of a systems approach to optimizing

compressed air system performance involve taking the following steps [3]:

1. Develop a basic block diagram of the system.

2. Measure the baseline performance of the system (kilowatts, system

pressure profiles, compressed air flows, and leak loads).

3. Work with a specialist to implement appropriate compressor control

strategies, which may include installing a VFD compressor, compres-sor sequencer, or compressed air storage.

4. After controls adjustment or improvements, re-measure system perfor-

mance through measurements of compressor power, system pressures,

and system flows; evaluate whether system pressure can be reduced;

implement the pressure reduction; and re-evaluate the system and

end-use performance.

5. Walk through to identify obvious preventive maintenance items, and

other opportunities for cost reduction (changing inappropriate air uses,

for example).

6. Implement a leak repair program, correct inappropriate air uses, and

then re-measure system performance and re-adjust controls as noted

in Steps 3 and 4 above.



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7. Evaluate the results from the previous steps and implement an aware-

ness and continuous improvement program.

Although not specifically outlined above, capital cost measures may be

appropriate to satisfy the broad principles of a systems approach. Brief

discussions of these basic principles, which underlie the steps above, are

presented below.

Audit the System and Establish Baseline Performance (Steps 1 and 2)

Auditing the existing performance of a compressed air system should be

one of the first steps taken to optimize system performance. The audit

should be focused on identifying problems in production and end uses of

compressed air, previous work-arounds for persistent issues, and under-

standing of the operations of the existing system. The process begins with

Step 1 above, drawing a block diagram of all system components. Next is

carrying out Step 2, by measuring compressor electrical demand, measur-

ing pressure at various points in the system, and if possible, determining

compressed air flow to various parts of the system. Useful work is only

accomplished with compressed air flow, and measuring only pressure and

power may not provide the right answers for complex system problems.

For example, increasing flows through a distribution system that is too

small can rapidly cause pressure drops through the line; evaluation of

flows allows the auditor to calculate compressed air velocities, the prima-ry determinant of distribution line-size problems.

Bringing in an experienced compressed air specialist with the specialized

training, knowledge, and the correct tools to make an accurate assessment

can be very cost-effective for facilities that lack in-house expertise. The

Compressed Air Challenge[9] has guidelines on selecting a compressed air

service professional for Level 1 Walk-Through Assessments through more

sophisticated Level 3 System Audits, which can include monitoring and

detailed measurement of system pressures, flows, and compressor power.

Reports of audit findings can guide facility owners to next steps in correct-

ing deficiencies and improving system performance. A list of qualified Best

Practices AIRMaster+ specialists is also available on DOEs Industrial

Technology Program website[10]. Energy auditing or engineering and design

assistance may also be available from the electric utility serving the facility.



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Implement Compressor and System Control Strategies (Step 3)

Adjusting, modifying, or replacing controls and control strategies are

among the more effective changes that can be made to a compressed air

system.

Individual Compressor Controls

With respect to individual compressor controls, older inlet modulation

compressors not equipped with a blowdown capability (i.e., the sump is

de-pressurized during unload periods) deliver the poorest efficiency of

any of the rotary screw compressors. Up to 60% of the rated power is

required to operate at no flow or unloaded conditions with un-relieved

sump pressures. Retrofitting these compressors with relatively simple

blowdown controls and valves allows an inlet modulation compressor to

reduce its unloaded energy consumption significantly. The modulation

capability is retained at higher part loads above about 60% of full flow, but

energy consumption for unloaded operation drops to 15% to 35% of rated

full load power.

Other types of individual compressor controls may not be as easily modi-

fied, and when this is the case, replacement or reassignment of a com-

pressor in the overall control sequence may be the best approach. A VSD-

controlled compressor is a fairly common retrofit strategy for controllingsystem capacity and meeting trim loads. In this case, other compressors

are designated as base-load compressors and are kept operating at high

part load ratios. A single VFD compressor, sized to meet variations in the

compressed air demands, serves to meet the trim load. The performance

curve of a VSD-controlled rotary screw compressor makes it ideal to run

at part loads while retaining high efficiency.

System Controls

From a system perspective, proper sequencing of individual compressors

can have a dramatic effect on the overall performance of the system at dif-

ferent load conditions. By ensuring that the control system keeps any base

load compressors operating at high load levels while the trim compressor

only serves the trim load (or the whole load if demand is low enough), a

facility can improve overall system performance. With an accurate pres-

system failures



25/35


26/35


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storage isolates the equipment and its intermittent demand from the dis-

tribution system such that the event has essentially no impact on the rest

of the system. Provided that the metered storage has sufficient capacity to

ride out the event, the system pressure responds only minimally as the

receiver refills before the next event.

The compressor performance curves for a load/unload compressor previ-

ously illustrated in Figure 5 showed the effect of differing amounts of stor-

age on the performance of a load/unload compressor. With greater system

storage, the compressor follows a more linear relationship for percent

power vs. percent flow. Conversely a load/unload compressor will tend to

consume additional energy with an insufficient amount of storage as the

compressor wastes additional energy cycling between load and unload

conditions.

Evaluate and Make System Adjustments (Step 4)

Once system improvements are implemented, evaluation of the changes

to system performance is a critical step in the process. The pressure pro-

file, compressor power, and flows should be measured again, to assess the

adjusted system performance. With lower pressure swings from addition-

al storage, re-sequencing of compressors, or addition of VSD compressor,

the operator should review whether the system is now operating at the

right pressure. Minor changes to the system pressure of just a few psig can

result in fairly significant energy savings, both for compressor power and

for reduced leakage in the system. Estimates are that for every 2 psig drop

in pressure, the compressors use approximately 1% less power[1]. Careful

assessment of potential changes to operating the system, combined with

evaluation of the results of those changes can lead to persistent gains in

system performance and energy savings.

Implement Preventive Maintenance, Eliminate Inappropriate Uses,

and Identify Other Opportunities (Step 5)

Good opportunities for system improvement are often related to timely,

regularly scheduled maintenance of system components: a prime example

is to change filter elements at regular intervalsboth at the compressor end

and at end-use pointsto minimize pressure losses from clogged filters.



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Eliminating inappropriate uses of compressed air, substituting electric-

powered tools and equipment, or correcting behavioral practices in com-

pressed air use are the among the most direct and cost-effective methods

of reducing compressed air usage and saving energy. With poor energy

conversion efficiency, compressed air should generally be utilized onlywhen: (1) safety considerations require doing so, (2) productivity gains

justify the expense, and (3) the associated reduction in labor costs justifies

the expense. In general, compressed air use should be continuously mon-

itored and evaluated to ensure that it is prudent and judicious.

Examples of potentially inappropriate uses of compressed air are:

I Open blowing

I Aspirating, atomizing, dilute-phase transport, padding, sparging

I Vacuum generation

I Ventilation

I Improperly sized and installed diaphragm pumps

I Cabinet or personal cooling

Among the continuous improvement opportunities that might be consid-

ered, relocation of the intake for inlet air is worthy of consideration. Since

less energy is required to compress cooler, denser air than warmer, lighter

air, compressors operate more efficiently when the intake air to the com-

pressor is drawn from outdoors, and not from within a hot engine room.

If an existing compressor arrangement draws its inlet air from inside a

warm building (such as a boiler room), there are energy savings available

just from installing ducts to bring in cooler air from the outside.

Implement a Leak Repair Program (Step 6)

A leak repair program should be undertaken after other system improve-ments have been completed. Leak loads in compressed air systems are

typically quite large, and can be at 20% of compressor production, with

some systems observed with over 50% losses[1]. Every system has leaks,

and no leak repair program is likely to correct all of the problems.

Additionally, the pressure stresses on the system will likely generate new

system failures



29/35

sources of leakage over time. Thus, leak repair should be an ongoing pro-

gram that is repeated at regular intervals.

The first step in any leak repair program is identifying the severity of the

problem, which can be done in several ways. For simple systems

equipped with a single load/unload compressor, the percentage of leak

load can be calculated by timing a load/unload cycle with no end-use

equipment operating. The proportion of loaded time to the total time in a

load/unload cycle is the percentage of air leakage in the system.

Another approach to estimating leak loads requires monitoring a pressure

gauge and the time required for a system with no end-use equipment

operating to drop to another pressure. The time the system takes to drop

to the lower pressure and the beginning and end pressures can be used

in Equation 2 to estimate the normal leak load.

Eq. 2

Where Leakage is in cfm

V = Storage volume, cfm

P1= Beginning pressure, psig

P2=Ending pressure, psig

T = Time, minutes

If the end-use equipment operates continuously, or leak loads cannot be

isolated, specialized ultrasonic equipment can locate the source and deter-

mine the general magnitude of leaks. Once the leaks are identified and

tagged, corrective actions can be taken to repair distribution piping leaks

or replace defective valves, leaking disconnects, and fittings for hose

drops. Reducing overall leakage in a system to 10% of total free air deliv-

ery is a reasonable goal that will typically be cost-effective, depending onthe initial state of the system.

It bears repeating that leak repair programs should not be undertaken

prior to other system improvements. Reductions in artificial demand

(excess air required by a systems unregulated uses from high system pres-

sure) and measures to minimize pressure losses in the system will tend to



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increase the rate at which leaks waste compressed air. Completing a leak

repair program first will likely result in a false total for leak repair savings

as the increased pressure in the system from other measures will only

accentuate the remaining leaks, and perhaps initiate new ones. Figure 10

illustrates the ongoing nature of leak repair programs.

Implement an Awareness and Continuous Improvement Program

(Step 7)

Some of the activities described above are iterative and focus on evaluat-

ing the results of system improvements between major steps to ensure

continued improvement. Performance evaluation and adjustments to sys-

tem controls, for example, should be repeated regularly to ensure that any

performance gains are maintained and possibly improved onespecially

if production changes occur in a plant. Benchmarking of the baseline sys-

tem performance and regular evaluation of ongoing system performance

can help to prevent degradation of system performance, and simultane-

ously provide opportunities for improving production and use of com-

pressed air. Unlike electricity and gas, compressed air has a relatively poorenergy conversion ratio into mechanical work to start with. Thus com-

pressed air system performance can easily degrade through poor mainte-

nance practices, changes in production, or even simple changes to system

controls by well meaning operators trying to solve production problems

by turning up the pressure just a bit. Regular evaluation and implementa-

system failures


Figure 10: Best Practices for Eliminating Leakage

Source: Adapted from U.S. DOE Best Practices Manual.


31/35

tion of improvements to a system can lead to dividends in both com-

pressed air costs and an improved production environment.

Best Practices Design Methods for a New Compressed Air

System

The U.S. Department of Energy has an established set of best practices for

compressed air systems. These practices provide end users with an effec-

tive roadmap for designing new systems that provide an efficient, reliable

source of power for operating pneumatic machines and tools.

Designing the System

The design methods recommended include five key design steps, which

are similar to the basic principles outlined above for optimizing an exist-

ing compressed air system:

Step 1: Create a Demand Profile Chart.

Step 2: Review the end users stated air requirement.

Step 3: Determine the expected average flow rates during each shift and

expected maximum demands.

Step 4: Consider the need for future increases in compressed air use.

Step 5: Determine the minimum operating pressure requirements.

DOEs Improving Compressed Air System Performance, a Sourcebook for

Industry[1], provides an in-depth analysis of these design principles, and

detailed step-by-step procedures.

Assessing Total System Costs

As with all energy efficiency projects, estimating capital costs when design-

ing a new compressed air system is an exercise in balancing competing

demands for scarce capital with the need to minimize future operating

costs. As indicated earlier, production of compressed air is expensive, with

lifetime electricity costs estimated at six times the initial capital investment

in the equipment[2]. Clearly, minimizing the life cycle electricity costs

should be a priority in the design phase and should be highlighted in pre-

senting the business case for making marginal improvements in the speci-

fication of high efficiency system components and integrated design.



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In addition to first-cost capital requirements, recurring costs associated

with compressed air should also be considered, including:

I Filter maintenance and replacements

I Compressor maintenance and rebuilds

I Rental equipment in case of breakdowns

I Spare parts

I Maintenance and leak repair programs by outside contractors or in-

house staff

I Disposal costs for lubricants and condensate

Calculating compressed air costs on a life cycle basisincluding all capi-

tal expenditures, recurring costs for maintenance and replacement of fil-

ters and other consumables, and especially electricity costs to produce

compressed airis key to understanding the true costs of owning and

operating these systems, and the importance of operating them at the

highest possible efficiency level. Recognition of the true cost of producing

compressed air is likely to help decision makers approve proposals for

best practices design of new compressed air systems.

Some costs can be offset by utility-sponsored incentive programs. The

compressed air system measures covered in a number of these programs

in California include a wide range of custom energy efficiency measures,

a sample of which includes:

I Direct replacement of one or more compressors with compressors of

higher efficiency, including those equipped with variable speed drives

I Installation of new compressors to service increased production capacity

I Sequence optimization of multiple compressors

I Installation or upgrade of system storage

I Installation of intermediate pressure/flow control valves

Up-to-date information about incentives and other assistance can be found

on the utilities websites or through utility representatives.

system failures



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Notes

1. Improving Compressed Air System Performance, a Sourcebook for Industry,

prepared for: U.S. Department of Energy, Office of Energy Efficiency and

Renewable Energy (EERE), Best Practices. November 2003. Prepared by

Lawrence Berkley National Laboratory and Resource Dynamics Corporation.http://www1.eere.energy.gov/industry/bestpractices/pdfs/compressed_air_so

urcebook.pdf

2. U.S. Department of Energy, Energy Efficiency and Renewable Energy, Industrial

Technologies Program,Energy Tips Compressed Air Tip Sheet #1, Determine

the Cost of Compressed Air for Your Plant, August 2004.

http://www1.eere.energy.gov/industry/bestpractices/pdfs/compressed_air1.pdf.

3. U.S. Department of Energy, Energy Efficiency and Renewable Energy,

Industrial Technologies Program, Compressed Airs Role in Productivity,Energy Matters, Fall 2002, http://www.nrel.gov/docs/fy03osti/32767.pdf.

4. Best Practices for Compressed Air Systems, The Compressed Air Challenge,

Inc., 2003, page 83.

5. California Energy Efficiency Potential Study, Volume 1. Itron. May 2006.

6. Compressed Air Best Practices, Smith Onandia Communications LLC,

Fairhope, AL, September 2006, page 40.

7. AIRMaster+, V1.2.0, U.S. Department of Energy, Energy Efficiency and

Renewable Energy, Industrial Technologies Program, DOE Industry Tools.

Available for download at

http://www1.eere.energy.gov/industry/bestpractices/software.html#air

8. Table C.4 Air Compressor Equipment, Minimum efficiency ratings for rotary

screw and reciprocating air compressors. http://www.spc-

nrrdr.com/download/2008SPCDocs/PGE/PG&E%20C%20Min%20Equipment

%20Efficiency.pdf

9. Guidelines for Selecting a Compressed Air System Provider, 2002,

Compressed Air Challenge, Inc.:

http://www.compressedairchallenge.org/library/guidelines.pdf

10. Qualified Best Practices AIRMaster+ Specialists: Full List, DOE Industrial

Technologies Program,

http://apps1.eere.energy.gov/industry/bestpractices/qualified_specialists/spec

ialists.cfm?software_id=1&display=Full_List



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For More Information

Compressed Air and Gas Institute (CAGI): www.cagi.org

Compressed Air Challenge: www.compressedairchallenge.org

Improving Compressed Air System Performance, a Sourcebook for

Industry: http://www1.eere.energy.gov/industry/bestpractices/pdfs/

compressed_air_sourcebook.pdf

U.S. Department of Energy Best Practices:

www.eere.energy.gov/industry/bestpractices



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Energy Design Resources provides information and design tools to archi-

tects, engineers, lighting designers, and building owners and developers.

Our goal is to make it easier for designers to create energy efficient new

nonresidential buildings in California. Energy Design Resources is funded

by California utility customers and administered by Pacific Gas and Electric

Company, the Sacramento Municipal Utility District, San Diego Gas and

Electric, Southern California Edison, and Southern California Gas

Company, under the auspices of the California Public Utilities

Commission.

To learn more about Energy Design Resources, please visit our website at:

www.energydesignresources.com.

This design brief was prepared for Energy Design Resources by Nexant,

Inc., San Francisco, CA.

Documents

EDR Design Briefs Compressed air