Compressed Air

Compressed Air

Walter BrightMAE406 – Energy Conservation in Industry

[email protected]/29/2013

C.A. BASICS

C.A. Basics• Compressed air is simply a medium to transmit

power, similar to electricity or steam to transmit heat

• Often referred to as the ‘fourth utility’ • Compressed air can be used for a multitude of

applications– Simple: Pumping up tires and blow-off nozzles– More Complex: Instrumentation, Vacuum generation,

Pneumatic tools, cylinders and valves• Ex: flow controllers, pumps, impact wrenches, nail guns, etc

– End-use equipment is cheap, lightweight, compact & powerful

– Explosive environments– Easy to control (solenoid valves, pressure

proportional to force)

Why Compressed Air?

C.A. Basics Basic Compressor

Specialized Bicycles/Popular Mechanics

C.A. Basics

$$$Typically the most expensive utility at a plantRule of Thumb: It takes 7 units of compressor

horsepower to provide one horsepower of useful work!Why is compressed air so expensive???

Ex: Cost of operating a 10hp motor for 1 year (8,760hrs) 10hp Electric Motor 10hp Pneumatic Motor

Why NOT Compressed Air?

$5,388

$32,818

C.A. Basics• Surely if it’s the most expensive utility at a

plant it’s being continuously managed…• Example:

– Foundry Sand Transport System– 350 hp of compressor power– Energy consumption reduced by 36%– $16,300 in annual savings– 1.3 year simple payback

• Substantial opportunity throughout industry to reduce compressed air usage and cost

• Plant personnel often think compressed air is free

Why Manage Compressed Air?

Compressed Air Challenge, www.compressedairchallenge.org

C.A. Basics Compression Thermodynamics

MOTOR

100 kW of electrical

energy input

COMP

Greg Harrell, Energy Management Services (EMS)

MOTOR


energy input

5-8 kW of thermal energy

loss

COMP

Greg Harrell, EMS


MOTOR


energy input


loss

# kW of loss??

COMP

Greg Harrell, EMS


MOTOR


energy input


loss

98-99% Efficient

COMP

Greg Harrell, EMS


MOTOR


energy input


loss

We want high-pressure air from the compressor…

98-99% Efficient

COMP

Greg Harrell, EMS


MOTOR


energy input


loss


What we get is high-pressure,

high-temperature air

98-99% Efficient

COMP

Greg Harrell, EMS


MOTOR


energy input


loss




98-99% Efficient*

COMP

Greg Harrell, EMS


MOTOR


energy input


loss




# kW of thermal energy loss??

98-99% Efficient*

COMP

Greg Harrell, EMS


MOTOR


energy input


loss




90 kW of thermal energy loss

98-99% Efficient*

COMP

Greg Harrell, EMS


MOTOR


energy input


loss





98-99% Efficient*

COMP C.A.MTR

#kW of shaft

energy from comp. air motor??

Greg Harrell, EMS


MOTOR


energy input


loss





98-99% Efficient*

COMP C.A.MTR

10 to 20 kW of shaft

energy from comp. air motor

Greg Harrell, EMS


MOTOR


energy input


loss





98-99% Efficient*

COMP C.A.MTR



What about the 1st Law of Thermo??

Greg Harrell, EMS


MOTOR


energy input


loss





98-99% Efficient*

COMP C.A.MTR



The 1st Law of Thermo is not

violated because the air discharged is

very cold

Greg Harrell, EMS


MOTOR


energy input


loss





98-99% Efficient*

COMP C.A.MTR





very cold

COMPRESSION EFF:10-20%Greg Harrell, EMS


MOTOR


energy input


loss





98-99% Efficient*

COMP C.A.MTR





very cold

COMPRESSION EFF:10-20%Greg Harrell, EMS

C.A. Basics Compression ThermodynamicsPROVE IT

THE C.A. SYSTEM

The C.A. System Typical System

Compressed Air Challenge

Supply Side Demand

Side

The C.A. System Supply Side

Types of Compressors


The C.A. System• Analogy: Car IC

Engine• How it works:• Oil and Oil-free• Single-acting and

double-acting• Single or multi-

stage, depending on pressure/size

• Typically smaller units (less than 30hp*)

Supply SideReciprocating

Compressed Air Challenge (pg. 129)

http://www.youtube.com/watch?v=ITCu7gNMicc



The C.A. System

• Originally THE compressor technology

• Many vintage reciprocating compressors operating today, some in excess of 1,000 hp

• THE most efficient compressor technology (double-acting)

• Not used much today in industry

• 22-24 kW/100 cfm (single-acting), 15-16 kW/100 cfm (double-acting)

Supply SideReciprocating

Belliss and Morcom

The C.A. System

• Analogy: Car turbocharger

• How it works:– Impeller spinning at

10,000+ rpm• Typically larger units

(300 hp to >4,500 hp)• All Oil Free• Multi-stage, typically 2-

4 depending on size/pressure

• Centrifugal Compressor Animation

Supply SideCentrifugal

http://www.youtube.com/watch?v=DnpLlOMha5M

http://www.youtube.com/watch?v=DnpLlOMha5M

The C.A. System

• Low vibration, don’t need a heavy concrete pad like reciprocating

• Still very efficient

• Favored by industry today for large applications

• Operating range limited• 16-20 kW/100 cfm

Supply SideCentrifugal


Rotary Screw• Analogy: Car supercharger• How it works:

– Two screws meshed together which squeeze air

• Typically medium sized units (20 hp to 300 hp) but can be as large as 600 hp

• Oil and Oil Free• Typically single stage, some

larger units 2 stage

http://www.youtube.com/watch?v=stjvbAO_6JQ

http://www.youtube.com/watch?v=stjvbAO_6JQ


Rotary Screw

Ingersoll Rand

• By far, most common industrial air compressor today

• Low first cost, good efficiency, large operating range

• Variety of control techniques and manufacturers

• 17-22 kW/100 cfm (single stage)

The C.A. System

Supply SideRotary Screw (Lubricant-

Injected)

Credit: Ponna Pneumatic

Compressed Air/Oil MixtureOil (Lubricant)“Oil-Free” Compressed Air(2-3 ppm)

The C.A. System Supply SideDryers


• Air dryers condense water out of compressed air• Air at 80°F and 50% = 60°F dewpoint and 0.01092 lbw/lba

• Compressed to 100 psig and 185°F, how much water in air?– Same! 0.01092 lbw/lba Squeeze water into space 8 times smaller (114.7/14.7=7.8)

• What is new dewpoint?– 125°F (Rule of Thumb: Double pressure, increase dewpoint by 20°F

• What happens if we send that air into a industrial plant that is 80°F ambient?– Rain inside compressed air pipes

The C.A. System

• Refrigerated dryers utilize a refrigerant circuit to condense moisture from the air stream

• Typical leaving dewpoint of 40°F• Cycling, non-cycling and head-unloading designs• 0.80 kW/100 cfm

Supply SideRefrigerated Dryers

The C.A. System

• Desiccant dryers use a desiccant to dry the air (via adsorption)

• Typical leaving dewpoint of -40°F to -100°F, depending on desiccant type

• Heatless, heat-assisted and blower-heat assisted designs• 2-3 kW/100 cfm

Supply SideDesiccant

Regenerative Dryers

The C.A. System• Storage (Air Receivers, piping, etc)• Pressure/Flow Controllers• After-coolers• Air/Lubricant Separators• Filters

– Particulate: Removes dirt/debris– Coalescing: Removes vapors (typically oil/lubricant

vapors)– Adsorption: Additional hydrocarbons and other

impurities• Traps and Drains

– Level operated– Timer operated– Zero-air loss

Supply SideAdditional

Components

The C.A. System• In a typical compressed air system,

how much air is used “appropriately” by production?

Demand SideUsage Breakdown


• Leaks: Compressed air which leaks from distribution• Inappropriate Uses: Anything that compressed air is

used for which could be replaced via a more efficient process

• Increased Demand from Excessive System Pressure: Better known as artificial demand

The C.A. System• Pneumatic tools, cylinders, valves• Automation equipment• Instrumentation Air• Baghouses• Blow-off (special cases)• Motors/Pumps (where appropriate)• Etc.

Demand SideEnd-Users (Normal

Production)

The C.A. System

• Higher the system pressure, higher the leak rate– <2 cfm leak: can’t feel, can’t hear– 3-4 cfm leak: can feel, can’t hear– >5 cfm leak: can feel, can hear

• Leaks do more than waste energy– Shortens life of supply equipment because of increased

runtime– Buy/add new compressor capacity that is not needed

• Leak Table for a ‘perfect’ orifice (values are cfm)

Demand SideLeaks


1/64” 1/32” 1/16” 1/8” 1/4” 3/8”70 psig 0.300 1.20 4.79 19.2 76.7 17380 psig 0.335 1.34 5.36 21.4 85.7 193

90 psig 0.370 1.48 5.92 23.8 94.8 213 100 psig 0.406 1.62 6.49 26.0 104 234

125 psig 0.494 1.98 7.90 31.6 126 284

The C.A. System

• An inappropriate use is anything that compressed air is currently used for, but has a more efficient alternative

Demand SideInappropriate Uses

DOE Tip Sheets

Potentially Inappropriate Uses

Suggested Alternatives/Actions

Clean-up, Drying, Process Cooling Low-pressure blowers, electric fans, brooms, nozzles

Sparging Low-pressure blowers and mixers

Aspirating, Atomizing Low-pressure blowers

Padding Low to medium-pressure blowers

Vacuum generator Dedicated vacuum pump or central vacuum system

Personnel cooling Electric fans

Open-tube, compressed air-operated vortex coolers without thermostats

Air-to-air heat exchanger or air conditioner, add thermostats to vortex cooler

Air motor-driven mixer Electric motor-driven mixer

Air-operated diaphragm pumps Proper regulator and speed control; electric pump

Idle equipment Put an air-stop valve at the compressed air inlet

Abandoned equipment Disconnect air supply to equipment

The C.A. System• If the pressure of the system is too

high, uncontrolled uses consume more air– For example, a system that is at 100 psig

has a leak load of 100 cfm. If the pressure is decreased, the leak rate is also decreased.

– An unregulated air cylinder• Reducing the pressure not only saves

energy because the compressor doesn’t have to work as hard, it also reduces the amount of air it has to generate

Demand SideArtificial Demand

The C.A. System• Compressed air systems are dynamic,

meaning that a spot check is not sufficient to determine how well it is operating

• Determining how a compressor is operating requires logging equipment

Measurements and Baselining

C.A. CONTROL STRATEGIES

C.A. Control Strategies• The simplest and most efficient control method• Turn compressor on and low pressure setpoint and

turn off at high pressure setpoint• Only practical for small motors

On/Off Control

C.A. Control Strategies• Compressor operates in a pressure dead-

band, similar to on/off• At upper band, instead of shutting off,

compressor “unloads”• Bleed off air/oil separator (~40 seconds)

– Only bleed down to ~40 psi– Why does it take 40 second to bleed sump?

• Wait for pressure to reach lower setpoint• Compress air/oil separator back to

operating pressure (~6 seconds)• Resume operation

Load/Unload Control

C.A. Control Strategies Lubricant-Injected Rotary Screw

Load/Unload Control



LoadedUnloadingUnloaded

C.A. Control Strategies Lubricant-Injected Rotary Screw

Load/Unload Control



LoadedLoadingUnloaded

C.A. Control Strategies• Storage plays a huge role in

load/unload power consumption

Lubricant-Injected Rotary ScrewLoad/Unload Control

C.A. Control Strategies Load/Unload Control


Capacity of

TRIM

compressor!

C.A. Control Strategies• A low and high pressure limit as with

load/unload• Inlet valve modulates flow rate into

compressor– System pressure increases, inlet valve

closes– System pressure decreases, inlet valve

opens– No blowdown valve, sump always

pressurized• Pressure drop across inlet valve

– inlet pressure at screws decreases– increases pressure ratio, increases work

• Results in competition between savings and costs

Modulating Control

C.A. Control Strategies Modulating Control


C.A. Control Strategies• Add variable speed drive to motor• Speed is proportional to capacity

– So at 80% speed, you produce roughly 80% of the rated capacity

• Less efficient than other types at 100% capacity– VFD drive consumes some power– Screws on constant speed machines can be

designed for a single speed. Screws on variable speed machines must pick a design point, typically about 80% of full speed.

Variable Speed Control


C.A. Control Strategies Variable Speed Control


C.A. ENERGY SAVINGS

55

C.A. Energy Savings• Compressed air leaks can be between 5

and 30% of system energy usage– Typically 20-30% for ‘unmanaged’ systems– Poorly maintained plants can be even more!

• Savings depend on type of compressor and control type, but applicable for all compressed air systems

Fix Compressed Air Leaks

C.A. Energy Savings• At $0.10/kWh, 8,760 hrs/yr• For a variable speed compressor:

Fix Compressed Air Leaks

Leak Volumetric Power Loss Demand Energy CostDiameter,

DFlow Rate,

VfL Reduction,

DRSavings Savings

(in) (cfm) (hp) (kW) (kWh/yr) ($/yr) 1/32 1.0 0.22 0.16 727 $72 1/16 4.0 0.88 0.66 2,908 $291 1/8 16.1 3.53 2.63 11,634 $1,163 1/4 64.6 14.11 10.53 46,535 $4,654• For a modulating compressor:Leak Volumetric Power Loss Demand Energy Cost

Diameter, D

Flow Rate, Vf

L Reduction, DR

Savings Savings

(in) (cfm) (hp) (kW) (kWh/yr) ($/yr) 1/32 1.0 0.22 0.048 218 $22 1/16 4.0 0.88 0.198 872 $87 1/8 16.1 3.53 0.789 3,490 $349 1/4 64.6 14.11 3.16 13,961 $1,396 • Ex: Reduce air leaks by 160 cfm, save $3,458/yr

C.A. Energy Savings• Pressure typically set at whatever compressor is

rated • Plant rarely needs that high of a pressure• Pressure should be set based on highest pressure

need– If the highest pressure need is 65 psig for a process line,

then the compressor should be set at a pressure to provide that 65 psig

– <100 psi is a typical header pressure• Rule of Thumb: 1% for every 2 psi reduction• Recall discussion about artificial demand• Example: Can we decrease pressure with system “as-

is?”– No, already below critical pressure at high demand!– Then modify the system

Reduce Compressor Pressure

C.A. Energy Savings• Screw compressors can switch from modulating to

load/unload very easily*– *Many compressors can do it at the flip of a switch, not

all– Storage is important (next slide)

• Difficult to retrofit from single speed to variable speed– Typically have to buy new compressor, even more pricy

• Interlink multiple compressors with network controls– Typically only useful for many compressor systems– Not as helpful in our example

• Overlapping control bands– With pressure fixed, control bands can be separated

Change Control Type/Setpoint

C.A. Energy Savings• Only for load/unload• Adding storage allows for compressor to run

unloaded for longer periods of time, resulting in lower overall energy usage

• Storage is expensive– $20,000 or more for 7,000 gal of storage

• What does 7,000 gallons look like?• What does 20,000 gallons look like?

• Add 4,000 gal of storage to system– Switch to Load/Unload– Lower control bands to appropriate points (18 psi

improvement)– Savings of $22,226/yr (this includes load/unload savings,

pressure reduction savings and overlapping control band savings)

Add Storage

C.A. Energy Savings• Cooling the air at the compressor inlet

reduces energy consumption• Doesn’t work for flooded-oil screw

compressors• Works well for reciprocating,

centrifugal and oil-free rotary screw compressors

Cool Compressor Inlet

C.A. Energy Savings• Baghouse pulse causes severe pressure

drops in system• 6 cubic foot pulse for 0.25 seconds every 1

minute• Instantaneous Flow:

– (6 ft3)/(.25 sec)x(60 sec/min) = 1,440 cfm• Average Flow

– (6 ft3)/(2 minutes)=6 cfm• Add secondary storage• Savings are hard to figure, likely from

productivity increase

High Volume, Intermittent Needs

C.A. Energy Savings• Air Knifes can be replaced with low

pressure blowers• Think about an industrial-strength hair

blower without the heat• High flow, low pressure

Low Pressure Blowers

C.A. Energy Savings• Use the 80-85% of energy wasted as

heat– If air cooled, use to heat areas during

winter– If water cooled, might be able to utilize

for boiler makeup water or other sources• Piping can get very expensive

• Low-grade heat, makes it difficult to capture

Utilize Compressor Waste Heat

C.A. Energy Savings• Facility not operating, still need

compressed air• Typically a result of a “dry” sprinkler

system• Compressed air pressurizes and fills

pipes, if a sprinkler head bursts the compressed air escapes and the water follows behind

• Typically need <5 hp to keep system pressurized

• Many companies run one compressor off-shift to pressurize system

• Savings for our example are $14,400/yr!

Off-Hours Compressed Air Use

C.A. Energy Savings• Savings are intertwined; cannot add savings

from other slides all together because effects are cumulative

• Summary– Eliminate Compressed Air Leaks– Reduce Pressure and Fix Control Bands– Add storage and switch to Load/Unload– Add low pressure blower for air knifes– Add new 5 hp compressor for sprinkler system

• Total savings: $66,331 or 65%!• Implementation cost likely below $30,000• Doesn’t include Productivity Increase!

Total Savings from Example

QUESTIONS???

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Compressed Air