Compressed Air
Walter Bright
MAE406 – Energy Conservation in Industry
wabright@ncsu.edu
10/29/2013
C.A. BASICS
C.A. Basics
Why Compressed Air?
• 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)
C.A. Basics
Specialized Bicycles/Popular Mechanics
Basic Compressor
C.A. Basics
Why NOT Compressed Air?
Typically the most expensive utility at a plant
Rule 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
$5,388
$32,818
C.A. Basics
Why Manage Compressed Air?
• 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
Compressed Air Challenge, www.compressedairchallenge.org
C.A. Basics
100 kW of
electrical
energy input
MOTOR
COMP
Greg Harrell, Energy Management Services (EMS)
Compression Thermodynamics
C.A. Basics
100 kW of
electrical
energy input
MOTOR
5-8 kW of
thermal
energy
loss
Greg Harrell, EMS
COMP
Compression Thermodynamics
C.A. Basics
100 kW of
electrical
energy input
MOTOR
5-8 kW of
thermal
energy
loss
COMP
# kW of
loss??
Greg Harrell, EMS
Compression Thermodynamics
C.A. Basics
100 kW of
electrical
energy input
MOTOR
5-8 kW of
thermal
energy
loss
COMP
98-99%
Efficient
Greg Harrell, EMS
Compression Thermodynamics
C.A. Basics
Compression Thermodynamics
100 kW of
electrical
energy input
MOTOR
5-8 kW of
thermal
energy
loss
COMP
98-99%
Efficient
Greg Harrell, EMS
We want highpressure air from
the compressor…
C.A. Basics
Compression Thermodynamics
100 kW of
electrical
energy input
MOTOR
5-8 kW of
thermal
energy
loss
COMP
98-99%
Efficient
Greg Harrell, EMS
We want highpressure air from
the compressor…
What we get is
high-pressure,
high-temperature
air
C.A. Basics
Compression Thermodynamics
100 kW of
electrical
energy input
COMP
We want highpressure air from
the compressor…
5-8 kW of
thermal
98-99%
energy
Efficient*
loss
What we get is
high-pressure,
high-temperature
air
MOTOR
Greg Harrell, EMS
C.A. Basics
Compression Thermodynamics
# kW of thermal
energy loss??
100 kW of
electrical
energy input
COMP
We want highpressure air from
the compressor…
5-8 kW of
thermal
98-99%
energy
Efficient*
loss
What we get is
high-pressure,
high-temperature
air
MOTOR
Greg Harrell, EMS
C.A. Basics
Compression Thermodynamics
90 kW of thermal
energy loss
100 kW of
electrical
energy input
COMP
We want highpressure air from
the compressor…
5-8 kW of
thermal
98-99%
energy
Efficient*
loss
What we get is
high-pressure,
high-temperature
air
MOTOR
Greg Harrell, EMS
C.A. Basics
Compression Thermodynamics
90 kW of thermal
energy loss
100 kW of
electrical
energy input
COMP
We want highpressure air from
the compressor…
5-8 kW of
thermal
98-99%
energy
Efficient*
loss
What we get is
high-pressure,
high-temperature
air
MOTOR
Greg Harrell, EMS
C.A.
MTR
#
kW of shaft
energy
from comp.
air motor??
C.A. Basics
Compression Thermodynamics
90 kW of thermal
energy loss
100 kW of
electrical
energy input
COMP
We want highpressure air from
the compressor…
5-8 kW of
thermal
98-99%
energy
Efficient*
loss
What we get is
high-pressure,
high-temperature
air
MOTOR
Greg Harrell, EMS
C.A.
MTR
10 to 20
kW of shaft
energy
from comp.
air motor
C.A. Basics
90 kW of thermal
energy loss
100 kW of
electrical
energy input
MOTOR
COMP
5-8 kW of
thermal
98-99%
energy
Efficient*
loss
Greg Harrell, EMS
Compression Thermodynamics
We want highpressure air from
the compressor…
What we get is
high-pressure,
high-temperature
air
C.A.
MTR
10 to 20
kW of shaft
energy
from comp.
air motor
What about the 1st
Law of Thermo??
C.A. Basics
90 kW of thermal
energy loss
100 kW of
electrical
energy input
MOTOR
COMP
5-8 kW of
thermal
98-99%
energy
Efficient*
loss
Greg Harrell, EMS
Compression Thermodynamics
We want highpressure air from
the compressor…
What we get is
high-pressure,
high-temperature
air
C.A.
MTR
10 to 20
kW of shaft
energy
from comp.
air motor
The 1st Law of
Thermo is not
violated because the
air discharged is
very cold
C.A. Basics
90 kW of thermal
energy loss
100 kW of
electrical
energy input
MOTOR
COMP
5-8 kW of
thermal
98-99%
energy
Efficient*
loss
Greg Harrell, EMS
Compression Thermodynamics
We want highpressure air from
the compressor…
What we get is
high-pressure,
high-temperature
air
C.A.
MTR
10 to 20
kW of shaft
energy
from comp.
air motor
The 1st Law of
Thermo is not
violated because the
air discharged is
very cold
C.A. Basics
90 kW of thermal
energy loss
100 kW of
electrical
energy input
MOTOR
COMP
5-8 kW of
thermal
98-99%
energy
Efficient*
loss
Greg Harrell, EMS
Compression Thermodynamics
We want highpressure air from
the compressor…
What we get is
high-pressure,
high-temperature
air
C.A.
MTR
10 to 20
kW of shaft
energy
from comp.
air motor
The 1st Law of
Thermo is not
violated because the
air discharged is
very cold
THE C.A. SYSTEM
The C.A. System
Compressed Air Challenge
Typical System
The C.A. System
Compressed Air Challenge
Supply Side
Types of Compressors
The C.A. System
•
•
•
•
Analogy: Car IC Engine
How it works:
Oil and Oil-free
Single-acting and doubleacting
• Single or multi-stage,
depending on
pressure/size
• Typically smaller units
(less than 30hp*)
Compressed Air Challenge (pg. 129)
Supply Side
Reciprocating
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 (singleacting), 15-16 kW/100 cfm
(double-acting)
Belliss and Morcom
Supply Side
Reciprocating
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 Side
Centrifugal
The C.A. System
Supply Side
Centrifugal
• 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
The C.A. System
• 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
Supply Side
Rotary Screw
The C.A. System
•
•
•
•
Supply Side
Rotary Screw
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)
Ingersoll Rand
The C.A. System
Supply Side
Rotary Screw (Lubricant-Injected)
Compressed Air/Oil Mixture
Oil (Lubricant)
“Oil-Free” Compressed Air
(2-3 ppm)
Credit: Ponna Pneumatic
The C.A. System
Supply Side
Dryers
• 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
Compressed Air Challenge
The C.A. System
Supply Side
Refrigerated Dryers
• 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
The C.A. System
Supply Side
Desiccant Regenerative Dryers
• 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
The C.A. System
•
•
•
•
•
Supply Side
Additional Components
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
The C.A. System
Demand Side
Usage Breakdown
• In a typical compressed air system, how much air is
used “appropriately” by production?
• 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
Compressed Air Challenge
The C.A. System
•
•
•
•
•
•
•
Demand Side
End-Users (Normal Production)
Pneumatic tools, cylinders, valves
Automation equipment
Instrumentation Air
Baghouses
Blow-off (special cases)
Motors/Pumps (where appropriate)
Etc.
Demand Side
Leaks
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)
70 psig
80 psig
90 psig
100 psig
125 psig
1/64”
0.300
0.335
0.370
0.406
0.494
Compressed Air Challenge
1/32”
1.20
1.34
1.48
1.62
1.98
1/16”
4.79
5.36
5.92
6.49
7.90
1/8”
19.2
21.4
23.8
26.0
31.6
1/4”
76.7
85.7
94.8
104
126
3/8”
173
193
213
234
284
The C.A. System
Demand Side
Inappropriate Uses
• An inappropriate use is anything that compressed air is
currently used for, but has a more efficient alternative
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
DOE Tip Sheets
The C.A. System
Demand Side
Artificial Demand
• 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
The C.A. System
Measurements and Baselining
• 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
C.A. CONTROL STRATEGIES
C.A. Control Strategies
On/Off Control
• 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
C.A. Control Strategies
Load/Unload Control
• 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
C.A. Control Strategies
Loaded
Unloading
Unloaded
Credit: Ponna Pneumatic
Lubricant-Injected Rotary Screw
Load/Unload Control
Compressed Air/Oil Mixture
Oil (Lubricant)
“Oil-Free” Compressed Air
(2-3 ppm)
C.A. Control Strategies
Loaded
Loading
Unloaded
Credit: Ponna Pneumatic
Lubricant-Injected Rotary Screw
Load/Unload Control
Compressed Air/Oil Mixture
Oil (Lubricant)
“Oil-Free” Compressed Air
(2-3 ppm)
C.A. Control Strategies
Lubricant-Injected Rotary Screw
Load/Unload Control
• Storage plays a huge role in load/unload power
consumption
C.A. Control Strategies
Compressed Air Challenge
Load/Unload Control
C.A. Control Strategies
Modulating Control
• 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
C.A. Control Strategies
Compressed Air Challenge
Modulating Control
C.A. Control Strategies
Variable Speed Control
• 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.
Compressed Air Challenge
C.A. Control Strategies
Compressed Air Challenge
Variable Speed Control
C.A. ENERGY SAVINGS
55
C.A. Energy Savings
Fix Compressed Air Leaks
• 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
C.A. Energy Savings
Fix Compressed Air Leaks
• At $0.10/kWh, 8,760 hrs/yr
• For a variable speed compressor:
Leak
Diameter, D
(in)
1/32
1/16
1/8
1/4
Volumetric
Flow Rate, Vf
(cfm)
1.0
4.0
16.1
64.6
Power Loss
L
(hp)
0.22
0.88
3.53
14.11
Demand
Reduction, DR
(kW)
0.16
0.66
2.63
10.53
Energy
Savings
(kWh/yr)
727
2,908
11,634
46,535
Cost
Savings
($/yr)
$72
$291
$1,163
$4,654
Leak
Diameter, D
(in)
1/32
1/16
1/8
1/4
Volumetric
Flow Rate, Vf
(cfm)
1.0
4.0
16.1
64.6
Power Loss
L
(hp)
0.22
0.88
3.53
14.11
Demand
Reduction, DR
(kW)
0.048
0.198
0.789
3.16
Energy
Savings
(kWh/yr)
218
872
3,490
13,961
Cost
Savings
($/yr)
$22
$87
$349
$1,396
• For a modulating compressor:
• Ex: Reduce air leaks by 160 cfm, save $3,458/yr
C.A. Energy Savings
Reduce Compressor Pressure
• 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
C.A. Energy Savings
Change Control Type/Setpoint
• 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
C.A. Energy Savings
Add Storage
• 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)
C.A. Energy Savings
Cool Compressor Inlet
• 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
C.A. Energy Savings
High Volume, Intermittent Needs
• 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
C.A. Energy Savings
Low Pressure Blowers
• Air Knifes can be replaced with low pressure blowers
• Think about an industrial-strength hair blower
without the heat
• High flow, low pressure
C.A. Energy Savings
Utilize Compressor Waste Heat
• 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
C.A. Energy Savings
Off-Hours Compressed Air Use
• 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!
C.A. Energy Savings
Total Savings from Example
• 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!
QUESTIONS???
67