1. MAE406 – Energy Conservation in Industry
Compressed Air
What is compressed air?
Anyone w/ compressed air experience?
Walter Bright
MAE406 – Energy Conservation in Industry
10/29/2013

2. C.A. Basics

3. 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)

4. C.A. Basics Basic Compressor Specialized Bicycles/Popular Mechanics
Talk about basic compressor
Specialized Bicycles/Popular Mechanics

5. Why is compressed air so expensive???
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

6. 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
DOE offers 6 days worth of compressed air training!
Compressed Air Challenge,

7. C.A. Thermodynamics

8. 100 kW of electrical energy input
C.A. Basics
Compression Thermodynamics
100 kW of electrical energy input
MOTOR
COMP
Demonstrate how much heat is created via a bike pump and road bike tire
Greg Harrell, Energy Management Services (EMS)

9. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
MOTOR
COMP
5-8 kW of thermal energy loss
Greg Harrell, EMS

10. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
MOTOR
COMP
5-8 kW of thermal energy loss
# kW of loss??
Greg Harrell, EMS

11. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
MOTOR
COMP
5-8 kW of thermal energy loss
98-99% Efficient
Greg Harrell, EMS

12. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
MOTOR
COMP
We want high-pressure air from the compressor…
5-8 kW of thermal energy loss
98-99% Efficient
Greg Harrell, EMS

13. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
MOTOR
COMP
We want high-pressure air from the compressor…
5-8 kW of thermal energy loss
What we get is high-pressure, high-temperature air
98-99% Efficient
Greg Harrell, EMS

14. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
MOTOR
COMP
We want high-pressure air from the compressor…
5-8 kW of thermal energy loss
What we get is high-pressure, high-temperature air
98-99% Efficient*
Greg Harrell, EMS

15. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
# kW of thermal energy loss??
MOTOR
COMP
We want high-pressure air from the compressor…
5-8 kW of thermal energy loss
What we get is high-pressure, high-temperature air
98-99% Efficient*
Greg Harrell, EMS

16. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
90 kW of thermal energy loss
MOTOR
COMP
We want high-pressure air from the compressor…
5-8 kW of thermal energy loss
What we get is high-pressure, high-temperature air
98-99% Efficient*
Greg Harrell, EMS

17. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
90 kW of thermal energy loss #
kW of shaft energy from comp. air motor??
MOTOR
COMP
C.A. MTR
We want high-pressure air from the compressor…
5-8 kW of thermal energy loss
What we get is high-pressure, high-temperature air
98-99% Efficient*
Greg Harrell, EMS

18. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
90 kW of thermal energy loss
10 to 20 kW of shaft energy from comp. air motor
MOTOR
COMP
C.A. MTR
We want high-pressure air from the compressor…
5-8 kW of thermal energy loss
What we get is high-pressure, high-temperature air
98-99% Efficient*
Greg Harrell, EMS

19. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
90 kW of thermal energy loss
10 to 20 kW of shaft energy from comp. air motor
MOTOR
COMP
C.A. MTR
We want high-pressure air from the compressor…
What about the 1st Law of Thermo??
5-8 kW of thermal energy loss
What we get is high-pressure, high-temperature air
98-99% Efficient*
Greg Harrell, EMS

20. C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
90 kW of thermal energy loss
10 to 20 kW of shaft energy from comp. air motor
MOTOR
COMP
C.A. MTR
We want high-pressure air from the compressor…
The 1st Law of Thermo is not violated because the air discharged is very cold
5-8 kW of thermal energy loss
What we get is high-pressure, high-temperature air
98-99% Efficient*
Greg Harrell, EMS

21. COMPRESSION EFF: 10-20% C.A. Basics Compression Thermodynamics
100 kW of electrical energy input
90 kW of thermal energy loss
10 to 20 kW of shaft energy from comp. air motor
MOTOR
COMP
C.A. MTR
We want high-pressure air from the compressor…
The 1st Law of Thermo is not violated because the air discharged is very cold
5-8 kW of thermal energy loss
What we get is high-pressure, high-temperature air
98-99% Efficient*
COMPRESSION EFF:
10-20%
Greg Harrell, EMS

22. PROVE IT COMPRESSION EFF: 10-20% C.A. Basics
Compression Thermodynamics
100 kW of electrical energy input
90 kW of thermal energy loss
10 to 20 kW of shaft energy from comp. air motor
MOTOR
PROVE IT
COMP
C.A. MTR
We want high-pressure air from the compressor…
The 1st Law of Thermo is not violated because the air discharged is very cold
5-8 kW of thermal energy loss
What we get is high-pressure, high-temperature air
Demonstration with air nozzle or just a open connector
7 units of compressor work for 1 useful unit of mechanical work = 14.3% compression efficiency
98-99% Efficient*
COMPRESSION EFF:
10-20%
Greg Harrell, EMS

23. C.A. Thermo An isothermal process is ideal
Equations
An isothermal process is ideal
That way, every portion of power goes into pressure energy instead of thermal and pressure energy
Compression is essentially adiabatic (no heat transfer)
Ideally, it would be reversible also, or isentropic

24. The C.A. System

25. The C.A. System Typical System Supply Side Demand Side
Compressed Air Challenge

26. The C.A. System Supply Side Types of Compressors
Compressed Air Challenge

27. The C.A. System Analogy: Car IC Engine How it works: Oil and Oil-free
Supply Side Reciprocating
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*)
Compressed Air Challenge (pg. 129)

28. The C.A. System Supply Side Reciprocating
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), kW/100 cfm (double-acting)
Belliss and Morcom

29. The C.A. System Analogy: Car turbocharger How it works:
Supply Side Centrifugal
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
Important content

30. The C.A. System Supply Side Centrifugal
Low vibration, don’t need a heavy concrete pad like reciprocating
Favored by industry today for large applications
Operating range limited
Still very efficient
16-20 kW/100 cfm
Saint Gobain in Wilson, NC. Picture of four 1000hp, 3 stage Centac units. Three more two stage 1000hp behind photographer to right. Also a ~250hp screw

31. The C.A. System Analogy: Car supercharger How it works:
Supply Side 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
How do I know the front of the picture is the intake and back is discharge? Answer: Size of hole, density of air is much greater at exit, need less space

32. The C.A. System By far, most common industrial air compressor today
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

33. 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

34. 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 lbw/lba
Compressed to 100 psig and 185°F, how much water in air?
Same! 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

35. 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

36. 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

37. The C.A. System Storage (Air Receivers, piping, etc)
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

38. 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

39. The C.A. System Pneumatic tools, cylinders, valves
Demand Side End-Users (Normal Production)
Pneumatic tools, cylinders, valves
Automation equipment
Instrumentation Air
Baghouses
Blow-off (special cases)
Motors/Pumps (where appropriate)
Etc.

40. The C.A. System Demand Side Leaks
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)
1/64”
1/32”
1/16”
1/8”
1/4”
3/8”
70 psig
0.300
1.20
4.79
19.2
76.7
173
80 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
Compressed Air Challenge

41. Potentially Inappropriate Uses Suggested Alternatives/Actions
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

42. 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

43. 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

44. C.A. Control Strategies

45. C.A. Control Strategies The simplest and most efficient control method
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

46. 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

47. C.A. Control Strategies Load/Unload Control Loaded Unloading Unloaded
Lubricant-Injected Rotary Screw
Load/Unload Control
Loaded
Unloading
Unloaded
Compressed Air/Oil Mixture
Oil (Lubricant)
“Oil-Free” Compressed Air
(2-3 ppm)
Credit: Ponna Pneumatic

48. C.A. Control Strategies Load/Unload Control Loaded Loading Unloaded
Lubricant-Injected Rotary Screw
Load/Unload Control
Loaded
Loading
Unloaded
Compressed Air/Oil Mixture
Oil (Lubricant)
“Oil-Free” Compressed Air
(2-3 ppm)
Credit: Ponna Pneumatic

49. Storage plays a huge role in load/unload power consumption
C.A. Control Strategies
Lubricant-Injected Rotary Screw
Load/Unload Control
Storage plays a huge role in load/unload power consumption
Graph of “ideal” unload curve, 50% on and 50% unloaded. Then draw actual with various amounts of storage

50. Capacity of TRIM compressor!
C.A. Control Strategies
Load/Unload Control
Capacity of TRIM compressor!
Compressed Air Challenge

51. A low and high pressure limit as with load/unload
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

52. C.A. Control Strategies
Modulating Control
Compressed Air Challenge

53. Add variable speed drive to motor Speed is proportional to capacity
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

54. C.A. Control Strategies Variable Speed Control
Compressed Air Challenge

55. C.A. Energy Savings

56. 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

57. C.A. Energy Savings At $0.10/kWh, 8,760 hrs/yr
Fix Compressed Air Leaks
At $0.10/kWh, 8,760 hrs/yr
For a variable speed compressor:
Leak
Volumetric
Power Loss
Demand
Energy
Cost
Diameter, D
Flow Rate, Vf L
Reduction, DR
Savings
(in)
(cfm)
(hp)
(kW)
(kWh/yr)
($/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
$4,654
For a modulating compressor:
Leak
Volumetric
Power Loss
Demand
Energy
Cost
Diameter, D
Flow Rate, Vf L
Reduction, DR
Savings
(in)
(cfm)
(hp)
(kW)
(kWh/yr)
($/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

58. 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

59. 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

60. 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)

61. 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

62. 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

63. 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

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

65. C.A. Energy Savings Facility not operating, still need compressed air
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!

66. 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!

67. QUESTIONS???

68. Notes Required Materials Road Bike Tire Hand Pump/Compressor
Pancake compressor
Air hose
Extension cord
Blow-off nozzle
Open connector
Compressed air tools
Nail gun
Two boards
Ammo
Air Wrench
Gatorade Bottle w/ soap & water
Loggers
HOBO logger with CT
HOBO logger with power meter
Fluke
Pressure transducer
Plastic model of screw compressor
Data log of air compressor