A common sense approach to its use and abuse
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Compressed air is free –
as a power source it is nine times more expensive to use than electricity.
Dryer is better – some
facilities install -40° F pressure dewpoint dryers in hopes of fixing their
moisture carry over problems. Often the problems have little to do with
dryer type. Installing this type of dryer can pose a whole new set of system
problems to be dealt with – like adding 15% more compressor power to
accommodate the dryer’s purge requirements!
Many of the assumptions listed above are real barriers to operating compressed air systems efficiently. Education is the best first-step measure that can be taken in improving compressed air system operating efficiency.
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Common
Inefficiencies in Compressed Air Systems
Many
compressed air systems waste as much as 40% of their total operating cost.
One reason for that is that
compressed air is often viewed as a “free” utility by the people that
consume the air.
Failure to store compressed air
energy for use during peak demand periods
Leaks at both point-of-use and
supply-side equipment
Severe fluctuation in pressure
Indiscriminate use of open
blowing
Inappropriate production use of
compressed air
Simple lack of maintenance,
including neglect of dirty filter cartridges
Non-existent system-wide
control and monitoring
By optimising your
compressed air system, you have the ability to increase production, and improve
quality.
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However, before you
attend a seminar, sign that predictive / preventive maintenance contract or call
your compressor manufacturer, read this article. This guide will provide you
with what could be considered common sense advice for compressed air system
placement and maintenance that could reduce the amount of downtime you
experience with your compressed air system. We will review compressor location,
power source, ventilation, piping, filtration, cooling systems, and preventive
maintenance.
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While proper maintenance can help
prevent complaints from compressor users, there are several issues that can be
addressed before the compressed air system is actually in use. Proper compressor
location, power sources, and ventilation can help prevent unscheduled downtime
and environment issues.
Poor air filtration is the leading cause
of early death for air compressors. Here are a few guidelines to help ensure
that your compressor will continue to produce clean, dry air:
Know Your Environment: One common
mistake that compressor users make is when they neglect to evaluate the quality
of the air that they will be using within the compressor. To get to know your
environment, evaluate the size and make-up of air-borne particulates and ask
yourself some questions regarding your surroundings:
Is chemical cleaning being done in the
area?
Are noxious fumes present?
Most environments fall into one of three
categories -- dusty, hostile and clean. Here is a brief description and the
potential problems:
Dusty:
Dusty conditions, on the other hand, may contain dust as well as dirt, casting
sand, and other airborne particulates. The hazards created by these conditions
can be reduced by using a high dust inlet filter. While it may not remove any
additional particles, it can reduce frequency of replacement.
Hostile:
A hostile environment is defined as having caustic gases/chemicals, chlorine,
ammonia, acids, in the air. With a hostile environment, one solution may be to
remove the problem by relocating the compressed air system or the caustic
materials. Another option is replacing standard materials of construction with
more tolerant materials; for instance stainless steel coolers vs. copper
coolers. In addition, to save money, evaluate the compressor fluid life in the
hostile environment. A smart move may be possible conversion to a more cost
effective fluid given the shortened life.
A proper evaluation of air quality at
the time of installation and at least once a year could help prevent a premature
failure of your compressor.
Confirm Inlet Filter Size: When inlet
filters are not sized properly, it allows micron size dust to enter the
compression system, which can decrease the life of the coolant and separator
filters. A basic guideline for maintenance personnel is to monitor the pressure
drop of filters and replace elements before the cost of increasing pressure
drop, due to dirt or dust build up, exceeds the cost of a replacement element.
Inlet and oil filters left too long before changing can literally choke a
compressor, reducing its flow. This will also accelerate the wear rate of
rotating elements, such as bearings, in rotary screw compressors.
In addition, you should remember that
the air filter that came with the compressor originally may no longer be
adequate for your changing facility. Systematically evaluate your air filtration
needs to fit your application.
Compressed air dryers help to reduce the
water vapour concentration and prevent liquid water formation in compressed air
lines. Dryers are a necessary companion to filters, aftercoolers, and automatic
drains for improving the productivity of compressed air systems.
Refrigerated and desiccant dryers are
the most commonly specified for correcting moisture related problems in a
compressed air system. Refrigerated dryers are normally specified where
compressed air pressure dew points of 330°F. to 390°F. are adequate. Desiccant
dryers are required where pressure dew points dip below 330°F.
Evaluate Your Cooling Water:
Aftercoolers are essential elements of air compressors. These aftercoolers are
heat exchangers that utilize either water or ambient air to cool the compressed
air. The compressed air is typically cooled to within 15°- 25 °F of the
cooling media. In addition, aftercoolers typically remove 60 percent of moisture
content in the air and help insure that the temperature of the air within the
piping system is not considered a safety hazard.
Just as clean cool air is important to
every compressor, clean cool water is critical to units fitted with water-cooled
heat exchangers.
At a minimum, water conditions should
meet the manufacturer’s requirements for flow, pressure and temperature;
however, one item that is often overlooked is the relevant "hardness"
of the water. Hard water deposits lead quickly to clogging and fouling of
coolers causing temperature shutdowns.
Water quality test kits are readily
available from hardware or even swimming pool supply stores. Once a
"bad" condition is identified, the cure could be as simple as
scheduled chemical treatments of your cooling tower or the addition of an
electro static or magnetic treatment system.
Regardless of what you do to maintain
your compressor, if you are not maintaining your piping system your efforts have
been wasted. All air/water inlet and discharge pipeworks are affected by
vibration, pulsations, temperature, pressure, corrosion and chemical resistance.
In addition, lubricated compressors will discharge small amounts of oil into the
air stream; therefore, you need to assure compatibility between discharge
piping, system accessories and software.
Nearly all of the compressed air system
manufacturers recommend that customers do not use plastic piping, soldered
copper fittings and rubber hose as discharge piping for compressed air systems.
Plastic piping is not recommended because some types might react with compressor
fluids, soften due to heat or shatter due to pressure or pulsation of the
compressor. Soldered, copper fittings will eventually work loose due to
pulsating caused by the compressed air system. Rubber hose piping is
unacceptable because it is easily attacked by today’s lubricants. In addition,
flexible joints and/or flex lines can only be considered for such purposes if
their specifications fit the operating parameters of the system.
Condensate Removal: After compressed air
leaves the compression chamber the compressor’s aftercooler reduces the
discharge air temperature well below the dew point (for most ambient
conditions), therefore, considerable water vapor is condensed. To remove this
condensation, most compressors with a built-in aftercoolers are furnished with a
combination condensate separator/trap. One concern when dealing with condensate
is the Clean Water Act, which forbids the routing of condensate to floor and
storm drains and to the ground outside even after condensate separation.
In situations such as this, a drip leg
assembly and isolation valve should be mounted near the compressor discharge. A
drain line should be connected to the condensate drain in the base. Keep in mind
that it is important that the drain line must slope downward from the base to
work properly. It is possible that additional condensation can occur if the
downstream piping cools the air even further and low points in the piping
systems should be provided with drip legs and traps. It is also important that
the discharge piping is as large as the discharge connection at the compressor
enclosure. All piping and fittings must be suitably rated for the discharge
pressure.
Careful review of piping size from the
compressor connection point is essential. Length of pipe, size of pipe, number
and type of fittings and valves must be considered for optimum efficiency of
your compressor.
Preventive Maintenance
If someone asked, "what is the key
to maintaining an efficient compressed air system," my answer would have to
be -- preventive maintenance. This is the one way the operator can actively help
prevent unbudgeted maintenance expenses from cropping up. One way to execute a
preventive maintenance program is by data trending.
Data trending is the recording of basic
operation parameters including pressures, temperatures, and electrical data. For
example, slowly increasing temperature indicates a variety of maintenance
requirements including cooler core cleaning, overloading of system and possible
mechanical problems. Another example might include slowly decreasing pressure,
indicating increased system flow requirements, reduced compressor performance or
increased system leakage.
Keep in mind, once a preventive
maintenance program has been implemented, a key element often overlooked is data
analysis. If the data is never reviewed, looking for trends, the benefit is
lost.
Finally, the operator should understand
that the same information used to evaluate and establish requirements for buying
a new compressor should be used to re-evaluated periodically to ensure your
compressor is still capable of doing the job. If not, there is a good chance you
may be asking it to do more than it can, which will inevitably lead to a short
life.
The compressor must be purchased with
the Outdoor Modification Option to provide NEMA 4 electric’s and a cabinet
exhaust on the end of the unit rather than the top to prevent re-circulation of
cooling air;
The compressor should be installed on a
concrete pad designed to drain water away. If the concrete pad is sloped, the
compressor must be leveled. In order to properly pull cooling air through the
aftercooler, the base/skid must be sealed to the concrete pad;
The roof of the shelter should extend a
minimum of 4 feet around all sides of the compressor to prevent direct rain and
snow from falling on the unit. Air-cooled machines must be arranged in a way
that prevents air re-circulation. (i.e. hot exhaust back to the package inlet).
If the installation includes more than
one compressor, the hot air exhaust should not be directed towards the fresh air
intake of the second unit or an air dryer.
Arrange the machine with
controller/starter enclosure facing away from the sun as radiant heat can affect
starter performance. In addition, direct sunlight and UV rays will degrade the
membrane touch panel.
Power disconnect switch should be within
line of sight and in close proximity to the unit operating panel.
Incoming power connections must use
suitable connectors for outdoor weather tight service.
A minimum of three feet clearance must
be allowed on all four sides of the unit for service access. If possible, access
by a forklift and or an overhead beam hoist should be kept in mind (for eventual
service to airend or motor).
Some type of protection such as a fence
or security system should be provided to prevent unauthorized access.
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Compressed
Air Systems Approach
Traditionally, the phrase “compressed
air system” is used to refer to compressors, dryers, coolers, filters, etc.
The problem with this very narrow system definition is that it overlooks the
interrelationship between supply side components and their demand side
counterparts. The supply and demand sides of a manufacturing facility do not
work independently of each other. They work (or often don’t work) together as
a system. The entire compressed air system should be analysed, monitored and
controlled.
Both sides must be coordinated by
suitable controls in order to work together.
The “traditional” compressed air
systems definition ignores the demand side and its point-of-use application
needs.
Optimisation
and Prevention Maintenance Through Advanced Control Systems
This section was originally written about the textile
industry but the same basic principals apply to all users of compressed air.
Why Worry About
Compressed Air?
During the last quarter the textile
industry has embraced new technologies, which have increased productivity and
improved quality. Many of these new technologies have brought with them a new
focus on an utility that has become as important as electricity and water -
compressed air. Compressed air makes today's air jet spinning, air jet weaving,
air jet texturising and air splicing possible.
Compressed Air Systems
The make-up of compressed air systems
vary from plant to plant. The different types of air compressors which make up
these systems are either positive displacement; reciprocating and rotary, or
dynamic; centrifugal.
In plant air needs such as blow down,
controls and operation of pneumatic cylinders, the compressed air does not come
into direct contact with the textile product. Therefore, reciprocating or rotary
air compressors have been commonly utilized.
For air jet weaving, spinning and
texturising, compressed air is in direct contact with he product, which mandates
the use of 100% oil-free air compressors to insure product quality. For these
applications and any applications with large plant air needs, centrifugal air
compressors are commonly utilized.
Cost of Compressed Air
Before we can investigate methods of
conserving compressed air we should review the factors which contribute to the
cost of compressed air.
Generally, these factors can be grouped
into the following categories:
Fixed Charges and Repairs -- Usually
about 15% of total cost
Operation - Usually about 20% of total cost
Utilities - Usually about 65% of total cost
While fixed charges, such as
depreciation, insurance and taxes, cannot typically be reduced, repairs can
provide an area of possible cost reductions. Major repairs can be often be
avoided with proper preventative maintenance. Advanced control systems can
provide the tools to utilise preventative maintenance to reduce repair costs.
Vibration analysis can also be utilised, either alone or in conjunction with an
advanced control systems, for further reduce major repairs.
Operational costs include the monitoring
of the compressed air systems and the parts and labor necessary for regular
maintenance. These costs are necessary and, typically, are kept to a minimum.
The cost of power to compress air is the
area in which most cost savings solutions exist. Most compressed air systems can
be made more efficient by simply operating at the lowest pressure the systems
can handle. Since it takes power to compress air to a higher pressure,
maintaining the lowest possible pressure uses the least power.
In order to keep the pressure low, air
leaks become more important. Not only do air leaks cause pressure drops, but
they also cost money. Table 1 indicates that the cost from a small leak in term
of dollars is considerable. Identification and repair of leaks can provide
another method of power savings.
|
Diameter
of Opening |
Cubic
Feet of Waste Per
Month |
Cost |
|
1/32" (0.75mm) |
45,400 |
£6.81 |
|
1/16"
(1.5mm) |
182,000 |
£27.30 |
|
1/8"
(3mm) |
729,000 |
£109.35 |
|
1/4"
(6mm) |
2,920,000 |
£438.00 |
To accurately determine the costs of
compressing air, measurements of power and compressed air flow are essential.
System efficiency, decay of that efficiency and incorrect usage of compressed
air can all be determined through these measurements. These measurements make it
possible to investigate cost savings through various methods of conserving
compressed air.
Advanced control systems can provide
various methods of power conservation, particularly in multi-unit installations.
Before we go in-depth to these methods we should first review the standard
control systems in use in many textile plants today.
All compressors are supplied with some
type of control system. These systems will typically monitor the compressed air
system and automatically adjust for demand. Additionally, the primary health
functions of the compressors are monitored to provide protection against
breakdown.
For centrifugal compressors an inlet
throttling device is utilized to throttle inlet flow to the compressor to
maintain a constant discharge air pressure. Inlet flow can be throttled to a
minimum point at which point air is bypassed to maintain the constant discharge
air pressure. The typical control package will control the throttling and
bypassing of air. It can even provide unloading of the compressor of low system
demand with reloading on falling system air pressure.
The control system will monitor
compressor temperatures, pressures and vibrations and compare these actual
values against alarm and shutdown settings. Additionally, most systems are
capable of providing alarms for basic preventative maintenance such as dirty
inlet air or oil filters.
While these standard control systems
provide efficient control for single compressor installations, they may not meet
the new needs of the modern textile manufacturer with multi-unit installations.
Modern Textile Control Needs
The textile mill of today has become
very flexible in order to operate under a wide variety of market conditions. For
this reason, most compressed air systems are made up of multiple compressors in
order to allow efficient operation at less than full plant production. Multiple
compressor installations also allow for effective planning for future plant
needs. These multiple compressor installations have created a new set of control
needs for the modern textile plant:
Optimisation of power usage
System dependability - avoiding
unplanned downtime
System reliability - planning
maintenance
These special control needs are not
typically provided in the standard controls provided with each compressor.
Optimising System Power Consumption
Multiple compressor installations, when
left their standard controls, will typically see the strongest compressor taking
the lead by operating at full load. While the weaker compressors make up the
remaining system needs by operating at partial loads. The problem with this
configuration is that one compressor is operating at a much less efficient
point.
A central energy management system
should be capable of forcing all of the compressors to share the load equally.
This can be accomplished by many methods, for example controlling all inlet
valves to the same throttle point. Systems, which do this, have shown savings of
up to 8-10%.
Checks can also be made to determine if
the optimum number of compressors for a certain load are operating. Basis these
checks, compressors can be started and stopped, within the motor starting
capabilities, to insure a minimum number of compressors are operating at any
point in time. Savings from these checks are dependent on the load variations of
a specific system.
System Dependability
Loss of compressed air pressure in
today's textile mills can result in hours of lost production and damage to
product in process. For these reasons, system dependability must be optimised to
provide a system, which can protect against unplanned outages. While each
compressor's standard control panel provides compressor protection, no system
protection is provided. A central energy management system can not only supply
system protection, it can also optimise it.
A central energy management system
should be capable of monitoring the health of each compressor in order to
determine an alert or shutdown status as soon a sit occurs. This will allow the
system to bring another compressor on-line before the air pressure reaches
problem levels. The system can the alert the compressor operator that a
compressor has encountered trouble so that maintenance can be completed.
An automatic system such as this allows
unmanned operation of a compressed air system. Thus, maintenance personnel that
have been required for years to monitor the compressed air system can now spend
time optimising system performance by repairing system air leaks, providing
preventative maintenance, and providing for other plant maintenance needs.
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The key to system reliability is a
strong preventative maintenance program. In past years this has meant taking
data by hand and then reviewing the data looking for specific trends. This
method often overlooked problems until it was too late to plan preventative
maintenance on a schedule that was acceptable to production. Therefore, an
automatic form of data collection with trending and indication of preventative
maintenance requirements was necessary.
To accomplish this task a method of data
collection must first be developed. Since digital data is best used for this
process, this means that temperatures, pressures and vibrations must be
collected via electronic devices such as RTD's, pressure transmitters and
vibration probes. This data is already collected on many modern compressors for
use on their standard control panels. On older compressors it may be necessary
to update the standard controls or provide direct signals to the central energy
management system.
Once the data is collected and
transmitted to the central energy management system, it must be analyzed.
Through data trending, potential problems can be detected far before they would
cause a compressor to fail. This will allow maintenance to be planned in
conjunction with production needs.
A central energy management system could
also provide an accounting system for routinely scheduled preventative
maintenance such as oil and filter changes. The system could simply schedule the
routine maintenance items and indicate to maintenance a daily schedule of items
to be completed. After each item is completed it is recorded into the system
thus updating the maintenance log for each compressor.
While an advanced control system does
reduce the workload on the personnel responsible for compressors, these
personnel are still necessary. A visual check of the compressor is still the
best method of identifying leaks, faulty condensate traps and many other
indications of problems.
Once the need for advances control
systems is recognized, there are several questions that must be considered.
These questions will help to define several questions that must be considered.
These questions will help to define the type of system that should be further
investigated. At this point it may be in your best interest to consult your
compressor manufacturer for assistance on adaptability of their compressors to
advanced control systems. This will impact your decision on the following
questions.
First, does your facility currently use
a distributed control system? A distributed control system, or DCS, is used to
control more than one system within a facility. For instance, it may control
compressors, pumps, lightning and air conditioning. If your facility does use a
DCS it may be beneficial to utilize it for advanced compressor control. The
benefit of this type of system is that is designed to exactly fit your unique
needs. Often though, this type of system is too expensive due to the cost of
programming of the DCS and the cost of transmitting the data to the DCS.
Additionally, much time must be spent to develop the algorithms necessary for
system optimisation.
Second, does your compressor
manufacturer have an advanced control system which fits your needs? Many
compressor manufacturers have developed advanced control systems for their
compressors. These vary from simple sequences, which simply start and stop
compressors to elaborate computer-based systems, which provide for modern
textile needs. Some of these systems can even be linked to an existing DCS to
pass on compressor data. In this way, the compressor vendor supplies the
programming and algorithms for compressor control while preventative maintenance
and compressor logs can be maintained on the DCS.
Finally, how sophisticated do you want
to get? It is important to define your unique system needs before you purchase a
central energy management system. The level of system sophistication varies with
the cost of the systems. These systems can cost any where from £5,000 to £500,000.
Without defining your specific systems needs it is very easy to end up with the
wrong system for the wrong price.
Summary
There are many effective methods of
identifying ways to reduce the costs of compressing air. Among these are
compressed air surveys, compressed air leak detection, vibration analysis,
maintenance contracts and advanced control systems.
Modern textile plants can utilize
advanced air compressor control systems for:
Efficient Energy Usage
Controlling System Dependability
Controlling System Reliability
These systems can be as simple or
complex as an individual plants needs. Determination of your unique needs can
lead to finding an advanced control system that will allow your facility to
operate without worry of loss of compressed air.
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A
Guide To Using Compressed Air Control Systems To Improve Efficiency For
Multiple Compressor Installations
Compressed air is considered a utility
used in a variety of plant functions from transporting material, to operating
production machinery and power tools. Because most facilities use multiple
compressors, an excellent opportunity for energy savings exists in the efficient
control of these multiple unit systems.
Since plant air demand is constantly
fluctuating there exists several operating options that can provide significant
savings during part load conditions. This article will review several options
for compressor control systems and help plant personnel address varying
compressed air demands.
The Basics
To understand the logic behind system
controls, a good place to starts is by reviewing some of the basic principles
associated with compressed air usage:
A compressor that is running at idle
will usually consume over 30% of its full load power. This is due in part to
degrading motor efficiency coupled with relatively high unloaded motor
horsepower.
Air flow in CFM is dependent on
pressure. As pressure decreases, air flow through an orifice, regulator, etc.
will also decrease. A 0.25 inch orifice will discharge 126 CFM at 125 PSIG and
only 95 CFM at 90 PSIG -- a reduction of 25%.
A single-stage rotary screw compressor
consumes 0.5% of its input power to produce each pound (PSI) of discharge
pressure. A two-stage compressor will consume 0.4% input power per pound per
pressure.
To help reduce the wasted costs of
unused compressed air, it is important to design a system that limits compressor
operations to meet plant demand. In addition, it is also important to reduce
discharge pressure since it will decrease both power and air consumption.
For our discussion, we will identify a
"typical" manufacturing facility that we can use to illustrate the
benefits and limitations to various control systems. Our typical manufacturing
facility will include:
Three air compressors (usually different
sizes and possibly different designs),
Varying compressed air demands, and
A poorly defined control hierarchy among
compressors.
The most critical step required for any
control scheme is an understanding of system demand.
The two key elements of system demand
are pressure and capacity. Any facility interested in improving its productivity
and efficiency must have an understanding of the amount of air pressure and
capacity that is required by their air-operated equipment.
Most air system audits reveal that plant
air requirements are typically lower in pressure than current compressor
discharge pressure. In addition, plant air capacity requirements vary
significantly over the course of a "typical" production day. A typical
demand profile is illustrated in Figure A, with two shift operation and capacity
requirements lower during the second shift.
Using this information, now we can look
at compressed air control scheme options. In order to remain brief, this article
will group all of the various control options into four categories:
Category 1 -- No
Control Scheme
Category 2 -- Local
Control Scheme
Category 3 -- Central
Control Scheme
Category 4 -- Global
Control Scheme.
Category 1 -- No
Control Scheme
Over 80% of facilities have no true
control scheme for their compressed air systems. Each compressor simply runs
constantly at its initial pressure setting. This can result in compressors
idling needlessly, sometimes for multiple hours each day.
Example: A single 100 hp air compressor
idling only three hours per day, 300 days-per-year, with a power cost of 0.06 £/kW
hr. equals an electrical cost of £1,400 per year.
Another result is that compressors may
operate at higher than required discharge pressures.
The simplest of control schemes,
Category 2 is defined when the individual controls of each compressor are
adjusted to operate in concert. Without a control scheme (Figure B) the
pressures are not complementary, nor do they support each other.
By adjusting the local controls (Figure
C), a more logical system is provided while at the same time overall system
pressure is reduced. The addition of automatic stop/start controls to each unit
allows those machines that are idling needlessly to stop, increasing the system
efficiency further. This type of system usually yields a 10-20% improvement in
efficiency.
Note: Most compressed air system
manufacturers do offer some version of the stop/start control. The simplest
version consists of a timer and a relay. The timer initiates as soon as the
compressor unloads. If the compressor continues to run unloaded until the timer
runs out, the relay is tripped, stopping the compressor. Should plant pressure
decrease to the low pressure set point, the compressor automatically comes back
on line.
Category 3 - Central Control Scheme
Category 3 is
the first option utilizing a true system controller. When utilizing a local
control system, each compressor is operating in concert, but independently. A
central control scheme replaces the local controls of the individual compressors
with a master or "central" controller.
The first advantage provided by a
central control scheme is an overall reduction in operating pressure. Figure
"D" illustrates the savings possible by replacing several
pressure switches with a programmable controller and a single pressure
transducer. All three compressors are now controlled using a 2 PSIG
differential. This provides substantial energy savings since it reduces total
pressure by 15 PSIG or more. In addition, this pressure signal can be located
downstream of the air clean up equipment, further increasing system efficiency.
Due to the variable pressure differentials of in line filter elements, system
pressure would automatically adjust to optimise efficiency.
Second, with a 2 PSIG differential, a
virtually steady system pressure can now be maintained. This can offer increased
production savings by reduction in scrap rate due to fluctuating pressure.
There is a considerable initial cost
when installing a global control scheme. However, the level of control provided
by this investment yields an even higher degree of energy efficiency as well as
a considerable reduction in operating costs. Some examples of these savings
include:
Remote monitoring and notification of
equipment alarm and shutdown setpoints.
Automatic data trending and low level
analysis.
Full integration with existing facility
controls.
In conclusion, there is no doubt that for most facilities upgrading controls to a Category 4 will provide substantial energy savings. However, the true justification must be tested on the incremental savings over lower level, less expensive options. For many facilities simply investing the time and effort in a Category 2 upgrade may generate the majority of energy savings. Keep in mind, each facility is unique and should be evaluated based on its current situation and specific requirements.
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Discharge pipe is to be the same size as
air compressor outlet.
Install a pipe tee in the discharge pipe
to blow to atmosphere if necessary for control and adjustment. This will also
serve as a convenient connection for a rental compressor if required.
Locate headers and sub-headers near air
uses and manufacturing equipment. A loop system is ideal, providing two way flow
distribution.
Slope piping so that condensate travels
with the flow of air and away from the compressor.
Take all drop lines from the top of main
pipe lines and locate them near main points of air use.
Do not connect multiple air users to the same drop.
Use one drop for each air user.
Use carbon steel pipe as discharge pipe
material. Never use PVC or ABS. Consider using
Schedule 40 black iron, galvanized, copper, stainless steel, or anodized
aluminum.
Size the pipe for maximum CFM required.
This will equal full load production plus future expansion plans.
Install an air receiver at intermittent
high demand points such as occasional sandblasting, air motors, etc.
Air receiver size should be one gallon
of storage per 1 CFM of air compressor output as a minimum in order to permit
the compressor controls to operate correctly..
Always consider leakage and future
expansion in order to eliminate compressed air system obsolescence. A 10% per
year growth rate is common.
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Compressor
sizing, Air treatment sizing,
Receiver
sizing
Point of use components
Compressor sizing – The
easiest, least costly and most common method for sizing a compressor is to
determine existing peak demand, add 20-30% for growth and add one compressor
that matches the resulting cfm calculation. Typically, peak demand periods are
of shorter duration than say, second, third or weekend shifts. Having one
compressor sized for peak demand will mean it is operating inefficiently an
average of 85% of the time. Installing multiple compressors to match this peak
while incrementally matching other lower compressed air demand periods will pay
for the additional procurement costs in energy savings.
The other end of the scale must be
considered as well. If the temperature is very low, ambient air contains very
little moisture for removal. The air dryer must not be so large that it freezes
up from lack of heat loading.
Receiver sizing – One gallon
per rated cubic foot? Never! This rule of thumb, though popular and easy to
remember, does not take system events into account. Receivers are sized to
accommodate large intermittent system air demand, a stand-by compressor start,
local high rate of flow requirements and insulation of critical pressure users.
The object of each case is: to prevent a compressor start, maintain pressure in
the system while a compressor starts, servicing demand from storage separate
from the main system, and to operate the system at lower pressures,
respectively.
In the central compressor configuration,
avoid placing any receiver capacity upstream of dryers or filters. Receiver
capacity should always be installed downstream of air treatment to avoid surges
across this equipment that might result in carry-over to the system.
Distribution piping – A good
rule of thumb that is commonly used is to limit pressure drop to less than 1
psiG per 100 linear feet. This applies to the rate of flow through any
particular section of piping, and has little to do with total compressor
capacity. Certain point of use applications may take compressed air at a rate of
flow greater than the capacity of available compressors for a short duration. If
the piping is sized per the capacity of the available compressor(s) rather than
the rate of flow, it might represent a restriction that causes pressure to drop
system-wide.
Point of use components – Rate
of flow considerations at the point of use is much more important than in sizing
distribution piping. When end users complain of low pressure the first thing
blamed is the piping because “the user is at the other end of the system” or
“the piping system has been expanded haphazardly over the years” (or
whatever the excuse), the real problem is almost never the piping. The real
source of problems described as “low pressure” usually resides in the choice
of installed point of use components.
Pipe drops, filters, regulators,
lubricators, quick disconnects, and hose must all be sized for the rate of flow
at the point of use. A common mistake is to buy a tool that uses 100 Scfm, apply
a 10% utilization factor to it and size all of the in-line components for 10
Scfm. The components need to be sized for the 100 Scfm rate of flow, not some
averaged demand level used to size compressors!
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Presented By N.E.M
Business Solutions
November 2001