Compressed Air 

A common sense approach to its use and abuse

Presented By N.E.M Business Solutions
Tel / Fax 01823 680119    Mobile 07768 981196

You can "speed jump" to sections in this document by clicking in the headings listed below.

Common Myths about Compressed Air

Common Inefficiencies in Compressed Air Systems 

Common Sense of Compressor Air System Maintenance

Compressor Placement

Compressed Air Systems Approach 

Preventative Maintenance

A Guide To Using Compressed Air Control Systems To Improve Efficiency For Multiple Compressor Installations

  Piping Rules Of Thumb

  The “How To’s” of Compressed Air


 

Common Myths about Compressed Air

 

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.

  Common inefficiencies include:

  Lack of integrated system control of multiple compressors

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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Common Sense of Compressor Air System Maintenance

  Compressed air is probably one of the most universal operations within a plant environment. It transcends industries, operations and applications. Compressed air is used to power tools, move conveyers, transport products and make process applications possible. Considered a power source, compressed air systems are increasingly more reliable and predictable. Simply, it is considered the fourth utility.

  Therefore, similar to electricity, disruption of the compressed air supply can cause costly production delays. With more and more companies eliminating capital budgets to purchase back-up compressed air systems, compressor downtime for repair, adjustments and maintenance becomes a critical issue.

  In a recent industry study, 20 percent of calls logged into equipment manufacturers’ help desks could have been avoided by proper installation and or maintenance procedure, which is why so many people -- from compressor manufacturers to consultants -- take time out to preach about compressed air reliability and efficiency.

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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Compressor Placement

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.

  Location Selection: One important consideration when utilising a compressed air system within an operation is where to physically locate the unit. While there isn’t just one way to install a compressor you should be aware of all the advantages and disadvantages to each system. For most plant environments, compressed air systems are designed to fit in a centralized area, adjacent or near the actual applications in which it serves or, in certain scenarios, in an outdoor installation (see sidebar for guidelines for outdoor installations).

  Again, each scenario will offer different sets of advantages and problems. Regardless if your company chooses a centralized, decentralized or outdoor installation, you should consider the advantages and shortcomings and prepare for potential problems.

  For instance, if a compressor is located indoors in a centralised compressor room, the compressed air system is protected from the weather, allows for easy access for maintenance and maximizes plant floor space. However, centralized compressed air systems usually requires additional space to provide adequate ventilation and additional piping to reach the actual operation, which can increase the potential of system pressure drops.

  A decentralised compressed air layout allows for compressors to be located closest to the largest air users and reduces pressure drop through air lines. However, this configuration can also result in the highest probability of incorrect filtration as well as noise and heat complaints.

  Another issue surrounding compressor location is the ambient temperature of the area. Compressed air systems subjected to low temperatures may deal with slow starting, possible control line freeze problems, a condensate freeze problem and/or a possible fluid misapplication. To remedy these issues, maintenance personnel can specify heaters and heat tracing key elements to minimize the freezing or simply relocate the unit to a warmer area of the plant.

  On the other end of the spectrum, compressed air systems exposed to extremely high temperatures can experience unscheduled shutdowns, increased maintenance, and decreased lubricant life. These factors can be reduced by adjusting ventilation, utilizing a higher performance lubricant or again, relocate the compressor to a better location.

  Power Ratings: The quality of the incoming power from your utility company will greatly affect the reliability of the electrical components of your compressor.

  For obvious reasons, the power supply should be free of any phase variation and voltage droops. For this reason many manufacturers offer phase and voltage monitors on their air compressors in order to help extend the life of the motor and any other electric/electronic components.

  A simple rule to keep in mind when selecting a power source is matching voltages -- the voltage emitted by the power source and the voltage needed to run the compressed air system. The closer the voltages, the longer your motor will last. During relocation "voltage matching" can be solved by having the original motor rewound or simply buying a new one.

Ventilation: One of the leading complaints by plant workers and causes of unscheduled shutdowns is heat. Because compressed air systems generate such large amounts of heat, require extensive ventilation is required. Contrary to popular belief, ventilation is equally important for all compressors, regardless if it is water-cooled or air-cooled.

  When there is insufficient ventilation, heated air from the compressor exhaust remains around the unit and is then ingested by the compressor increasing the operating temperature of the unit. This will cause the unit temperature to spiral upward and eventually shutdown.

  It is important to plan for ventilation and access when deciding compressor placement. Plant designers need to be aware that they need to allow for three feet around the entire compressor package for maintenance and approximately 42-inches at the motor starter access panel. In addition, you need to avoid areas that are extremely humid or whose ambient temperatures exceed 115°F.

  In addition to the actual ventilation area around the compressor, it is helpful to duct the cooling air exhaust of a compressed air system to either an outdoor area or an energy recovery system. Regardless on how you decide to duct the exhaust heat, addressing this issue at installation time can help extend the life of your compressor. Specifically, by exhausting the heat, you can increase the life of your coolant, heat exchanger, bearings and hoses.

  Filtration

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 the compressor near a chemical process?

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:

Clean: A clean environment is defined as having low dust and debris. This type of environment does not require much more beyond what would be considered standard maintenance. A common problem, however, is that many people think that because they are situated in a "cleanroom" environment their compressed air system is safe from air quality issues. However, cleanroom environments often contain gases that are incompatible with the cooler lubricant. One solution to this problem may be to add additional ducting that will bring in ambient air from outside the facility.

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.

Evaluate Your Compressed Air Dryer Needs: Liquid water occurs naturally in air lines as a result of compression. Additional condensation occurs downstream as the compressed air continues to cool. Moisture in compressed air is responsible for costly problems in almost every application that relies on compressed air. Some common problems caused by moisture are rusting and scaling in pipelines, clogging of instruments, sticking of control valves, and freezing of outdoor compressed air lines. Any of the se could lead to downtime of your compressed air system.

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.

  Piping

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.

  Many times a compressor must be installed outside due to jobsite conditions or limited space within a manufacturing facility. When this occurs there are certain items that should be incorporated into the installation to help ensure trouble free operation, including:

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.

 

Pre-Comp Air Diagram

 

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.

  Once thought of as a powerhouse utility, with no thought to the cost of producing compressed air or on methods of optimising these costs, today's textile managers have realized that an efficient, reliable compressed air system is a necessity.

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.

The requirement of air jet technology for oil-free compressed air created separate compressed air systems in many plants. These plants utilize older, lubricated air compressors for plant air needs while using oil-free air compressors for their air jet needs. A large number of plants, however, have taken advantage of the higher efficiency of newer centrifugal, oil-free air compressors to provide air for both plant and air jet needs. For this reason the focus of this discussion will be on centrifugal compressors. It should be noted, however, that the ideas put forth can be carried over to other types of compressors.

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.

  TABLE 1

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

  Based on sharp-edged orifice continuously at 100 psig with air costs at 15 pence per 1000 cubic feet.

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.

  Standard Controls

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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Preventative Maintenance

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.

  Where To Start

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.

  Example: The same 100 hp air compressor operating at 125 PSIG may be capable of fully meeting system demand operating only at 110 PSIG. This additional 15 PSIG pressure translates into a potential power cost of £3,200 per year. In addition, the same 15 PSIG increases air consumption by 11% due to increased air flow at the higher pressure.

Category 2 - Local Control Scheme

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.

  Category 4 - Global Control Scheme

  In a Category 3 system, compressor control is typically centralized in the powerhouse. In a Category 4, compressor control now becomes part of the overall plant programmable logic controller (PLC) system.

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:

  Load sharing ability utilising "smart" system selection of on line equipment based on demand characteristics and equipment specifications (i.e., selection of a 200 CFM unit over an 800 CFM unit to provide air during a 150 CFM demand period).

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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  Piping Rules Of Thumb

  Use large enough carbon steel pipe so as not to exceed 3 PSIG pressure loss through the entire line. (The target is a maximum 10% pressure drop through the entire system, i.e. from air compressor to farthest drop.)

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.

  Install pressure gauges throughout the system for troubleshooting. Locations should include the receiver, headers, tools, production equipment and the end of plant piping system.

  Use long radius elbows. Try to use flow resistant fittings and valves. Use ball or butterfly valves.

  Slope main lines approximately 100mm per metre of pipe away from air compressor. Install drop legs for condensate removal.

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.

  Be sure to read and understand equipment instruction and installation manuals and discuss the layout and piping requirements before installation.

 

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The “How To’s” of Compressed Air

Compressor sizing,        Air treatment sizing,           Receiver sizing ,    Distribution piping,   
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.

  Air treatment sizing – Central heat exchangers, filters and dryers must be sized for compressor displacement at extreme ambient conditions. Most compressors are capable of running in a high ambient but little consideration is given to the air treatment components in this regard. If a 1,000 Acfm air cooled compressor operates in a 110° F ambient, the effect of ambient pressure, temperature and relative humidity rates the compressor at 900 Scfm with an inlet temperature to an air dryer of at least 120° F. This means the dryer must be capable of handling nearly 1,600 Scfm or dewpoint will not be maintained.

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