A compressed air system audit (see below) should be conducted to highlight the true costs of compressed air and identify simple opportunities to improve efficiency and productivity. Then actions should be taken in the areas identified in the audit for efficiency and productivity improvements. Generally, actions in the following areas are the most important:
Compressed air system users should consider using an independent auditor to examine their compressed air system. A local energy utility may even help finance such audits. Several firms exist that specialize in compressed air system audits. Audits are also performed by energy utilities, equipment distributors and manufacturers, energy service companies, and engineering firms. An informed consumer should be aware that the quality and comprehensiveness of audits can vary. Independent auditors should provide recommendations that are systems-neutral and commercially impartial. Independent auditors should neither specify nor recommend any particular manufacturer's products.
A comprehensive compressed air system audit should include an examination of both air supply and usage and the interaction between supply and demand. Auditors typically measure the output of a compressed air system, calculate energy consumption in kilowatt-hours, and determine the annual cost of operating the system. The auditor may also measure total air losses due to leaks, and locate those that are significant. All components of the compressed air system are inspected individually and problem areas are identified. Losses and poor performance due to system leaks, inappropriate uses, demand events, poor system design, system misuse, and total system dynamics are calculated, and a written report with a recommended course of action is provided. Important aspects of a basic compressed air system audit are discussed below.
System issues go beyond examining the performance of an individual compressed air system component and, instead, examine how components on both the supply and demand side of the system interact. Auditors typically address a number of systems issues. These are discussed below, and many are addressed in more detail in other Compressed Air Systems Fact Sheets.
Level of Air Treatment
The auditor typically examines the compressed air applications and determines the appropriate level of air treatment required for proper operation of the equipment. Actual air quality levels are then measured. If the air is not being treated enough, alternative treatment strategies are recommended. If the air is being over-treated (an indication of energy being wasted), recommendations are made to modify the system. In some cases, only certain end-use equipment requires highly treated air, and the auditor may recommend a system that allows for different treatment levels at different points in the system.
The auditor should identify and quantify leaks in the system and recommend a leak management program.
An auditor also typically determines the lowest possible pressure level required to operate production equipment effectively. In many cases, system pressure can be lowered, thereby saving energy. Most systems have one or more critical applications that determine the minimum acceptable pressure in the system. In some cases, the application of dedicated storage or differential reduction on the critical applications will allow a reduction in overall system pressure.
The existing control system is evaluated to determine if it is appropriate for the system demand profile. Performance gains available from operating the system in a different mode or using an alternative control strategy should be estimated.
Auditors will identify potential applications for the use of heat recovery, if it is not already being used.
Demand Side Issues
The demand side of the system refers to how compressed air is actually used in the plant.
The overall layout of the distribution system (piping) is examined. Pressure drop and efficiency are measured or estimated, and the effectiveness of the condensate removal system is evaluated. Simple changes that can enhance system performance are suggested if appropriate.
Auditors typically estimate the compressed air load profile in terms of how the demand in cubic feet per minute (cfm) changes over time. A facility with a varying load profile will likely benefit from advanced control strategies. A facility with short periods of heavy demand may benefit from implementing storage options. To establish the load profile, an auditor will measure system flow and pressure across the system under different demand conditions, while observing the loading effect on the existing compressors. This may require a number of measurements over a 24-hour period (or even several days) if demand varies significantly over time. Auditors may use data logging equipment to obtain both demand and power consumption profiles
The equipment and processes that use compressed air will also be examined. In some cases, recommendations such as specifying equipment that operates at a lower pressure will be made. An auditor may also recommend replacing existing compressed air-powered equipment with equipment that uses a source of energy other than compressed air (see the Fact Sheet titled Inappropriate Uses of Compressed Air). Critical pressure applications are examined in detail. Local storage and other modifications may be recommended.
Supply Side Issues
The supply side refers to how the compressed air is generated and treated.
The compressors are evaluated in terms of appropriateness for the application and general appearance and condition. Compressor efficiency is usually estimated based on manufacturer-supplied data, corrected to site conditions. The installation is also evaluated in terms of its location, connection to cooling water, and ventilation. A general appraisal and any recommendations for alternative systems are also made.
Filters are examined for cleanliness and suitability for the application. Pressure drop across the filters is measured to estimate energy losses from the filter. A maintenance schedule for changing the filters, and possibly higher performance filters, may be recommended.
Aftercooler and separator efficiency, cooling effectiveness, and condensate separation effectiveness are all measured and evaluated, and feasible modifications or alternative systems are recommended.
Dryer appropriateness is assessed based on the facility's end-use applications. Dryer size, pressure drop, and efficiency are measured and evaluated. Modifications and replacements are recommended if needed.
Location, application, and effectiveness of both supply-side and demand-side drains are evaluated and alternatives recommended if necessary.
The effectiveness of the receiver tank is evaluated in terms of location and size, and the receiver drain trap is examined to see if it is operating properly. Storage solutions to control demand events should also be investigated.
More Comprehensive Evaluations
System audits are designed to identify system inefficiencies. If a system is found to be poorly designed, in unsatisfactory operating condition, or in need of substantial retrofit, a more detailed analysis of the system may be recommended. A comprehensive evaluation may also include extensive measurements and analysis of supply and demand interactions. Some auditors will also prepare a detailed systems flow diagram. A financial evaluation may also be performed, including current and proposed costs after retrofits are taken.
Pressure drop is a term used to characterize the reduction in air pressure from the compressor discharge to the actual point of use. Pressure drop occurs as the compressed air travels through the treatment and distribution system. A properly designed system should have a pressure loss of much less than 10% of the compressor's discharge pressure, measured from the receiver tank output to the point of use.
Excessive pressure drop will result in poor system performance and excessive energy consumption. Flow restrictions of any type in a system require higher operating pressures than are needed, resulting in higher energy consumption. Minimizing differentials in all parts of the system is an important part of efficient operation. Pressure drop upstream of the compressor signal requires higher compression pressures to achieve the control settings on the compressor. The most typical problem areas include the aftercooler, lubricant separators, and check valves. This particular pressure rise resulting from resistance to flow can involve increasing the drive energy on the compressor by 1% of the connected power for each 2 psi of differential.
An air compressor capacity control pressure signal normally is located at the discharge of the compressor package. When the signal location is moved downstream of the compressed air dryers and filters, to achieve a common signal for all compressors, some dangers must be recognized and precautionary measures taken. The control range pressure setting must be reduced to allow for actual and potentially increasing pressure drop across the dryers and filters. Provision also must be made to prevent exceeding the maximum allowable discharge pressure and drive motor amps of each compressor in the system.
Pressure drop in the distribution system and in hoses and flexible connections at points of use results in lower operating pressure at the points of use. If the point of use operating pressure has to be increased, try reducing the pressure drops in the system before adding capacity or increasing the system pressure. Increasing the compressor discharge pressure or adding compressor capacity results in significant increases in energy consumption.
Elevating system pressure increases unregulated uses such as leaks, open blowing and production applications without regulators or with wide open regulators. The added demand at elevated pressure is termed "artificial demand", and substantially increases energy consumption. Instead of increasing the compressor discharge pressure or adding additional compressor capacity, alternative solutions should be sought, such as reduced pressure drop, strategic compressed air storage, and demand/intermediate controls. Equipment should be specified and operated at the lowest efficient operating pressure.
What Causes Pressure Drop?
Any type of obstruction, restriction or roughness in the system will cause resistance to air flow and cause pressure drop. In the distribution system, the highest pressure drops usually are found at the points of use, including in undersized or leaking hoses, tubes, disconnects, filters, regulators and lubricators (FRLs). On the supply side of the system, air/lubricant separators, aftercoolers, moisture separators, dryers and filters are the main items causing significant pressure drops.
The maximum pressure drop from the supply side to the points of use will occur when the compressed air flow rate and temperature are highest. System components should be selected based upon these conditions and the manufacturer of each component should be requested to supply pressure drop information under these conditions. When selecting filters, remember that they will get dirty. Dirt loading characteristics are also an important selection criteria. Large end-users that purchase substantial quantities of components should work with their suppliers to ensure that products meet the desired specifications for differential pressure and other characteristics.
The distribution piping system often is diagnosed as having a high pressure drop because a point of use pressure regulator cannot sustain the required downstream pressure. If such a regulator is set at 85 psig and the regulator and/or the upstream filter has a pressure drop of 20 psi, the system upstream of the filter and regulator would have to maintain at least 105 psig. The 20 psi pressure drop may be blamed on the system piping rather than on the components at fault. The correct diagnosis requires pressure measurements at different points in the system to identify the component(s) causing the high pressure drop. In this case, the filter/regulator size needs to be increased, not the piping.
Minimizing Pressure Drop
Minimizing pressure drop requires a systems approach in design and maintenance of the system. Air treatment components, such as aftercoolers, moisture separators, dryers, and filters, should be selected with the lowest possible pressure drop at specified maximum operating conditions. When installed, the recommended maintenance procedures should be followed and documented. Additional ways to minimize pressure drop are as follows:
Controlling System Pressure
Many plant air compressors operate with a full load discharge pressure of 100 psig and an unload discharge pressure of 110 psig or higher. Many types of machinery and tools can operate efficiently with an air supply at the point of use of 80 psig or lower. If the air compressor discharge pressure can be reduced, significant savings can be achieved. Check with the compressor manufacturer for performance specifications at different discharge pressures.
Demand controls require sufficient pressure drop from the primary air receiver into which the compressor discharges, but the plant header pressure can be controlled to a much narrower pressure range, shielding the compressor from severe load swings. Reducing and controlling the system pressure downstream of the primary receiver can result in a reduction in energy consumption of up to 10% or more, even though the compressors discharge pressure has not been changed.
Reducing system pressure also can have a cascading effect in improving overall system performance, reducing leakage rates, and helping with capacity and other problems. Reduced pressure also reduces stress on components and operating equipment. However, a reduced system operating pressure may require modifications to other components, including pressure regulators, filters, and the size and location of compressed air storage.
Lowering average system pressure requires caution since large changes in demand can cause the pressure at points of use to fall below minimum requirements, which can cause equipment to function improperly. These problems can be avoided with careful matching of system components, controls, and compressed air storage capacity and location.
For applications using significant amounts of compressed air, it is recommended that equipment be specified to operate at lower pressure levels. The added cost of components, such as larger air cylinders, usually will be recouped quickly from resulting energy savings. Production engineers often specify end-use equipment to operate at an average system pressure. This results in higher system operating costs. Firstly, the point of use installation components such as hoses, pressure regulators, and filters will be installed between the system pressure and the end-use equipment pressure. Secondly, filters will get dirty and leaks will occur. Both result in lower end-use pressure. This should be anticipated in specifying the available end-use pressure.
If an individual application requires a higher pressure, instead of raising the operating pressure of the whole system it may be best to replace or modify this application. It may be possible to have a cylinder bore increased, gear ratios may be changed, mechanical advantage improved, or a larger air motor may be used. The cost of the improvements probably will be insignificant compared with the energy reduction achieved from operating the system at the lower pressure.
It is also important to check if manufacturers are including pressure drops in filters, pressure regulators, and hoses in their pressure requirements for end-use equipment, or if the pressure requirements as stated are for after those components. A typical pressure differential for a filter, pressure regulator, and hose is 7 psi, but it could be much higher in poorly designed and maintained systems.
When demand pressure has been successfully reduced and controlled, attention then should be turned to the compressor control set points to obtain more efficient operation, and also to possible unloading or shutting off a compressor to further reduce energy consumption.
Leaks can be a significant source of wasted energy in an industrial compressed air system, sometimes wasting 20-30% of a compressor's output. A typical plant that has not been well maintained will likely have a leak rate equal to 20% of total compressed air production capacity. On the other hand, proactive leak detection and repair can reduce leaks to less than 10% of compressor output.
In addition to being a source of wasted energy, leaks can also contribute to other operating losses. Leaks cause a drop in system pressure, which can make air tools function less efficiently, adversely affecting production. In addition, by forcing the equipment to cycle more frequently, leaks shorten the life of almost all system equipment (including the compressor package itself). Increased running time can also lead to additional maintenance requirements and increased unscheduled downtime. Finally, leaks can lead to adding unnecessary compressor capacity.
While leakage can come from any part of the system, the most common problem areas are:
Estimating Amount of Leakage
For compressors that use start/stop controls, there is an easy way to estimate the amount of leakage in the system. This method involves starting the compressor when there are no demands on the system (when all the air-operated end-use equipment is turned off). A number of measurements are taken to determine the average time it takes to load and unload the compressor. The compressor will load and unload because the air leaks will cause the compressor to cycle on and off as the pressure drops from air escaping through the leaks.
Total leakage (percentage) can be calculated as follows:
Leakage (%) = [(T x 100)/(T+t)]
where: T= on-load time (minutes)
t = off-load time (minutes)
Leakage will be expressed in terms of the percentage of compressor capacity lost. The percentage lost to leakage should be less than 10% in a well-maintained system. Poorly maintained systems can have losses as high as 20-30% of air capacity and power. Leakage can be estimated in systems with other control strategies if there is a pressure gauge downstream of the receiver. This method requires an estimate of total system volume, including any downstream secondary air receivers, air mains, and piping (V, in cubic feet). The system is then started and brought to the normal operating pressure (P1). Measurements should then be taken of the time (T) it takes for the system to drop to a lower pressure (P2), which should be a point equal to about one-half the operating pressure.
Leakage can be calculated as follows:
Leakage (cfm free air ) =
(V x (P1-P2)/T x 14.7) x 1.25
where: V is in cubic feet
P1 and P2 are in psig
T is in minutes
The 1.25 multiplier corrects leakage to normal system pressure, allowing for reduced leakage with falling system pressure. Again, leakage of greater than 10% indicates that the system can likely be improved. These tests should be carried out quarterly as part of a regular leak detection and repair program.
Since air leaks are almost impossible to see, other methods must be used to locate them. The best way to detect leaks is to use an ultrasonic acoustic detector, which can recognize the high frequency hissing sounds associated with air leaks. These portable units consist of directional microphones, amplifiers, and audio filters, and usually have either visual indicators or earphones to detect leaks. A simpler method is to apply soapy water with a paint brush to suspect areas. Although reliable, this method can be time consuming.
How to Fix Leaks
Leaks occur most often at joints and connections. Stopping leaks can be as simple as tightening a connection or as complex as replacing faulty equipment such as couplings, fittings, pipe sections, hoses, joints, drains, and traps. In many cases leaks are caused by bad or improperly applied thread sealant. Select high quality fittings, disconnects, hose, tubing, and install them properly with appropriate thread sealant. Non-operating equipment can be an additional source of leaks. Equipment no longer in use should be isolated with a valve in the distribution system.
Another way to reduce leaks is to lower the demand air pressure of the system. The lower the pressure differential across an orifice or leak, the lower the rate of flow, so reduced system pressure will result in reduced leakage rates. Stabilizing the system header pressure at its lowest practical range will minimize the leakage rate for the system. For more information on lowering system pressure, see the section on Pressure Drop and Controlling System Pressure. Once leaks have been repaired, the compressor control system should be re-evaluated to realize the total savings potential.
A Leak Prevention Program
A good leak prevention program should include the following components: identification (including tagging), tracking, repair, verification, and employee involvement. All facilities with compressed air systems should establish an aggressive leak prevention program.