Abstract
This article will provide you with an complete action plan to optimize your compressed air system including compressor optimization, demand management, density management, and storage in a variety of different applications.
Compressed air represents one of the most critical utilities in most production and process environments. The efficiency of a compressed air system is 100% energy in and, when perfect, produces 11% useful work out. Understanding this, it will cost more to operate a compressor in the first year than it costs to buy and install. Despite this harsh information, power is thrown at symptoms of undefined problems every day. The opportunities of reducing operating cost and energy in air systems is typically more than 50%. This session will carve out a plan of attack to optimize the supply and demand systemically and yield the lowest demand at the highest rate of standard cubic feet per kilowatt of energy.
There are a number of essential actions that need to be taken to optimize the compressed air systems. You need to minimize demand, control the expansion of the air, distribute it while minimizing energy loss, store potential energy, and compress the air efficiently. Other than operating the compressors, as efficiently as possible, everything else seems to elude most everyone. This work cannot be done theoretically on each piece of equipment only in the compressor room. It must be done systemically. More efficient compressors make more air with the same amount of power. They cost more and can be an important part of a well-operated system. On the other hand, if you throw a more efficient compressor at a highly inefficient system, you will waste more air at the same operating cost and save nothing.
CONTROLLING DEMAND IN THE SYSTEM
1. Control the expansion of the compressed air to the point of use. You must control 100% of users with regulation, which is adjusted lower than the lowest supply pressure. If it is not possible to achieve this with operator discipline, then you must use a demand controller or expander at a central location adjusted in the same manner.
2. Reduce the pressure differentials on installation components such as filters, regulators, lubricators, tube, hose, and disconnects on the demand side of the system. The intent is to operate demand at the lowest possible supply pressure on critical high-pressure applications.
3. Flat line the high rate of flow, intermittent applications with dedicated storage and metered recovery. This is much like a battery charger or water tower application. This can also be a pressure driver for the operating protocol. You will slightly increase the base usage and eliminate peaks.
4. Review and add as necessary general and control storage to slow the rate of change in the system. This will allow you to maintain a higher point of use pressure if necessary without increasing the supply pressure. If there is any diligence used, you can normally reduce the supply pressure simultaneously.
5. Upgrade the quality of information to track progress and improve decision making. This should include a flow meter and demand pressure monitor at the discharge of the demand controller or the expander. If you do not use a demand controller, recognize that demand is only accurately displayed when the demand exceeds the supply. This is referred to as a negative rate of change. When supply exceeds demand, which is a positive rate of change, you are measuring supply response to demand. The system will take whatever supply power you throw at it. A 450 scfm negative rate of change will recover to the original pressure in 1 min, if we respond with a 200 hp compressor. If we throw a 400 hp compressor at the event, it will recover in 15 s at a more rapid rise in pressure. The inefficiency is the part load energy of the larger compressor for the balance of the 45 s. If we match the event with a 100 hp compressor, the pressure will hold at the load pressure of the compressor until the event stops, at which time, the pressure will recover at the same rate of rise as the initial rate of decay.
6. Review and add as necessary general and control storage to slow the rate of change in the system. This will allow you to maintain a higher point of use pressure, if necessary, without increasing the supply pressure. If there is any diligence used, you can normally reduce the supply pressure simultaneously.
REDUCING DEMAND IN THE SYSTEM
1. Develop a leak benchmarking program on a gradual reduction of the tolerance volume. Select a level at a known low load, and repair your way to that level. Every several weeks, check the low load and scan the system using an ultra sonic leak detector. Find and repair the largest leaks found to bring the system back into benchmark. When you are comfortable with this level, lower the level and begin again. You will reach a point where there are so many small leaks to fix during the benchmarking period, the labor hours cannot be justified. At this point return to the previous higher tolerance value.
Record the types and nature of the leaks that you are fixing, so that you can leverage this information into buying more leak resistant components and improving best practices installations. Note that it is important that the reduction of demand does not cause the demand pressure to rise. If it does, then other unregulated users will increase at the elevated pressure. That is why it is so important to have demand controls installed before you become aggressive in demand reduction. It is also important to off load a linear amount of supply energy for the demand reductions.
2. Eliminate all open compressed air blowing applications and replace with low pressure centrifugal or positive displacement blowers, if at all possible. If it is not possible to use blowers, apply specialty air volume reducing nozzles for the application. Take your time with these applications developing the thrust per square inch as close as possible to the open blowing application. You will also need to filter the air for specialty nozzles, as they will easily plug up with pipe debris. Whenever possible, use a solenoid valve to shut of the air on cyclical applications.
3. Replace all applications, which are poor users of compressed air. Focus on operating cost alternatives. Use electricity whenever possible for its better wire to work energy relationship.
4. Reduce the size of demand events as seen by the system including high ramp applications. This can be accomplished by slowing down the introduction of these events into the system. This can be done by opening the demand valve slower manually or automatically. This reduces the “ramp in” rate of flow, so that the supply including control storage can match the event limiting the ultimate pressure drop, which would result.
5. Regulate all points of use, even if you have installed a demand controller or expander in the main supply system’s piping. Make sure that the set points on the regulators are equal to the minimum supply pressure minus the point of use filter and regulator pressure drop or less. If you allow for a 2-3 psig margin below this value, small leaks and filter dirt loading will not cause frequent changes in process performance.
6. Limit the coincidence of events that cause peak demands in the system. This includes minimizing the blow duration on timer drains and adjusting intervals seasonally for relative humidity. Move large events to low load times where possible.
7. Shut off all air using equipment when not in use. Make sure that the shut off valves are ergonomically installed, so that operators can easily reach them. If this does not work, install solenoid shut off valves that are tied into the electrical shut off on the machine, work station, or process.
STORE POTENTIAL ENERGY TO SUPPORT TRANSIENT EVENTS INCLUDING A COMPRESSOR FAILURE IN THE SUPPLY SYSTEM
1. Convert enough kinetic energy to potential energy so that you can handle largest event without turning on another compressor during normal operation. If you do this, you will also handle all of the smaller transient events that are not controlled from downstream. This can include the coincidental impact of a third to first shift startup. Remember that storage is a function of the capacity to store air times the useful differential across it. If you are operating constant pressure compressor controls and they operate correctly, no amount of capacitance will generate any useful storage.
2. Store enough air on the supply side of the system to manage a desired pressure drop, while bringing up a backup compressor to replace a failed one. The intent would be that the event will have no impact on the process or production serviced by the system. The intent is to operate only the supply that is required at any time with everything else off.
Example: largest compressor= 1600 scfm, maximum allowable pressure drop from the load pressure on the back up compressor = 10 psid, permissive time to load the compressor from a cold start signal to full load= 15 s, atmospheric pressure = 14.3 psia, gallons per standard cubic feet = 7.48 gal
1600 X (15/60) X (14.3 X /10) X 7.48
= 4278.6 gal
3. Create enough storage to control the maximum load cycles per time period on any trim compressor. It is safe to say that 3 min load-unload cycles or longer would be desirable on any positive displacement compressor. This can get trickier on large dynamic compressors, but it is not impossible.
4. If the size of any event or compressor is too large to handle with control storage or you want to protect the system and production against an electrical outage, single phase, or brown out, offline high-pressure peak shaving would be the most desirable approach to minimize on board power. It would not be unusual to store 30-40,000 ft1 of air in a 100 psig differential supported by a 20 hp compressor offline. You would then introduce the air back into the system on variety of different cues or logic patterns to support the various events.
Note that it is the intent of all potential energy applications to either prevent the normal operation of an additional compressor, extend the mechanical life of a compressor or compressors, or both. Well applied storage will increase the base load in the system slightly, and eliminate the requirement for added compressors during peak plus the inefficient part load in between peaks.
DISTRIBUTE THE COMPRESSED AIR, WHILE MINIMIZING ENERGY LOSSES
1. The concept of design or redesign should be to minimize the highest amount of air mass or volume of air and the distance that the air must flow to support any part of the system from supply to demand.
2. Resistance to flow is necessary in the system.Without resistance to flow there is no flow. As the system is open on both ends of the system all of the time to a larger or lesser degree, resistance to flow and storage keeps it functioning. Mass flow restrictions are differential pressures in the system, which change as a square function of flow change. It is important to design or retrofit your system for a maximum differential at highest flow, highest temperature, and lowest inlet pressure. This will produce the highest differential pressure across the components being evaluated. Although we are recommending a conservative approach towards this process, the piping distribution system should not be made intentionally oversized or all the same size for convenience. Oversized piping will not provide economical storage and will make it difficult for supply to see demand efficiently. A reasonable differential pressure would be 1-2 psid from the discharge of the cleanup equipment at the supply or the discharge of the demand controller, as it applies to your system, to the farthest point in the demand system at the previously discussed design conditions.
3. In most systems that have distribution problems, you should minimize waste and flat line transient users with dedicated storage and metered recovery at the point of use before considering making changes in the piping distribution system.
As little as a 10%-20% demand reduction at the peak condition can be sufficient to eliminate the most distribution losses and the requirement for piping retrofits.
REDUCE SUPPLY ENERGY WHEREVER POSSIBLE
1. When 100% of demand is at a lower pressure than the lowest supply pressure, set up the supply pressure to optimize the pound per kilowatt of compressed air energy for the on board compressors. Operate all compressors that need to be on flat out and optimized except one compressor trimming and all other compressors off regardless of inlet conditions or relative demand load. You must optimize the compressor and the motor simultaneously. Optimal means the most pounds or standard cubic feet at the optimal density (pressure and temperature), while managing the highest power factor and motor efficiency simultaneously. In this scenario, the trim compressor is the only compromise to “optimal” assuming you can maintain a range of supply pressure across the range of load conditions that relates to optimal on the base load compressors. Another option is to trim with variable frequency drive compressors using storage continuously, while adding and subtracting base load compressors. The Variable frequency drive (VFD) compressor or compressors will displace or fill in the removal or addition of a base. In this case, you will optimize both the base load compressors and the trim compressors at the same time.
Note that this is called a “Bellows Effect” operating protocol.
2. Base compressors should always be selected based on the best energy efficiency. Trim compressors should be selected first on operating speed to cold or hot start and shut off capabilities, and secondly on their flexibility for automation interface. If you are trimming with VFD/s, the same requirements are applicable. This typically translates into smaller, less permissive compressors. You must be certain that the total trim capacity (one, two, or three trim compressors) is equal to or larger in capacity than the largest base compressor in the supply arrangement. This will assure that there are no gaping holes in supply, so that you can make smooth transitions from one power level to the next. Supply systems that do not have this capability end up running too much power part loaded all of the time to support the transitions. Remember that bigger is more expensive.
3. Develop an operating profile for the supply system, which optimizes the compressors based on a full range of usage and conditions. In most systems, the only time the system is remotely efficient is during peak load. It generally goes down hill during lighter or low load. Also evaluate the full range of system’s usage against the full range of ambient inlet and cooling conditions to determine how the system will work before you make any final plans on equipment selection. Make every attempt to manage peaks with potential energy instead of on line power. You must also evaluate the risk of a unit failure in order to have a solid curtailment plan. If brown outs or black outs are common, you must include this in your plan.
4. Unload all unnecessary ancillary power, such as dryers, pumps, fans, etc. through the use of more efficient controls and motor drivers. Size all filtration and dryer equipment for a total differential of 3.5-5 psid. The differential should be at the highest inlet flow, highest inlet temperature, and the lowest inlet pressure. The differential on the filtration should be in a wet and clean condition. Plan the additional differential from dirt loading when selecting the compression equipment, so that you do not overload the motor drives as you will absorb the added differential at the air end discharge. We would recommend no more than an additional 1.5-2 psid on the total filters. There are filters available to accomplish this with a change every 5-6 years at this dirt loading rate. The total differential across all cleanup equipment should not influence the total connected horsepower on the compressors by more than 4% at the worst case maintenance condition.
5. Use a master signal for the compressors located in the dry clean storage downstream of the contaminant control equipment. If the signals are in the compressors upstream of the cleanup equipment, the compressors controls will respond to the demand interpreted through the differential pressure, which changes as a square function of flow change. This causes the compressors to over shot and under shot, which results in hunting. This requires excess energy to compensate.
Please note in the illustration that we have installed a three-way valve so that you can return to local control signals when you wish to isolate the compressor from the system. It is also important to note that the adjustment of the compressor controls, with a master control signal, should be based on controlling downstream of the cleanup. If the pressure across the cleanup equipment is 10 psid, when you moved the signal, you would also want to reduce the control set points on the compressor/s an additional 10 psid. This is because you will absorb the differential when you move the signal and without adjusting the operating set points for the compressor/s, you may overload the motor.
6. Develop an operating profile which takes control storage, set points of the compressors, signal locations, and differentials into account. Put it down on paper prior to implementing it and check the range of conditions to make sure it will work. Do not put fudge factors into the profile. This is not an art form. It is a science. If you are not sure of what you are doing, contact a technology firm who can assist you. Literally, 95% of all compressor profiles are not set up correctly. Most engineering firms that design systems select the equipment and never think through the operating protocol or profile prior to installation.
7. Finally, you must get the system to operate effectively and efficiently before you automate it. More than 90% of the time, users try to apply automation to a system to get it to work properly. If you automate a system that does not work, you will have an automated mess. You must be able to get it to work correctly on the local controls first. When and if you automate, keep in mind that their purpose is to refine the operating cost and reliability issues across all conditions unattended. Automate the operation based on at least rate of change, storage, time, and pressure. You may even wish to add a selective rate of change protocol, which chooses the correct compressor for the situation. Take your time and test your concept prior to making the decision by preparing algorithms including transitions of power and demand including failure scenarios. Keep in mind that you do not have to match the event in the system. You only need to slow it down so you can wait longer. The essence of a masterfully designed system is the ability to control demand by matching transient events as quickly as possible with an expander or demand controller serviced with potential energy.
Once this is accomplished, the compressors’ control job is managing control storage by replenishing it as slowly as possible. The longer you can take, the less energy you will use.
SUMMARY
A compressed air system is a highly interactive configuration with all aspects affecting all other aspects. Developing an action plan to improve the efficiency and reduce the operating cost can be rewarding, but must be done in the correct order to enjoy the success and avoid production inconvenience. It is a process of black and white with a lot of gray in between. Far too many owners want to buy a solution, rather than apply one. Problem definition, metrology, and carefully planning are all essential. When you have completed the action plan, do not forget to measure the results. Validation is necessary to support you return on investment strategy.
