Here are the major types:. These air compressors are ideal for plants with large, steady air demands. They can easily handle high capacities and are actually better and more efficient in that range. Their simple, oil-free design also requires less maintenance, but their controls and monitoring systems are a little more intricate.
Because they have limited speed control, they offer limited air-flow flexibility. For facilities with multiple compressors, a centrifugal compressor is a good choice for the lead compressor. Centrifugal compressors are also an excellent choice for aerobic fermentation processes that require a constant volume of oil-free, sterile air. Rotary screw compressors are the compressors of choice for medium-to-large-capacity plants with unsteady air requirements. The compressor itself is fairly compact and provides smooth, pulse-free output.
Standard rotary screw compressors are not oil-free. Oil is injected into the screw to lubricate the screws and maintain an air seal. That oil mist then needs to be caught before it enters the plant, which presents an additional challenge. The air ends on these compressors are also not field serviceable, which complicates maintenance.
These are very common, but not typically used for plant air. Positive Displacement and Reciprocating compressors are both piston-type compressors. Most plant-wide air compressors fall into the industrial category and are designed to handle robust manufacturing processes.
Plant air is a relatively expensive utility, so proper system design is critical to ensuring efficiency. Here are some suggestions for getting those key design items right from the start:. The size of your system will be driven by the size of the plant and the breadth of the need. A properly managed supply side will result in clean, dry, stable air being delivered at the appropriate pressure in a dependable, cost effective manner.
Controls serve to adjust the amount of compressed air being produced to maintain constant system pressure and manage the interaction between system components. Air filters and air dryers remove moisture, oil and contaminants from the compressed air. Compressed air storage wet and dry receivers can also be used to improve system efficiency and stability. Accumulated water is manually or automatically discharged through drains.
Optional pressure controllers are used to maintain a constant pressure at an end use device. The Demand side includes distribution piping, secondary storage and end use equipment. A properly managed demand side minimizes pressure differentials, reduces wasted air from leakage and drainage and utilizes compressed air for appropriate applications.
Distribution piping systems transport compressed air from the air compressor to the end use point where it is required. Compressed air storage receivers on the demand side can also be used to improve system pressure stability. The highest degree of efficiency is generally obtained from water cooled installations where the compressor discharge cooling water can be connected directly to a continuous process heating requirement.
For example, the heating boilers- return circuit. Surplus energy can then be effectively utilized all year round. Most new compressors from the major suppliers can be adapted to be supplemented with standard equipment for recovery. Air-cooled, packaged, rotary screw compressors are very amenable to heat recovery for space heating or other hot air uses. As a rule, approximately 50, British thermal units Btu per hour of energy is available for each cfm of capacity at full-load. Recovery efficiencies of 80 to 90 percent are common.
Caution should be applied if the supply air for the compressor is not from outside and the recovered heat is used in another space, because this can cause a decrease in the static pressure in the cabinet or room and reduce the efficiency of the compressor.
If outside air is used, some return air may be required to avoid damaging the compressor with below freezing air. Because packaged compressors are typically enclosed in cabinets and already include heat exchangers and fans, the only system modifications needed are the addition of ducting and possibly another fan to handle the duct loading and to eliminate any back pressure on the compressor cooling fan.
These heat recovery systems can be modulated with a simple, thermostatically controlled hinged vent. When heating is not required—such as in the summer months—the hot air can be ducted outside the building. The vent can also be thermostatically regulated to provide a constant temperature for a heated area. Hot air can be used for space heating, industrial drying, preheating aspirated air for oil burners, or any other application requiring warm air.
Energy recovery from air cooled compressor installations will not always give heat when it is required and perhaps not in sufficient quantities. The quantity of recovered energy will vary if the compressor has a variable load. In order for recovery to be possible a corresponding energy requirement is needed, which is normally met through an ordinary system supply.
There should be good primary storage on the supply side, properly sized for the air compressors. Secondary storage may also be needed throughout the plant, as well as at some high-demand locations. Demand side: Piping distribution system. Consider the overall layout of the distribution system piping. Ensure that the piping size and arrangement match the compressor sizes on the supply side. Sometimes, a loop distribution pipeline makes more sense than a straight-line distribution pipe.
Specifying a 3-in. The main compressed air headers should be sloped to allow condensate to drain. The compressed air piping that is being fed to the end user should come out of the top of the header to further eliminate entrained moisture in the air.
The effectiveness of the condensate removal system should also be evaluated. The main compressed air headers should be slightly sloped, with a pitch of about one inch per ten feet of pipe to allow water and condensate to drain out.
Drains should be located at low points in the header. Feeder pipes to equipment should come out of the top of the supply header to help prevent moisture from reaching the point of use Figure 6. Demand side: Load profile.
Estimate the compressed air load profile, i. 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.
Demand side: Air treatment. Compressed air systems require an appropriate level of air treatment for proper operation of the end-use equipment. Air treatment includes filtration and drying. Different manufacturing processes require different levels of air treatment. Air quality levels should be measured at critical points in the system. Air that contains excessive moisture or that is not filtered properly can damage equipment and reduce system efficiency.
In some cases, only certain end-use equipment requires highly treated air, so you might need a system that provides different levels of treatment at different points in the system 3. Matching the compressed air system supply and demand can be a tricky problem.
Ideally, the air compressors would be controlled and optimized to run absolutely as little as possible, while at the same time providing high-quality air at the correct pressure to all demand-side end users. Air compressors are generally most efficient when they are running fully loaded.
Running multiple air compressors at partial loading is very inefficient. An ideal situation is to have one air compressor or several fully loaded and one additional air compressor with a variable-frequency drive VFD to handle the variable load on the demand side. Information on the demand load profile is very valuable for setting up the air compressor control system and determining the feasibility of using a VFD. The existing control system should be evaluated to determine whether it is appropriate for the system demand profile.
Estimate possible performance gains that could be achieved by operating the system in a different mode or using an alternative control strategy 3. For example, check to see if your control system could be optimized to operate in one mode on first and second shift and in another mode on third shift and the weekends. Your compressed air control system should be based on your operational schedule and your compressed air demand profile. Maintenance is key to sustaining successful long-term operation and maximizing equipment life.
Just as you change the oil filter in your car at scheduled intervals to ensure optimum performance, be sure to change the filters in your air compressor and air system regularly to ensure air quality and to prevent excessive pressure drops 2. Condensate drains. Are your condensate drains stuck open? If so, you could be wasting compressed air.
Go one step further and replace timer drains with zero-loss drains to save more air 2. Timer drains use a solenoid valve that is opened by a timer for a few seconds every hour or so to bleed out moisture. This type of drain opens directly to the compressed air lines and air is lost during each cycle. Zero-loss condensate drains operate similarly, but do not release any air during each drain cycle — they only release the collected moisture and are therefore much more economical.
For both types, if something prevents the solenoid valve from reseating, or if the open signal stays engaged, a significant amount of air can be lost. Both types should be checked frequently. Cooling water quality. Various cooling water sources are found in industrial plants.
Where city water is used, the quality of the water is generally good, but in some areas, treatment for hardness may be needed to avoid fouling of cooling surfaces. Water analysis should be conducted to determine any treatment needs 3. It is a source of pressure drop, and if not maintained properly, pressure drop will increase, requiring more energy from the air compressor motor.
Heat exchangers and coolers. All coolers and heat exchangers on the supply side should be cleaned periodically.
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