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Types of Compressed Air Systems

There are two main types of compressed air systems: oil-free and lubricated. One design will be chosen over the other, depending on purification and industry requirements. Air purification requirements include general purity, instrument quality, breathing air, medical air, pharmaceutical, and clean dry air.

Oil-Free Systems: Applications that cannot tolerate a lubricant require an oil-free system. It is critical to remove unwanted oil aerosols and vapors from compressed air, not just moisture. These aerosols and vapors are found in ambient air and can be generated by the compressor. This oil ends up degraded and oxidized by the heat of compression. Once heated, the oil can carbonize and form a solid, varnish-like substance on downstream equipment, causing valves and air tools to malfunction. If the oil is mixed with water, it forms a sludge that can gum up components of the air line. Downstream from the compressor, an air receiver stabilizes system pressure, serves as a demand reservoir, and holds some moisture. Downstream from the receiver, an air dryer, which will provide the correct pressure dew point, traps the remaining moisture. If either of these fail, there is still a coalescing filter after the dryer to provide protection. A dry receiver can also be installed after the coalescing filter to stabilize pressure and serve as a reservoir for times of high demand.

Lubricated Systems: These types of systems use a lubricant to alleviate friction between moving parts. In rotary screw compressors, the lubricant also seals clearances and removes heat of compression. The viscosity of the lubricant used depends largely on the operating ambient temperature range. It must offer adequate lubrication for bearings and rotors at operating temperature. In addition, it must have a pour point low enough to provide fluidity at low starting temperature. A modern, lubricated rotary screw compressor and high-efficiency purification system can produce compressed air with very high purity. These systems are very similar to the oil-free system, consisting of a wet receiver, an air dryer, and a coalescing filter. There is, however, a charcoal filter between the coalescing filter and the dry receiver that removes any leftover oil vapors.

 Compressed Air System Design and Configuration

There are eight basic elements that must be considered in designing your compressed air system: demand, compressed air quality, supply, storage, distribution, installation, maintenance, and condensate management

Demand
One of the most important and most difficult things you can do when designing your compressed air system is to determine the true demand in your system. Air demand will fluctuate beyond the predetermined average demand. If the actual demand is known, storage and distribution systems can be designed to meet demand without the installation of additional compressors.

The most precise way to determine demand in the system is to monitor the air flow using a flow meter, which would normally be positioned in the main headers. For small, simple systems, the ratio between loaded and unloaded compressor running time can be indicative of average demand over a long periods of time.

Often, leakage and artificial demand represent a substantial portion of the overall demand. There are various methods to stop leaks. Excess volume of compressed air created for unregulated users is called artificial demand. It occurs when greater line pressure than necessary was supplied. It includes the following:

  • all unregulated consumption, including appropriate and inappropriate production usage
  • open blowing
  • leaks
  • point of use with regulators adjusted to their maximum setting
  • tooling

These applications track the supply pressure as though no regulators were being used. The artificial demand challenge can be resolved by positioning a regulator at the point of use or at the beginning of the distribution network. Operating pressure requirements, compressed air requirements, and the duty cycle of individual equipment must all be considered when establishing demand for your system.

Air quality: Different applications demand different levels of compressed air quality. With each level, the cost to produce the compressed air increases. Therefore, it is essential to meet, but not exceed the level required by your particular application. If different levels are required for different applications within the plant, it is more cost-effective to treat smaller amounts of compressed air for the application with the highest level of quality requirements, than to treat the whole air supply.

Levels of Compressed Air Quality
Level Application Air Treatment Components Function
1 Shop Air Filtered Centrifugal Separator Removes solids 3 microns & larger, 99% of water droplets, & 40% of oil aerosols
2 Air Tools, Sand Blasting, Pneumatic Control Systems(indoor) Refrigerated Compressed Air Dryer, Air Line Filter Removes moisture producing a 35° to 50°F (-1.67° to 10°C) pressure dew point, removes 70% of oil aerosols, and all particles 1 micron and larger
3 Instrument Air, Paint Spraying, Powder Coating, Packing Machines Refrigerated Compressed Air Dryer, Oil Removal Filter Removes moisture & produces a 35° to 50°F (-1.67° to 10°C) pressure dew point, removes 99.999% of oil aerosols, and all particles .025 microns and larger
4 Food Industry, Dairy Industry, Laboratories Refrigerated Compressed Air Dryer, Oil Removal Filter, & Oil Vapor Adsorber Removes moisture & produces a 35° to 50°F (-1.67° to 10°C) pressure dew point, removes 99.999% of oil aerosols, all particles .025 microns and larger, oily vapor, oily smell, & oily taste
5 Outdoor Pipelines, Pneumatic Transport of Hygroscopic Material, Breweries, Chemical & Pharmaceutical Industry, Electronics Industry Air Line Filter, Oil Removal Filter, Low Dew Point Desiccant Dryer, Air Line Filter Removes moisture producing a -40° to -150°F (-40° to -101.11°C) pressure dew point, removes 99.999% of oil aerosols, and all particles .025 microns and larger
6 Breathing Air Breathing Air System (Continuous or Portable) Removes harmful compressed air contaminants and will produce Grade D breathing air Figure CAS1-1: Levels of Compressed Air Quality

Supply
The compressed air supply must always meet the compressed air demand by utilizing sufficient storage and correct distribution. Properly sized compressors and purification equipment will aid in meeting demand with supply. If the supply, storage, and distribution are not in sync, excessive pressure fluctuation will occur. Most compressors are controlled by line pressure. A drop in pressure normally signifies a demand increase. This is corrected by increased compressor output. A rise in pressure usually indicates a decrease in demand, which causes a reduction in compressor output. To accommodate the fluctuating demand, a load/no load or constant speed control can be used to run the compressor at full load or idle. Either a single compressor or a multiple compressor installation, which can be centralized or decentralized, can provide the entire plant supply. There are three other types of compressor control systems:

  • Auto-dual control: Most traditional modulating controls throttle the capacity 30%-50% before fully unloading the compressor. This type of modulation is known as auto-dual control. It combines start/stop and constant speed control into a single control system. Auto-dual control automatically selects the most desirable control method and runs the compressor in constant speed control. When the compressor unloads, an unloaded run timer energizes, which usually has a time range of 5 to 60 minutes. If the compressor does not reload, the timer will shut the compressor off. The compressor will restart and reload when the pressure switch senses low pressure.

 

  • Sequencing: Sequencing is also known as a central controller. This has the advantage of little cost per compressor and is usually available for systems with up to 10 compressors. A sequencer should have a single pressure transducer in the air header. Logic should maintain a target pressure within +/- 5 psi. The sequencer should automatically start and stop compressors, as well as load and unload them. The control should be set to rotate the order of loading and unloading to optimize compressor combinations for different demand conditions.

 

  • Lead/Lag: Lead/lag controls are typically found on reciprocating compressors. When there are two compressors in the system, one compressor can be set as the lead compressor, and the other as the lag compressor. When the pressure drops to a certain point on the lead compressor, the lag compressor will then take over. These can also be switched so that the other compressor is the lead compressor.

 

Storage
All devices containing compressed air make up the storage system. Adequate storage is essential. It represents available energy that can be released or replenished at any time it is needed. The air receiver tank normally makes up the majority of the total storage capacity of the system. If this tank is properly sized, excessive cycling will be prevented, and adequate storage capacity for any peaks in demand will be provided. In the distribution system, there will periodically be large volume demands, which will rapidly drain the air from surrounding areas, and cause pressure levels to fall for surrounding users. However, strategically located receivers in the system can supply these abrupt demands and still provide a consistent air flow and pressure to the affected areas. The total storage capacity needed is dependant upon the amount of excess demand in cubic feet, the available pressure differential between the flow controller, the system and compressor start-up time, and the time available to replenish stored compressed air.

 

Distribution
The distribution system is the link between supply, storage, and demand. Ideally, the distribution system will allow the required air to flow with minimum pressure drop. It will supply an adequate amount of compressed air at the required pressure to all of the locations where compressed air is needed. The compressed air travels through a network of pipelines, but the flow creates friction and results in pressure drop. The pressure drop should never exceed 1-2 psi (0.07 - 0.14 bar). The longer and smaller diameter the pipe is, the higher the friction loss. To reduce pressure drop effectively, a loop system with two-way flow can be used. Pressure drop caused by corrosion and the system components themselves are important issues. These typically range from 5-25 psid (0.34 - 1.7 bar) and their control is essential for the efficiency of the system.

 

Installation
To effectively control and manage the compressed air system, the system layout must be considered. Sufficient ventilation, foundation and compressor room requirements must be met, and appropriate piping materials used. Compressor intakes located outdoors should be a minimum of 10 feet (3 meters) above grade. Proper ventilation may be achieved through natural ventilation, forced ventilation with an exhaust fan, ducted ventilation to the outside with or without a recirculating damper, as well as by mixing warm air with cold intake air, or venting an exhaust air duct to the outside during summer (space heating during winter). Foundation requirements apply only to larger reciprocating compressors, but all compressors should have their own clean, cool room. Piping must be durable enough for existing work conditions, provide minimum possible pressure loss and leakage, and be easy to maintain.

 

Maintenance
Preventive maintenance is the most important step you can take. Leaks are one of the biggest maintenance issues and can be very expensive. For example, one ¼" (6.35 mm) diameter opening equals 100 CFM (2.8 m3/min) at 90 psig (6.2 bar). This is equivalent to running a 25 horsepower (18 kW) compressor. However, developing a formal program to monitor and repair leaks can control or prevent them. If a leak goes undetected, it can eventually cause the entire system to have to be shut down. A well-maintained compressor, in addition to having less downtime and repairs, will save on electrical power costs as well.

 

Condensate Control
Moisture in the form of liquid and vapor is in compressed air as it leaves the system. The system can lose productivity and require significant maintenance if the moisture and other contaminants are not removed properly. Purification devices have been developed to help remove some of the contaminants from the system. As pneumatic applications and compressed air systems become more sophisticated, the proper selection of these devices is crucial. The most critical devices for condensate control are the coalescing filter, drain valve, air dryer and after filter.

Note: All compressor condensate is to be disposed of in accordance with all local, state and federal regulations.

 

Energy efficiency

Compressed air costs are a significant component of most companies' utility costs. In many cases, companies are paying much more than they have to. This is because they are not operating their compressed air systems at the greatest efficiency. There are six steps that can be taken to reduce energy waste and increase energy savings:

  • Evaluate your costs for compressed air. To do this, add up all the compressor horsepower, calculate the average air demand, and determine the percentage of full load power.
  • Identify the volume of wasted air. This is accomplished by checking the leakage rate during off periods, determining required point of use pressure, and calculating wasted air through "over" pressurization.
  • Calculate specific performance at rated pressure, compare it with different brands, and select the most efficient compressor control. Turn the control selector switch to Dual Control, or check with the manufacturer for retrofit.
  • Reduce the pressure drop in your compressed air system. You can do this by measuring the pressure drop at the maximum flow across all of the system components. After that, increase the pipe size of the loop piping system, properly maintain the filters, drain valves, dryers, and compressors.
  • Stabilize and/or reduce the system pressure downstream of the air drying equipment. Installing a flow controller in conjunction with additional air receivers will accomplish this. Use the 2-4 gallon receiver capacity/CFM, and install a sequencer in multiple compressor installations.
  • Evaluate the potential for heat recovery. Explore applications that involve heating, analyze existing costs for these applications, and implement a compressor duct system or liquid/oil heat exchangers.

 

Compressed Air Challenge:

The Compressed Air Challenge is a voluntary cooperative of many organizations that deal with compressed air systems in somecapacity, such as users, manufacturers, distributors, system operators, consultants, state research agencies, energy efficiency organizations, and other utility companies. Their purpose is to provide the consumer with information that will improve the performance of their compressed air systems, resulting in higher overall operating efficiency and lower energy costs. Ultimately, net profits may be increased through compressed air system optimization.

 

Air System Maintenance:

Maintenance

Preventive maintenance is the most important step you can take. Leaks are one of the biggest maintenance issues and can be very expensive. For example, one ¼" (6.35 mm) diameter opening equals 100CFM (2.8 m3/min) at 90 psig (6.2 bar). This is equivalent to running a 25 horsepower (18 kW) compressor. However, developing a formal program to monitor and repair leaks can control or prevent them. If a leak goes undetected, it can eventually cause the entire system to have to be shut down. A well-maintained compressor, in addition to having less downtime and repairs, will save on electrical power costs as well. Selection and purchase of the compressor and necessary purification equipment can be easily done on the eCompressedAir site. Our application engineers are ready to answer all of your questions and to assist you in placing your order.

Industry's Fourth Utility:

Compressed air is considered to be industry's fourth utility. It is an energy source that, like electricity, water, and natural gas, allows people to operate equipment, tools, and processes safely and efficiently. Many businesses would experience losses in productivity and profitability without dependable pneumatic power.

Rules of thumb:

There are a few rules of thumb regarding the efficiency of compressed air systems:

  • At 100 psig (7 bar) discharge pressure, most air compressors deliver 4-5 CFM per horsepower (0.11 - 0.14 m3/min per kW).
  • Every 2 psig (0.137 bar) of pressure changes the power draw of a compressor by 1%.
  • Efficiency is affected by about 1% for every 10°F change in inlet air temperature. Warmer temperature decreases and colder temperature increases efficiency.
  • A 50 hp (67 kW) compressor ejects about 126,000 Btu per hour. It is possible to regain approximately 119,000 Btu per hour of this.
  • The power cost for 1 horsepower for three shifts, seven days a week (8,760 hours) at $.10/kWk equals approximately $750/year.
  • The control air receiver located after the compressor should be sized for about 1 gallon capacity per CFM of compressor capacity.
  • To ensure an effective demand side control management system, the storage air receiver should be sized for about 2-4 gallon capacity per CFM of compressor capacity.
  • Total pressure drop should not exceed 15 psi (1 bar) across all compressed air system components, including piping.

The following schematics may be viewed for a visual representation of these systems:

Pharmaceutical facility air systems
Compressed air systems used in pharmaceutical facilities have to meet the requirements of ISA-S7.0.01-1996 and cGMP for validated systems. Proper design of your compressed air system will meet these requirements and reduce your utility cost up to 30 % by optimizing the system. Proper selection of compressor, receiver, filter, dryer, drain valve, piping and maintenance is key. There are several types of air compressors and air dryers, which are the heart of the system. They include non-lubricated positive displacement (reciprocating and rotary), dynamic (centrifugal) compressors and heat-less, heated (internal and external) dryers. ECompressedAir can design and supply an AIR COMPRESSOR DRYER PACKAGE that will meet your specific requirement.

Pharmaceutical fermentation air systems
Fermentation air is a validated system that meets the requirements of cGMP. Redundancy and by-pass systems are required to maintain continuous flow, pressure (20 to 40 psig), and dew points (-20° to -60°F, or -28.89° to -51.11°C). Due to variable flow conditions, energy management controllers are required. By optimizing the design of your system, this requirement can be met and reduce your utility costs by up to 50 %. The proper selection of compressors, chiller packages, dryers, filters, drain valves, piping, instrumentation, and good maintenance can help you realize this savings. There are several types of non-lubricated compressors, air chillers, and air dryers. They include:

  • non-lubricated positive displacement (reciprocating and rotary)
  • dynamic (centrifugal) compressors
  • air or water cooled chillers
  • heated (internal or external) air dryers.

eCompressedAir can design and supply all components that will meet your specific requirements.

Pharmaceutical solvent batch drying system
The solvent drying system is used to dry batches of solvents from bulk transfer trucks or storage tanks. The system includes a single sieve, drying column and closed loop, nitrogen regeneration system consisting of a condenser, coolant pump, separator, filter, re-circulation blower, and heater all piped and mounted on a common skid. All pressure vessels meet the provisions of the ASME Boiler and Pressure Vessel Code, Section VIII, Division I. All process wetted components and piping are constructed of 316 L SS. The drying and regeneration operations are completely automated and sequenced by a microprocessor-based control system. The drying cycle is started manually. All electrical components are suitable for installation in a Class I, Division II, Group C & D location. eCompressedair can design and supply solvent drying systems that will meet your specific requirements.

Typical Performance Conditions
Solvent Batch Size: 5000 gallons
Solvent to dry: THF
Solvent Flow Rate: 10 gpm
Inlet H2O: 2000 ug/ml
Outlet H2O: 50 ug/ml
Number of Batches: One batch before regeneration
Cycle Time: 48 hours (adsorption and regeneration)
Purge Rate: 10 scfm nitrogen

 

Other solvents to be dried at the same conditions with specific limitations:
Acetonitrile Hexanes
Chlorobenzene Isopropyl Alcohol
Cyclohexane Isopropyl Acetate
O-Dichlorobenzene Methycyclohexane
Dimethoxymethane Methyl Tert Butyl Ether (MTBE)
Dimethylformamide (DMF) N-Ethyl Pyrrolidone
Ethanol N-Methyl Pyrrolidone
Ethyl Acetate Toluene
Heptanes Xylenes