The document discusses different types of air compressors, including reciprocating, rotary screw, and rotary centrifugal compressors. It provides details on the operation and characteristics of each type. Reciprocating compressors use pistons to compress air, while rotary screw compressors use two meshing helical screws. Rotary centrifugal compressors rely on an impeller to impart angular momentum and compress the air. Each compressor type has different applications depending on factors like pressure and capacity needs.
The document discusses different types of air compressors, including reciprocating, rotary screw, and rotary centrifugal compressors. It provides details on the operation and characteristics of each type. Reciprocating compressors use pistons to compress air, while rotary screw compressors use two meshing helical screws. Rotary centrifugal compressors rely on an impeller to impart angular momentum and compress the air. Each compressor type has different applications depending on factors like pressure and capacity needs.
The document discusses different types of air compressors, including reciprocating, rotary screw, and rotary centrifugal compressors. It provides details on the operation and characteristics of each type. Reciprocating compressors use pistons to compress air, while rotary screw compressors use two meshing helical screws. Rotary centrifugal compressors rely on an impeller to impart angular momentum and compress the air. Each compressor type has different applications depending on factors like pressure and capacity needs.
The document discusses different types of air compressors, including reciprocating, rotary screw, and rotary centrifugal compressors. It provides details on the operation and characteristics of each type. Reciprocating compressors use pistons to compress air, while rotary screw compressors use two meshing helical screws. Rotary centrifugal compressors rely on an impeller to impart angular momentum and compress the air. Each compressor type has different applications depending on factors like pressure and capacity needs.
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The main types of air compressors discussed are reciprocating, rotary screw, and rotary centrifugal compressors. Reciprocating compressors use pistons while rotary screw compressors use intermeshing rotors. Centrifugal compressors use an impeller to impart angular momentum to the air.
The main types are reciprocating, rotary screw, and rotary centrifugal compressors. Reciprocating compressors use pistons while rotary screw compressors use intermeshing rotors to compress air internally.
Rotary screw compressors are more efficient, produce less noise and vibration, require less maintenance over time, and do not need to be oversized for intermittent duty cycles.
Types of Air Compressors
Reciprocating, rotary screw and rotary centrifugal air compressors
Sponsored Links The three basic types of air compressors are reciprocating rotary screw rotary centrifugal These types are further specified by: the number of compression stages cooling method (air, water, oil) drive method (motor, engine, steam, other) lubrication (oil, Oil-Free where Oil Free means no lubricating oil contacts the compressed air) packaged or custom-built Reciprocating Air Compressors Reciprocating air compressors are positive displacement machines, meaning that they increase the pressure of the air by reducing its volume. This means they are taking in successive volumes of air which is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by a piston within a cylinder as the compressing and displacing element. Single-stage and two-stage reciprocating compressors are commercially available. Single-stage compressors are generally used for pressures in the range of 70 psig to 100 psig. Two-stage compressors are generally used for higher pressures in the range of 100 psig to 250 psig. Note that 1 HP ~ 4 CFM at 100 psi and that 1 to 50 HP are typically for reciprocating units. Compressors 100 hp and above are typically Rotary Screw or Centrifugal Compressors. The reciprocating air compressor is single acting when the compressing is accomplished using only one side of the piston. A compressor using both sides of the piston is considered double acting. Load reduction is achieved by unloading individual cylinders. Typically this is accomplished by throttling the suction pressure to the cylinder or bypassing air either within or outside the compressor. Capacity control is achieved by varying speed in engine-driven units through fuel flow control. Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non-lubricated configurations and provide a wide range of pressure and capacity selections. Rotary Screw Compressors Rotary air compressors are positive displacement compressors. The most common rotary air compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These compressors consist of two rotors within a casing where the rotors compress the air internally. There are no valves. These units are basically oil cooled (with air cooled or water cooled oil coolers) where the oil seals the internal clearances. Since the cooling takes place right inside the compressor, the working parts never experience extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, air cooled or water cooled compressor package. Rotary screw air compressors are easy to maintain and operate. Capacity control for these compressors is accomplished by variable speed and variable compressor displacement. For the latter control technique, a slide valve is positioned in the casing. As the compressor capacity is reduced, the slide valve opens, bypassing a portion of the compressed air back to the suction. Advantages of the rotary screw compressor include smooth, pulse-free air output in a compact size with high output volume over a long life. The oil free rotary screw air compressor utilizes specially designed air ends to compress air without oil in the compression chamber yielding true oil free air. Oil free rotary screw air compressors are available air cooled and water cooled and provide the same flexibility as oil flooded rotaries when oil free air is required. Centrifugal Compressors The centrifugal air compressor is a dynamic compressor which depends on transfer of energy from a rotating impeller to the air. Centrifugal compressors produce high-pressure discharge by converting angular momentum imparted by the rotating impeller (dynamic displacement). In order to do this efficiently, centrifugal compressors rotate at higher speeds than the other types of compressors. These types of compressors are also designed for higher capacity because flow through the compressor is continuous. Adjusting the inlet guide vanes is the most common method to control capacity of a centrifugal compressor. By closing the guide vanes, volumetric flows and capacity are reduced. The centrifugal air compressor is an oil free compressor by design. The oil lubricated running gear is separated from the air by shaft seals and atmospheric vents.
ROTARY SCREW AIR COMPRESSOR TROUBLESHOOTING AND REPAIR You are now at the rotary screw air compressor section of the largest online troubleshooting and repair guide for air compressors. This section deals with problems with stationary rotary screw air compressors. The other sections are reciprocating air compressor troubleshooting and portable air compressor troubleshooting. Over the years, I have repaired and did troubleshooting on hundreds of rotary screw air compressors. Here is a list of the most frequently asked questions and problems. Common rotary screw air compressor problems Compressor will not start You press the start button but nothing happens. Either an electrical problem, or the compressor has tripped on a safety device. Check supply power. Check for errors on display. Check emergency stop is out. Check and reset the overload relay. If it's a new installation, check the phase sequence. Compressor shuts down on high-temperature If your compressor trips on over temeprature, it could be any of the following: Ambient temperature too high or not enough ventilation. Too low oil level Wrong type of oil Dirty oil cooler Thermostatic valve not not working Dirt / obstruction in oil lines Compressor runs but will not load A screw compressor can run loaded ('pumping air') or unloaded ('idle'). The inlet/loading valve opens and closes according to air demand. The inlet valve is controlled by a solenoid valve that supplies control air to the inlet/loading valve. Check electrical power to solenoid valve Check solenoid valve coil and solenoid valve operation. Check working of inlet/loading valve Low capacity / not enough pressure First, check that there isn't a very high air demand, or air leak somewhere. If the capacity of the air compressor is really too low, check the following: Does the inlet valve fully open Check differential pressure over oil separator. Replace separator when necessary. Check if inlet filter is clean Check and replace compressed air filters (if installed). Safety valve blows / too high pressure Compressor does not unload. Check if pressure switch is correctly set and working. Check inlet valve and loading solenoid for good operation. If the safety valve is located before the oil separator, check differential pressure of oil separator. Oil in compressed air Oil in compressed air can have various causes: Oil separator old / saturated Scavenge line plugged Too high running temperature Too high oil level Wrong type of oil used Minimum pressure valve not working Water in compressed air Water is a natural by product of air compression. There will always be water in compressed air, unless we remove it. Check the condensate trap for good operation. There should be water coming out every few minutes. If you open up the manual drain, there should only a little water be coming out. If you have a compressed air dryer, check the dewpoint. Compressor overload relay trips Check the current draw with a current clamp meter. If the motor draws exeessive current: Try to turn the compressor by hand. It should be possible to turn it around. It should turn around smoothly, without any 'hickups' (be sure to completely shut down the air compressor!). Check the isolation of the motor windings. Should be in the mage-ohms (you need an isolation tester / high voltage ohm meter for this). Check the voltage when the compressor is running. If the voltage drop significantly when the compressor starts/runs, you have a bad connection somewhere. Check all relays, fuses and electrical connections. Check if all phases are present If the motor draws it's normal current, but still trips on overload, replace the overload relay with anew one (they are known to sometimes become too sensitive when they get old) Troubleshoot Your Rotary Screw Compressor Today!
My new eBook Your Rotary Screw Compressor: Troubleshooting and Maintenance will show you: How to troubleshoot problems yourself. How to prevent the 6 common mistakes people make. How to Prevent costly down time of your air compressor. How to properly take care for your air compressor to avoid downtime. Rotary screw compressor From Wikipedia, the free encyclopedia
Rotary screw air compressor internal view
Rotary screw air compressor in a housing for sound attenuation A rotary screw compressor is a type of gas compressor which uses a rotary type positive displacement mechanism. They are commonly used to replace piston compressors where large volumes of high pressure air are needed, either for large industrial applications or to operate high- power air tools such as jackhammers. The gas compression process of a rotary screw is a continuous sweeping motion, so there is very little pulsation or surging of flow, as occurs with piston compressors. Contents [hide] 1 Operation 2 Size 3 Applications o 3.1 Oil-free o 3.2 Oil-flooded 4 Control schemes o 4.1 Start/stop o 4.2 Load/unload o 4.3 Modulation o 4.4 Variable displacement o 4.5 Variable speed 5 Superchargers o 5.1 Comparative advantages o 5.2 Related terms 6 See also 7 References Operation[edit] Rotary screw compressors use two meshing helical screws, known as rotors, to compress the gas. In a dry running rotary screw compressor, timing gears ensure that the male and female rotors maintain precise alignment. In an oil-flooded rotary screw compressor, lubricating oil bridges the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threads as the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws. [1][2]
The effectiveness of this mechanism is dependent on precisely fitting clearances between the helical rotors, and between the rotors and the chamber for sealing of the compression cavities. Size[edit] Rotary screw compressors tend to be compact and smooth running with limited vibration and thus do not require spring suspension. Many rotary screw compressors are, however, mounted using elastomer vibration isolating mounts to absorb high-frequency vibrations, especially in rotary screw compressors that operate at high rotational speeds. Rotary screw compressors are produced in sizes that range from 10 cubic feet per minute to several thousand CFM. Rotary screw compressors are typically used in applications requiring more airflow than is produced by small reciprocating compressors but less than is produced by centrifugal compressors. Applications[edit] Typically, they are used to supply compressed air for general industrial applications. Trailer mounted diesel powered units are often seen at construction sites, and are used to power air operated construction machinery. Additionally, they are becoming increasingly popular in municipal wastewater treatment facilities, for their increased efficiency and thus, lower power consumption. [citation needed]
Oil-free[edit] In an oil-free compressor, the air is compressed entirely through the action of the screws, without the assistance of an oil seal. They usually have lower maximum discharge pressure capability as a result. However, multi-stage oil-free compressors, where the air is compressed by several sets of screws, can achieve pressures of over 150 psig, and output volume of over 2000 cubic feet (56.634 cubic meters) per minute (measured at 60 C and atmospheric pressure). Oil-free compressors are used in applications where entrained oil carry-over is not acceptable, such as medical research and semiconductor manufacturing. However, this does not preclude the need for filtration as hydrocarbons and other contaminants ingested from the ambient air must also be removed prior to the point-of-use. Subsequently, air treatment identical to that used for an oil-flooded screw compressor is frequently still required to ensure a given quality of compressed air. Oil-flooded[edit]
Diagram of a rotary screw compressor In an oil-flooded rotary screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling sink for the gas charge. The oil is separated from the discharge stream, then cooled, filtered and recycled. The oil captures non-polar particulates from the incoming air, effectively reducing the particle loading of compressed air particulate filtration. It is usual for some entrained compressor oil to carry over into the compressed gas stream downstream of the compressor. In many applications, this is rectified by coalescer/filter vessels. [3] In other applications, this is rectified by the use of receiver tanks that reduce the local velocity of compressed air, allowing oil to condense and drop out of the air stream to be removed from the compressed air system via condensate management equipment. Control schemes[edit] Among rotary screw compressors, there are multiple control schemes, each with differing advantages and disadvantages. Start/stop[edit] In a start/stop control scheme, compressor controls actuate relays to apply and remove power to the motor according to compressed air needs. Load/unload[edit] In a load/unload control scheme, the compressor remains continuously powered. However, when the demand for compressed air is satisfied, instead of disconnecting power to the compressor, the inlet valve is closed, unloading the compressor. This reduces the number of start/stop cycles for electric motors over a start/stop control scheme in electrically-driven compressors, improving equipment service life with a minimal change in operating cost. This scheme is utilised by nearly all industrial air compressor manufacturers. When a load/unload control scheme is combined with a timer to stop the compressor after a predetermined period of continuously unloaded operation, it is known as a dual- control or auto-dual scheme. [4]
Modulation[edit] Instead of starting and stopping the compressor or actuating the inlet valve between two distinct positions, a modulation control scheme proportionally adjusts the inlet valve open and closed, altering the compressor discharge according to demand. While this yields a consistent discharge pressure over a wide range of demand, power consumption is significantly higher than with a load/unload scheme, resulting in approximately 70% of full-load power consumption when the compressor is at a zero-load condition. This control scheme was popularized in industrial air compressor applications by KOBELCO, Kaeser and Gardner Denver. [5]
Due to the limited adjustment in compressor power consumption relative to compressed air output capacity, modulation is a generally inefficient method of control when compared to variable speed drives. However, for applications where it is not readily possible to frequently cease and resume operation of the compressor (such as when a compressor is driven by an internal combustion engine and operated without the presence of a compressed air receiver), modulation is suitable. Variable displacement[edit] Utilized by compressor companies Quincy Compressor, Kobelco, Gardner Denver, and Sullair, variable displacement alters the percentage of the screw compressor rotors working to compress air by allowing air flow to bypass portions of the screws. While this does reduce power consumption when compared to a modulation control scheme, a load/no load system can be more effective when large amounts of storage (10 gallons per CFM). If a large amount of storage is not practical, a variable displacement system can be very effective, especially at greater than 70% of full load. [6]
One way that variable displacement may be accomplished is via the use of multiple lifting valves on the suction side of the compressor, each plumbed to a corresponding location on the discharge. In automotive superchargers, this is analogous to the operation of a bypass valve. Variable speed[edit] While an air compressor powered by a variable speed drive can offer the lowest operating energy cost without any appreciable reduction in service life over a properly maintained load/unload compressor, the variable frequency power inverter of a variable speed drive typically adds significant cost to the design of such a compressor, negating its economic benefits if there are limited variations in demand. However, a variable speed drive provides for a linear relationship between compressor power consumption and free air delivery. In harsh environments (hot, humid or dusty), variable speed drives may not be suitable due to the sensitivity of the equipment. [7]
Superchargers[edit]
Lysholm screws. Note the complex shape of each screw. The screws run at high speed and with closely engineered tolerances. The twin-screw type supercharger is a positive displacement type device that operates by pushing air through a pair of meshing close-tolerance screws similar to a set of worm gears. Twin-screw superchargers are also known as Lysholm superchargers (orcompressors) after their inventor, Alf Lysholm. [8] Each rotor is radially symmetrical, but laterally asymmetric. By comparison, conventional "Roots" type blowers have either identical rotors (with straight rotors) or mirror-image rotors (with helixed rotors). The Whipple-manufactured male rotor has three lobes, the female five lobes. The Kenne-Bell male rotor has four lobes, the female six lobes. Females in some earlier designs had four. By comparison, Roots blowers always have the same number of lobes on both rotors, typically 2, 3 or 4. The working area is the inter-lobe volume between the male and female rotors. Its larger at the intake end, and decreases along the length of the rotors until the exhaust port. This change in volume is the compression. The intake charge is drawn in at the end of the rotors in the large clearance between the male and female lobes. At the intake end the male lobe is much smaller than its female counterpart, but the relative sizes reverse proportions along the lengths of both rotors (the male becomes larger and the female smaller) until (tangential to the discharge port) the clearance space between each pair of lobes is much smaller. This reduction in volume causes compression of the charge before being presented to the output manifold. Comparative advantages[edit] The rotary screw compressor has low leakage levels and low parasitic losses vs. Roots type. The supercharger is typically driven directly from the engine's crankshaft via a belt or gear drive. Unlike the Roots type supercharger, the twin-screw exhibits internal compression which is the ability of the device to compress air within the housing as it is moved through the device instead of relying upon resistance to flow downstream of the discharge to establish an increase of pressure. [9]
The requirement of high-precision computer-controlled manufacturing techniques makes the screw type supercharger a more expensive alternative to other forms of available forced induction. With later technology, manufacturing cost has been lowered while performance increased. All supercharger types benefit from the use of an intercooler to reduce heat produced during pumping and compression. A clear example of the technology applied by the twin-screw in companies like Ford, Mazda, Mercedes and Mercury Marine can also demonstrate the effectiveness of the twin screw. While some centrifugal superchargers are consistent and reliable, they typically do not produce full boost until near peak engine rpm, while positive displacement superchargers such as Roots type superchargers and twin-screw types offer more immediate boost. Variable-speed air compressor From Wikipedia, the free encyclopedia A variable-speed drive (VSD) air compressor [1] is an air compressor that takes advantage of variable-speed drive technology. This type of compressor uses a special drive to control the speed (RPM) of the unit, which in turn saves energy compared to a fixed speed equivalent. The most common form of VSD technology in the air compressor industry is a variable-frequency drive, which converts the incoming AC power to DC and then back to a quasi-sinusoidal AC power using an inverter switching circuit. The variable-frequency drive article provides additional information on electronic speed controls used with various types of AC motors. Contents [hide] 1 Benefits 2 Industry 3 See also 4 References 5 External links Benefits[edit] The benefits of this technology included reducing power cost, reducing power surges (from starting AC motors), and delivering a more constant pressure. The down side of this technology is the heavy expense associated with the drive, and the sensitivity of these drives specifically to heat and moisture.. Industry[edit] Typically, a fifth of a factory's electricity bill is attributed to the production of compressed air. The majority of modern factories are heavily involved in cutting costs, and energy awareness should be a key concern. For example, 1012% of all power generated in the UK is dedicated to the production of compressed air, and a portion of this power is wasted energy. Large electrical cost savings can be achieved by installing a variable speed drive compressor in place of an existing rotary screw or piston machine. Because of this, many governments, including the US and UK, are pushing the industries to move towards this technology in hopes of reducing wasted energy. Governments offer various incentives, such as tax rebates or interest free loans, to cover the upgrades. However, variable speed drive compressors are not necessarily appropriate for all industrial applications. If a variable speed drive compressor operates continuously at full speed, the switching losses of the frequency converter result in a lower energy efficiency than an otherwise identically sized fixed speed compressor. Where demand remains constant within 515% of the total free air delivery flow rate, dual-control compressors configured in a split solution can provide higher efficiency than a VFD. A professional air audit is the best way to identify if a VFD compressor is most appropriate for any given compressed air application. [2] These audits are available from various companies that specialize in the implementation of compressed air equipment, which can determine the most efficient controls for a compressed air system including variable speed, variable capacity or using storage and flow controllers. COMPRESSOR TYPES: DIFFERENT WAYS TO COMPRESS AIR Buy the right type of compressor, with our informative articles, that teaches you everything about different air compressor type. There are a lot of types of compressor to choose from. Choosing the right type of air compressor for your needs is very important, it will save you a lot of trouble and money in the long run. You will learn on this page the advantages and disadvantages of the different kinds of compressors. Which compressor type is right for you? I can't tell you! It all depends what you use it for! You will find here a short list with the most popular types of compressors and I will discuss the pros and cons of each one, including some typical uses. After reading this page, you should have a fairly good idea what kind of compressor is best for you. The difference between these compressors is both the PRESSURE and the FLOW (cubic meters per minute, or cubic feet per minute). Most common pressure is 7 to 8 bars. Reciprocating piston compressors are most often seen for high-pressure / low flow applications (up to 30 bars), while rotary screw compressors are used for continuous applications (high flow, 7 or 8 bar).. Reciprocating piston compressor Example of a reciprocating piston compressor. Photo: Atlas Copco This compressor type uses a piston, which moves inside a cylinder, to compress the air. Reciprocating simply means 'moving back-and-forth'. Usually it's a piston that move back-and-forth, but it can also be a rubber membrane (membrane compressor). Two sets of valves take care of the air intake and exhaust. Piston compressors are available as lubricated and oil-free, and most of the time with 2 cylinders in V-shape. Very small piston compressors use only 1 cylinder. The piston compressor is nowadays available from 1 HP to about 30 HP. They are used a LOT for general-purpose applications ('workshop-air'), where the air is used for hand-tools, cleaning dust, small paint jobs, etc. Together with the rotaryscrew compressor, it's one of the most used compressor types. Pros: Relatively cheap Easy maintenance (easy to understand the inner working) Suitable for high pressures Cons: Very noisy! You really need to put this type of compressor in a sound-isolating box or in it's own room. High outlet temperature of compressed air High oil content in air piping. More information about reciprocating compressors. Rotary screw compressor (oil-injected or oil-free) Rotary screw compressor element. Photo: Atlas Copco Another one of the popular compressor types is the rotary screw compressor. It uses two rotors (helical screws) to compress the air. The rotors have a very special shape and turn in opposite directions with very little clearance between them. Air sucked in at one end gets trapped between the rotors, and get pushed to theother side of the rotors (the pressure-side). There are two basic types of rotary screw compressors: oil-injected and oil-free. The oil-injected type is most common, because it has a much lower price-tag than the oil-free one (which you should only use if your application requires 100% oil-free air). Pros: Low noise level . You can just put in in your workshop without wearing ear-protection. These are the work-horses of the compressors and can supply a large amount of compressed air. Good energy-efficiency compared to piston-type compressors Relatively low end temperature of compressed air Possible to use energy recovery Cons: Purchase price is much higher than piston-type compressors More complex design, good maintenance very important. Minimal air use (per day/week) is required to prevent water condensate forming (will create a lot of problems with rust!) More information about rotary screw compressors. Scroll compressor Inside of a scroll compressor. Photo: Atlas Copco I like scroll compressors... they are 'elegant'.. They run smoothly, with almost no noise, no vibrations and use a clever design principle to compress the air. They are however one of the less seen compressor types. How do they work? They compress the air using two spiral elements. 1 is stationary (it doesn't move), and the other one moves in small eccentric circles inside the other spiral. Air gets trapped and because of the way the spirals move, gets transported in small air-pockets to the center of the spiral. It takes about 2.5 turn for the air to reach the pressure output in the center. I haven't seen too many scroll compressors 'in the wild' (as compared to other compressor types). Normally they are used in places where a small amount of oil-free and clean compressed air is needed (for example drinking water-treatment facilities, specialized factories or laboratories, etc). They don't product a high air flow. If you need a lot of oil-free compressed air, go for an oil-free rotary screw compressor. Pros: Very quiet. Really very quiet! Compact. A scroll compressor is very small. Simple design, not so many parts Low maintenance (hardly any) Oil-free design Cons: Low capacity (flow, liters/minute or cfpm). Relatively expensive When the scroll-element fails, there's a very big chance you just have to buy a whole new element. The compressed air gets very hot! Much hotter than compared to other types of compressors More information about scroll compressors. What kind of compressor is right for you? It all depends what you use it for! Do you have any specialized needs? Do you need a lot of air, or just a little bit? Always check your tools, machines and other air-users and calculate how much air (cfpm = cubic feet per minute, you need. If you just need a small amount of air (power some tools, operate a small machine), the most obvious choice would be to buy an oil-lubricated piston-type compressor. Do you need oil-free air? Use special filters to filter out the oil, or buy an oil-free piston-type compressor (more expensive, maintenance and repairs will also be more expensive compared to oil lubricated compressors). Do you need more air? For example, do you use compressed air to sand-blast or operate one or more machines that use a lot of air, then the obvious choice would be a rotary-screw compressor. They range from small (5 kW) to very big (1000 or more kW). The Right Compressor For You Reciprocating vs Rotary Screw This is a common question asked by most people looking for a new air compressor. Either compressor depends in your application. If the application requires air all day or has multiple shifts that use air on a consistent basis, then a rotary screw is probably a better fit for the application due to its ability to run all day every day. Rotary Screw compressors are thermally controlled through an air cooled oil cooler. Some rotary screw compressor packages have built in timers that will turn the compressor off if it runs unloaded for an extended period of time. (Everyone goes to lunch, or it does not get turned off at the end of the day.) Reciprocating compressors are best suited for applications that require air for shorter durations of timeand might have 15-30-45 minutes before they need air again. Because a Reciprocating compressor is air cooled, they need to enter an application with a duty cycle of 70% or less. Either package depends on making sure you purchase a properly sized compressor for your application. If you have any questions, you can always contact CAS or one of our local distributors to make sure you have the right unit for your application. Noise of Reciprocating vs Rotary Screw The noise of an air compressor package depends on how it is constructed.. Most rotary screw compressors sold in the world today are built in sound dampening enclosures. These does not add to theperformance of the compressor package, all they do is lower the DBA of the compressor package. Most reciprocating compressors are built open on a tank with no sound proof enclosure, which is why they sound louder than a rotary screw in a sound dampening enclosure. If you take a reciprocating compressor and build it in the same enclosure as a rotary screw compressor you will get a similar DBA reading. At CAS we manufacture a reciprocating compressor muffler that can lower the DBA of the compressor by up to 10DBA depending on the acoustics or the compressors environment. This would put the DBA of a CAS reciprocating compressor in the range of 74-82DBA. At CAS we are one of the few compressor companies to build an open tank mounted rotary screw compressor while the DBA range is 70-84 depending on the size of the compressor, an enclosed rotary screw compressor package is between 64-78DBA depending on the size. The enclosure for a rotary screw compressor is an added expense up front and to the yearly maintenance cost. It takes longer to service and enclosed rotary screw compressor than an open tank mount version. At Compressed Air Systems we have several sizing guides to help you pick the right compressor. Or give us a call and we'll put you in touch with a local distributor who can size a compressor for your application.
Piston vs Rotary Screw Compressors: A Short Comparison for the Collision Market Michael Camber, Marketing Services Manager, Kaeser Compressors, Inc. Automotive Remanufacturers Piston compressors are still the most common type of compressor found in the automotive service industry, which includes gas stations, general service, quick lube shops, tire stores, fleet maintenance facilities, dealer fixed operations, and collision repair. For most of these facilities, the relatively low air demand and quality needed make the piston a cost-effective choice. Collision repair shops, however, typically use much more compressed air and have higher air quality needs than other automotive service businesses. In these respects, collision repair is similar to manufacturing. In fact, many modern body shops might be more accurately referred to as automotive remanufacturers. These days, many body shop owners are finding out they have similar compressed air needs to larger industrial facilities and that rotary compressors offer significant operational benefits. Rotary compressors provide an extremely reliable supply of clean, dry compressed air. This may not be as critical for general repair, but collision repair stands apart because the end product is directly affected by air quality. When deciding between rotary and piston compressors, it is important to consider duty cycle and performance, energy efficiency, air quality, maintenance, and installation costs.(Right, collision repair shops are unique in the automotive service industry because they often need larger volumes of higher quality air for body work, as shown here, and applying high quality automotive finishes.) Duty Cycle and Flow An important difference between piston and rotary compressors is their duty cycle. Duty cycle is the percentage of time a compressor may operate without the risk of overheating and causing excessive wear. A piston compressor may provide adequate flow for a short period, but its allowable duty cycle must be considered. Most small piston compressors have an allowable duty cycle of 60 to 70%. For this reason, piston compressors are usually oversized to allow the compressor to periodically shut down and cool off because of the relatively high operating temperatures. Even with adequate air storage this can cause capacity problems during peak operating hours. Further, if the shop expands or business increases, lack of air capacity can become even more of an issue. (Left, although routine maintenance for piston compressors is inexpensive, they have much higher oil carry- over and have higher operating temperatures.) Rotary screw compressors have a 100% allowable duty cycle and operate continuously if the need arises. This is possible because rotary compressors are fluid cooled. The fluid performs four important functions: Lubricates the bearings in the pump, Removes contaminants from the air, Forms a non-wearing seal between rotors and casing, Removes the heat generated by compression as part of a thermostatically controlled fluid circuit. Heat and Moisture All of these benefits are important, but this last point is very relevant to the body shop. Piston compressors operate at internal temperatures of 300 to 400F, while a rotary compressor runs at much lower internal temperatures (between 170 and 200F). Just as hot summer air holds more humidity, hotter compressed air can hold more moisture and requires additional components to dry and clean it. A rule of thumb is that every 20 degree (F) increase in temperature doubles air's ability to hold moisture. (Right, duty cycle comparison between rotary and piston compressors.) Modern rotary screw compressors now come with built-in aftercoolers designed with ample surface area and a powerful fan to lower the compressed air's temperature as it exits the compressor. By comparison, the air exiting a piston compressor is very hot and hard to dry. Even with an aftercooler and a specially designed high temperature dryer, it is difficult to reach the same dew point as a rotary screw compressor. Lower operating temperatures make it easier to remove moisture and other contaminants, which is very beneficial for facilities with expensive tools, paint spray booths, and other moisture-sensitive applications. Oil Carry-Over As pistons, cylinders, rings, and valves wear, the piston compressor delivers less air. A side effect is that more lubricating oil gets past the rings into the compressed air piping and down to the points of use. This is often referred to as oil "carry-over." Even new piston compressors pass several times more oil than rotary compressors. This is highly undesirable if you are spraying finishes. (Left, rotary screw compressors have a higher initial purchase price, but can be a long term cost effective solution.) With rotary screw compressors, there is little or no change in performance over time because the rotors do not touch each other or the rotor housing, so they don't wear down. The compressor fluid acts as a non-wearing sealant. It is captured, filtered, cooled, and recirculated. This greatly extends the life of the compressor pump and very little lubricant gets downstream. Energy Efficiency Energy efficiency may not matter much for a repair or tire shop that intermittently runs a 5-10 hp unit, but collision repair typically requires more volume. Many shops have compressors as large as 30 hp. At these sizes, energy efficiency becomes a competitive advantage, especially where electricity is expensive. Rotary compressors typically deliver more air per unit of input energy than piston compressors. Piston compressors generally deliver 3-4 cfm per hp. Rotaries deliver 4-5 cfm per hp. Of course, you pay for kWh used so it is more practical and accurate to compare efficiencies in terms of kW and cfm. The Compressed Air and Gas Institute (CAGI) has created a form for manufacturers to state their energy efficiency for better "apples-to-apples" comparison. Most manufacturers make the form available on their websites. Maintenance Routine maintenance for piston compressors is simple and inexpensive. Drive belts, inlet air filters, and lubricating oil should be checked and replaced on a routine schedule. It is also common to add "make-up" oil due to the oil carry-over, and doing so frequently will slow wear on the machine. Let there be no mistake: rotary screw compressors have more maintenance points than piston compressors, including the fluid filter and separator. The routine annual service costs will be higher. Piston units will, however, eventually wear to the point that they need major service (rebuild) to reverse the gradual loss of flow and increase in oil carry-over. This expense must be considered in a lifecycle cost comparison. Noise Levels and Vibration Typical shop piston compressors have a well-earned reputation for high noise and vibration that may be heard and felt throughout the shop. For these reasons, they are often put in separate rooms, in forgotten corners, or outside - exposed to the elements. Where you put a compressor directly impacts air quality and compressor life. A hot stuffy room, for example, will increase operating temperature, shorten compressor life, and make it harder to remove moisture and oil from compressed air. Also, the cost of building separate rooms or enclosures for the compressor must be considered in an accurate cost comparison. Rotary compressors are far quieter and produce far less vibration. They don't need special rooms built and they don't need to be bolted to the floor to keep them in one place. The sound is low enough to have a normal conversation near the machine (a convenience and a plus for safety). Being relatively quiet and vibration free, they offer more flexibility where you put them. This usually results in a placement with better ventilation, lighting, and service access. The Real Cost The main reason cited for selecting piston compressors is often lower purchase price. But the actual cost comparison really extends beyond the initial transaction. Consider all the facts when setting up a new shop or retrofitting an existing facility. Rotary screw compressors do not need to be oversized to compensate for limited duty cycle and are more efficient that piston models. A 7.5 hp rotary will often do the job of a 10 hp piston. The smaller horsepower unit will use less electricity and reduce operating costs. Better compressed air quality creates significant savings in labor, paint, and other materials. Better compressed air quality will extend air tool and equipment life. More reliable air compressors keep employees working and productive, not waiting for the compressor to catch up or be repaired. Lower heat, noise, and vibration eliminate the need for a separate room or enclosure. Each of these advantages contributes to the positive ROI for a rotary compressor. Some of them will very quickly make up the difference in initial price. Think about what you spend on labor and finishing materials each month. The savings will pay for the investment many times over.
At-a-glance differences between a rotary screw and piston compressor