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Reciprocating or Piston compressors

Reciprocating or Piston compressors are the most common machines available on the market. They are positive displacement compressors and can be found in ranges from fractional to very high horsepowers.

Positive displacement air compressors work by filling an air chamber with air and then reducing the chamber’s volume (Reciprocating, Rotary Screw and Rotary Sliding Vane are all positive displacement compressors).

Reciprocating compressors work in a very similar manner as does as internal combustion engine but basically in a reverse process. They have cylinders, pistons, crankshafts, valves and housing blocks.

Rotary Screw Compressors

Rotary Screw Compressors work on the principle of air filling the void between two helical mated screws and their housing.

As the two helical screws are turned, the volume is reduced resulting in an increase of air pressure. Most rotary screw compressors inject oil into the bearing and compression area. The reasons are for cooling, lubrication and creating a seal between screws and the housing wall to reduce internal leakage. After the compression cycle, the oil and air must be separated before the air can be used by the air system.

Rotary Sliding Vane Compressors

Rotary Sliding Vane Compressors like Reciprocating and Rotary Screw compressors are positive displacement compressors.

The compressor pump consists primarily of a rotor, stator, and 8 blades. The slotted rotor is eccentrically arranged within the stator providing a crescent shaped swept area between the intake and exhaust ports. As the rotor turns a single revolution, compression is achieved as the volume goes from a maximum at the intake ports to a minimum at the exhaust port.

The vanes are forced outward from within the rotor slots and held against the stator wall by rotational acceleration. Oil is injected into the air intake and along the stator walls to cool the air, lubricate the bearings and vanes, and provide a seal between the vanes and the stator wall.

After the compression cycle, the oil and air must be separated before the air can be transferred to the air system.

Centrifugal Compressors

Centrifugal Compressors are not positive displacement compressors like the Reciprocating, Screw or Vane Compressors. They use very high speed spinning impellers (up to 60,000 rpm) to accelerate the air then diffuser to decelerate the air.

This process, called dynamic compression, uses velocity to cause an increase in pressure. In most Centrifugal compressors, there are several of these impeller/diffuser combinations. Typically, these machines have intercoolers between each stage to cool the air as well as remove 100% of the condensate to avoid impeller damage due to erosion.

If you are looking for an air compressor, you have come to the right place. At your supplier we have the air compressor that fits your needs. From gas powered to oil-less and everything in between you have the air compressor that is right for you.

This page will briefly talk about a few of the different air compressor types.

A two-stage air compressor is normally used in industrial, commercial and automotive applications where a reliable source is critical. A two-stage air compressor compresses to a higher pressure than single stage air compressor.

This allows the air compressor to store more air for future usage. The efficiency in a two-stage air compressor in much higher that in a single stage air compressor. A two-stage air compressor produces more cubic feet of air per horsepower that a single stage air compressor which results in lower operating costs.

Also with a two-stage air compressor less heat is generated which reduces the wear and results in a longer life for you air compressor.

Another type of air compressor is portable electric air compressors. With a portable electric air compressor, you can plug-in anywhere for a convenient alternative in areas where gas fumes are unacceptable.

Whether on the construction site or in the shop for power air tools such as framing, roofing, trim and finish nailers, an electric air compressor makes a great choice

If you are interested in purchasing an air compressor, you may also want to look into an air compressor pressure switch.

Two-stage and portable electric air compressors are only two of the many different air compressor types. In addition to our air compressor offerings

There are many other industrial supplies as well. From a whole house fan to simple green cleaner or a chain hoist, A supplier has the industrial supplies you are looking for.

To learn more about the air compressor selection from your supplier, or to browse through all the other industrial supplies we offer, please explore the your supplier online store today by choosing a link on this page.

Rotary-Vane-Reciprocating-Screw

What is the difference between these various types of compressors?

What is the difference between the miniature 12 VDC mini-piston type compressor you can purchase at your local tire store and a 40 HP rotary screw compressor?

Obviously, there are giant differences in appearance and size, but the difference that is really important to the compressed air user is the capacity that is offered by these various types of compressors.

One of these two will over deliver substantially if all you need a compressor for is to blow up a basketball, a bicycle tire, or perhaps an air mattress.

The discharge rate of a 40 HP compressor would quickly over inflate these items with unfortunate results. The other, plugged into the power outlet in your automobile, would not be the logical choice to provide air to your shop.

Yes, the 12 VDC unit plugged into your car's power supply can give you 120 PSI air (depending on the brand of course) just like the 40 HP compressor can, but with flow levels such that they are only suitable for low flow applications.

If you tried to use this type of air compressor for a high-demand application, the compressor could never catch up (compressed air outflow would always exceed the compressor's ability to compress it) and it would never reach cut out pressure to allow it to shut down.

Since these small compressors have a limited duty cycle it would run at full capacity until it self-destructed; and, you would never be able to get any work done. A few seconds with your air tools and the miniature receiver would be emptied, and thereafter there would never be enough air flow at the pressure you required for common industrial tasks to get any work done.

Compressor capacity!

It’s critical to your operation. All compressors do the same thing. They “gather’ free air, and compress it up to the pressure limit that is specified by the unit.

Therefore, you’ll select the type of compressor that delivers the capacity you need, both in terms of pressure and the flow of compressed air at the pressure your application requires.

Reciprocating (piston) air compressors are the 'work-horse' compressors with which you'll likely be most familiar. You'll see them at the corner garage, on the shelves at the hardware stores, in residential garages, many home basements, darn near everywhere, and their uses are numerous too.

Careful though. Reciprocating compressors often have the lowest up-front cost, but the highest operating cost! If you're planning on using a lot of air in your shop or in your home, a different style of compressor may give you better value over the long haul.

Here's a link to more information on the reciprocating type air compressors. Vane compressors use "air-tool" type technology to compress air. These compressors are used in a variety of applications.

See information on vane compressors through this link. The Rotary Screw compressor manufacturers state that their technology is the right choice for many industrial applications.

Many times you'll need compressed air at a site where there aren't any air mains, and where the common plant electrical supply is not available to run the compressor motor. That being the case, you'll want to look at this page, that details information about portable air compressors. And for the hobbyist or professional spray artist, please check out this link for information about air brush compressors .

Portable air compressors

Portable air compressors vary by size and power level, and there are a few different models available from which to choose depending on the tools you will use.

Be sure to check the power requirements of your tools to ensure that you purchase a model that has sufficient power to run your strongest tool. Regardless of the compressor you purchase, it is always very important to follow the safety precautions including safety goggles, protective clothing and proper footwear.

Portable rotary screw compressors are one of the more commonly used compressors. They range from 65 to 1,600 cubic feet per minute, with pressure ratings ranging from 100 to 350 PSI.

As discussed above, the compressor you need depends on your tools and the required power level. Contractors often use 185 CFM because they are able to power two tools simultaneously. One benefit to this compressor is its suitability for both the lighter and heavy duty jobs.

The truck mounted compressor is an oil less air compressor and can be mounted in the bed or under the hood of a truck. These compressors are excellent space savers, a great advantage for those with limited space available.

Their power source is from the engine of the truck, which allows them to be a low-maintenance compressor. However, one disadvantage is that the truck must be running to provide the compressor the power it needs. Finally, the deck-mounted compressor is mounted in the bed of the truck but can be removed and left at the job site. This compressor does not rely on the truck for power since it has its own engine.

However, it does require fuel to power the engine, unlike the truck-mounted compressor, which runs from the truck. The engine also requires regular maintenance.As you see, the power source of compressors varies with some powered electronically and others requiring fuel.

With either type, though, the air is stored in the holding tank and the tools are to be attached with a hose. A valve regulates the pressure which is measured by a gauge.

Quincy and Husky are two of the major brands of compressors. Quincy’s rotary screw compressors are very durable, reliable and quiet with its power level varying from 10 to 350 horsepower.

Quincy’s available air compressors range in size from the smaller tank models to the larger, stationary styles. Husky’s line of small air compressors are generally made for home and personal use. The 1.75 gallon tank compressor has 135 PSI power has an oil free pump for easy maintenance and is easy to transport with its telescope handle.

This model of Husky compressors is good for running tools such as nailing guns, for example, and is also useful for insulation purposes.

Husky’s four gallon model has 125 PSI power and is a good choice for running small tools, spraying and even inflating tires and other recreational items. The four-gallon model is a compressor for homeowners.

Equipment, which produces, compressed air from ambient air. Compressed air is often a source of power and motion for packaging equipment.

Compressor equipment or compressors are used to compress ambient air and give the air a higher pressure. Compressors can employ different methods to compress ambient air. A reciprocating air compressor uses reciprocating pistons and valves to compress the air.

A centrifugal compressor uses two close tolerance centrifugal turbines to create compressed air. A screw type compressor uses twin rotating interlock screws with a cavity between the two screws, which is progressively decreasing in volume.

Most industrial type air compressors will produce compressed air pressurized to approximately 100 pounds per square inch (psi). Most packaging machines utilize compressed air pressurized to 60 to 80 pounds per square inch (psi). (Because of air pressure regulators attached to most machines, a 100-psi air feed from a compressor will working for an 80-psi machine.

Compressed air is used in conjunction with pneumatic actuators also called air cylinders. Air cylinders are designed with a small metal rod with a diaphragm at one end. An air cylinder can be used as a linear actuator because the metal rod extends beyond the cylinder and the use of compressed air can turn into actual motion.

General Information

There are many different types of Air Compressor Equipment, and Frain can help you find the right Air Compressor machine to meet your needs. We have Centrifical Air Compressor Equipment, Reciprocating Air Compressor Equipment, and Screw Air Compressor Equipment.

Reciprocating compressors - In these machines, also known as piston-driven compressors, pistons compress the air in a cylinder and force it into a high-pressure storage tank. Piston-driven machines are the oldest and most affordable type of air compressor, and are most commonly found in portable applications or home workshops.

Rotary screw air compressors - Instead of using pistons, rotary screw compressors use twin screws, like two oversized drill bits next to each other, to force air up into higher pressures. Until recently, rotary screw compressors were considerably more expensive than reciprocating models - now, prices are becoming more and more competitive.

Both types are available in two-stage models that compress the air a second time to allow for increased pressure and air flow.

An important way to decide between these two is to consider whether your application is continuous use or intermittent. A nail gun, firing a burst of air every few seconds, is an intermittent tool, while a spray painter is used continuously. If your application calls for continuous use, you'll want a rotary compressor.

That's due to the rated "duty cycle:" the amount of time each hour that a compressor is able to work. A duty cycle of 75% means the air compressor needs to rest for 15 minutes out of every hour to cool down. Rotary compressors have a duty cycle of 100%, thanks to fewer moving parts and robust oil cooling systems. In contrast, reciprocating compressors are designed to run only part of the time. A third major type is the centrifugal compressor.

These high-end compressors are essentially turbine engines: they use rotating blades to create high pressure. They are almost exclusively used in power plants and huge industrial applications, simply because they begin to become economically competitive at a scale well beyond that of most users.

Breaking it Down

Every compressed-air system begins with a compressor - the source of air flow for all the downstream equipment and processes. The main parameters of any air compressor are capacity, pressure, horsepower, and duty cycle.

It is important to remember that capacity does the work; pressure affects the rate at which work is done. Adjusting an air compressor's discharge pressure does not change the compressor's capacity - even though many people seem to believe it will.

There are a number of basic air compressor designs - and variations of them - on the market today. They all fall into two general categories: positive displacement and dynamic.

Although the operating specifications for two different types of air compressors may be very similar on the surface, other installation and performance factors can make one design superior to the other in a real-world application. Let's review some of the basic designs and terminology.

Reciprocating compressors

Reciprocating compressors are positive-displacement units that trap a charge of air and then physically reduce the space that confines it, causing its pressure to increase.

Reciprocating units, commonly called piston compressors, use a piston, cylinder, and valve arrangement. Their operation is very similar to the familiar internal-combustion engine, but they simply trap and compress the air without adding fuel to explode it.

Note that whenever air is compressed, heat is generated. Proper cooling of the internal parts of any air compressor is a critical part of its design.

There are three basic selection decisions that must be made about reciprocating compressors:

•     single- or double-acting operation,

•     single- or multi-stage configuration, and

•     air or water cooling.

In a single-acting piston compressor, the piston only compresses air in one direction of its stroke.

In a double-acting model, the piston compresses air with both directions of its stroke. Obviously, because both strokes perform work, a double-acting compressor is more efficient (in moving a volume of air per input hp) than a comparable-size single-acting unit.

A single-stage unit compresses air from inlet to discharge pressure in one operation. A multi-stage unit compresses from inlet to discharge pressure in two or more operations - generally passing the air through an intercooler to remove some of the heat of compression between each stage.

This saves power and keeps the compressor's internal operating temperatures lower.

In air-cooled compressors, ambient air circulates around the compressor cylinders and finned heads to provide cooling. Heat transfers through the metal to the air. Air-cooled units are generally designed for 50% to 75% duty cycles, depending on the particular units and their application.

In water-cooled compressors, integral water jackets surround the cylinders and heads. Heat transfers through the metal to the water - more effectively than through metal to air. Thus, water-cooled reciprocating units reduce internal temperatures more efficiently than comparable air-cooled units.

Most air-compressor manufacturers promote the two-stage compressor as the optimum machine for producing 100-psi class air - the base pressure level in most industrial plants - providing the best efficiency per dollar cost with adequate reliability of internal working parts.

For a reciprocating compressor to be categorized as continuous duty, it is generally agreed that it must be double acting and water cooled. Double-acting, water-cooled reciprocating compressors are offered in a variety of styles that combine efficient air compression with durability and reliability. However, they also are heavy and bulky, making them relatively expensive to install.

They generally have more-significant unbalanced forces, which combines with their size to require a special foundation and support.

When they meet selection criteria such as capacity, weight, size, and price, single- and two-stage single-acting reciprocating units are a good choice - particularly in the 50- to 150-psig pressure ranges. (Three-stage reciprocating units are offered, but generally are used for pressures above 250 psig.)

Oil-cooled rotary-screw compressors

The rotary-screw compressor is another positive-displacement machine. In an analogy with the reciprocating compressor, Figure 1, the male rotor is like a piston, pushing air along the female rotor, which is like the cylinder. The sealing strips are like piston rings, and air is compressed against the stationary end plate, which is like the bottom of the cylinder.

This design has been around for about 50 years. However, until the mid 1970s, it was considered suitable only for engine-driven portables and small-horsepower electric-motor units because of low efficiency (the ratio of compressed-air delivery to power cost).

In the 1970s, development began on two-stage rotary screw compressors for pressures up to 250 psi. Rotor-profile development during the 1970s, 1980s, and early 1990s has led the oil-cooled rotary-screw design to become the significant choice in electric-motor-driven, lubricated, industrial air compressors, particularly in sizes from 20 to 300 hp.

Then, a significant breakthrough in air-end design occurred. The introduction of the unsymmetrical profile resulted in an efficiency improvement of approximately 15%. This improvement was significant enough to make the oil-cooled rotary-screw compressor competitive in the larger-horsepower sizes for continuous duty. It has almost the same efficiency as the single-stage double-acting units and smaller centrifugal compressors.

Two-stage rotary-screw compressors can approach and sometimes equal the full-load performance of two-stage reciprocating units in 100-psig class service.

Today, two-stage oil-cooled rotary-screw compressors are frequently used in the 150- to 400-psia pressure range. They also are used for 100-psi service with significant power savings. Two stages offer advantages associated with lower compression ratio per stage.

Reduced pressure differential across the rotors minimizes blow-by and significantly reduces thrust-bearing loads. (Obviously two-stage units require two air ends, which increase the initial cost.)

The unique characteristic of this compressor is that it is cooled by oil. Oil injected into

the air stream absorbs the heat of compression while it is being generated.

The heated oil then is taken to an air- or water-cooled heat exchanger for cooling. Because the cooling takes place right inside the compressor, the working parts are never subjected to extreme operating temperatures. The cooling oil never is cracked nor burnt.

No matter what the load on the compressor is, there are no hot spots inside the airend. The resulting absence of wear produces trouble-free service and high efficiency.

In other words, oil-cooled rotary-screw compressors can run at full load and full pressure -twenty-four hours a day, seven days a week. This compressor's useful life in operating hours and its maintenance cost per hour will be the same as under any other load condition.

Continuous duty

The availability of continuous-duty air-cooled compressors (particularly in large sizes) offers a great deal of flexibility for installing them.

Such compressors can be mounted on any surface that will support their static weight. In many facilities, great savings also are available in piping cost, compared to other types of systems.

These compressors lend themselves to either the central- or departmental-compressor system concept. Units are available with electric motor and engine drives - on bases, on skids, on wheels, etc.

Compared to other types of continuous-duty air compressors, oil-cooled rotary-screw compressors offer a number of advantages:

•     Oil cooling holds internal temperatures to an optimum level. As a result, discharge air is relatively cool -no more than about 180° F higher than ambient.

•     Discharge air is clean - free from burned oil or carbon.

•     The rotary design lends itself to higher speeds, particularly in the larger sizes. Consequently, larger flow capacity is available from compressors with physically smaller envelopes - providing significant savings on floor space and foundation requirements.

•     Because of their compact size and inherent quiet-running characteristics, it is relatively easy to suppress noise. Electric-motor-driven models are commercially available rated from 75 to 85 dB at one meter per the CAGI Pneurop Test Code.

•     Most models have fewer moving parts, and those parts run under more ideal conditions - resulting in lower temperatures and less vibration.

•     Fewer parts make it easier to stock them for the rotary designs, and the machines are easier to work on.

In summary, oil-cooled rotary-screw compressors offer users a continuous-duty source of compressed air in a neat, compact package that has low initial cost, maximum flexibility of installation, and easy maintenance.

Non-lubricated rotary screw and lobe

In addition to the non-lubricated reciprocating compressors that have become so common over the years, there are several versions of non-lubricated positive-displacement lobe or screw rotary compressors.

These units are referred to as clearance-type compressors because the internal parts do not contact each other, so they require no lubrication in the compression chamber. Cooling is accomplished through the cylinder walls via water jackets.

The lobes or screws do not drive one another either; they are driven by some type of gear arrangement instead. This drive system also acts as a timing gear to maintain the rotor or lobe profile relationship accurately. Lubricant for the drive train must be confined to the bearing and gear area - and not allowed to get into the compression chamber.

In this basic design, there is a constant leakage rate for any fixed set of conditions. The critical internal clearances are between end covers and the rotor, between the rotor lobes, and between the rotor OD and the cylinder ID.

These gaps, combined with no injected oil to help with sealing, are the main reasons why two stages are required for these units to produce acceptable efficiencies in 100-psi class applications.

Because these are rotary units, they enjoy all the advantages of rotaries over similar-sized non-lubricated reciprocating units:

•     compact size,

•     smooth delivery of cool air,

•     ease of installation, and

•     simple (but critical) maintenance

They also have some disadvantages, depending on the specific type of compressor and its duty cycle:

•     more sensitive to dirty inlet air,

•     lower efficiency - resulting in higher power cost, and

•     any repair work is more sophisticated and requires specialized training, which the user may not have nor want to have. This means repair work will probably have to be performed by the distributor or the manufacturer.

Sliding-vane rotary compressors

Oil-cooled sliding-vane compressors, Figure 2, operate as other positive-displacement compressors do by trapping a charge of intake air - in this case, between the vanes. As the eccentric rotor turns, the vanes are forced into the rotor slots, shrinking the size of the cell holding the trapped air.

The air is compressed to full discharge pressure when it reaches the outlet port. The heat of compression is removed by cooling oil sprayed right into the air while it is being compressed. The same oil helps with sealing the vane tips.

For decades, oil-cooled, sliding-vane rotary compressors have been popular for continuous-duty applications. Their design has a number of unique characteristics:

•     light weight - yet continuous rating,

•     integrated and compact configuration,

•     efficient production of compressed air at relatively low rotary speeds,

•     smooth operation with little vibration,

•     extremely quiet operation,

•     coolest possible discharge air, and

•     few wearing parts, making the machine easy and economical to repair.

However, the oil-cooled rotary-vane design in its single-stage configuration is limited in capacity. Bending stress applied to the vanes is the problem.

The speed, size, and weight of the vanes must be limited for the machine to be durable. Because of this, oil-cooled rotary-vane compressors generally are applied only in a size range between 2 and 100 hp.

Lubricated or lube-free?

Two fundamental groups of compressor types are lubricated and lube-free. Lubricated compressors use oil to reduce friction between moving parts.

As a result, some oil is entrained in the air being compressed. The entrained oil must be removed from or tolerated by the downstream system.

Lube-free compressors use no oil in the airend, and thus add no oil to the compressed air they produce.

Power and efficiency

Brake horsepower is the input power required at the compressor input shaft for a specific speed, capacity, and pressure condition.

Motor or engine horsepower is the nominal rating of the prime mover.

The service factor is the additional power built into an electric motor above its nominal rating - expressed as percent. Within the service factor, the brake horsepower driving an air compressor can be higher than the motor's nominal horsepower.

The power efficiency of a compressor is the ratio of the air delivered by the compressor and its input electrical requirements. Efficiency usually is expressed as brake horsepower per 100 cfm of delivered air.

Water-cooled rotary screws

Another version of oil-free rotary-screw compressors is a single-stage design that uses water injection to cool and seal the rotors during compression.

The bearings and drive gears are lubricated with oil and sealed from the compression chamber. These units serve a selected market and are a special design. In some applications, care must be taken to avoid the build-up of bacteria in the water.    

Approximately 70 percent of all manufacturers have a compressed air system. These systems power a variety of equipment, including machine tools, material handling and separation equipment, and spray painting equipment.