Compressed air system.docx

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Compressor is a device used to increase the pressure of the compressible fluid. The Inlet and outlet pressure are related,

corresponding with the type of compressor and its configuration.

Compression Method:

There are two basic mode of compression: Intermittent and continuous.

The Intermittent mode of compression is cyclic in nature, in that a specific quantity of gas is ingested by the compressor,

acted upon, and discharged, before the cycle is repeated. The compressors using the intermittent mode of compression

are referred to positive Displacement compressor.

The Continuous compression mode is one in which gas is moved into the compressor, is acted upon, moved through the

compressor, and discharged without interruption of the flow at any point in the process. The compressors using the

continuous mode of compression are referred to Dynamic Compressor.

The below chart shows the classification of compressor.

Positive Displacement Vs. Dynamic Displacement:

Positive displacement compressors increase the pressure of the gas by reducing the volume. Positive displacement

compressors are usually of the reciprocating piston type, in which the gas is drawn in during the suction stroke of the

piston, compressed by decreasing the volume of the gas by moving the piston in the opposite direction, and, lastly,

discharged when the gas pressure exceeds the pressure acting on the outlet valve.

Dynamic Displacement Compressor increases the kinetic energy of the gas with a high-speed impeller and then converts

this energy into increased pressure in a divergent outlet passage called the diffuser. As gas passes through the

compressor, its velocity is alternately increased and decreased. During each increase in velocity the kinetic energy of the

gas is increased, and during each decrease in velocity this kinetic energy is converted into an increase in pressure.

http://www.britannica.com/EBchecked/topic/187171/energy

http://www.britannica.com/EBchecked/topic/187171/energy



Reciprocating Compressor:

It consists of a mechanical arrangement in which reciprocating motion is transmitted to a piston which is free to move in

a cylinder. The displacing action of the piston, together with the inlet valve causes a quantity of gas to enter the cylinder

where it is in turn compressed and discharged. Action of the discharge valve to prevent backflow of gas into the

compressor from the discharge line during the next intake cycle. When compression takes place on one side of the

piston only, the compressor is said to be single acting. The compressor is double acting when compression takes place

on each side of the piston. Configuration consists of a single cylinder or multiple cylinders on a frame. When singlecylinder is used or multiple cylinders on a common frame are connected in parallel, the arrangement is referred to as

single stage compressor. When multiple cylinders on a common frame are connected in series, the arrangement is

referred to as multistage compressor.

Rotary Compressor:

The Rotary compressor portion of the positive displacement family is made up of several compression configurations.

The features these compressors have in common are:

They impart energy to the gas being compressed by way of an input shaft moving a single or multiple rotating

elements.

They perform the compression in an intermittent mode.

They do not use inlet and discharge valve.



The helical and spiral lobe compressors are generally similar and use two intermeshing helical or spiral lobes to

compress gas between the lobes and the rotor chamber of the casing. The compression cycle begins as the open part of

the spiral form of the rotors passes over the inlet port and traps a quantity of gas. The gas is moved axially along the

rotor to the discharge port where the gas is discharged into the discharge nozzle of the casing. The volume of the

trapped gas is decreased as it moves toward the outlet.

The spiral lobe version is the more limited of the two and is used only in the lower pressure application.

The helical lobe compressor is further divided into a dry and a flooded form. The dry form uses timing gears to hold a

prescribed timing to the relative motion of the rotors; the flooded form uses a liquid media to keep the rotors from

touching. This compressor is usually referred to as the Screw Compressor .

The application range of the helical lobe compressor is unique in that it bridges the application gap between the

centrifugal compressor and reciprocating compressor. The capacity range of the dry configuration is approximately 500

to 35000cfm. Discharge pressure is limited to 45 psi in single stage configuration with atmospheric suction pressure. On

supercharged or multistage applications, pressures of 250psi are attainable. The spiral lobe version is limited to

10000cfm flow and about 15psi discharge pressure.

Screw compressors are generally two types:

Oil-free: 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 cfm (measured at 60 °C and atmospheric pressure).

Oil-flooded: 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 ofcompressed air particulate filtration. It is usual for some entrained compressor oil to carry over into the compressed gas

stream downstream of the compressor.



The Straight lobe compressor is similar to the helical lobe machine; it has two untwisted or straight lobe rotors that

intermesh as they rotate. Normally, each rotor pair has a two lobe rotor configuration, although a three lobe version is

available. All versions of the straight lobe compressor use timing gears to phase the rotors. Gas is trapped in the open

area of the lobe as the lobe pair crosses the inlet port. There is no compression as gas is moved to the discharge port;

rather it is compressed by the backflow from the discharge port. Four cycles of compression take place in the period of

one shaft rotation on the two lobe version. The operating cycle of the straight lobe rotary compressor is shown in below

figure.

Volume range of straight lobe compressor is 5 to 30000cfm. Pressure ranges are very limited with the maximum single

stage rating at 15psi. In few applications, the compressors are used in two stages from where the discharge pressure is

extended to 20psi.

The sliding vane compressor uses a single rotating element. The rotor is mounted eccentric to the center of the cylinder

portion of the casing and is slotted and fitted with vane. The vanes are free to move in and out within the slots as the

rotor revolves. Gas is trapped between a pair of the vanes as the vanes cross the inlet port. Gas is moved and

compressed circumferentially as the vane pair moves toward the discharge port. The port location controls the pressure

ration.

The sliding vane compressor is widely used as a vacuum pump as well as a compressor with the largest volume

approximately 6000cfm. The lower end of the volume ranges is 50cfm. A single stage compressor with atmospheric inlet

pressure is limited to a 50psi discharge pressure. In booster service, the smaller units can be used to approximately

400psi.

Continuous Compression Compressors:

Continuous compression compressors are two types: Ejector and dynamic

The Ejector can first identify as having no moving parts. It is used primarily for that future as it is not as efficient as most

of the mechanical compressors. Ejector is widely used as vacuum pump, where it is staged when required to achieve

deeper vacuum levels.



The Dynamic compressor , energy is transformed from a moving set of blades to the gas. The dynamic compressors are

further subdivided into three categories, based primarily on the direction of flow through the machine. These are radial,

axial and mixed flow.

The radial flow or centrifugal compressors are widely used compressor. The compressor uses an impeller consisting of

radial or backward leaning blades and a front and rear shroud. The front shroud is optionally rotating or stationary

depending on the specific design. As the impeller rotates, gas is moved between the rotating blades from the area near

the shaft and radially outward to discharge into a stationary section, called diffuser. Energy is transferred to the gas

while it is travelling through the impeller. The part of energy converts to pressure along the blade path while the balance

remains as velocity at the impeller tip where it is slowed in the diffuser and converted to the pressure. The fraction of

the pressure conversion taking place in the Impeller is a function of the backward leaning of the blade. The more radial

the blade, the less pressure conversion in the impeller and the more conversion taking place in the diffuser. Centrifugal

compressors are quite often built in a multistage configuration, where multiple impellers are installed in one frame and

operate in series.

Centrifugal Compressors range in volumetric size from approximately 1000 to 1,50,000 cfm. In single wheel

configuration, pressure varies considerably. A common low pressure compressor may only be capable of 10 to 12 psi

discharge pressure. In higher head models, pressure ratios of 3 are available, which on air is a 30psi discharge pressure

when the inlet is at atmospheric condition. Centrifugal are in service at relatively high pressure up to 10000psi either as

a booster or as the result of multiple compressors operating in series.



Axial Compressors are large volume compressors that are characterized by the axial direction of the flow passing

through the machine. The energy from the rotor is transferred to the gas by blading. Typically, the rotor consists of

multiple rows of enshrouded blades. Before and after of each rotor row is a stationary (stator) row. For example, a gas

particle passing through the machine alternately moves through a stationary row, then a rotor row, then another

stationary row, until it complete the total gas path. A pair or rotating and stationary blade rows define a stage. One

common arrangement has the energy transfer arranged to provide 50% of the pressure rise in the rotating row and the

other 50% in the stationary row. This design is referred to as 50% reaction.

Axial compressors are smaller and are significantly more efficient than centrifugal compressors when comparison is

made at an equivalent flow rating.

The volume range of the axial starts at approximately 70000 cfm. One of the largest sizes built is 1,000,000 cfm, with the

common upper range at 300,000cfm. The axial compressor, because of a low pressure rise per stage, is exclusively

manufactured as a multistage machine. Axial compressors are an integral part of large gas turbine where the pressure

rations normally are much higher. In gas turbine service, discharge pressures up to 250psi used.

Mixed flow compressor is relatively uncommon form. Mixed flow compressor very much resembles the radial flow

compressor. A bladed impeller is used, but the flow path is angular in direction to the rotor; that is , it has both radialand axial components. Because of the stage spacing is wide; the compressor is used almost exclusively as a single stage

machine. The energy transfer is same as described for the radial compressor.

Comparison between reciprocating and rotary compressor:

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 possiblebecause 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:

Piston compressors operate at internal temperatures of 300 to 400°F, while a rotary compressor runs at much lower

internal temperatures (between 170 and 200°F). Just as hot summer air holds more humidity, hotter compressed air canhold 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 after coolers 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 after cooler 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 flexibilitywhere 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.

http://www.cagi.org/

http://www.cagi.org/



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:

Note: Reciprocating compressor uses for small industries, shop, workshop etc, but Rotary screw compressor used in big

plant.

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Compressed air system.docx