Air compressors

Roots blowers

2012-02-28T00:57:00.002-08:00

Roots blowers

Roots blowers is named for the brothers Philander and Francis Marion Roots of Connersville, Indiana, who first patented the basic design.

Roots blowers features

A Roots blower is a positive displacement lobe compressor without internal compression. Once the compression chamber comes into contact with the outlet, compressed air flows back into the housing from the pressure side. Subsequently, further compression takes place when the volume of the compression chamber further decreases with continued rotation. Accordingly, compression takes place against full counter-pressure, which leads to low efficiency and a high noise level.

Roots blowers work by two contra rotating lobes in a housing, synchronized by means of a set of gear wheels. Roots blowers cannot achieve high pressures, they are usually air cooler, reliable and produce oil-free air. Low efficiency of roots blowers limits them to very low pressure applications and compression in a single stage, even if two- and three-stage versions are available. Roots blowers are frequently used as vacuum pumps and for pneumatic conveyance.

Diaphragm compressor

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Diaphragm compressor

Diaphragm compressors form another group with backup and piston rings and rod seal. Their diaphragm is actuated mechanically or hydraulically by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in touch with pumped gas. For this reason the mechanical diaphragm compressors construction can be used with a small flow and low pressure or as vacuum pumps for pumping toxic and explosive gases. The membrane has to be reliable enough to take the strain of pumped gas. It must also have adequate chemical properties and sufficient temperature resistance.

A diaphragm compressor is also called as a membrane compressor.

Scroll compressors

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Scroll compressors structure and operation

A scroll compressor is one type of oil-free orbital motion, positive-displacement compressor. The compressor element consists of two interfitting, spiral-shaped scroll spirals, a stator spiral is fixed in a housing and the other is motor-driven eccentric, orbiting spiral. The spirals are mounted with 180° phase displacement or mirror displacement. The orbiting spiral is driven by a short-stroke crankshaft and runs eccentrically around the center of the fixed spiral. The inlet is situated at the top of the element housing. This movement forms air pockets with a gradually varying volume. By this way, scroll compressor compresses a specific amount of air into a continuously decreasing volume.

Scroll compressors features

This motion of spirals provides the scroll elements with radial stability. Leakage can be minimized because the pressure difference in the air pockets is lower than the pressure difference between the inlet and the outlet.

When the orbiting spiral moves, air is drawn in and is captured in one of the air pockets, where it is compressed gradually while moving towards the center where the outlet port and a non-return valve are situated. The compression cycle is in progress for 2.5 turns, which virtually gives constant and pulsation-free air flow. Scroll compressors have many distinctly appealing qualities. They are efficient, quiet, and reliable and vibration-free, as the element has hardly any torque variation as compared to a piston compressor, for example.

Tooth compressors

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Tooth compressors structure

The compression element in a tooth compressor consists of two rotors that rotate in opposite directions inside a compression chamber.

Tooth compressors compression process

The compression process consists of intake, compression and outlet. During the intake phase, air is drawn into the compression chamber until the rotors block the inlet. During the compression phase, the drawn in air is compressed in the compression chamber, which gets smaller as the rotors rotate.

The outlet port is blocked during compression by one of the rotors, while the inlet is open to draw in new air into the opposite section of the compression chamber.

Discharge takes place when one of the rotors opens the outlet port and the compressed air is forced out of the compression chamber.

Both rotors are synchronized via a set of gear wheels. The maximum pressure ratio obtainable with an oil-free tooth compressor is limited by the limiting temperature difference between the inlet and the discharge. Consequently, several stages with inter-stage cooling are required for higher pressures.

Screw compressors

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Screw compressors

In the 1930s, a rotating compressor with high flow rate and stable flow under varying pressure conditions was required. Then the principle for a rotating displacement compressor in twin screw form was developed. it took nearly 75 years until the wide ranging opportunities for this type of compressor were recognized.

The twin screw element’s main parts are the male and female rotors, which rotate in opposite directions while the volume between screws and the housing decreases. Each screw element has a fixed, build-in pressure ratio that is dependent on its length, the pitch of the screw and the form of the discharge port. To attain maximum efficiency, the build-in pressure ratio must be adapted to the required working pressure.

The screw compressor is generally not equipped with valves and has no mechanical forces that cause unbalance. This means it can work at a high shaft speed and can combine a large flow rate with small exterior dimensions. An axial acting force, dependent on the pressure difference between the inlet and outlet, must be overcome by the bearings.

Oil-free screw compressors

The first twin screw compressors had a symmetric rotor profile and did not use any cooling liquid inside the compression chamber. These were called oil-free or dry screw compressors. Modern, high-speed, oil-free screw compressors have asymmetric screw profiles, resulting in significantly improved energy efficiency, due to reduced internal leakage.

External gears are most often used to synchronize the position of the counter-rotating rotors. As the rotors neither come into contact with each other nor with the compressor housing, no lubrication is required inside the compression chamber. Consequently, the compressed air is completely oil-free.

The rotors and housing are manufactured with ultimate precision to minimize leakage from the pressure side to the inlet. The build-in pressure ratio is limited by the limiting temperature difference between the inlet and the discharge. This is why oil-free screw compressors are frequently built with several stages and inter-stage cooling to reach higher pressures.

Liquid-injected screw compressors

In liquid-injected screw compressors, a liquid is injected into the compression chamber and often into the compressor bearings. Its function is to cool and lubricate the compressor element’s moving parts, to cool the air being compressed internally, and to reduce the return leakage to the inlet.

Today oil is the most commonly injected liquid due to its good lubricating and sealing properties, however, other liquids are also used, for example, water or polymers. Liquid-injected screw compressor elements can be manufactured for high pressure ratios, with one compression stage usually being sufficient for pressure up to 14 and even 17 bar, albeit at the expense of reduced energy efficiency.

Screw compressors market situation

Screw compressors are manufactured from small up to medium output volume ranges and thus overlap market areas which, until recently, were still reserved for turbo compressors. Only a few manufactures can supplier screw compressors above 300kW. The smaller ranges of screw compressors are supplied in air cooled form whereas the larger ones are constructed either in air cooled or in water cooled versions.

Vane compressors

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Vane compressors are one type of rotary displacement compressors

The rotor of vane compressors is eccentrically mounted relative to the axis of the casing. Longitudinal slots for holding the vanes are cut into this rotor. Upon rotation, the centrifugal force presses the vanes against the internal wall of the housing.

Trailing rings is formed by the service liquid rotating concentrically to the axis of the casing, around the periphery of the impeller rotor. The internal diameter of the trailing rings used, is made somewhat smaller than the internal cylinder or housing diameter. Vane compressors exist in oil lubricated and oil flooded designs. In both cases, the compressor oil serves not only for lubrication but also as a sealant between individual vanes and the housing inner wall.

Oil flooded vane compressors

For oil flooded vane compressors, a considerable amount of oil is introduced into the compression chamber. This amount of oil is used to conduct away the heat generated by compression so that the compressor output temperature amounts to only about 80-90°C. The injected oil is filtered through separators after compression and channeled back to the circuit after separation.

oil lubricated vane compressors

On the other hand, for oil lubricated vane compressors, compressor output temperature reaches very high temperatures in the compression chamber, depending on the final pressure. Compression causes the compressor oils to crack to such an extent that the oil residues in the compressed air may still be filtered out, however, only at the expense of reduced service life time of the downstream filter elements.

How does the vane work

The individual vanes, manufactured either from phenolic resin impregnated plastics or hardened steel, form cells within the rotor which, upon rotation, expand on one side and shrink on the other. On the intake side, the ambient air is drawn in through the enlargement of the cells and, through further rotation, conveyed to the pressure side. There, shrinking of the cells leads to continuous compression of the air.

Compact units of vane compressors

Vane compressors are supplied as ready to connect compact units. These single-stage, single shaft compressors can be installed without foundations. This type of compressor is usually fully equipped with aftercooler, separator and all necessary safety devices. Compressors of this design have no valves but, are fitted with an output control adapted to the desired output pressure.

Power consumption of vane compressors

The power consumption of the drive motor is harmonized with the operating conditions. The less the compressed air is consumed, the more the system pressure rises. Once the high pressure limit value is reached, the regulating system opens an integrated discharge valve and the pressure in the compressor falls to low pressure limit.

Connection of vane compressors

The compressor is separated from the distribution network of points of use by a check valve. On the intake side, the drawn in volume is set to the lowest value through restriction. The compressor now operates in a closed circuit or idle running, with minimum power consumption, which amounts to about 22% of the nominal consumption. Vane compressors, whether air or water cooled, find diminishing use for industrial applications. Maximum pressure is usually up to 10 bar gauge. The output is suitable for medium requirements.

Piston compressors

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What is piston compressor?

The piston compressors are the oldest and most common industrial compressors. In principle, the piston or reciprocal compressors consist of a crank case with crank shaft and connecting rod, piston and cylinder, as well as intake and pressure valve.

The rotary motion of the crank shaft results in the reciprocating motion of the piston. When the piston strokes backward, the air is drawn from the atmosphere via the open intake valve into the cylinder space. When, piston strokes upward, the piston compresses the drawn in ambient air inside the cylinder chamber, the intake valve is closed and the air in the cylinder undergoes compression.

The air is therefore compressed to a fraction volume of its original volume while, at the same time, the pressure in the cylinder is increased. The relationship of volume and pressure changes can be explained with Gas Laws. Once the pressure enclosed by the cylinder exceeds the spring force of the pressure valve, this valve opens and the compressed air is expelled out from the cylinder.

Piston compressor structures.

Compression types for piston compressors can be separated as single-stage or multi-stage. With single-stage compression, the compressed air is immediately pushed into the compressed air system directly. With multi-stage compression, the air will pass through further stages of compression follow the first stage, then is pushed in to the compressed air system.

A two-stage compressor generally consists of two single-stage compressors mounted in series upon a common crank shaft casing. With two stage or multi-stage compression, the mechanical strain on the pressure bearing parts is thus diminished, achieving a longer service life of the piston compressor.

An intercooler is installed between the first and the second stage of the compressor. Without an intercooler, the work of compression would, in the course of the second stage, continue to heat and expand the air volume and have an effect upon the compressor output temperature. Then the compression effect is same as single-stage compression. Cooling by means of the intercooler permits compression towards the final pressure in the face of reduced air volume. It is a further advantage of two- stage compression that the final temperature of the air after compression is lower as a result of cooling after compression.

Two-stage compression to pressure at 9 bar causes a delivery temperature of 110°C whereas, if the same pressure had been reached by single-stage compression, the delivery pressure would be 240°C. Such a high temperature resulting from single-stage compression would naturally cause problems, regarding the lubrication of the compressor.

Piston compressor working theory

Piston compressors have various forms. The standard form is the complete compressor assembly, with an electric motor as a drive unit and the fundamental accessories such as intake filter, inter and after-cooler, automatic water drain and safety release valves.

An integrated control system is very important, because it must be able to optimize the loading, idle running and off-loading of the compressor. Control depends on a signal provided by monitoring the pressure transmitter set for the operating with a lower and upper pressure limit value.

If the lower limit value is reached, the compressor automatically and continuously displaces compressed air into the system until the upper switching point has been reached, thus causing the switching off of the compressor. Off-loading control switches off the compressor when the upper switching limit has been reached. This type of control is applied if the compressed air use is subject to great variations and is irregular. This type of control is the most economical, but the highest permitted switching frequency of the electric motor must not be exceeded in its application.

Idle running control is used when frequent short off-loading times leading to high frequency of switching of the compressor. When the upper switching frequency limit is reached, the throughput is set to “zero”. The compressor then continues running idly, using little current.

At present, standards provide the combination of off-loading and idle running control, with the additional refinement, the duration of the idle running can be set. This makes it possible to have an automatic adaptation to operating conditions when these involve high air consumption with a correspondingly high frequency of switching.

Now, nearly all manufacturers of compressors provide custom designed integrated control systems for different operating conditions.

Piston compressor products

Single-stage compressors are usually for the lower pressure ranges and small outputs. For higher pressures, two stage and multi-stage compressors are usually installed.

Up to medium range output quantities, piston compressors are normally supplied solely in the air cooled types, except this, manufacturers provide the options of air and also water cooled compressors.

Piston compressor effective shaft power

Diagram below shows the shaft output power of piston compressors as a function of the throughput and the operational gauge pressure (delivery pressure). This diagram is based on typical piston compressors with a rotary speed of 1450 r.p.m.

The shaft power levels given in the diagram do not refer to a particular type but are mean values based on a uniform design series.

Most common Types of compressors, displacement compressors and dynamic compressors

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There are two generic working principles for the compression of air (or gas), so compressors can be subdivided into two groups: The first group operates in the principle of dynamic compression; and the second group of compressors operate in positive displacement principle:

In dynamic compression group, compressors are oil free compressing systems. The effective quantity delivered by a dynamic compressor depends on the operating pressure.

For examples: compressors using the dynamic compression principle are turbocompressors of axial or radial design. In a dynamic compressor, the pressure increasing takes place while the gas flows. Single or multi bladed wheels are used to impart a high velocity of flow to the air drawn in. The flow rate of the gas is subsequently transformed into static pressure when it is forced to decelerate under expansion in a diffuser. Thus converting the kinetic energy of the flow velocity into pressure. All dynamic compressors are designed for large volume flow rates.

The second group consists of compressors which operate in accordance with the positive displacement principle. The air is drawn into one or more compression chambers, which are then closed from the inlet. Gradually the volume of each chamber decreases and the air is compressed internally. So static pressure is built up through this positive displacement process, later the compressed air can be discharged to compressed air distribution system.

Positive displacement compressors include piston compressors and rotary compressors (vane compressors and screw compressors).

These displacement compressor systems are supplied in three versions: oil free (“non lubricated”), oil lubricated and oil flooded versions.

Displacement compressors by far are the most widely used types, and the common types are listed as follows:

Piston compressors: oil free and oil lubricated versions.
Vane compressors: oil lubricated and oil flooded versions.
Screw compressors: oil free and oil flooded versions.

Oil free compressors are very commonly used in clean industries, such as food, pharmaceutical, electronics and semiconductor industries, etc.

But in other industries it is also a better choice to use oil free compressors, in where it is possible, to keep oil pollution away from the compressed air.

It is also not correct to say that the oil free compressors make the compressed air oil free, so the compressed air purification could be completely removed. In most cases, the atmosphere air is not clean enough, it contains solid particles, air borne molecules, oil vapor and oil aerosol. But the treatment of compressed air is much simpler than for compressed air from the other type of compressors.

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2012-02-06T02:06:00.000-08:00

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