A displacement compressor is characterised by enclosing a volume of gas or air and then increasing the pressure by reducing the area of the enclosed volume.
Dynamic compressors are available in axial and radial designs. The latter are frequently called turbo or radial turbo and the former are called centrifugal compressors. A dynamic compressor works with a constant pressure, unlike for example a displacement compressor, which works with a constant flow. The performance of a dynamic compressor is affected by external conditions, for example, a small change in the inlet pressure results in a large change in the capacity.
A booster compressor is a compressor that works with air that has been compressed and compresses it to a higher pressure. It is used to compensate the pressure drop in long pipelines or in applications where a higher pressure is required for a sub-process. Compression may be single or multi-staged and the compressor can be of a dynamic or displacement type, but piston compressors are the most common. The power requirement for a booster compressor increases with a rising pressure ratio, while the mass flow drops. The curve for power requirement as a function of the intake pressure has the same general form as the curve for a vacuum pump.
Pressure intensifiers increase the pressure in a medium, for example, for laboratory tests of valve, pipes and hoses. A pressure of 7 bar can be amplified in a single stage to 200 bar or up to 1700 bar in multi-staged equipment. The pressure intensifier is only available for very small flows. When the high pressure chamber is filled, the low pressure piston is lifted. When the propellant flows in, the piston is pressed downwards and forces the medium out under high pressure. The intensifier can work in a cycling process, up until a preset pressure level. All inert gases can be compressed in this way. Air can also be compressed in a pressure intensifier, but must be completely oil-free to avoid selfignition.
A vacuum means a lower pressure than atmospheric pressure. A vacuum pump is a compressor that works in this pressure range. A typical characteristic of a vacuum pump is that they work with a very high pressure ratio, however despite this, multi-stage machines are common. Multi-stage compressed air compressors can also be used for vacuums within the pressure range 1 bar(a) and 0.1 bar(a).
An Axial compressor has axial flow, the air or gas passes along the compressor shaft through rows of rotating and stationary impellers. In this way the velocity of the air is gradually increased at the same time as the stationary blades convert the kinetic energy to pressure. The lowest volume flow rate through such a compressor is about 15 m3/s. A balancing drum is usually built into the compressor to counterbalance axial thrust. Axial compressors are generally smaller than equivalent centrifugal compressors and work ordinarily with about a 25% higher speed. They are used for constant high volume rate of flow at a relatively moderate pressure. With the exception of gas turbine applications the pressure ratio is seldom higher than 6. The normal flow is approx. 65 m3/s and effective pressure up to approx. 14 bar(e). (Picture right; Axial compressor example).
A blower is not a displacement compressor as it works without internal compression. When the compression chamber comes into contact with the outlet, compressed air floods in from the pressure side. It is first here that compression takes place, when the volume of the compression chamber decreases with continued rotation. Accordingly, compression takes place against full counter-pressure, which results in low efficiency and a high noise level. Two identical, normally symmetrical, counter-rotating rotors work in a housing with flat ends and a cylindrical casing. The rotors are synchronised by means of a gear wheel. Blowers are usually air cooled and oil-free. The low efficiency limits the blowers to low pressure applications and compression in a single stage, even if two and three stage versions are available. Blowers are frequently used as vacuum pumps and for pneumatic conveyance.
The centrifugal compressor is characterised by the radial discharge flow. Air is drawn into the centre of a rotating impeller with radial blades and is thrown out towards the periphery of the impeller by centrifugal forces. Before the air is led to the centre of the next impeller, it passes a diffuser and a volute where the kinetic energy is converted to pressure. The pressure ratio across each stage is determined by the compressors final pressure. This also gives a suitable velocity increase for the air after each impeller. The air temperature at the inlet of each stage has a decisive significance for the compressors power requirement, which is why cooling between stages is needed. Centrifugal compressors with up to six stages and pressure up to 25 bar are not uncommon. The impeller can have either an open or closed design. Open is the most common with air applications. The impeller is normally made of special stainless steel alloy or aluminium. The speed is very high compared with other types of compressor, 15,000-100,000 r/min are common. This means that journalling on the compressor shaft takes place using plain bearings instead of rolling bearings. Rolling bearings are used on single stage compressors with a low pressure ratio. Often multi-stage compressors have two impellers mounted on each end of the same shaft to counteract the axial loads caused by the pressure differences. The lowest volume flow rate through a centrifugal compressor is primarily determined by the flow through the last stage. A practical limit value of 160 l/s in the outlet from a horizontal split machine is often a rule-of-thumb. Each centrifugal compressor must be sealed in a suitable manner to reduce leakage along the shaft where it passes through the compressor housing. Many types of seal are used and the most advanced can be found on compressors with a high speed intended for high pressures. The four most common types are labyrinth seals, ring seals, (usually graphic seals that work dry, but even sealing liquids are used), mechanical seals and hydrostatic seals.
The piston compressor is the oldest and most common of all compressors. It is available as single or double acting, oil lubricated or oil free with a different number of cylinders in different configurations. With the exception of really small compressors with vertical cylinders, the V configuration is the most common for small compressors. On double acting, large compressors the L-type with vertical low pressure cylinder and horizontal high pressure cylinder, offers immense benefits and is why this is the most common design. Oil lubricated compressors normally work with splash lubrication or pressure lubrication. Most compressors have self-acting valves. A self-acting valve opens and closes through pressure differences on respective sides of the valve disk.
The principle for a rotating displacement compressor a screw was developed during the 1930s, when a rotating compressor with a high capacity and stable flow in varying conditions was required.
The screw element's main parts are the male and female rotors, which move towards each other while the volume between them and the housing decreases. Each screw element has a fixed, integrated pressure ratio that is dependent on its length, the pitch of the screw and the form of the discharge port. To attain the best efficiency the pressure ratio must be adapted to the required working pressure.
The screw compressor is not equipped with valves and has no mechanical forces that cause unbalance. This means it can work at a high shaft speed and 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 taken up by the bearings.
The screw, which originally was symmetrical, has now been developed in different asymetrical helical profiles.
LUBRICATION FREE SCREW COMPRESSOR
The first screw compressors had a symetric profile and did not use liquid in the compression chamber, so-called oil-free or dry screw compressors. At the end of the 1960s a high speed, oil-free screw compressor was introduced with an asymetric screw profile. The new rotor profile resulted in significantly improved efficiency, due to reduced internal leakage. An external gear is used in dry screw compressors to synchronise the counter rotating rotors. As the rotors neither come into contact with each other nor with the compressor housing, no particular lubrication is required in the compression chamber. Consequently the compressed air is completely oil-free. The rotors and housing are manufactured with great precision to minimise leakage from the pressure side to the inlet. The integrated pressure ratio is limited by the temperature difference between the intake and the discharge. This is why oil-free screw compressors are frequently built with several stages.
OIL INJECTED SCREW COMPRESSOR
An oil injected screw compressor is cooled and lubricated by oil that is injected to the compression chamber and often to the compressor bearings. Its function is to cool and lubricate the compressor element and to reduce the return leakage to the intake.
Today oil is the most common liquid due to its good lubricating properties, however, other liquids are also used, for example, water. Liquid injected screw compressor elements can be manufactured for high pressure ratio, which why one compression stage is usually sufficient for pressure up to 13 bar. The elements low return leakage also means that relatively small screw compressors are efficient.
- Air enters the compressor through the intake air filter . The air intake filter is there to protect the compressor from airborne contamination, its size and filtration medium quality will affect both the reliability and energy efficiency of the compressor.
Airborne contamination can: contaminate the oil in the compressor reducing its lubricating quality resulting in scoring on the compression element, bearing failure, and wear on gear drive compressors. Block the separator causing increased pressure drop and the premature failure of the element, causing the oil to enter the compressed air system, the resulting low oil level will cause overheating of the compressor. Block the oil filter element reducing oil flow and causing overheating of the compressor.
When airborne contamination blocks the air intake filter, Pressure drop increases, resulting in increased power Consumption of the compressor. For every 0.5 bar pressure drop, power consumption is increased by 3% Airborne contamination can also block the oil separator and oil filter causing pressure drop within the compressor further increasing power consumption.
- The Intake valve allows air to enter the compression element, On load / off load control , the valve is fully open or fully closed. The vale is set to open at a minimum pressure setting and closes at a preset maximum pressure setting Modulating control system. The valve gradually opens and closes to maintain a preset pressure within the compressed air system.
- Air and oil are drawn into the compression element simultaneously, The oil provides three vital tasks, Cooling of the element, a seal between the rotors, and lubrication. Oil quality is vital for maintaining the efficient operation of the oil injected rotary screw compressor. The incorrect viscosity or low quality oil can increase power consumption and cause premature wear on internal components.
- The air oil mixture enters the oil separator vessel , and the air is separated by mechanical impact within the separator, centrifugal action, by weight and finally by the oil separator cartridge. The quality of the filter medium is essential to reducing oil carryover into the compressed air network.
- When sufficient pressure is generated within the separation chamber, the minimum pressure valve opens and air is delivered to the air net
- Air passes through the after-cooler and is cooled to 10 C above ambient causing moisture to condense into water droplets
- The moisture droplets in the air are separated by centrifugal action within the moisture separator. Compressed air now enters the customers
- Oil that has collected in the separator vessel is forced by the pressure in the vessel to enter the oil cooler
- The oil is cooled by cold air being forced across the radiator , or in the case of water cooled machines by an oil/water heat exchanger
- The oil temperature is controlled by means of a thermal bypass valve , Prior to re-injection to the rotary screw element the oil is filtered to remove any harmful contaminants
A scroll compressor is a rotating displacement compressor, i.e. it compresses a specific amount of air in an ever decreasing volume. The compressor element consists of a fixed spiral in an element housing and a motor powered eccentric, moveable spiral. The spirals are mounted with 180° phase displacement to form air pockets with a varying volume. This provides the elements with radial stability. Leakage is minimised as the pressure difference in the air pockets is less that the pressure difference between the inlet and the outlet. The moving spiral is driven by a short stroke crankshaft and runs eccentrically around the centre of the fixed spiral. The intake is situated at the top of the element housing. When the moving spiral runs clockwise air is drawn in, and is captured in one of the air pockets and compressed variably in towards the centre where the outlet and a non-return valve are situated. The compression cycle is in progress for 2.5 turns, which virtually gives constant and pulsating free air flow. The process is relatively silent and vibration free, as the element has hardly any torque variation compared to, e.g. a piston compressor.
The operating principle for a vane compressor is the same as for many compressed air motors. The vanes are usually manufactured of special cast alloys and most compressors are oil lubricated. A rotor with radially movable blades is eccentrically mounted in a stator housing. When it rotates the vanes are pressed against the stator walls by centrifugal force. Air is drawn in when the distance between the rotor and stator is increasing. The air is captured in different compressor pockets, which decrease in volume with rotation. The air is discharged when the vanes pass the outlet port.