How to choose a screw compressor

25.08.2019

Screw compressors: design and principle of operation

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Machines producing compressed air are indispensable equipment in manufacturing enterprises. The domestic market represents this equipment quite widely. Screw compressors are very popular.

The screw compressor operates silently, without creating vibration, so it can be installed in any room. This model has a long service life. It is economical and highly efficient, far exceeding the parameters of piston compressors.

Maintenance can be done by changing the oil and filters. These compressors have a limited compression ratio. Typically they do not exceed a pressure of 15 bar. As will be seen below, they find their usefulness in the production of Nitrox by the semi-permeable membrane method.

This is an alternative bending of the membrane that provides suction and compression of the transport gas. Part A of the compressor consists of a hydraulic pump driven by an eccentric, which moves the fluid against the diaphragm, producing an oscillating motion.

Design Features

This device has 2 rotors. One rotor is the driving one, the other is the driven one. The body is made of durable material. The rotors are located in the housing. A gear is used to connect the rotor; in some electric models it is directly connected.

Compared to a piston compressor, the design of a screw compressor is more complex. The surfaces of the rotors of this equipment are created by working chambers with a housing. As the rotors rotate, the protrusions move away from the recesses and the chambers become larger.

A plate plate distributes the liquid evenly under the membrane, and on the opposite side, another plate includes the suction and discharge valves. The compensating pump allows the required amount of oil to be sent so that the dead space is reduced to a maximum, resulting in easily obtaining compression ratios close to 20, allowing adjacent pressures of 400 bar to be obtained in just 2 floors.

The detector allows you to instantly stop the compressor in the event of a diaphragm rupture. The compressed gas contains no contaminants. There is no compressed gas leak. All gases can be compressed. Filling efficiency is high. You can get high pressures. The mobile parts are well lubricated, giving them excellent reliability.

Video: operating principle and design

When the volume of the chambers reaches its maximum, air suction stops. They are separated by the walls of the device and the pipe covers of the Fubag compressor. As the rotors rotate, the leading screw is inserted into the cavity of the driven screw. There is a connection in one plane. After which the volume of the chambers becomes smaller. Thus, there is a continuous creation of compressed air. This is the operating principle of screw compressors.

Energy efficiency is excellent. Maintenance is kept to a maximum and therefore to a low cost. Its only drawback is its price, which is 2 - 3 times higher than a piston compressor. This type of compressor is mainly used in the professional field to compress all types of gas. In diving it is used to load buffers using Nitrox or Trimix mixtures.

In this group of compressors, the engine and the compressor are not in the same housing. The drive shaft exits the compressor housing. It can be connected to an electric, diesel or gas engine. The coupling is made either by a clutch or by a belt.

Screw refrigeration compressor. Operating principle and device.

The use of screw compressors in refrigeration technology is due to the need for large values ​​of the required cooling capacity. Screw refrigeration compressors are successfully used as part of medium-, high- and low-temperature refrigeration units. They are popular among equipment manufacturers due to their reliability and high efficiency. The operating principle of a screw refrigeration compressor is based on the continuous movement and compression of refrigerant vapors, thanks to the operation of a screw pair.

Operating principle of a screw refrigeration compressor.

The working parts of a screw refrigeration compressor are screws, the so-called rotors. Compressors with two rotors in design are mainly common. One of them is the leader, the second is the slave. They rotate relative to each other in a closed housing and are “engaged” with each other by teeth made in the form of spirals. The torque to the drive rotor is transmitted by an electric motor, which can be located in one compressor housing, or connected to it through a clutch or other transmission.

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The compression process occurs in the space between the teeth of the screws. With mutual rotation in different directions, the tooth of one rotor enters the cavity of the other, correspondingly reducing the volume of the working area. As gas moves from the suction cavity to the discharge, the volume decreases and the pressure increases. At the end point, the volume of the working area is reduced to zero, this indicates minimal dead (harmful) space and the efficiency of the compressor. In screw compressors, the process of suction, compression and discharge occurs continuously.

The device of a screw refrigeration compressor.

Let's consider the design of a screw refrigeration compressor using the example of a semi-hermetic compressor ]Bitzer[/anchor]. The main elements of a screw compressor are shown in the figure.

The shapes of the profiles of the rotor teeth are adjusted so that during the compression process, constant contact is maintained between the screws, to prevent gas flows from the high pressure area to the low pressure area.

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Screw refrigeration compressors operate with large amounts of oil. Its use is necessary to lubricate the screw pair, reduce wear on working elements, seal the gaps between the screws, and also to remove heat generated by compressing the refrigerant. This condition requires the installation of oil separators and oil coolers as part of a refrigeration machine based on screw compressors. As a result of oil injection into the gas compression zone, friction in the screw compressor is minimized and there is no mechanical contact between the rotors.

To increase the cooling capacity of screw compressors, especially when used for low-temperature purposes, manufacturers have made it possible to use a subcooler (economizer). The use of an economizer (refrigerant subcooler) makes it possible to obtain significantly greater performance characteristics from the same compressor.

Screw refrigeration compressors are widely used and have the ability to regulate performance without the use of frequency inverters. Multi-stage control of refrigeration capacity is ensured by changing the volume of suction gas.

Today, such manufacturers of screw compressors as Bitzer, Refcomp, Frascold are represented on the Russian market, including Chelyabinsk.

Types and uses of compressors

This equipment has two groups of devices:

• dry compression device;
• device filled with oil.

The first ones do not fill the working surfaces with oil during operation (Fubag model).

Access to all compressor components is possible. Power is adjusted by stopping certain cylinders or by changing the speed of the drive motor. Open circuit compressors are used in installations with a cooling capacity of up to 500 kW.

Piston compressors, semi-hermetic design

The compressor and drive motor are housed in a common housing. The drive is usually provided by an electric motor. It is usually cooled by cold coolant gases, sometimes by a fan or a coil wound around the engine frame. For repairs, you can access all parts of the machine and even separate the compressor from the engine.

Fubag screw models are used by enterprises that require high purity of compressed gas: food, pharmaceutical, microbiological, which produce electronic devices.

Oil filled compressors (Abac model) have high performance characteristics. Oil enters the working cavity of the device. The oil film formed between the rotors is a lubricant. In addition, oil activates heat removal and transports air. The existing separation system reduces residual oil in the produced mixture. As a result, clean air comes out.

Piston compressors

Power is adjusted by turning off some cylinders or by changing the speed of the drive motor.
Semi-hermetic compressors are used in installations up to 100 kW or, using several compressors, up to 400 kW. The compressor and electric motor are placed in a welded envelope. They are usually supported by springs to prevent vibration transmission. The rotating joint disappears, and with it the risk of leakage. But new restrictions are emerging. The coolant and, of course, the lubricating oil must be compatible with the materials that make up the engine. The engine is cooled by the refrigerant itself and this heating is harmful to the refrigeration cycle, the compressor suction increases and therefore a higher discharge. In addition, if the engine is to be damaged, the entire refrigeration circuit will be contaminated: a complete cleaning of the circuit must be carried out if no further problems can be avoided. The engine is cooled by the intake gases and the engine is switched off when the engine overheats. In principle, the cooling capacity cannot be changed, except by changing the frequency of the supply current.

Screw compressors have a belt or direct drive. The first are equipped with a controller and microprocessor. They control the actions of the unit or pneumatic networks. They are also used in modular compressor stations as an additional device.

Abac brand direct drive compressors are equipped in the same way as belt drive ones. Instead of a drive, they have an elastic coupling. This type of compressor has some advantages: low-noise connections, low energy losses, minimal impact on the operation of engine bearings. This equipment does not require adjustment or adjustment.

Reciprocating piston compressors are installed in small appliances or systems up to 30 kW. A piston compressor must be constantly lubricated. The lower part of the crankcase forms an oil reservoir. The housing pressure is the suction pressure. The oil pump provides a pressure 5-4 bar higher than the pressure in the crankcase.

The piston compressor is very sensitive to the flow of liquid fluid: if a few drops of liquid penetrate the valves, they cause slow wear, if liquid liquid penetrates in large quantities, immediate destruction of the valves. There, anti-shock protection is adopted.

Advantages and disadvantages of screw compressors

Thanks to their excellent technical characteristics, screw compressors have become a worthy replacement for piston equipment. When comparing these two devices, it was found that the Abac model has the following advantages:

Dead space is the volume that remains between the piston and the cylinder head when the piston is at its highest position, this space is necessary to avoid shock when the piston is at its end. 4% of the cylinder volume and should be reduced to the maximum to increase the volumetric efficiency of the compressor.

The purpose of a compressor in a refrigeration unit is to create high pressure on one side and low pressure on the other to circulate fluid in the pump's refrigerant circuit. heat. But whoever says liquid does not always speak in its liquid phase.

• the design includes twin turbines that operate almost silently, without creating vibration;
• they have compact dimensions and light weight;
• if there is an automatic control system, they do not require constant presence of maintenance personnel;
• economical;
• cooled by air.

However, despite their advantages, Fubag screw compressors are not ideal. They, like any other equipment, have disadvantages. The oil separator and cooler, as well as other high-cost components, are considered a disadvantage of this equipment.

Refrigeration compressor families

The compressor sucks in and always discharges the refrigerant in the gas phase. There are three main families of refrigeration compressors.

• Hermetic compressors.
• Semi-hermetic compressors.
• Open the compressors.

Hermetically sealed compressors are compressors whose motor and compression parts are enclosed in a sealed, permanent bell housing.
If one of the parts does not work, the compressor has failed and cannot be repaired. Its electrical part is cooled by low pressure refrigerant vapor during compressor operation. Semi-hermetic compressors are so called because the motor part is decoupled from the compressor part.

Advantages of screw compressors

Due to their advantageous characteristics, they have practically replaced other types of devices in production. The compressors presented on compressor.org.ua have proven themselves to be reliable units with a long service life. Additionally, they have a number of advantages:

• The design is such that they do not create much noise during operation;
• They have compact dimensions and relatively light weight (compared to other systems of a similar principle of operation).

Screw installations are distinguished by the fact that they can operate in autonomous mode, which reduces energy costs.

Most models are equipped with a digital control panel. This simplifies working with the unit. Using the panel, you can set a timer for the process cycle, adjust energy consumption, change pressure, and so on.

The design with rotors works cleaner, since it has minimal oil consumption, which is only 3 milligrams per cubic meter. This configuration reduces the cost of installing additional filters. The air-operated device eliminates the water return system and allows the compressor heat to be reused.

Minimum energy consumption, practical noiselessness and compactness have made stations with a screw design leaders in their segment.

Selection of screw compressors

Manufacturers from different countries offer equipment of this class. Advertising brochures indicate that they are leaders in this field. What do you need to know to choose the right screw compressor? All Abac screw models look the same.

The body of the model is rectangular, with a controller display installed on it. There is a minor difference - color and size. When choosing equipment, it is worth finding out which products are the most popular, so as not to invest money in repairs.

Therefore, they are repaired by disassembling two parts assembled one against the other. For hermetic compressors, part of the electric motor is cooled by low-pressure refrigerant vapor during operation. Open compressors are so named because the motor portion is separated from the refrigerant compressor portion.

Various types of compressors

There are two types of connections between two parties. Piston compressors: This is a common model that is mainly used in domestic refrigerators and freezers. Compact and compact, it works on the same principle as car engines. During the intake phase, the piston sucks in fluid, then compresses it and then pumps it out into the circuit. It is also possible to satisfy this type of compressor in industrial refrigeration equipment, such as chilled water production plants or air purification plants. Scroll Compressors: These are also called scroll compressors. They are recognized by their elongated shape. They use two coils intercalated as trays to pump and compress liquids. Often one of the coils is fixed while the other moves eccentrically, not rotating enough to capture pockets of fluid between the coils. Their scope of application is rather for air conditioning due to their reasonable discretion. Screw compressors: These are also called helicopters. They consist of two synchronized rotating screws that allow the fluid to be compressed. Volume reduction is used to compress a liquid. Indeed, the impression of the screws is more excavated at the entrance than at the exit. Depending on the models and the distance between the two screws, an oil film is used to provide a seal. Centrifugal compressors: Also called turbochargers, these are radial-type machines with a rotor that rotates at the same speed so the fluid flows constantly. Depending on the refrigerant used, a centrifugal compressor can be single-wheel or multi-wheel. In the case of a multi-wheel wheel, the wheel diffuser is expanded by a return channel, which brings the compressed fluid to the inlet of the next wheel.

• Belt drive.
• Drive using a clutch.
• And again a new cycle begins.

From small machine shops to large oil refineries to automobile factories, this essential accessory can be found in absolutely every industry.
Surveys have shown that the leading leader is the Ingrsoll-Rand company, which participated in the creation of Fubag screw models. This company has been producing such equipment for 100 years. It is reliable and has excellent performance.

The parameter you should pay attention to is the load. Screw compressors differ:

• productivity;
• power;
• existing or non-existent drive;
• built-in air preparation system.

The load of a screw compressor depends on the type of work:

Today, screw compressors offer more than just compressed air: flexible and designed for maximum efficiency, they meet the highest application requirements and are reliable and economical. The most common outlet pressures are between 4 and 14 bar. Some can go much higher, for example to about 25 bar for compressing process gases.

Also known as a rotary positive displacement compressor, it operates with two screws rotating in the opposite direction. The rapid rotation gradually reduces the space between the two screws and creates suction and compression of the gas trapped between them prior to delivery. Depending on the length, profile of the screws and the shape of the outlet, the pressure is more or less intense. The outlet pressure is also adjustable. Gas is transported along the propeller, from the suction port to the exhaust port continuously.

• episodic;
• constant.

In the first case, a low-quality and cheap model is suitable. In the second case, you need reliable equipment that can operate 3000 hours a year.

How to choose the right screw compressor?

Quite a few businesses use compressors to supply compressed air.
This includes the food industry, machine factories, dental offices and many, many other places where compressors are used. Of course, good equipment for an enterprise is like air. Of course, price is also an important factor, so you should find the optimal solution between quality and price. The main types of compressors are piston and screw. Unfortunately, the former have a number of disadvantages that make them not the most attractive offer on the compressor market. But screw compressors have long won the trust of customers with their performance and reliability. These are machines that are ideal for industries where a constant supply of compressed gas is needed. Among all the variety, you can always choose a model.

Operation and Maintenance

The equipment will last a long time if you follow the rules established by regulatory documentation. It contains a clause on timely maintenance. You should not neglect this requirement so as not to pay for expensive repairs in the future.

This type of compressor must be heavily oiled. The oil provides sealing by forming a film and also ensures proper cooling of the rotors. There are also screw compressors whose compression chambers are not lubricated. Synchronized screws do not touch each other. The resulting compressed gas is then completely oil-free. This type of compressor is generally less powerful at the same price as the piston model, but has many advantages over the latter. It has no valve, no imbalance, its flow is important, it is more compact and especially much quieter.

If the equipment is used correctly, no repairs are required. Although the compressor design is reliable, it can be damaged if oil or refrigerant is added. Liquid entering the device during startup will provide high hydraulic loads. This will lead to adverse effects on the bearings. It is necessary to scrupulously approach filling the system with workers.