This article is about air compressor, its types and selection criteria for industrial use. More technical specification of air compressor used for plants and oil refineries.
What is Air Compressor?
A compressor is a mechanical device used to increase the pressure of a gas or air by reducing its volume. It works by drawing in a low-pressure gas and then compressing it to a higher pressure, which can be used for various applications.
Compressors are widely used in industries and everyday life. They play a crucial role in providing compressed air for pneumatic tools, air conditioning, refrigeration systems, gas pipelines, and many other applications. There are various types of compressors, including reciprocating compressors, rotary compressors, centrifugal compressors, and scroll compressors, each with its own specific design and applications. Compressors are essential for many processes that require high-pressure gas or air, making them an integral part of modern industrial and commercial systems.
In this article, we will mainly refer to air compressors when using the terms “compressors” and “air compressors.” However, in some specialized cases, we will also discuss compressors used for specific gases.
Compressor Types
Types of Air Compressor:
- Piston
- Diaphragm
- Helical Screw
- Sliding vane
- Scroll
- Rotary Lobe
- Centrifugal
- Axial
Piston Compressor
Piston compressors, also known as reciprocating compressors, use pistons to compress gas within cylinders and then discharge it into high-pressure tanks through valving. They are commonly used to provide compressed air as an energy source and in natural gas transmission by pipeline operators. These compressors are selected based on the required pressure and flow rate. Typical plant-air systems deliver compressed air at 90-110 psi with volumes ranging from 30 to 2500 cfm, which can be achieved through off-the-shelf units. To achieve higher air pressures, two-stage units are available, and an intercooler is used to reduce heat generated during the first-stage cycle.
Piston compressors often operate within duty cycles rather than continuously to allow generated heat to dissipate through air-cooled fins. They are available in both oil-lubricated and oil-free designs, with the latter better suited for applications requiring oil-free air of the highest quality.
Diaphragm Compressors
Diaphragm compressors are a specialized type of reciprocating compressor that uses a motor-mounted concentric to oscillate a flexible disc. This disc expands and contracts the compression chamber, creating compression. Like a diaphragm pump, the drive is sealed from the process fluid, ensuring no contact between lubricant and gas. These compressors are ideal for applications requiring very clean air, such as in laboratories and medical settings. They are relatively low capacity machines with specific usage due to their clean air requirement.
Helical Screw Compressors
Helical-screw compressors are rotary machines that excel in 100% duty cycle operations, making them ideal for trailerable applications like construction or road building. They use meshing male and female rotors to draw gas in at the drive end, compress it as the rotors form a cell, and then discharge the compressed gas through a port on the non-drive end. The rotary screw design reduces vibration, making it quieter compared to reciprocating compressors. Additionally, the screw compressor provides discharge air free of pulsations, unlike piston compressors. These units can be oil- or water-lubricated and can even deliver oil-free air for critical applications.
Sliding Vane Compressors
Sliding Vane Compressors, also known as rotary vane compressors, operate by using vanes mounted on a rotor to sweep along the inside wall of an eccentric cavity. As the vanes rotate, they reduce the volume of space they pass by, compressing the gas trapped within. An oil film on the cavity wall provides a seal for the vanes, ensuring efficient compression.
Advantages:
- Pulsation-Free Compression: Sliding vane compressors deliver compressed air free of pulsations, providing a steady and consistent air supply for various applications.
- Forgiving of Contaminants: These compressors use bushings instead of bearings, making them more tolerant of contaminants in their environment. This feature enhances their reliability and durability.
- Quiet Operation: Sliding vane compressors are relatively quiet during operation, making them suitable for noise-sensitive environments.
- 100% Duty Cycles: They can handle continuous operation at full capacity without overheating, making them reliable for constant air supply.
- Used in Non-Air Applications: While they are less common in air-compressor applications compared to screw compressors, sliding vane compressors are widely used in non-air applications in the oil and gas and process industries.
Limitations:
- Not Oil-Free: Sliding vane compressors cannot provide oil-free air, which may be a critical requirement in certain industries like pharmaceuticals or food processing.
- Overtaken by Screw Compressors: In some applications, screw compressors have become more prevalent due to their efficiency and oil-free capabilities.
Overall, sliding vane compressors are valued for their steady performance, reliability, and quiet operation, making them a viable choice for various industrial and non-air applications. However, their inability to provide oil-free air and the growing popularity of screw compressors in specific applications have led to a shift in their usage in the air-compressor sector. Nonetheless, they remain indispensable in other process industries where their unique characteristics are highly beneficial.
Scroll Compressors
Scroll Compressors, or scroll-type compressors, utilize stationary and orbiting spirals that create a reduction in the volume of space between them as they move. Gas is drawn in at the outer edge of the scrolls and compressed gas is discharged near the center. A notable advantage of scroll compressors is that the scrolls do not come into contact with each other, eliminating the need for lubricating oil and making the compressor inherently oil-free.
Advantages:
- Inherently Oil-Free: Scroll compressors require no lubricating oil as the scrolls do not make contact, making them ideal for applications where oil-free air is essential, such as in the pharmaceutical or food industry.
- Low Noise and Vibration: Scroll compressors are known for their quiet operation and low vibration levels, making them suitable for noise-sensitive environments.
- Compact and Lightweight: These compressors have a compact design, allowing for easy installation and integration into various systems.
- Efficient Performance: Scroll compressors deliver high efficiency, making them suitable for low-end air compressors and home air-conditioning units.
Limitations:
- Limited Capacity: Scroll compressors have somewhat limited capacities compared to other compressor types due to the absence of oil for heat removal during compression.
- Specific Applications: While they are widely used in low-end air compressors and residential air-conditioning units, scroll compressors may not be suitable for high-demand industrial applications that require higher capacities.
Overall, scroll compressors are well-regarded for their oil-free operation, low noise, and compact design. They find extensive use in residential air-conditioning systems and applications where clean, oil-free air is essential. However, their limited capacity may restrict their use in certain high-demand industrial settings.
Rotary Lobe Compressors
Rotary Lobe Compressors, also known as blowers, are devices designed for high-volume, low-pressure applications. They are more commonly classified as blowers rather than compressors.
Centrifugal Compressors
Centrifugal compressors are a type of mechanical device that uses high-speed impellers to increase the velocity of gases and subsequently raise their pressure. They are commonly used in applications requiring high-volume gas compression, such as commercial refrigeration units (with power ratings of 100+ horsepower) and large industrial processing plants (with power ratings reaching up to 20,000 horsepower) that handle vast volumes of gas, often exceeding 200,000 cubic feet per minute (cfm).
The construction of centrifugal compressors is quite similar to centrifugal pumps. The gas is propelled outward by the action of spinning impellers, causing it to expand in a casing volute, where its velocity slows down, and its pressure rises. This process allows for significant increases in pressure and energy transfer within the system.
Compared to displacement compressors, centrifugal compressors have lower compression ratios. However, they excel in handling large gas volumes efficiently, making them ideal for applications requiring high flow rates.
To achieve even higher compression ratios, many centrifugal compressors utilize multiple stages in their design. In these multi-stage compressors, the gas passes through intercoolers between stages to reduce the temperature and improve compression efficiency. The use of intercoolers prevents excessive heat buildup and enables better control over the compression process.
Overall, centrifugal compressors play a crucial role in various industries where high-volume gas compression is required. Their efficient and reliable performance makes them an essential component in large-scale refrigeration systems, industrial processing plants, and other applications demanding powerful gas compression capabilities.
Axial Compressors
Axial compressors are known for their ability to deliver exceptionally high volumes of air, ranging from 8000 to an astounding 13 million cubic feet per minute (cfm) in industrial applications. These compressors are widely used in jet engines, where they produce even higher volumes across a broader range.
Unlike centrifugal compressors, axial compressors are more inclined towards multi-stage designs, mainly due to their relatively low compression ratios. The multi-stage configuration allows for gradual increases in pressure, making it well-suited for applications requiring significant air compression.
Similar to centrifugal compressors, axial compressors also rely on increasing the gas velocity to raise its pressure. However, the distinctive feature of axial compressors lies in the way they slow down the gas after velocity increase. This is achieved by passing the gas through curved, fixed blades, which effectively boosts its pressure.
Axial compressors are essential in various industries that demand large volumes of compressed air, such as industrial machines and jet engines. Their capability to handle immense airflow efficiently makes them a vital component in high-performance applications where substantial air compression is required.
a. Layout and piping arrangement
1. The head specification will be verified when a certified performance curve for the compressor or fan and the approved piping/equipment layout of system is available.
2. For packaged or skid mounted units, the performance specification will be based on package or skid inlet and outlet flange conditions.
3. Permanent strainers shall be provided on centrifugal compressor suction lines 24” and smaller. Temporary strainers to be provided for lines larger than 24” and design shall be agreed upon with SABIC. Strainers will be designed in order to withstand both forward and reverse flow under surging conditions.
4. Permanent strainers will be installed at the inlet hood of centrifugal air compressors.
5. Check valves shall be provided for individual discharge line of compressors when discharging into a system from which gas may flow backward through the compressor and where any back flow may cause hazard or damage to equipment.
6. When computing pressure drop in compressor suction lines, the conversion of static pressure to kinetic energy shall be considered. Because of the size of compressor suction lines, they are frequently swaged down well upstream of the compressor to match compressor case nozzle size. Include the swage and reduced line size in the piping size calculations. The compressor guarantees shall be based upon the static pressure specified at the compressor flanges.
7. Provide two ejectors for hot well tank in compressor turbine condensate vacuum system. One ejector for normal use and the other one as stand-by.
8. Reciprocating compressors shall be provided with an installed spare. For high capacity compressor, spare compressor shall be decided on case to case basis.
9. The potential for compressor surge and the need for an anti-surge control system shall be clearly established.
10. Refer to API-617 & 618 for Centrifugal & reciprocating compressor general control system guidelines.
11. For reciprocating compressors above 75 KW, a combination of re-circulating bypass control and step control may be used, as recommended by the manufacturer.
12. The volume between the compressor discharge flange and the check valve on the discharge side of the compressor should be kept at a maximum.
13. Reciprocating compressor discharge temperature shall be limited to a maximum of 150 ºC for most gases and up to a max. 135 ºC for hydrogen rich gases (MW 12 or less) as recommended by API-618.
14. Check valve with damper required on large volume machines.
b. Drains
All compressor piping will have adequate drains and vents piped to a safe disposal area.
c. Valving and Isolation
Block valves will be provided at the following locations in pump, turbine and compressor piping: