Compressors, Fans & Blowers – Basic Understanding
Compressors, fans and blowers are widely used in various industries. These devices are quite suitable for complex processes and have become indispensable for some specific applications. They have been defined in simple terms as below:
- Compressor: A compressor is a machine which reduces the volume of gas or liquid by creating a high pressure. We can also say that a compressor simply compresses a substance which is usually gas.
- Fans: Fan is a machine used to move fluid or air. It is operated through a motor via electricity which rotates the blades that are attached to a shaft.
- Blowers: Blower is a machine to move air at a moderate pressure. Or simply, blowers are used for blowing air/gas.
The basic difference between the above three devices are the way they move or transmit air/gas and induce system pressure. Compressors, Fans & Blowers are defined by ASME (American Society of Mechanical Engineers) as the ratio of the discharge pressure over the suction pressure. Fans have the specific ratio up to 1.11, blowers from 1.11 to 1.20 and compressors have more than 1.20.
Types of Compressors
Compressor types can be mainly grouped into two: Positive Displacement & Dynamic
Positive displacement compressors are again of two types: Rotary and Reciprocating
- Types of Rotary compressors are Lobe, Screw, Liquid Ring, Scroll and Vane.
- Types of Reciprocating compressors are Diaphragm, Double acting and Single acting.
Dynamic Compressors can be categorized into Centrifugal and Axial.
Let us understand these in detail.
Positive displacement compressors use a system which induces in a volume of air in a chamber, and then reduce volume of the chamber to compress the air. As the name suggest, there is a displacement of component that reduces volume of the chamber thereby compressing air/gas. On the other hand, in a dynamic compressor there is a change in velocity of the fluid resulting in kinetic energy which creates pressure.
Reciprocating compressors use pistons where discharge pressure of air is high, quantity of air handled is low and which has a low speed of compressor. They are suitable for medium and high pressure ratio and gas volumes. On the other hand, rotary compressors are suitable for low and medium pressures and for large volumes. These compressors do not have any pistons and crankshaft. Instead these compressors have screws, vanes, scrolls etc. So they can be further categorized on the basis of the component they are equipped with.
Types of Rotary compressors
- Scroll: In this equipment, air is compressed using two spirals or scrolls. One scroll is fixed and does not move and the other one move in circular motion. Air gets trapped inside the spiral way of that element and gets compressed at middle of the spiral. These are often with oil-free designs and require low maintenance.
- Vane: This consists of vanes that move in and out inside an impeller and compression occurs because of this sweeping motion. This forces the vapor into small volume sections, changing it into high pressure and high temperature vapor.
- Lobe: This consists of two lobes which rotate inside a closed casing. These lobes are displaced with 90 degrees to one another. As the rotor rotates, air is drawn into inlet side of the cylinder casing and is pushed with a force out from the outlet side against the system pressure. The compressed air is then delivered to delivery line.
- Screw: This is equipped with two inter-meshing screws which traps air between the screw and the compressor casing, which results in squeezing and delivering it at a higher pressure from the delivery valve. The screw compressors are suitable and efficient in low air pressure requirements. In comparison to a reciprocating compressor, the compressed air delivery is continuous in this type of compressor and it is quiet in operation.
- Scroll: The scroll type compressors have scrolls driven by the prime mover. The scrolls outer edges trap air and then as they rotate, the air travel from outwards to inwards thus getting compressed due to reduction in the area. The compressed air is delivered through central space of the scroll to the delivery airline.
- Liquid ring: In this type of compressor vanes are built inside a cylindrical casing. When the motor rotates, gas gets compressed. Then liquid mostly water is fed into the device and by centrifugal acceleration, it forms a liquid ring through the vanes, which in turn forms a compressing chamber. It is capable of compressing all gases and vapors, even with dust and liquids.
- Single-Acting Compressors: It has piston working on air only in one direction. The air is compressed only on the top part of the piston.
- Double-Acting Compressors: It has two sets of suction/intake and delivery valves on both sides of the piston. Both sides of the piston are utilized in compressing the air.
The main difference between displacement and dynamic compressors is that a displacement compressor works at a constant flow, whereas a dynamic compressor such as Centrifugal and Axial works at a constant pressure and their performance is affected by external conditions such as changes in inlet temperatures etc. In an axial compressor, the gas or fluid flows parallel to the axis of rotations or axially. It is a rotating compressor that can continuously pressurize gases. The blades of an axial compressor are relatively closer to each other. In a centrifugal compressor, fluid enters from center of the impeller, and moves outward through the periphery by guide blades thereby reducing the velocity and increasing pressure. It is also known as a turbo compressor. They are efficient and reliable compressors. However, its compression ratio is lesser than axial compressors. Also, centrifugal compressors are more reliable if API (American petroleum Institute) 617 standards are followed.
Types of fans
Depending on their designs, the following are main types of fans:
- Centrifugal fans: In this type of fan, airflow changes direction. They can be inclined, radial, forward curved, backward curved etc. These kinds of fans are suitable for high temperatures and low and medium blade tip speeds at high pressures. These can be effectively used for highly contaminated airstreams.
- Axial Fans: In this type of fan, there is no change in direction of air flow. They can be Vanaxial, Tubeaxial, and Propeller. They produce lower pressure than the Centrifugal fans. Propeller-type fans are capable of high-flow rates at low pressures. Tube-axial fans have low/medium pressure and high flow capability. Vane-axial fans have an inlet or outlet guide vanes, exhibit high pressure and medium flow-rate capabilities.
The air flow required in the process along with required outlet pressure are key factors determining the selection of type and size of a fan. Fan enclosure and duct design also determine how efficiently they can work.
Blower is equipment or a device which increases the velocity of air or gas when it is passed through equipped impellers. They are mainly used for flow of air/gas required for exhausting, aspirating, cooling, ventilating, conveying etc. Blower is also commonly known as Centrifugal Fans in industry. In a blower the inlet pressure is low and is higher at the outlet. The kinetic energy of the blades increases pressure of the air at the outlet. Blowers are mainly used in industries for moderate pressure requirements where the pressure is more than the fan and less than the compressor.
Types of Blowers: Blowers can also be classified as Centrifugal and Positive displacement blowers. Like fans, blowers use blades in various designs such backward curved, forward curved and radial. They are mostly driven by electric motor. They can be single or multistage units and use high speed impellers to create velocity to air or other gases.
Positive displacement blowers are similar to PDP pumps, which squeezes fluid that in turn increases pressure. This kind of blower is preferred over a centrifugal blower where high pressure is required in a process.
Applications of Compressors, fans and blowers
Compressors, Fans and blowers are mostly used for processes such as Gas Compression, Water Treatment Aeration, Air Ventilation, Material Handling, Air Drying etc. Compressed air applications are widely used in various fields such as Aerospace, Automotive, Chemical Manufacturing, Electronics, Food and Beverage, General Manufacturing, Glass Manufacturing, Hospitals/Medical, Mining, Pharmaceuticals, Plastics, Power Generation, Wood Products and many more.
The main benefit of an air compressor includes its usage in the water treatment industry. The waste water treatment is a complex process that requires breaking down millions of bacteria as well as the organic waste.
Industrial fans are also used in a variety of applications such as chemical, medical, automotive, agricultural, mining, food processing, and construction industries, which can each utilize industrial fans for their respective processes. They are mainly used in many cooling and drying applications.
Centrifugal blowers are routinely used for applications such as dust control, combustion air supplies, on cooling, drying systems, for fluid bed aerators with air conveyor systems etc. Positive displacement blowers are often used in pneumatic conveying, and for sewage aeration, filter flushing, and gas boosting, as well as for moving gases of all kinds in the petrochemical industries.
Therefore, compressors, fans and blowers, largely cover Municipal, Manufacturing, Oil & Gas, Mining, Agriculture Industry for their various applications, simple or complex in nature.
A detailed study of all the designs and specifications is required to buy an appropriate compressor, fan or a blower that is available in the market, so that it can match the requirements of your process and ensure reliability and durability at the same time.
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