Compressed Air: Industry's Fourth Utility

• Air is one of the most important utilities in the process industry, essential in modern industrial facilities due to the widespread use of compressed air.

Compressed Air

• Compressed air is known as the industry's fourth utility.
• The process involves compressing air into a smaller volume.
• The generation of compressed air requires air receivers and air compressors.
• Applications of compressed air include cement industry clinker cleaning, spraying, atomizing, pressure filtration, drying granular materials, classification of solids, palletizing plants, combustion reactors, rubber industry mold operation, and casting industry.

Composition and Properties of Air

• Chemical formula is a mixture of N₂ and O₂.
• Molecular weight: 28.97.
• Specific Heat Ratio (γ) at 15.55 °C is 1.40.
• Critical pressure (absolute): 37.7 bar.
• Critical temperature (absolute): 132.8K.
• Mean specific heat, C’pm (kJ/kmol.K) at 0 °C: 29.05.
• Mean specific heat, C’pm (kJ/kmol.K) at 10 °C: 29.32.
• Air Composition includes 78% Nitrogen, 21% Oxygen, and 1% other gases and impurities, such as carbon dioxide, carbon monoxide, water vapor, ammonia, nitrogen oxides, hydrogen sulfide, pollen, volcanic ash, dust, sea salt, mold, and bacteria spores.

Air Receivers

• A small tank after the compressor in the installation is known as an air receiver.
• Air receivers dampen air pulsation from delivered by the compressor, act as coolers, and are used as storage for air and removal of oil and moisture.

Types of air receivers

• Air Receivers come in vertical and horizontal models.
• The primary receiver is located near the compression unit, after the cooler and before filtration and drying equipment.
• The secondary receiver is located close to points with larger intermittent air consumptions.

Air Compressors

• Air compressors increase the pressure of a compressible fluid.
• They convert power into kinetic energy by compressing and pressurizing air, released on command.
• Inlet pressure can vary from deep vacuum to high positive pressure with discharge pressure ranging from sub-atmospheric to high values.
• The relationship between inlet and outlet pressures depends on the compression type and its configuration.
• Compressible fluids can be either gas or vapor.
• Applications range from consumer products like home refrigerators.

Air Compressor Classification

• Type of Displacement:
  • Positive displacement compressors for high pressures.
  • Non-positive displacement compressors for lower pressures.
• Staging: Number of compression steps.
• Type of Drive System: Classified by prime mover (mostly electric motors).
• Operating Discharge Pressure:
  • Low: <151.
  • Medium: 151-1000.
  • High: >1000.
• Capacity: Volume of air compressed, measured in CFM (cubic feet per minute).

Main Components in Compressed Air Systems

• Intake air filters.
• Inter-stage coolers.
• After coolers.
• Air dryers.
• Moisture drain traps.
• Receivers.
• Distribution system.

Types of Compressors and Air Movers

• Compressor Classification:
  • Positive Displacement:
    • Rotary (Lobe, Screw, Scroll, Liquid Ring, Vane)
    • Reciprocating (Diaphragm, Double Acting, and Single Acting)
  • Dynamic:
    • Centrifugal
    • Axial

Positive Displacement: Reciprocating Piston Compressors

• Reciprocating Piston Compressors have a piston assembly with valves to control flow and can be single or multistage.

Reciprocating Compressors

• Used for air and refrigerant compression.
• Operate like bicycle pumps, reducing cylinder volume and increasing pressure with pulsating output.
• Configurations include single and double acting units.
• Widely used for providing high-pressure air in pneumatic conveying systems.
• Reciprocating compressors generally offer the best thermodynamic efficiency for air movers.

Rotary Compressors

• As the roller rotates, it reduces the fluid volume, compressing it.
• The high-pressure fluid pushes open the exhaust valve for air release.
• The act of fluid intake and compression occur simultaneously, and the cycle repeats
• Rotary compressors are very efficient because intake and compression occur simultaneously

Rotary Compressor - Lobe-Type

• This compressor uses two intermeshing rotors on parallel shafts, with each rotor having two lobes in a twin-lobe design or three lobes in a tri-lobe machine.
• Two rotors rotate in opposite directions.
• Each rotor traps a definite volume of gas as it passes the blower inlet, carrying it around the case to the blower outlet.
• With constant speed operation, the displaced gas volume remains approximately the same independent of inlet temperatures, pressures, and discharge pressures.
• As each rotor passes the blower outlet, the gas compresses to the system pressure and is expelled.
• clearances allow operation without lubrication inside the air casing and are used for low pressure applications.

Rotary Compressor - Sliding Vane Type

• Well-suited for medium to high-pressure systems.
• It is a single-rotor device with the rotor eccentric to the casing.
• Compression usually occurs within the machine and air is delivered without marked pulsation.

Rotary Compressor - Screw Type

• High efficiency and capacity compressors that require oil injection into the compression chamber to reduce internal air leakage.
• The oil helps to cool air during compression, but it is then necessary to remove the oil from the compressed air.
• It consists of intermeshing rotors mounted on a parallel shaft.

Non-Positive Displacement or Dynamic Compressors

• Include Centrifugal Compressors, Radial flow, Axial Compressors, and Axial Flow.
• Centrifugal compressors and multiple stage axial flow compressors are capable to handling very high volumetric flow rates

Dynamic – Centrifugal Compressors

• Use rotation of an impeller wheel to exert centrifugal force on air inside a round chamber (volute).
• Fluid is sucked into the impeller wheel through a large circular intake and flows between the impeller.
• The impeller forces fluid outward, exerting centrifugal force on the fluid to pressurize as the fluid is forced against the volute sides.
• They compress large volumes of fluid to relatively low pressure.
• The compressive force generated by an impeller wheel is small.
• For high efficiency, these compressors often use multiple impeller wheels arranged in series.

Cyclone Separators

• Cyclone separators disengage solid particles from the conveying gas by imparting spinning motion to the incoming stream, leading the particles to migrate outward and downward due to centrifugal and gravitational forces.
• Collection efficiency is affected by the density difference between solid particles and air, the solid concentration, and the cyclone dimension.
• Increased entry velocity and cylinder diameter increase the collection efficiency up to a limit of 10-micron particle size.
• Tangential entry to the cylindrical upper part of the device affects rotation of gas.
• Solid dust particles are collected at the base, while clean gas is at the top.

Bag Filters

• Universally adopted filtration methodology used as bin vents by blowing solid material into a hopper and venting clean air off at the top through the filtering unit.
• The collected material usually discharges at the base.
• Bag Filters are suitable for continuous and batch conveying operations.
• Filter size is based on the air-to-fabric ratio, defined as the volumetric air flow rate divided by the effective area of filter fabric, and is expressed dimensionally as velocity.
• Power requirement is low due to the lower volumetric flow rate.

Pneumatic Conveying Systems

• Well-suited for transporting powdered and granular materials.
• Air is used for transportation of items in most cases.
• Different gases are used when special conditions are present for example the risk of explosion, fire, and health hazards.
• The pneumatic conveyor system requirements are a source of compressed gas.
• The systems are completely enclosed.
• Dry air can be utilized for some hygroscopic material while an inert gas, such as nitrogen, can be used when potentially explosive material involved
• Systems have different zones such as, prime mover, feeing mixing and acceleration zone, conveying zone, gas and solid separation zone.

Pneumatic Conveying System Zones

• Prime Mover: Essential, consists of a compressor, fan, or blower to provide energy for conveying gas.
• Feeding/Mixing/Acceleration Zone: Solids are introduced into the flowing gas.
• Large change in momentum occurs when mixing solids and gas with acceleration needed over a certain pipe length to achieve steady-state flow.
• Conveying Zone: The solids have entered after the acceleration zone consisting of piping with the selection depending on abrasiveness, length and pressure requirements, and more.
• A number of bends and diverters are in the wall for which flow direction might change. Solid are accelerated during these movements.
• Gas and Solids Separation Zone: The primary factor in selection is the size of solids for separation from gas streams.
• Different configurations separate gas and solids in pneumatic conveying systems.