Compressed Air: Properties and Applications

• Air is a critical utility in the process industry.
• Air compressors are essential in modern industrial facilities due to the widespread use of compressed air.

Compressed Air

• Often referred to as the industry's fourth utility.
• Compressing air involves reducing the volume a given mass of air occupies.
• Equipment needed includes air receivers and air compressors.

Uses of Compressed Air

• Cement industry for clinker cleaning
• Spraying and atomizing
• Pressure filtration
• Drying of granular materials
• Classification of solids
• Palletizing plants
• Combustion reactors
• Operating molds in the rubber industry
• Casting industry

Properties of Air

• Chemical formula: Mixture of N₂ and O₂
• Molecular weight: 28.97
• Specific heat ratio (γ) at 15.55 °C: 1.40
• Critical pressure (absolute): 37.7 bar
• Critical temperature (absolute): 132.8K
• Mean specific heat (C’pm):
• At 0 °C: 29.05 kJ/kmol.K
• At 10 °C: 29.32 kJ/kmol.K
• Air Composition: Approximately 78% Nitrogen, 21% Oxygen, and 1% other gases and impurities.

Air Receivers

• A small tank installed after the compressor.
• Functions include surge tank to dampen air pulsation, cooler, air storage, and removal of oil/moisture.
• Types: Vertical and horizontal models.
• Primary location is near the compression unit, before filtration and drying equipment.
• Secondary location is close to intermittent air consumption points.

Air Compressors

• Increases the pressure of a compressible fluid.
• Converts power to kinetic energy by compressing and pressurizing air for release.
• Inlet pressure can range from a deep vacuum to high positive pressure.
• Discharge pressure can range from sub-atmospheric to high values.
• Inlet and outlet pressures depend on compression type and configuration.
• Can compress any compressible fluid (gas or vapor).
• Applications include consumer products like refrigerators.

Air Compressor - Classification

• Type of displacement: Positive (high pressures) and Non-positive (lower pressures)
• Staging: Based on the number of compression steps.
• Type of drive system: Classified by prime mover (often electric motors)
• Operating Pressure: Low (<151), Medium (151-1000), High (>1000)
• Capacity: Air volume compressed (CFM).

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

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

Positive Displacement Compressors

• Includes Reciprocating Piston Compressors.
• Reciprocating Piston Compressors: Use pistons and valves to control flow and can be single or multistage.

Reciprocating Compressors

• Used for air and refrigerant compression.
• Cylinder volume reduces with pulsating output, similar to bicycle pumps.
• Single or double-acting configurations are available.
• The most widely used machine for high-pressure air in pneumatic conveying systems.
• Potentially have the best thermodynamic efficiency of air movers.

Rotary Compressors

• Volume of the area is reduced and fluid is compressed as the roller rotates.
• High-pressure fluid forces the exhaust valve to open when exposed.
• Very efficient because fluid intake and compression occur simultaneously.

Rotary Compressor – Lobe-type

• Incorporates two intermeshing rotors on parallel shafts.
• Twin-lobe compressors have two lobes per rotor (four total); tri-lobe have three lobes per rotor (six total).
• Rotors rotate in opposite directions.
• As each rotor passes the blower inlet, it traps a definite gas volume and carries it around the case to the blower outlet while maintaining constant speed.
• Operation allows definite clearances without lubrication inside the air casing.
• Used for low-pressure applications.

Rotary Compressor – Sliding vane-type

• Well-suited for medium to high-pressure.
• Single-rotor device with rotor eccentric to the casing.
• Air delivered without marked pulsation, as compression occurs inside the machine

Rotary Compressor – Screw type

• High efficiency and high capacity
• Require oil injection to reduce internal air leakage.
• Used to cool air during compression, but the oil needs to be removed from the compressed air.
• Consists of intermeshing rotors mounted on a parallel shaft

Non-Positive Displacement or Dynamic Compressors

• Includes Centrifugal and Axial Compressors.
• Centrifugal and multiple-stage axial compressors operate with very high volumetric flow rates.

Dynamic – Centrifugal Compressors

• Use an impeller wheel's rotating action to exert centrifugal force on air inside the volute.
• Fluid is sucked into the impeller through a large circular intake and flows between the impeller.
• Low ratio of compressive force is generated by an impeller wheel.
• Due to such low ratios, centrifugal compressors usually employ more than one impeller wheel arranged in series to increase effectiveness.
• Suited for compressing large fluid volumes to relatively low pressure.

Cyclone Separators

• Forces disengage solid particles from the conveying gas by imparting spinning motion.
• Incoming stream migrates outward and downward due to centrifugal and gravitational forces.
• Collection efficiency depends on particle/air density difference, solid concentration, and cyclone dimensions.
• Increased entry velocity increases collection efficiency up to a 10-micron particle size limit.
• Tangential introduction of gas affects rotation.
• Solid dust particles collect at the base while clean gas exits at the top.

Bag Filters

• Universally used filtration method.
• Used as bin vents where solid material is blown into the hopper and clean air is vented off through a filtering unit.
• Collected material typically discharges from the base.
• Suitable for continuous and batch conveying operations.
• Measure filter size by the air-to-fabric ratio (volumetric air flow rate / effective filter fabric area).
• Have dimensions of velocity.
• Power required to use bag filters is low due to lower volumetric flow rate of operation.

Pneumatic Conveying System

• Suitable for the transportation of powdered and granular materials.
• Typically uses air.
• Other gases are used when special considerations exist such as risk of explosion or health hazards.
• Systems are enclosed entirely.
• Dry air can be used for hygroscopic material. Inert gases (like nitrogen) can be used for potentially explosive materials.
• Zones consist of:
• Prime mover
• Feeding, mixing, and accelerating zone
• Conveying zone
• Gas and solid separation zone.

Pneumatic Conveying System Zones

• Prime mover: Provides necessary energy for gas conveyance; includes compressor, fan, or blower.
• Feeding, mixing, acceleration zone: Crucial for introducing solids into the flowing gas. As solids are introduced in the moving gas, momentum change occurs.
• Accelerating Zone: Pipe section designed to allow solids to reach steady-state flow.
• Conveying zone: Piping and piping selection depending upon a number of factors like the abrasiveness of the product, pressure requirement, etc for solids transport. Change in the direction of flow from bends provides acceleration to the solids.
• Gas and solids separation zone: Size of separated solids helps to select what type of gas-solid separator is required.. Different gas-solid separation systems configurations used.