A small presentation on “COMPRESSED AIR SYSTEM”

Why do we need a Compressed Air System?

Compressed Air (CA) is a major prime-mover for the modern industry. Compressed Air is referred to as the fourth utility after electricity, gas, and water. A properly managed Compressed Air system can:

  • save energy
  • reduce maintenance & decrease downtime
  • increase throughput
  • improve product quality depending on its end-use

Compressed Air Quality

CA quality ranges from Plant air to Breathing air depending upon the end-use. Quality is determined by the dryness & the contaminant level. Higher the quality, the higher the cost. The following figure (Fig. 1) gives the applications of compressed air with respect to quality.

Application of Compressed Air.
Fig. 1: Application of Compressed Air.

Components of a Compressed Air system

Compressed air systems consist of:

  • a supply-side which includes compressors and air treatment
  • a demand side, which includes distribution and storage systems and end-use equipment.

A Compressed Air system broadly consists of:

  • Compressor
  • Prime mover
  • Controls
  • Treatment equipment and accessories
  • Distribution system.

Air Compressor Types (Fig. 2)

  • The Air compressor is the heart of any CA system.
  • There are two basic compressor types: positive-displacement and dynamic.
Basic Compressor Types
Fig. 2: Basic Compressor Types

Reciprocating Compressor (Fig. 3)

This is a very versatile type of compressor and can be used for nearly all industrial applications. These are constant-capacity machines & deliver the air in pulses.

Characteristics are:

  • High discharge pressures & relatively low to moderate volumetric flows (600-3000 cfm with a pressure range of 2000-5000 PSIG)
  • Generally more maintenance intensive due to the many wearing parts
  • It can be single-acting or double-acting, single-stage or multi-stage, air-cooled or water-cooled and lubricated or non-lubricated.
  • High efficiencies
  • Occupy larger footprint
  • Higher capital cost
  • Control is usually by Load-unload with 3 or 5 step capacity control
A typical reciprocating compressor
Fig. 3: A typical reciprocating compressor

Multi-staging

Multistage machines are used in place of single-stage ones for the following reasons:

  • Single-stage compression would generate excessive heat of compression
  • MOC would have to be of high grade and hence expensive
  • Power consumption of a single machine would be higher
  • Better efficiency

Rotary Compressor (Fig. 4)

The most common type of rotary compressor is the helical-twin, screw-type. Less common types include sliding-vane, liquid-ring, and lobe.

Characteristics are:

  • Not usually suited for high discharge pressures & are most efficient for moderate air flows & low pressures (3000-6000 cfm with a pressure range of 300-400 PSIG)
  • Low initial cost, compact size, low weight, and are easy to maintain.
  • Dry or oil-flooded type
  • Lower efficiency
  • VSDs provide good capacity control
A typical rotary compressor
Fig. 4: A typical rotary compressor

Centrifugal Compressor (Fig. 5)

Centrifugal compressor develops pressure by increasing the velocity of the air going through the impeller & then recovering the velocity in a controlled manner to achieve the desired flow and pressure.

Best suited for continuous air flows in large quantities.

Characteristics are:

  • The heat generated & power consumption is lower.
  • The space requirement & maintenance is minimum.
  • Inherently non-lubricated.
  • Available for flows ranging from 300 to more than 100,000 cfm, but the common ones are 1200-5000 cfm with a pressure range up to 125 PSIG.
  • Capacity control by inlet valve/guide vane throttling
  • Surge/choke phenomena
A typical Centrifugal Compressor
Fig. 5: A typical Centrifugal Compressor

Characteristic curves

The characteristic curve (Fig. 6) of a compressor plots its discharge pressure as a function of flow.

A typical characteristic curve
Fig. 6: A typical characteristic curve

Selection criteria

  • Flow rate
  • Discharge pressure
  • End-use of the air
  • Energy efficiency
  • Reliability

Compressor sizing

Estimation of Compressed air consumption:

  • Instrument air requirement
    • 5 nm3/hr per CV
    • 0 nm3/hr per ROV
  • The plant (Utility) air requirement
  • Compressed air requirement for pneumatic equipment, etc. operating in full load condition

Compressor Discharge pressure

  • end-use pressure (for instrument air, minimum 7 bar g)
  • plus all the pressure drops in the system.

Compressor Controls

Compressed air system controls serve to match compressor supply with system demand. Proper control is essential to efficient operation & high performance.

System controls include:

  • Start/Stop
  • Load/Unload
  • Dual control
  • Modulating
  • Speed variation
  • Pressure/Flow controls

Minimum Instrumentation required

Indications/alarms/trips consist of 3 major systems: Compressor, Lube oil & Cooling Water

Lube Oil system

 Minimum alarms

  • Low oil pressure
  • Low oil level in the reservoir
  • High oil filter differential pressure
  • High oil temperature
  • The high thrust bearing metal temperature

Temperature gauges

  • Oil piping to & from coolers
  • The outlet of each radial & thrust bearing

Pressure gauges

  • Discharge of the oil pump
  • On bearing header
  • On control oil line & seal oil line

Compressor

  • Pressure indicator at inlet, inter-stage & discharge
  • Pressure switch at discharge
  • Temperature indicator/alarm at inlet, inter-stage, and outlet
  • The temperature gauge on bearings
  • Vibration switches
  • A safety valve on each stage (for reciprocating type only)
  • Flowmeter (if required)

CW System

  • Pressure & temperature gauge on CW inlet
  • The temperature gauge on CW outlet
  • The thermal relief valve on CW outlet

Contaminants in Compressed air

The 3 major contaminants in Compressed Air are:

  • Water
  • Oil
  • Dirt

Compressed Air System Components/Accessories

The standard components/accessories include:

Prime Mover

The prime mover is the main power source providing energy to drive the compressor. This power can be provided by any of the following sources:

  • Electric motors: Economic, reliable, efficient
  • Diesel/natural gas engines: Fuel availability, higher maintenance, high cost/uncertainty of power
  • Steam turbines & combustion turbines: Inexpensive steam availability

 Intercoolers/Aftercoolers

  • Air-cooled/water-cooled
  • Minimum pressure drop
  • Regular maintenance

 Moisture/oil separators

  • Separation of condensed moisture/oil
  • Types – Impingement baffle type, Centrifugal type, with Demister pads.

 Pulsation Dampeners

  • Reduce/eliminate pulsations of reciprocating machines
  • Installed at the outlet of each stage

 Receivers

  • Storage for utilization at peak load
  • Draining of condensed water
  • Reduce pulsations from reciprocating machines
  • Receiver sizing is based on hold-up for a drop in pressure level, say 10 minutes for a pressure drop of 3 bar.
  • Provided with standard accessories.

 Air filters

Suction filters or Post-compression filters.

Felt cloth filters are used for suction. Compressed air filters can be:

  • Coarse particle filters (filter media can be a ceramic candle, felt cloth, etc)
  • Coalescing & activated carbon filters
  • Microfilter (high efficiency for special uses such as breathing air, etc) Minimum pressure drop

 Drain traps

  • Manual
  • Mechanical float type
  • Electronic timer operated
  • Auto drain traps – condensate sensing
  • Regular maintenance required to avoid CA wastage

 Lube oil coolers

  • To remove heat from the lube oil
  • Usually Shell & tube type with CW

 Air distribution piping

  • Least pressure drop in the system to reduce operating costs. The maximum pressure drop between the compressor and farthest end of compressed air consumption should be around 0.3 bar
  • Velocities between 6-10m/s in air mains; this will limit the DP & thus energy consumption and also allow moisture to precipitate
  • Minimum bends & joints (long radius bends to be used)
  • Arrangement for draining of moisture at regular intervals, slope provision
  • Minimum expanders/reducers
  • Leakproof joints, proper piping supports
  • Gauges to be provided at different locations to monitor the system pressure & temperature.

Compressor Cooling system

Cooling plays an important role in energy efficiency, two types are:

  • Air-cooled – In-efficient, preferred only for low capacity compressors
  • Water-cooled – Efficient, used for high capacity compressors

Water is circulated in a jacket around the cylinders to remove the heat resulting from compression & friction due to sliding of pistons. Water is also required for the inter/aftercoolers and lube-oil coolers.

CW consumption for inter/aftercoolers can be estimated based on the compression ratio per stage assuming an adiabatic efficiency.

Generally, per thumb rule,

Power Consumption

Compressed air plant layout and distribution

Plant layout:

  • Centralized layout
    • All compressors installed in a single house, cost-effectiveness as maximum plant space utilization
    • More pressure drop expected
  • Decentralized layout
    • Suitable for large industry, different levels/pressures of air
    • Compressor situated at the maximum user location, less pressure drop, max energy utilization
    • Can lead to noise and heat inside the plant

 Compressor location

  • The location should be such that compressor can induct clean, dry and cool air
  • Interesting fact:- every 4°C reduction air intake temp reduces power consumption by 1%
  • Points to be remembered while selecting the location of the air compressor:
    • Low humidity to reduce water entrainment
    • Adequate ventilation especially for air-cooled units
    • Minimum suction piping
    • Minimum bends

Compressed air distribution (Fig. 7)

Different compressed air layouts
Fig. 7: Different compressed air layouts

Codes

  • API 617 Centrifugal compressors
  • API 618 Reciprocating compressors
  • API 619 Rotary type positive displacement compressors
  • API 672 Packaged integrally geared centrifugal air compressors
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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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