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.

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.

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

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

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

Characteristic curves

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

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,

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)

Codes

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