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
