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
Compressed Air quality ranges from Plant air to Breathing air depending upon the end-user. 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:
- Prime mover
- 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.
- 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
Multistage machines are used in place of single-stage ones for the following reasons:
- Single-stage compression would generate excessive heat from compression
- MOC would have to be of high grade and hence expensive
- The 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.
- 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 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.
- 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 of up to 125 PSIG.
- Capacity control by inlet valve/guide vane throttling
- Surge/choke phenomena
The characteristic curve (Fig. 6) of a compressor plots its discharge pressure as a function of flow.
- Flow rate
- Discharge pressure
- End-use of the air
- Energy efficiency
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.
Compressed air system controls serve to match compressor supply with system demand. Proper control is essential to efficient operation & high performance.
System controls include:
- Dual control
- Speed variation
- Pressure/Flow controls
Minimum Instrumentation required
Indications/alarms/trips consist of 3 major systems: Compressor, Lube oil & Cooling Water
Lube Oil system
- Low oil pressure
- Low oil level in the reservoir
- High oil filter differential pressure
- High oil temperature
- The high thrust bearing metal temperature
- Oil piping to & from coolers
- The outlet of each radial & thrust bearing
- Discharge of the oil pump
- On bearing header
- On control oil line & seal oil line
- Pressure indicator at inlet, inter-stage & discharge
- Pressure switch at discharge
- Temperature indicator/alarm at inlet, inter-stage, and outlet
- The temperature gauge on the bearings
- Vibration switches
- A safety valve on each stage (for reciprocating type only)
- Flowmeter (if required)
- 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:
Compressed Air System Components/Accessories
The standard components/accessories include:
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
- Minimum pressure drop
- Regular maintenance
- Separation of condensed moisture/oil
- Types – Impingement baffle type, Centrifugal type, with Demister pads.
- Reduce/eliminate pulsations of reciprocating machines
- Installed at the outlet of each stage
- 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.
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
- Mechanical float type
- Electronic timer operated
- Auto drain traps – condensate sensing
- Regular maintenance is 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 the 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 are 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 the 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 the thumb rule,
Compressed air plant layout and distribution
- 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 industries, different levels/pressures of air
- Compressor situated at the maximum user location, less pressure drop, max energy utilization
- This can lead to noise and heat inside the plant
- The location should be such that compressor can induct clean, dry, and cool air
- Interesting fact:- every 4°C reduction in 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)
- API 617 Centrifugal compressors
- API 618 Reciprocating compressors
- API 619 Rotary-type positive displacement compressors
- API 672 Packaged integrally geared centrifugal air compressors
One thought on “Overview of Compressed Air System (PDF)”
Thanks for helping me understand that compressor air systems should be controlled properly to ensure that they are efficient and would perform well. I guess this would be applicable as well for compressor system overhaul projects. They must be in great condition after the process for the safety of the operator and the device as well once it is used for various purposes.