Compressed air systems are an essential component in many industrial and commercial operations. They provide a versatile and reliable source of energy that powers various tools and processes.
What is a Compressed Air System?
A compressed air system is a network of equipment designed to convert atmospheric air into a high-pressure source of energy. This system consists of several key components, including air compressors, storage tanks, air treatment equipment, and distribution piping. Compressed air can be used for a multitude of applications, ranging from powering pneumatic tools to actuating machinery and performing process operations.
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.
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:
Air Compressor
The heart of the compressed air system, the air compressor is responsible for drawing in ambient air and compressing it to a higher pressure. There are several types of compressors, including:
Reciprocating Compressors: Utilize a piston mechanism to compress air.
Rotary Screw Compressors: Use two interlocking screws to compress air, providing a continuous flow.
Centrifugal Compressors: Use a rotating impeller to compress air, suitable for high-capacity applications.
Air Storage Tanks
These tanks store compressed air, helping to balance the supply and demand. They allow for peak demand management and reduce the frequency of compressor operation, leading to increased efficiency and extended equipment life.
Air Treatment Equipment
To ensure the quality of the compressed air, treatment equipment is necessary. This includes:
Filters: Remove dirt, moisture, and other contaminants.
Dryers: Reduce humidity in the air to prevent corrosion and other issues in pneumatic tools.
Regulators: Control the pressure of the air delivered to various tools and machinery.
Distribution System
A network of pipes and hoses transports compressed air from the compressor and storage tanks to the point of use. Proper design and maintenance of this system are critical to minimize pressure drops and energy losses.
Control Systems
Advanced compressed air systems may incorporate control systems that monitor and adjust the operation of the compressor, storage, and treatment equipment to optimize performance and energy efficiency.
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.
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
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 from compression
MOC would have to be of high grade and hence expensive
The power consumption of a single machine would be higher
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 easy to maintain.
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 of up to 125 PSIG.
Capacity control by inlet valve/guide vane throttling
Surge/choke phenomena
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.
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 the bearings
Vibration switches
A safety valve on each stage (for reciprocating type only)
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 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
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 industries, different levels/pressures of air
Compressor situated at the maximum user location, less pressure drop, max energy utilization
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)
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
Applications of Compressed Air Systems
Compressed air systems have a wide range of applications across various industries. Some common uses include:
1. Manufacturing and Assembly
Pneumatic Tools: Tools such as drills, wrenches, and hammers are powered by compressed air, allowing for greater efficiency compared to electric counterparts.
Material Handling: Compressed air is used in conveyor systems and automated guided vehicles (AGVs) for transporting materials.
2. Food and Beverage Industry
Packaging: Compressed air is used for sealing and packaging products, ensuring hygiene and efficiency.
Process Control: Many processes in food and beverage production require precise control of air pressure and flow.
3. Automotive Industry
Paint Spraying: Compressed air systems are crucial for paint application in vehicle manufacturing, providing a smooth and even coat.
Assembly Line Operations: Pneumatic tools are widely used in assembly lines for tasks like fastening and lifting.
4. Healthcare
Medical Equipment: Compressed air is utilized in various medical devices, such as ventilators and dental chairs, ensuring reliability and safety.
5. Construction
Heavy Machinery: Tools like jackhammers and nail guns rely on compressed air for their operation, improving efficiency on job sites.
Benefits of Compressed Air Systems
Compressed air systems offer numerous advantages that make them a preferred choice for many industries:
1. Versatility
Compressed air can power a wide array of tools and equipment, making it suitable for various applications across different sectors.
2. Safety
Unlike electricity, compressed air poses minimal fire and explosion risks, making it a safer alternative in hazardous environments.
3. Clean Energy Source
When properly treated, compressed air is clean and free from contaminants, making it ideal for applications in sensitive environments, such as food production and pharmaceuticals.
4. Reliability
Compressed air systems can provide a consistent and uninterrupted power source, crucial for maintaining productivity in manufacturing and other operations.
5. Energy Efficiency
Advanced control systems and energy-saving technologies can help optimize compressed air systems, leading to significant reductions in energy consumption and operational costs.
In conclusion, the compressed air systems are a vital component of many industrial processes, offering versatility, reliability, and safety. Understanding their components, applications, and challenges is essential for optimizing performance and efficiency.
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.
One thought on “Compressed Air System: A Detailed Guide”
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.
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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.