There are several uses for compressed air in a plant. Many valves and instruments are pneumatic. Maintenance and construction tools often require compressed air as a power source. Air is required in some processes. It is used for decoking furnaces and for regenerating some catalysts and desiccants.
Figure 1 shows a typical compressed air system with two air compressors. Each compressor has inlet air filters and silencers and outlet air coolers. There is an air receiver where some moisture is removed. The receiver acts as a surge drum in the system to maintain system pressure during short outages.
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Downstream of the air receiver, the system splits into two systems. One is called the utility air system. This air is distributed without drying. The other is called the instrument air system. This air is filtered and dried before distribution.
The equipment included in a compressed air system consists of the air compression package, air coolers, air receivers, air dryers, and distribution piping.
Air Compression Package
Components that are typically included in an air compression package are:
• An air filter and silencer
• A compressor and gear if required
• A driver with a coupling
• Intercoolers with moisture separators and automatic water removal
• A lube-oil system
• Suction throttle valve and discharge check valve
• Discharge blowoff valve and silencer
• Vibration monitoring system
• Controls and instrumentation
Inlet Air Filter and Silencer
An inlet air filter removes dust, dirt, sand, and other abrasive or gritty particles. Filtering prevents damage to the compressor and minimizes maintenance and downtime. The silencer removes the objectionable air inrush noise.
There are two ways to increase the pressure of a gas. One is to reduce the volume of the gas. The other is to increase the velocity of the gas. Positive displacement compressors reduce the gas volume. There are several different types of positive displacement compressors. They include:
• Rotary or helical screw, or rotary lobe
• Sliding vane
• Liquid piston
Of these, reciprocating compressors and rotary screw or helical screw compressors are most often used in gas plant and refinery compressed air systems.
Centrifugal compressors and axial compressors increase pressure primarily by increasing the gas velocity.
Centrifugal compressors are more often used in compressed air systems.
Capacity and discharge pressure are the key factors in selecting a compressor. Helical screw compressors can have a high capacity but are limited in discharge pressure to about 250 psig. Reciprocating compressors can have a high discharge pressure but are limited in capacity. A typical guideline is to consider reciprocating compressors up to 1,500 SCFM and centrifugal or screw compressors above 2,000 SCFM.
Other factors influence the selection of a type of compressor. These include:
• Sensitivity to fouling and solids
• Availability or reliability
• Maintenance requirements and costs
Compressor Type Characteristics
Reciprocating compressors tend to have higher maintenance costs, lower availability, and the poorest sensitivity to fouling and solids. However, they are lower in cost in smaller sizes.
Centrifugal compressors tend to have lower maintenance costs, higher availability, higher efficiency, and lower noise levels. They are also lower in cost in large sizes.
The final selection depends on the result of an over all economic analysis.
Controls are provided in an air compressor package to:
• Regulate the flow of air to match the demand
• Prevent surging of a centrifugal compressor
• Regulate the discharge pressure
• Automatically start spare compressors
• Protect the compressor and driver from damage
Two methods are commonly used for regulating the compressor flow and discharge pressure. The conventional
method with reciprocating compressors is a multistep load and unload method.
With a centrifugal compressor, a two-step load and unload operation is also possible. However, capacity
modulation with suction throttling has several advantages. Most variations in air demand can be
accommodated within the throttling capacity of the control system. A constant discharge pressure can be
maintained. Power demand is lower because no air is wasted down to the surge point, or about 70% of
capacity. There are power savings down to zero air delivery.
The function of an aftercooler is to cool the compressed air after it has been compressed. the compressed air must be cooled to 140°F or below. This cooling will condense up to 60% of the incoming water vapor. It will reduce the volumetric flow of air to downstream equipment and protect downstream equipment from overheating.
Two types of aftercoolers are in common use. One is shell-and-tube exchangers using cooling water or air fins
where ambient temperatures are low enough. Water-cooled exchangers are more common than air-cooled.
They can be used in any climate, are usually less expensive, take less space, and are less noisy.
Air cooled exchangers save water and generally require less maintenance. However, they are more expensive.
Also they are limited in cooling to about 15°F above ambient temperature or higher.
An air receiver provides continuity of air flow during surges in demand, compressor trips, and loading and unloading. An air receiver provides a large volume to entrap and remove condensed water vapor and oil. It will also provide some time (usually a minimum of one minute and often two to three minutes) for operators to take corrective action following loss of all compressors.
A typical standard for sizing an air receiver is to provide a minimum of one minute of base instrument air load while the air receiver pressure decays from 100 psig to 50 psig. This standard applies to systems that have standby compressors started automatically. For systems with manual-start compressors, this time can increase to as much as 15 minutes.
It is common to have only one or two air receivers at an air compressor house where several compressors are installed. However, in some plants, air receivers are installed at other locations.
Air Dryer Installation
An air dryer installation usually includes a prefilter, a dryer, and an afterfilter. The total installation pressure drop should be less than 5 psi at maximum design flow rates. The prefilter removes liquid oil and water and solids to protect the dryer desiccant. The prefilter is usually designed to remove 98% of all oil droplets greater than 1 mm in diameter. Oil retention is usually 2 lb for each 100 SCFM of design capacity. The filter material typically is activated carbon or alumina. The filter should have an automatic drain.
The afterfilter removes fragmented or pulverized desiccant from the dried air. It is designed to remove 100% of
all particles larger than 1 mm in diameter. The afterfilter is usually a dual cartridge filter to allow cleaning without shutdown or bypassing.
An air dryer keeps compressed air, particularly instrument air, free of water. This is necessary to avoid instrumentation malfunction and damage. Dry air keeps maintenance low, reduces pneumatic equipment downtime, and minimizes upsets in temperature controls, flow, and other process units.
The four main types of air dryers are heat-regenerated absorption, non-heat-regenerated absorption, refrigeration, and rotary absorption dryers.
The heat-regenerated and non-heat-regenerated absorption dryers are similar except for the regeneration system.
Both types consist of two desiccant filled chambers connected in parallel. The desiccant in one of the chambers
dries the air stream while the desiccant in the other chamber is being regenerated.
In the heat-regenerated dryer, air is heated with medium-pressure steam or other heat medium. The hot air then
is used to purge moisture from the spent desiccant. Silica gel or alumina is used as a desiccant. These dryers are relatively expensive to purchase and to operate.
In the non-heat-regenerated dryer, regeneration is carried out under vacuum using dry air for purging. Purging can require from 3 to 15% of the dryer capacity. Saudi Aramco prefers heatless regeneration, desiccant-type dryers.
The rotary absorption dryers dry by the chemical reaction of desiccants to form hydrates or hydroxides. They are rarely if ever used to dry plant air.
The design dewpoint for air from a dryer is often specified as 20°F below the minimum recorded temperature at a plant.
In most industrial plants with compressed air systems there are at least two separate systems. One supplies instrument air, which is dried. The other supplies plant or utility air, which is not dried.
Normally, instrument and plant air are distributed at the same pressure. There are exceptions at some plants for
different specific reasons. The distribution system usually is sized to provide a minimum of 75 psig at the most distant consumer from the compressed air source at maximum demand flow rate. The total pipeline pressure
drop should not exceed 5 psi at this maximum demand flow rate.
Plant air balances are very useful for designing, operating, and analyzing a plant air system. Balances should be prepared for all normal and extreme situations including:
• Normal operation
• Peak demand
• Upset and emergency situations
• Seasonal variations, if any
• Turnarounds and regenerations
An air balance summarizes the air production and air consumption unit by unit. The air balances are used for:
• Establishing or verifying the capacity of various system components
• Developing operating techniques to handle upsets and emergencies
• Establishing a basis for selecting a load-shedding scheme
• Optimizing the use of air within a system
• Checking the operating flexibility of a system