From Fig. The standby Instrument Air IA system can be diesel driven or steam driven, subjected to the utility available. Since most industrial facilities operate with IA air at about 7 barg to 8 barg, the set pressure of the relief valve RV must be set higher accordingly but must not exceed the vessel design pressure.
The IA distribution lines must be sized such that the line pressure drop is not more than 1 bar between the air dryer outlet and the farthest user of IA.
Typically, a user can be considered to use 0. Air purge regeneration is a commonly used method where the packed column of molecular sieves is dried by diverting a fraction of the dry air from the active air dryer vessel enabling adsorption of the moisture and expelling via a purge line. The hot air heats the regenerated dryer till the moisture reaches boiling point and is subsequently expelled through the purge line.
The recommended cycle time between regeneration cycles for normal operation is 6 hours for regeneration and 2 hours of standby. The maximum allowable cycle time between regeneration cycles is 6 hours for regeneration and 4 hours standby. Hence the air dryers must be designed to be capable of drying for at least 10 hours without increase in dew point.
After coolers can be air-cooled type or water- cooled type. Compression increases the partial pressure of the water vapour present.
If the water vapour partial pressure is increased to the saturation water vapour pressure, condensation occurs. If the saturation water vapour pressure is reduced to the partial pressure of the water vapour present, water or ice will result. Therefore, moisture removal is a major consideration of instrument air treatment systems. Water droplets can also obstruct the discharge ports on solenoid air pilot valves thus reducing their ability to function properly.
Therefore an automatic drain e. In cold climates, water extracted from the atmospheric air accumulates at the low points in the IA system. Hence, in such cold climates, insulation and steam tracing should be provided to both piping as well as up to a sufficient height from the bottom portion of the air receiver.
On the down side, they cause reduced flow rates, increased pressure loss, and allow places for particulates to collect causing valve leakages. Air Compressor Selection. Air Purge Regeneration - a common regenerative dryer system where the wet tower is dried by diverting a small flow of the dry air from the active drying tower back through the saturated tower. This dry air adsorbs the moisture and expels it to the atmosphere. Heater Regeneration - another common regenerative dryer system where the wet tower is dried by a heater element that is incorporated in the towers.
When in the regeneration mode, the heater element heats the tower to the point of boiling the entrained water to a vapor that is expelled from the wet tower into the atmosphere. Both dryer types accomplish the required dew point reduction and add some inefficiencies to the Instrument Air Unit by either utilizing some of the systems horsepower or adding extra electrical power requirements to the system.
The consideration for this regeneration cost must be applied to the initial sizing of the compressor unit. Air Dryer Principle. In most systems, the air compressor is sized to exceed the site air flow requirements.
As mentioned earlier, the compressed gas industry uses Now consider actual site conditions at The moisture saturation vapor pressure at 68F is 0. In turn, this reduced pressure value at compressor suction decreases the mass flow capability of the reciprocating compressor or decreases the discharge pressure in case of a centrifugal compressor.
The air receiver provides a storage volume of compressed air to be used when the compressor is off-line or when the air demand temporarily exceeds the compressor output. If the users require psig in the air receiver and the compressor is set to provide psig air to the receiver, then there is no hold-up or buffer. For the air receiver to be effective, it must therefore operate within a pressure band.
For example, assume that the demand for compressed air requires psig pressure in the air receiver and the compressor is set to operate in a pressure band to load at psig and unload at psig.
This means that in case the compressor is off-line, or if air demand increases, a storage volume corresponding to air receiver pressure ranging from psig to psig is always available. The air receiver volume can be calculated from the following equation which shows the time taken for the air receiver to drop from the higher pressure point to the lower pressure point within the operating pressure band:. If air is supplied to the air receiver during the time interval being evaluated, then the value of C must be reduced by the rate of air supplied.
Centrifugal compressors are cost effective in large sizes only, can provide oil-free air delivery and have the characteristic pressure curve with pressure decreasing as capacity increases. The reciprocating compressors have effective multistep capacity control but has a high first cost with special foundations for vibrations and needs routine maintenance.
The rotary screw compressor is popular in instrument air service since it is a compact package at a relatively low first cost and provides oil-free air. The rotary screw compressor capacity can be controlled by a variable speed drive. Therefore during periods of low air consumption, the compressor will need to be unloaded with the discharge-to-suction bypass open and if an over-run timer is fitted, it can stop the compressor in case it runs in unloaded condition for a pre-set period of time.
The atmospheric humidity entering the air compressor ends up in the air receiver which is usually at a temperature below the dew point of the compressed air.
Note that the pressure dew point is higher than the atmospheric dew point resulting in water accumulation at the bottom of the air receiver. This water is usually drained out through a liquid drain trap which drains the water while preventing escape of compressed air.
The balance line allows air which has entered the trap to be discharged back to the receiver. Without the balance line, air binding can occur in the liquid trap. The size of the liquid trap depends on the differential pressure across the trap and the required discharge flow rate.
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