The compressed air from compressor discharge is sent to the vessel “receiver”.
This vessel is having sufficiently large volume to store the pressurized air.
Loading and unloading of the compressors maintain receiver pressure. The pressure fluctuations due to this will not be transmitted to header because the volume of receiver absorbs them.
The receiver is provided with a relief valve, a pressure gauge, moisture drain trap and a drain.
Where the discharge header of compressors enter the vessel, on the pipe; a pressure sensing device is mounted to send pressure signals to the control logic that controls loading and unloading.
Air from receiver goes through a filter assembly.
Filter assembly comprises of two filter elements mounted in parallel, and can be put in line or isolated one by one. This is to allow changing the element.
A differential pressure (across the filter) indication is installed to know when the element gets clogged.
Another filter assembly is provided after the dryer too, to make sure any dryer desiccant particles do not travel to air header.
Air then passes through drier.
The drier works on the principle “adsorption and desorption”.
Air after dryer is sent to instrument air header.
Regeneration is done automatically.
4. Air Dryers
The principle of the adsorption dryer (Regeneration by Air) is based on the physical property of the desiccant to adsorb water vapor.
Absorption and adsorption sounds similar but are different process terms.
Absorption means to include or incorporate integrally some thing else, as part of itself.
In process terminology, absorption occurs when the smallest sample (molecule) of any one substance behaves like a sponge to water.
Adsorption is to hold (molecules of gas or liquid) to its surface allowing the forming of a thin film (layer) on the surface. In our application, water vapor is held to the surface of the alumina while wet air flows through alumina granules in the drier.
In our context, removing the water vapor attached to alumina by the high velocity flow of dry air in opposite direction.
The dryer consists of two drying towers, one of which is in drying service whilst the other is under regeneration treatment.
During drying, air passes through the desiccant (activated alumina) and goes to the outlet.
In regeneration, bulk airflow is stopped and a small quantity of air (purge air) is allowed to flow through the desiccant in the opposite direction to vent. This stream of air takes away the moisture from the adsorber regenerating it for use again. In some units purge air is through a fixed orifice and in others a regulator is provided.
In this way the drying process is made continual.
Before the drying capacity of the desiccant is fully used in the tower while in drying service, it is put to regeneration.
A timer mounted in the instrument panel controls the drier change over.
The timer output electrical signal operates a solenoid and the air signal to valves V1 & V3 is removed.
V1 opens allowing air to drier adsorber -1. V3 closes the vent line to exhaust.
At the same time a second solenoid is energized to allow air supply to V2 & V4. V2 closes the inlet to adsorber II.
V4 opens and vent the air pressure in adsorber-II.
Discharge air at the top of adsorber 1 pass through an orifice, a throttle valve and a second orifice to purge the adsorber II.
The throttle valve and orifice together controls the purge airflow.
The pressure gauges will show the drier pressure and we can know which one is in line. The drier which is regenerating will be vented and carry less pressure.
Now drier number 1 (Adsorber I) is in drying mode and the other is in vent mode or regeneration time.
The timer is set at few minutes (5-8) will change over the signal to solenoids.
Air from V2 & V4 is removed, V2 opens and V4 closes thereby bringing drier-II back in drying mode.
Then air is fed to V1 & V3. V1 closes the inlet and V3 vent the air in adsorber I.
Now drier I is in regeneration mode.
The cycle continues by changing over the dryers.
This is because the desiccant (activated alumina) after absorbing moisture for some time gets saturated with moisture. Desiccant can be made usable again to absorb moisture, by blowing hot air over the desiccant. This process is regeneration.
Regeneration is done automatically.