6
6. PRINCIPLES OF OPERATION
6.1
INTRODUCTION
As described in Section 1, water vapor is removed from compressed air by
diverting air flow alternately between two towers filled with desiccant. While one
tower processes the compressed air stream, adsorbing water vapor, the opposite
tower gets regenerated by desorbing the water vapor and venting it to atmosphere.
Refer to the Process and Instrumentation Diagram (P&ID) or the flow diagram for
visual representation of the drying and regenerating cycles.
6.2
DRYING CYCLE
Saturated compressed air enters the dryer and is diverted to the appropriate tower
by the Inlet Flow Valves. Referring to the Process and Instrumentation Diagram
(P&ID), the Left Inlet Flow Valve is actuated to a closed position to prevent air flow
from entering the regenerating tower. Simultaneously, the Right Inlet Flow Valve is
actuated to an open position, allowing air flow to the right hand tower. During this
time, the Right Tower Purge Valve is actuated to a closed position, preventing the
compressed air from venting to atmosphere. As the compressed air flows through
the desiccant bed at pressure, removal of water vapor from the air stream begins to
occur through adsorption. In the adsorption process, the desiccant beads draw
water vapor out of the compressed air and “hold” it until the right tower drying cycle
is complete. Compressed air flows out of the tower for delivery to the process use.
The Outlet Flow Check Valves provide air flow diversion to the outlet air connection
of the dryer. The Right Outlet Flow Check Valve allows air flow through to the outlet
connection of the dryer while The Left Outlet Flow Check Valve meters the purge
air to the regenerating tower.
6.3
REGENERATION CYCLE
Previously adsorbed moisture, removed from the process air stream, gets stripped
or desorbed from the desiccant material in the regeneration process. The first stage
of regeneration is tower depressurization. After the Inlet Flow Valves are switched
to divert air flow away from the regenerating tower, the appropriate Purge Valve will
be opened and the tower will be depressurized. Through rapid depressurization, a
significant portion of the previously adsorbed water vapor is stripped off of the
desiccant material and exhausted to atmosphere.
The second stage of regeneration uses a portion of the dry, compressed air,
expanded to atmospheric pressure to complete the desorption process. As shown
on the P&ID, the compressed air exits the drying tower and a portion of the air
flows through the fixed orifice located in the Outlet Check Valves. Once the air has
passed through this orifice, it expands to atmospheric pressure and continues the
regeneration process. Desorption occurs as the desiccant releases water vapor
into the regeneration air and is exhausted through the Purge Muffler.
6.4
SETTING THE REGENERATION AIR FLOW (Optional)
MPS Dryers are equipped with a fixed orifice to provide the appropriate amount of
purge air to the regenerating tower, however an adjustable purge option is
available. With this option, it is essential that the purge rate be properly set to
achieve proper dryer performance. Setting the purge flow too high will waste
compressed air and if set too low, the dryer will not achieve proper dew point
performance.
