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7.1 INTRODUCTION
As described in Section 1, water vapor is removed from compressed air
by diverting air flow alternately between two towers filled with activated
alumina desiccant. While one tower processes the compressed air
stream adsorbing water vapor, the opposite tower regenerates by
desorbing the water vapor and venting it to atmosphere.
The Microprocessor Controller provides the ability to select between
heated or heatless regeneration. Both heated and heatless regeneration
methods are described in the following sections.
NOTICE
The Microprocessor Controller must be in the OFF position prior to
changing the mode (heated purge / heatless) of the dryer. After the
change is made and the Microprocessor Control is subsequently
turned ON, the dryer will be in the new mode of operation.
7.2 DRYING CYCLE
Saturated compressed air enters the dryer and is diverted to the
appropriate tower by the Inlet Flow Valves. (Refer to the Process and
Instrumentation Diagram.) The Right Tower Flow Valve is actuated to a
closed position to prevent air flow from entering the regenerating tower.
Simultaneously, the Left Tower Flow Valve is actuated to an open
position allowing air flow to the drying tower. During this time, the Left
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 material in the left tower at pressure,
removal of water vapor from the air stream begins to occur through
adsorption. In the adsorption process, the desiccant material draws
water vapor out of the compressed air and "holds" it until the left 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.
7.3 REGENERATION CYCLE
Previously adsorbed moisture removed from the process 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 depressurization, a significant
portion of the previously adsorbed water vapor is stripped off the
desiccant material and exhausted to atmosphere.
CAUTION
Any time the dryer is switched between two operating modes, care
must be taken to ensure the purge adjustment valve is adjusted
correctly. Refer to the specification sheet in this manual for proper
gauge setting.
7.3.1
SETTING THE REGENERATION AIR FLOW
Proper setting of the purge flow is necessary to achieve
proper dryer performance in the heatless and externally
heated modes. Setting the purge flow too high will waste
compressed air and if set too low, the dryer will not achieve
dew point performance.
consists of the purge adjustment valve, purge pressure
gauge, and the purge orifice. The purge pressure gauge is
located between the purge adjustment valve and purge
orifice. Manually adjust the purge adjustment valve until the
reading on purge pressure gauge matches the purge
pressure setting listed on the tag attached to the gauge. Note
that there are two purge gauge values, corresponding to each
mode of operation.
4
7.0 PRINCIPLES OF OPERATION
The purge adjustment manifold
EH Series Desiccant Dryer Models 150-8000
http://www.ingersollrandproducts.com
7.3.2
HEATED PURGE REGENERATION
In the heat reactivated mode, the dryer uses 7.5% of dry
compressed air expanded to atmospheric pressure.
However, after air expansion through the Purge Orifice, the
purge air is passed through the purge heater. This expanded,
heated purge air is then passed through the regenerating
tower and exhausted out to atmosphere. After a three minute
delay, the heating process occurs for 2 hours and 57 minutes.
During the heating process the electric heater is cycled on
and off by Ingersoll Rand' temperature controller Solid State
relay. This advanced controller precisely monitors purge air
temperature and adjusts the heater temperature accordingly.
This results in a regeneration air temperature that remains
within 1.5 °F from the heater setpoint for the entire
regeneration
cycle,
associated with contactor-based heating system.
completion of the three-hour heating period, the electric
heater turns off. The dry regeneration air continues to flow for
57 minutes in order to cool down the desiccant bed.
7.3.3
HEATLESS PRESSURE SWING REGENERATION
In
the
heatless
regeneration uses approximately 15% 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 purge adjustment valve and the Purge Orifice. Once the
air has passed through the Purge 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
Outlet Purge Valves.
7.4 TOWER REPRESSURIZATION
Upon completion of tower regeneration, and prior to changing the Inlet
Flow Valve position to switch towers, the regenerated tower must be
repressurized. Repressurization is accomplished by closing the
appropriate purge valve.
Closing the Purge Valve allows the
regeneration air to pressurize the tower.
NOTICE
Failure to repressurize prior to tower switchover will result in shocking
the desiccant material and cause premature desiccant dusting.
7.4.1
HEATED PURGE REPRESSURIZATION
Three minutes prior to tower switch-over, repressurization is
accomplished by closing the appropriate Purge Valve. When
the Purge Valve closes, the regeneration air begins to
pressurize the tower. If the dryer is supplied with the optional
repressurization piping, the Repressurization Valve opens,
allowing some additional air from the outlet of the dryer to
assist the purge air and to ensure adequate pressurization.
During normal tower regeneration, the Repressurization Valve
is held closed so that the only source of air for regeneration
passes through the purge adjustment assembly.
7.4.2
HEATLESS PRESSURE SWING REPRESSURIZATION
45 seconds prior to tower switch-over, repressurization is
accomplished by closing the appropriate Purge Valve. When
the Purge Valve closes, the regeneration air begins to
pressurize the tower. If the dryer is supplied with the optional
repressurization piping, the Repressurization Valve opens,
allowing some additional air from the outlet of the dryer to
assist the purge air and to ensure adequate pressurization.
During normal tower regeneration, the Repressurization Valve
is held closed so that the only source of air for regeneration
passes through the purge adjustment assembly.
eliminating
temperature
mode,
following
depressurization,
swings
Upon
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