07773B DCN6940
19
4
OPERATION
4.1 Overview
Refer to Figure 4-1 for a diagram of the 751H pneumatic flow.
The 751H dries and scrubs ambient air to produce zero air. To accomplish this, first
the compressor draws air in through the intake filter and compresses it. At the
compressor outlet, the air is under pressure and hot from the compression. The
relative humidity is high as a result of the high pressure.
The air is conducted through the cooling coil where heat is removed by transfer to the
cooling fan air. With the high pressure and the temperature reduced to ambient level,
the relative humidity is at its highest. At this point, the air is usually supersaturated.
From the coil, the wet air passes through a coalescing filter where the excess water is
separated and settles in the bottom of the filter. The controller periodically opens the
solenoid drain valve allowing the water to be expelled through a drain fitting.
The partially dried air enters the Regenerative Dryer which removes essentially all the
remaining water and a portion of the other contaminants. The pressure relief valve
may open occasionally and can be loud but this is a normal part of the operation for
the 751H.
As the air leaves the dryer, its pressure is controlled to 30 psig by an air pressure
regulator mounted on the instrument panel. This maintains a constant pressure at the
calibrator inlet and is displayed by the pressure gauge on the 751H instrument panel.
For a final clean-up, the dry, regulated air enters the specific scrubbers/filters as
follows:
1. The Hydrocarbon scrubber where Hydrocarbons are catalytically converted to
CO
2
and water.
2. The NO scrubber where NO is oxidized to NO
2
.
3. The activated charcoal scrubber where the NO
2
, O
3
, SO
2
and H
2
S are absorbed.
4. The CO scrubber, where CO is catalytically oxidized to CO
2
.
5. The clean dry air passes through a fine particulate filter and leaves the 751H
through the instrument panel bulkhead union (Zero Air Out).
Summary of Contents for 751H
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