GAS SCRUBBING
The purpose of the Model 111 is to supply pollutant-free air (zero air) from the ambient
air to allow for proper zeroing, and to provide clean diluent air for spaning ambient air
analyzers. The components to be removed are SO2 NO, NO2, CO and hydrocarbons.
There is no consensus as to what extent zero air should have water vapor removed. Since
many analyzers have longer response times if super dry air (dew point less than -30o
C) is
used for zero and span, and since water vapor is not a pollutant, the Model 111 does not
have a drying system. However, the dew point is reduced as a result of compression of
the ambient air.
Figure 4-1 is the gas flow schematic for the Model 111. Room air enters the compressor,
where it is raised to a pressure of approximately 80 to 90 psi (4560 mm). At 25o C the
saturation water vapor pressure is approximately 24 mm. Therefore, most of the water
condenses out, and falls to the bottom of the tank. Out of the 4560 mm of pressure in the
tank, only 24 mm is due to water vapor. When this air is later expanded to atmospheric
pressure (760 mm) the water vapor pressure is reduced to approximately 4 mm. This
corresponds to a dew point of slightly less than 0o
C.
To keep any condensation from occurring in the tubing between the compressor and the
Model 111, the output of the compressor contains coalescing filter and a pressure
regulator where the pressure is reduced to 70 psig