Micro-GASS Operation
The Heated Filter
The first step in conditioning the sample if filtering
out particulate. The standard Micro-GASS system filter is a disposable
1µm "encapsulated micro-fiber" filter element housed in a Kynar™ filter
body for corrosion resistance. Surrounding the filter body is a heated
aluminum shell. The filter is thermostatically controlled at 95°C to
prevent aerosols from forming in the sample stream. The heated filter
and the MD-Series Nafion membrane dryer together form the Micro-GASS
Module.
The Nafion Membrane Dryer
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Click box
for schematic |
The MD-Series dryer is installed
downstream of the filter. The dryer is a shell-and-tube configuration in
which the sample gas flows in the inner tube, or "element", with the
purge gas in the shell flowing counter to the sample. O-ring seals
isolate the sample from the purge gas. As the sample flows through the
dryer water is continually removed, reducing the sample dew point as it
travels from inlet to outlet. The water vapor is evacuated with the
purge exhaust.
The sample inlet portion of the dryer is heated
to provide rapid removal of the water and to prevent condensation.
Nafion transfers water vapor with a higher efficiency at elevated
temperatures. Yet, not all of the water-of-hydration molecules can be
removed, and the Nafion always retains some water. The amount of water
retained is greater at higher temperatures. Since water is removed only
to the extent that the membrane is dry, the dryer should be kept at the
lowest temperature possible without causing the sample to condense. As
the sample dew point constantly drops as it travels through the dryer, a
temperature gradient that is just above the sample dew point is the most
desirable. This is achieved by heating only the sample inlet, where the
sample dew point is highest, and cooling the sample outlet with purge
air where the sample dew point is the lowest. Efficiency throughout the
dryer is achieved by using this process.
If liquid water is introduced into the dryer,
efficiency will decline and the dryer could fail to perform altogether.
The Nafion tube when wet will elongate approximately 10% over its dry
length. This elongation will cause the element to kink inside the dryer
housing, creating a restriction in the flow.
Dry Sample as Purge Gas and Temperature Control
The dry sample is
piped to the sensor and back to the Micro-GASS for use as the purge gas.
It is expanded through a needle valve in the flow meter and placed under
slight vacuum. The vacuum increases the difference in partial water
vapor pressure between the sample and purge gas and enhances the drying
efficiency of the system. The purge flow enters the dryer at the sample
outlet end of the dryer and performs two functions: First, it provides a
medium for the water vapor to be carried away. Second, the cool gas
creates a temperature gradient along the length of the dryer. The purge
gas is heated as it traverses the heated section of the dryer, reaching
its maximum temperature at the purge exhaust port. It is in the purge
exhaust that the dryer temperature is monitored and controlled.
System Pump
The system pump
draws sample from the source through the dryer and analyzer, and
re-circulates the sample through the purge side of the dryer under
vacuum. Pump has capacity is up to 1 lpm and is
made from materials
highly resistant to
chemical attack. The
pump power is
controlled by the
temperature
controller in order
to prevent drawing
the wet sample into
the system when the
system is cold.
Controls
-
System Power
-
The system power
switch controls
power to the
temperature
controller, and
the filter,
dryer, and
heated-line
heaters. System
power is on a 2
Amp fuse,
located on the
front panel.
-
Pump Power
–
The pump
power is wired
through the
temperature
controller. In
order for the
pump to engage
the system must
be heated to 40°
C, and the pump
power switch
must be in the
on position.
Sample flow is
controlled by
the flow meter.
-
Temperature
Control
- The
dryer heaters
are controlled
by a PID
electronic
temperature
controller.
Set-point
adjustment is
made by pressing
the ".
" key and either
the up arrow key
or the down
arrow key
simultaneously.
When the desired
temperature is
displayed,
release both
keys. The
allowed
set-point range
and the pump
cut-off
temperature are
pre-programmed
by PPI. Refer to
APPENDIX C for
the temperature
controller
configuration,
or to the manual
included with
the
documentation
package.
-
Flow Control
– System
flow is measured
after the
analyzer in the
dry sample at
atmospheric
pressure and
temperature. The
flow meter
should be
adjusted to
provide a flow
of between 0.5
to 1.0 lpm,
depending on the
water content of
the sample.
Lower flow-rates
result in dryer
samples.
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