Perma Pure Micro-GASS UG - Manual (Page 6)

6 Micro-GASS User Manual
2.2 Perma Pure Dryer
A Nafion tube is housed inside a dryer shell. As wet compressed air flows through the tube, water
vapor is absorbed through the membrane and carried off by a counter-current purge air flow at a
reduced pressure. The driving force for this process, is the difference in partial vapor pressure of
water between the product and purge gas flows. In this system, a portion of the dried air is
expanded and used as the purge gas.
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.
2.3 Pump
The system pump draws sample from the source through the dryer and analyzer, and
recirculates the sample through the purge side of the dryer under vacuum. Pump has
capacity up to 1 lpm and is made from materials highly resistant to chemical attack.
Pump power is controlled by the temperature controller in order to prevent drawing the wet
sample into the system when the system is cold.
2.4 Dry Sample as Purge Gas
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 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. The dryer temperature is monitored and controlled through
the purge exhaust.