Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 98)

The PD625 is similar to the PD1000, but it is designed for lower flow rates. Its 50-tube, 24" Nafion® dryer element has
a drying capacity one eighth that of the PD1000. The PD625's inlet filter, tubing connections, and purge flow meter
range are also smaller than for the PD1000. See
to compare specifications for the two dryers. The PD625 is
normally used in the two-dryer configuration, so it does not include the sample flow meter or sample needle valve
included in the PD1000.
Table 14
Table 14. Sample Air Dryer Specifications
Specification PD625 PD1000 Units
Length 76 (30) 142 (56) cm (in)
Weight 4.4 (9.7) 6.5 (14.4) kg (lbs)
Connections: Sample Inlet
Sample Outlet
Purge Outlet ¼" Swagelok
¼" Swagelok
⅜" Swagelok ⅜" Swagelok
⅜" Swagelok
½" Swagelok
Pressure drop (at standard temperature and pressure) 10 5 mbar per l min -1
Flow rate for -15 °C dew point 2 16 l min -1
Sample volume (internal volume of dryer tubing) 10 80 ml
Purge flow meter range 0 to 5 0 to 10 l min -1
Sample flow meter range NA 0 to 50 l min -1
9.4.2 Theory of Operation
The dryers work by forcing the humidity in the sample air through the walls of the Nafion® tubing, where it is carried
away by the purge flow. The sample air flows through the insides of the bundle of tubes, and the purge air flows on the
outside of these tubes, in the opposite direction. The water vapor is forced through the wall of the tubes by a difference
in partial pressure. The sample becomes progressively drier as it travels down the dryer, while the purge air becomes
progressively more humid. For best performance, the purge flow should be very dry (-40 °C dew point), and should
have an actual flow rate of at least twice the sample flow. Although the purge flow could be supplied by air from a
compressed air tank or by ambient air dried with a chemical desiccant, for most TGA applications it is provided by the
dryer itself. A portion of the sample flow is split off at the outlet of the dryer, and its pressure is reduced by connecting
the purge outlet to the TGA100 sample pump. Dropping the pressure reduces the partial pressure of the water vapor and
increases the actual flow rate, allowing the purge requirements to be met with just a fraction of the sample flow. More
information on the dryer can be found at
www.permapure.com .
9.4.3 Installation Instructions
9.4.3.1 Mounting the Dryer
The dryer is configured at the factory for horizontal installation, but it may be operated in any orientation. The dryer
mounting bracket may be attached to a horizontal or vertical pipe with a maximum diameter of 1.75 in (4.5 cm), with
the U-bolts included with the dryer. For vertical installation, the tee at the sample outlet may be rotated so that the
sample outlet connection is downward (along the dryer axis), and the flow meter may be rotated to be vertical (along
the dryer axis), as shown in
. Figure 9-3
98