Perma Pure dryers require a purge gas with a partial water vapor pressure that is significantly lower than the sample gas being analyzed. This can be provided by either supplying the dryer with air that already has the moisture removed from it or developing a vacuum-pump or educator based sub –system that generates the dry purge gas (water vapor pressure differential) on the fly.
Choice #1 – External Supply of Dry Air
Many customers use our dryers with external sources of dried air. There are a number of options here that our customers have used:
- Instrument Quality Compressed Air – Dried compressed or “Instrument” Air is typically found at most industrial plants and in many labs and serves as an excellent purge gas. The dew point of this purge gas is typically around -40° C. Although the air might already be free of lubricating oil and dirt, the air must be filtered to eliminate dirt and hydrocarbons being sucked in to the dryer purge inlet and deposited on the outside of the Nafion tubes. Over time this contamination will reduce the performance of the dryer by blocking its water transfer pathways. Most of our performance curves have been made from test data using instrument air as the purge gas.
- Standard Quality Compressed Air – Standard quality compressed air – which is not dried and may carry an oil mist – is often available but caution must be advised as dirt and oil in the air will collect on the dryer tubes and can cause it to lose performance over time.
- Local Air Compressor – An air compressor with an ultra-low dew point rating may be deployed locally in an analyzer station to provide clean and dry purge gas.
- Nitrogen or other Cylinder gas – Using ultra-dry cylinder gas with a typical dew point of -60° or -70°C can yield even lower results than shown on our performance curves. Often used for lab applications, this method might not be cost effective for continuous or high flow applications. If the target dew point is -5C or higher, ultra-low purge gas is only required when you NEED to get to a low dew point below -20 C.
Note: We recommend starting your performance tests with a purge gas flow at 2x of the sample gas flow. Adjust the purge gas flow based on target performance required – you might find you can still get good performance with a flow as low as 0.5x in your application. Because using options C and D are much more expensive, reducing the flow can lower costs while still providing effective performance.
Note: The dew point of your purge gas does not have to be at -40 C to have water vapor pressure differential low enough to dry your sample gas effectively, especially if your target dew point is moderate (-0 to +5c), or you just want to prevent condensation in your system.
Caution: We recommend that the purge gas pressure never exceeds the sample gas pressure. Otherwise, the tubing may collapse and restrict the sample gas flow, especially if any condensation occurs in the tubing. In some applications this may be unavoidable and in those cases it is especially critical to prevent condensation.
Choice # 2 – Using Nafion Dryers with a Vacuum Pump or Eductor to dry sample gas
Most of our OEMs use our dryers with the purge under a vacuum to drive the differential in water vapor pressure across the membrane, for two main reasons. First, no external source of dry, clean air is required. Second, when set up correctly, it is a very cost effective and reliable solution. There are four ways you can set up Perma Pure Dryers to work under vacuum.
For best dryer system performance, the optimal performance point of flow and vacuum level should be achieved as shown in the diagram below. Every combination of flow rate, dryer and vacuum pump or eductor will have its own unique performance characteristics – for any given combination, this point must be found. This is done by continually restricting the flow of the purge glass by closing the needle valve while watching the moisture level at the DP Sensor located at the sample gas outlet. The “sweet spot” or optimal performance occurs when the vacuum is at its highest point (the lowest pressure) and the flow rate is still high enough to sweep the water vapor away.
Note: During testing, it might take up to 30 minutes or more for the dryer performance to stabilize in order to make a performance determination – you may not see immediate changes and this could be confusing.
Figure 3: Optimum Performance Point for Dryers Working Under Vacuum