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[Art Montemayor]

karthik:

You don’t mention the name of the manufacturer nor the type of adsorbent used in your adsorption dryers, but rest assured; your dryer is probably working above expectations with regards to dew point. This isn’t because you got more for your money or more than you bargained for. It’s simply because the adsorbent is new and being used for the first time. Allow me to explain certain aspects about adsorption dryers:

1. All adsorbers have to be designed with an inherent amount of over-capacity. This is analogous to the “fouling factor” used in heat exchangers. As the adsorbent is used and regenerated, it undergoes, aging, contamination, attrition, by-passing, etc. etc.. In short, it starts to lose a portion of its sorptive capacity. Eventually, it will have to be replaced - usually after 2-3 years, depending on the operation. This is normal and to be expected. Consequently, at the onset of its operation, you will detect a “super” efficiency. It isn’t super efficient; it simply has inherent over-capacity. No one can practically design an adsorption bed to deliver a given purity consistently. It can’t practically be done. What one does is to design an inherent over-capacity and deliver a higher purity than expected.

2. You are describing your dewpoint test and tester totally wrong. Either you don’t have correct instructions or you are not interpreting the dewpoint test correctly. The dewpoint cup is filled with a solvent – usually acetone – and small chips of dry ice (@ -109 oF) are dropped into the bath while the person looks at the outside of the cup (which is enclosed in glass and has a highly polished, chromed surface; this glass chamber is connected to the test gas source and is also vented to atmosphere) and waits until the first sign of a water dew-drop is formed on the polished surface. The temperature of the acetone is noted at this moment and is taken as the “dewpoint” – the temperature at atmospheric pressure that determines 100% humidity. The lower the dewpoint, the lower the content of water moisture in the gas sample. -40 oF is very, very easy to achieve and is no challenge for an adsorption dryer. I usually design (and obtain) for a product air dewpoint of -90 to -100 oF . For air, this means approximately 1 or 5 ppm(vol) of water. This is the quality of instrument air I am used to designing for and using. These type of dewpoints are readily achieved with activated alumina, which is my preference for air dryers. I do not recommend silica gel as an adsorbent.

3. You cannot possibly condense dry ice. Dry ice is a solid. Your terminology or your understanding is mixed up.

4. The weather has absolutely nothing to do with the dewpoint you are obtaining. It plays no factor in your drying operation. Again, your understanding or your terminology is not correct.

5. I have never heard of an adsorption drying period of only 3 minutes. Conventional, industrial-type air adsorption dryers are usually designed to regenerate once every worker’s shift (approximately once every 8 hours). I have designed and operated this type of dryer for periods of up to a day between regenerations. I consider 3 minutes as impractical, but everyone has a choice and preference.

I cannot offer more comments because you haven’t furnished more basic data, so I’ll close for now. If you need more specifics, please furnish more data – such as process type (TSA – temperature swing; or PSA-pressure swing), fabricator, size of beds, type of adsorbent, type of regeneration, temperature of regeneration, etc. Then maybe I can furnish more information.

You are doing the correct thing in routinely checking your exit dewpoint. I normally expect this to be done 3 times a day and logged to keep a trending record.

Art Montemayor