In the internal type, heaters are embedded in the molecular sieve bed. This reduces the amount of purge air required for regeneration to less than 10%. The purge air plus radiant heat is used to regenerate the off line molecular sieve bed. It is also important to consider a cooling cycle to decrease the temperature of the bed prior to bringing it back on line to prevent elevated air temperatures going downstream where the excess heat could damage components. It should be known that in all heater regenerative dryers, when the off line tower is brought back on line there will normally be hotter than average air moving into the process.
Even when the air temperature would not cause a problem, the elevated temperatures cause a dew point spike because of the higher temperatures. These can be eliminated with engineering foresight to allow adequate cool down time or by utilizing a period of time that allows dry air to continue to flow through the bed, known as a cooling sweep. Another factor with internal heated regenerative dryers with molecular sieve is where earlier stated that activated alumina was typically used in regenerative dryers with molecular sieve, this is not the case with an internally heated unit. Due to the heaters being in such close proximity (although normally contained within wells) to the molecular sieve the temperatures right on the molecular sieve reaches extremes that do no make activated alumina ideal for this application.
Normally a silica gel molecular sieve is used in these units which can better withstand the increased temperatures. However, silica gel is very sensitive to liquid water and will rupture if subjected to this condition where the pre-filter might not remove all water in a liquid form. To mitigate this potential a layer of activated alumina would be used at the inlet side of the molecular sieve bed. While performing well, the layered bed arrangement is more difficult when molecular sieve changes are required and the silica gel molecular sieve is more expensive than it's activated alumina counterpart.
"Note：This is normal specification, in case of a particular application, specification or requirement, please contact us by email at firstname.lastname@example.org for technical information."
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