Dessicant type air dryers

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Vegman
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Dessicant type air dryers

Postby Vegman » Sat Jun 27, 2015 1:25 am

Has any one sailed with control air dryers of the dessicant type.
There are two "towers" or vessels filled with dessicant, while one is on line , the other is regenerating.
The on line vessel has the control air passing through it usually from the receiver to the users. The air has to flow through a vertical vessels filled with small beads of some hydroscopic susbtsance which reduces the moisture content of the air . The other vessel or "tower" is on standby and a portion of the dry air ( about 10% I think) is passed through it in order to regenerate the beads. A timer and solenoids alternates between the on line and standby towers .
What I cant figure out is how the standby tower can possibly have all the moisture removed from it when it only has a fraction of the dry air passing though it, because it presumably soaked up the moisture from the full stream of moist air. That is to say how can 10 % soak up the moisture from 90%?
It doesn't seem to add up.
Any ideas anyone?

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JK
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Re: Dessicant type air dryers

Postby JK » Sat Jun 27, 2015 2:11 am

Interesting thought. I suppose the timer is set so the desiccant doesn't get completely soaked through.
On one ship I was on, we used to take the desiccant out of the one air drier and dry it in the galley oven. I wonder if they'd let you do that anymore.

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Big Pete
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Re: Dessicant type air dryers

Postby Big Pete » Tue Jun 30, 2015 6:24 am

The tower that is absorbing the moisture is at a higher pressure (7.0 bar, 100 psi for control air) while the one that is regenerating is at Atmospheric Pressure so the moisture just evaporates off into the air flow, as the pressure is reduced.
When air is compressed it has less ability to absorb moisture, which is why there is condensation in compressor drains and air receivers, reduce the pressure of the air and it can absorb more water. You could use some of the wet air (pre drier) to drive an eductor to create a vacuum which would enable you to use even less dry air, to flush out the moisture.

Simples!

BP
It is always better to ask a stupid question than to do a stupid thing.

Vegman
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Re: Dessicant type air dryers

Postby Vegman » Mon Jul 06, 2015 2:32 am

Ahhh- that makes sense!

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JK
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Re: Dessicant type air dryers

Postby JK » Mon Jul 06, 2015 12:34 pm

BP tp the rescue with his good sense, once again!

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Merlyn
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Re: Dessicant type air dryers

Postby Merlyn » Tue Jul 21, 2015 1:59 am

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VARIANTS OF MOISTURE EXTRACTION. It is interesting to see that the Dessicant used back in the 1960's is virtually the same as used today. In the midsixies I bought a secondhand diving compressor that had a screw on brass container that was filled with silica gel and charcoal and through which the breathable air was pumped and a manual drain valve was fitted.
image.jpg
This, as JK stated was , after so many pump ups was placed alongside the Sunday lunch in the oven to remove all traces of water present. I have seen this done many times more in the past than now because a lot of installations now have screw on filters rather than loose silica gel and perhaps a little charcoal added. Here in the photos of the compressor I now have, you will observe two manual drain valves, ( operated every 15 minutes running time ) are fitted. The properties of this silica gel are something else. It seems it attracts water from the atmosphere to the extent you would never believe unless you have ever played around with it. I depict a photo of a kit bought to reduce the moisture present in order to store electrical test equipment. The top tray contains, as shown, silica gel. The lower container contains the water drawn without fan assistance or any help at all the water content present in the atmosphere at all times. This was placed in a cupboard and left for several days. The silica gel seems to contain a unique property, the water appears to attach itself to the gel absorbing a minimal amount only. Gravity does the rest. This silica gel I left out overnight outside in about 80 degrees and the next morning the container had half an inch of water in it from being empty the night before. I have found in handling the crystals it is a very different substance to anything I have ever handled, from Macerator pumps upwards. It's a horrible sensation on your hands, you wash and wash but it's still there. Slimy weird stuff indeed. Amazing the properties of it, compressed air blown over the crystals seems to take nearly all the moisture away. I have worked on several different types of air compression systems as well as the standard two tower system over the years. Some air control systems were several HP receivers running at approx 130 psi and were linked lol together via check valves. As well as the Silica Gel installation cartridge each receiver had on the bottom of it a dump valve / condensate valve. Every time the max blow off pressure was reached the valve would lift and exhaust to atmosphere thereby dumping all the condensate , for that reason some people used to refer to it as the dump valve. ( It was a diaphragm valve ) Some systems had a sight glass filled with Glycerine to carry over into the system to prevent operating mechanisms and valves freezing up thereby bringing the whole installation to a halt which I have seen many times because the sight glass was allowed to get too low. The blowoff pressure was adjustable Upto about 150 psi. I enclose photos of a compressor I bought new in 1981 which has disposable screw on air filters fitted, these are supposed to be renewed every approx 240 bottle pump ups but you can cheat and weigh them to disclose any water content present. Hot sun or the oven treatment will prolong the filter life, and guess what's in them? Silica Gel. ( Plus a load of O rings ) I enclose a shot of the compressor being set up for a bottle being pumped up. The bottle needs to go in water really as the compressed air being pumped in generates heat, especially through the restriction of the bottle neck. This compressor splits in the middle with no tools for carrying purposes and although it may be small is not to be underestimated. No 450 psi here, it will pump Upto 3200 psi and if the bottle is faulty/ pitted/ rusted inside and it explodes it is more than capable of razing a house to the ground, I have seen pictures in old diving magazines of workshops/ houses flattened and people killed. Like the HP receivers on ships regular inspections are necessary. When I was about 10 years old I had a weekend job assisting a local man who established a diving centre in our area. I used to help him clear underwater sea suctions ( not running ones )on ships that had been laid up for 3 years swinging around destroyer buoys in our local harbour. Grating / Grill types ( not like all those holes drilled in a plate like you folks in Canada have, ) I think an air drill would be in order to clear all those drilled holes underwater and for many hours to boot it would appear. These diving bottles would, every two years be subject to a detailed internal inspection and a hydrostatic test. Pumping up the bottle with water to 3200 psi and the expansion measured and compared to a when empty situation. Caliphers were used for this pump up measurement test. Furthermore in writing I would perhaps consider that there could be some form of extraction similarity to my earlier article but can only give you my best assurances that any similarities are completely unintentional , but I leave it to you, the reader as to the how you would perhaps associate same. So you can clearly see from all the foregoing that this is the correct way to extract the moisture BP. I enclose an engine plaque quoting cubic feet per minute outputs etc for those of you who like a challenge, how long to pump up your smallest HP receiver to 450 pounds per square inch on your ship? ( No 100,000 hp quotes here please. ) so we can see various, different methods of extracting the moisture, but it would appear Ally based Silica Gel has not, to date been superceded . Variants indeed of Moisture Extraction.
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.

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Merlyn
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Re: Dessicant type air dryers

Postby Merlyn » Tue Jul 21, 2015 2:12 am

Sorry about the pictures, must have fell off my posting
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.

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Merlyn
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Re: Dessicant type air dryers

Postby Merlyn » Tue Jul 21, 2015 2:17 am

image.jpg
Herewith a bag of Silica Gel which pulled out half an inch of water overnight, the air we breathe!
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.


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