Thread: Inline air filters

Originally Posted by gallegos

The moisture holding capacity of air reduces with increased pressure as well as reduced temperature so even with moderate humidity conditions outside once its compressed and cooled back to ambient water can condense out. To get water to condense out you need to cool the air to its dew point (the point at which it has 100% humidity) and as the temp drops and water condenses the humidity of the air remains at 100%. So my point is if you are using an after cooler or just relying on the air naturally cooling in the receiver the air will be at 100% humidity at the end, and if the air cools further in the lines down stream it will condense more water out.

A refrigerated dryer system works by cooling the air below ambient temp, draining off the condensed water then allowing the air to warm back up. Dessicant obviously absorbs the water vapour directly from the air. Both mean you end up with air with a dew point below ambient (less than 100% humidity) so less risk of water appearing due to temperature changes.

Just out of interest we aim for a dew point of -25 celcius in our instrument air system at work... so the air can be cooled to -25 before water will start to condense.

Hope this makes sense... I don't think i'm great at explaining this sort of thing.

No I'm afraid I don't understand. I can't see it matters what system is used to remove the moisture from the compressed air, the end result should be the same. Some systems are undoubtably more efficient at doing so than others, ignoring desiccants for the moment as they use a completely different process. In the case of what I use, the walls of the copper tubing are "cold" in relation to the saturated compressed air, ie below the dew point, of that air, hence the water condenses on the walls of the tube. A refrigerated dryer does a very similar thing, except further downstream where the air is cooler, therefore it has to cool the tubes/air considerably colder still, hence why it requires refrigeration. They are undoubtably more efficient at extracting the moisture if they are cooling the whole air mass instead of relying on condensing just on the cold walls, but unfortunately are beyond most private people to buy and run. I'm not suggesting for a moment that a coil of copper tube (as you correctly point out, an after-cooler) works as well as a refrigerated air drier, but it does remove quite a remarkable amount of water with each compressor cycle. I can't see how you're determining that the air in my system is therefore still at 100% humidity regardless of how much water I've pulled out of it? In other words you're suggesting the system does nothing?

Pete

It's common sense isn't it? The aftercooler takes a give volume of water out of a given volume of air. The delivery pressure, temperature and volume for all intents and purposes is the same (at the end of the hose), so there obviously will be less water delivered if an aftercooler is in place compared to doing nothing. From what I can work out from gallegos is the point is whether enough water has been taken out to ensure that no further condensation takes place before use in the tool. It would seem to me that as long as the bottom end of the aftercooler and the receiver are 'cool' (i.e. at ambient temperature), then the risk of the air cooling further (and allowing condensation) prior to release from the tool is virtually zero. True?

Originally Posted by WelderMick

the risk of the air cooling further (and allowing condensation) prior to release from the tool is virtually zero. True?

Temperature is not the only consideration.

When air goes from a high pressure area (air line) to a low pressure area (air tool exhaust) condensation also occurs if vapour is present. You can see this demonstrated by air contrails being formed by pressure differences with an aircraft wing.

If air through a basic condensor was still at 100% humidity (as stated) then condensation would occur/be obvious at the air tool on exit - which is not happening.

So I can not see how the statement by Gellegos is correct.


Rob

I didn't realise uploading a photo of a piece of scrap copper would create so much discussion

I need to point out that no wheels are in danger of being reinvented around this workshop. I'm always quite happy to swipe any established good idea, and quite openly did on this occasion. Ok the execution is sometimes "unique", but nothing new to see here that isn't already established practice.

As has been pointed out, what I made is technically called an aftercooler. I didn't think it was necessary to go into any technical details, thinking nobody would be interested, but it seems maybe they are Having re-read a number of times the comments by Gallegos I think I now understand what is being said, but please correct me if I'm mistaken. It should be pointed out that all most of us here are interested in is not having any free water in our lines, particularly at the end of them! Super dry "instrument quality" air is typically not the goal. I believe what was being said was that if the lines drop below the outlet temperature (ie close to ambient temperature) of the aftercooler, then condensation may occur in the lines. That is true in theory and I'd agree. However in practice we get handed a rare free get out of jail card. As the air moves further from the receiver it's pressure is dropping. While it's true that should also cause the temperature to drop, in practice we're not starting jet engines here! It's a relatively small flow of air, hence the lines themselves will maintain the air at roughly ambient temperature. Nevertheless the pressure dew point will continue to fall, especially after passing a regulator, such that condensation is extremely unlikely to occur down at the business end of the equation. The exception may be, as I commented earlier, on an extremely cold morning with previously compressed air.

It was some time back I cobbled this together so hopefully I'm not too vague on the details and have any technical information roughly correct. I believe commercial versions of these systems are said to remove roughly 75% of the water and I expect my little add-on would be achieving roughly the same, that will do me.

Pete