Filling and Humidity

[tmassey]

You are still missing the point that I am trying to make and what the article makes....

Forget about relative humidity as it is a stupid measure....

The PMV (if the article is to be believed) will only remove 99.3% of the water (and the maths makes sense as 139/140 is around 99.3%). So there will always be 0.7% of the water that was in the original air left after the PMV. This means that the more water there was in the air to start with, the more water that will be left AFTER the PMV but before the filter stack.

The molecular sieve has to deal with this 0.7%. It can only do this by absorbing it. If there's more water going in there's more water for it to absorb - so it will take less time before it has absorbed all it can.

So at the very least, the humidity of the air at intake will have an affect on how often you need to change the MS in the filter.

The percentage is not an inherent feature. It's derived. In other words, it removes 99.30% of 50% RH moisture, and 99.35% of 75% moisture. This was not put there so you'd use it as a magic absolute perfect linear factor of water removal. That's not how it works. At all.

And RH is an *incredibly* dumb measurement. But it's the key to understanding this. It's the fact that we can *only* start removing moisture using our drains *after* the RH exceeds 100%.

This is the point you seem to be missing. No matter what we start with, our drains will not separate moisture until after the gas exceeds 100%. And once it starts doing that, it will *always* leave our gas *at* 100%. So no matter what our intake RH is (in the realm of real life on Earth), we will end up reaching 100% RH before we get to our filter stacks. At a specific temperature and pressure, the amount of water by mass entering our filter stack will be *identical* no matter what the intake RH was. The different amount of water that entered (based on incoming RH) will simply be the different amount of water leaving our *drain*.

And I'm done with this. Maybe someone else will have better success.

 

[ATJ]

The percentage is not an inherent feature. It's derived. In other words, it removes 99.30% of 50% RH moisture, and 99.35% of 75% moisture. This was not put there so you'd use it as a magic absolute perfect linear factor of water removal. That's not how it works. At all.

And RH is an *incredibly* dumb measurement. But it's the key to understanding this. It's the fact that we can *only* start removing moisture using our drains *after* the RH exceeds 100%.

This is the point you seem to be missing. No matter what we start with, our drains will not separate moisture until after the gas exceeds 100%. And once it starts doing that, it will *always* leave our gas *at* 100%. So no matter what our intake RH is (in the realm of real life on Earth), we will end up reaching 100% RH before we get to our filter stacks. At a specific temperature and pressure, the amount of water by mass entering our filter stack will be *identical* no matter what the intake RH was. The different amount of water that entered (based on incoming RH) will simply be the different amount of water leaving our *drain*.

And I'm done with this. Maybe someone else will have better success.

So you are saying the article is wrong.

The article states, by example, that the amount of water in the air reaching the filter stack is relative (0.7%) to the water in air at the intake of the compressor. You are saying this is not true.

 

[rjack321]

So you are saying the article is wrong.

The article states, by example, that the amount of water in the air reaching the filter stack is relative (0.7%) to the water in air at the intake of the compressor. You are saying this is not true.

The gas entering the filter stack will be 100% humidity at whatever temperature that gas is (say ~75C). The rest of the water will have dropped out in your separators. If you started with 50% RH at 10C that will be a fairly small amount. If you started at 90% RH at 30C it will be quite a bit of water. No matter how much you start with, the gas going into the MS is at 100% RH for whatever its temperature. The biggest and really only factor controlling how long your MS lasts is the temperature of the gas leaving the 3rd stage (or 4th in some compressors) going into the MS. The temperature is what dictates exactly how much water its 100% RH will actually be. If its hot because the ambient temperature is hot and the 3rd stage is hot then it will carry a lot of water mass and saturate your MS faster. If that gas is cold because the ambient temp is only 8C and the 3rd stage isnt hot at all then the gas leaving your 3rd stage is still 100% RH but its only 45C and thus carries a smaller mass of water to saturate your MS.

So yes, that article is giving you the wrong image because its using percent units.

 

[kelemvor]

Are you quite sure that is exactly what he said. If it is then he is mistaken.

However the reason I ask and offer a possible alternative suggestion is that 75% RH water vapour is a "magic number" it is also the calibration set standard that all relative humidity sensors are calibrated to. 75% at 70F
Why 75% I guess is the big question and precisely why 75%

It turns out that 75% is the relative humidity generated from table salt dissolved in warm water.
And for those who like to learn something new each day. I guess today is no exception.

May I also suggest you buy a compressor from somone who knows what they are doing.

If you ever need to calibrate your RH sensor (we did this some years back on SB or DS with the Rix SA-6 oil free exped compressor).

Load a small 6 inch bowl with ordinary table salt to the brim, add small amounts of warm distilled water a table spoon at a time and until the small grains appear to form a heavy white paste. Place the sensor 100mm above the salt level and place both bowl and sensor in a bigger plastic bag and seal it all up overnight in a room box or cupboard at 70F. Next day the magic number of 75% RH on your sensor will appear. If not your sensor is out of calibration and can be adjusted. Cheap effective accurate and magic.

While you wait it also helps to use some of the spilt dry salt rather than waste it with a freshy squezed lime ice and a Reposado or Blanco of your choice. Iain

I thought the RIX compressors were discontinued? In fact, I thought it was you who told us all that.

 

[Miyaru]

The article is not about relative humidity in percentages. It's about physics, removing gaseous H2O from compressed air.

Temperature plays of course a big role. Compare running a compressor in wintertime, with subzero temperatures, against running a compressor in the tropics. The filter will last longer in the first case, the same laws of physics apply in both environments.

 

[ATJ]

This is the part of the article I'm talking about and this is at odds with what people are saying in this thread.

This is clearly saying that the amount of water remaining in the air after the PMV has done it's job and before the filter stack is proportional to the amount of water that was present in the air at the intake. This is not a misunderstanding. It's right there in the text.

The author uses that information to calculate how much water is present assuming 100% humidity and 45ºC. i.e resulting in 0.476g/m3.

The author then goes on to calculate how long the molecular sieve will last based on that amount of water calculated to be present in the air.

It's all there in the article.

Everyone else is saying that the amount of water remaining in the air after the PMV has done its job and before the filter stack will be the same regardless of the amount of water in the air at the intake, and will only be dependent on the temperature of the filter stack. Don't worry, I get that. I understand exactly what people are saying.

I'm not arguing with what people are saying. I'm not trying to be a dick about it. I'm pointing out that what the article says (water content before filter stack is proportional to intake water content) is not the same as what people are saying in the thread (water content before the filter stack is only dependent on temperature). Both of these can't be true.

 

[icechip]

Funny I found this thread today. I just got a compressor and was filling tanks for the 1st time Sat AM with probably more than 75%. I purged the moisture trap more often and for a longer time, seemed like the logical thing to do with high humidity. I was wondering if it would reduce the life of the filter and now this thread.

Good thing here in northern New England we don't get many +75% humidity days!

 

[ATJ]

Here's another issue I have with the article. The calculations for the life of the molecular sieve assume that the pressure in the condensation stack/separator and the filter stack will be maintained at 140bar throughout a tank filling.

If I understand the role of the PMV correctly, it is to set the MINIMUM pressure in the two stacks. i.e. when you start the compressor, no air will flow to the whip until the pressure is at the pressure the PMV is set for.

Using the example in the article, filling an AL80 tank, the pressure in the stacks should be 140bar until the tank is at 140bar and after that it will be at whatever pressure the tank is up to 207bar. So, using the logic in the article, once the pressure is higher than 140bar, the separator will remove MORE than the 99.3% indicated. e.g. at 200 bar it should be removing 99.5%, i.e. leaving only 0.3325g/m3.

 

[ATJ]

OK, I found a better explanation of what happens under pressure and yes, there is a flaw in the original article.

This article only uses 8 bar but the principle applies.
Water in Compressed Air Calculations – Air Compressor Guide

And, yes, one you compress the air 140bar if there was any moisture in the air it will be at 100% relative humidity.

The PMV (@140bar) DOES NOT remove 99.3% of the water in general as suggested in the article. It will only remove 99.3% of water at 100% relative humidity. The remaining water will be always be 0.7% of 100% relative humidity no matter what the original relative humidity was (unless the air was perfectly dry).

Sorry for wasting everyone's time.

 

[RayfromTX]

You did not waste our time. In fact you gave Rack321 an opportunity to give an excellent explanation of why the relative humidity of the ambient air is unimportant to the life of the filter and why the temperature of the air leaving the compressor is what matters entirely. Unfortunately it appears that not everyone understands that concept or perhaps it is just my perception that they don't. I am happy that you understand it. Thanks for your thread. Understanding physics and physical sciences is perhaps the most important thing we can do to help us grasp what we see around us. Your thread moved that forward and will for many years as others find it.