Compressed air and air drying

Hey, thought I would share a few tips on compressed air and humidity after reading some users comments on humidity in their systems and controlling it for dry processes like sand blasting.

Water traps are great, I use a home built water trap that pulls air from the compressor via the drain outlet and uses a mobile piston to force incoming air into the already trapped water and blows this water up along the inside of the collection tank, which rapidly cools it to ambient temperature. This has the funny effect of decreasing the relative and total humidity by decreasing temperature from the compressor.

When it comes to compressed air, the humidity in the tank is always going to be at 100% relative humidity because air's capacity to hold water is *pretty much* flat across the pressures used in home systems. This means that if I use a compressor to take air that is at only 25% relative humidity and compresses it to half it's volume, the humidity is now 50%.

In an air compressor the pressure increase is more likely in the range of14PSIa to over 120PSIg. The head temperature is high enough, due to adiabatic compression, that water does not condense in the cylinders, but excess water does come out in the tank and where the air is cooled.

The point of all this is that there are only four readily available ways for us to decrease the humidity of compressed air; isothermal expansion, decrease in temperature, deliquescents and desiccants.

When air is expanded isothermally, say a slow flow through a regulator where the temperature does not change, the relative humidity is reduced even though the total water volume remains the same.

When air is cooled to below the dew point, the relative humidity and total humidity is reduced to the dew point, this is how heat pump air driers work.

Deliquescents like magnesium or calcium chloride both chemically absorb the water to produce hydrated forms of their respective salts.

Desiccants like silica gel or Alumina rely on the generally reversible process of adsorption where the water is captured and held in pores.


Anytime air is rapidly expanded there will be a decrease in air temperature and a rapid spike in relative humidity, if the temperature hits the dew point of held humidity, you will get liquid water. This clogs air blasting nozzles, shorts plasma cutters etc.

So long story short, water traps are great but are only for liquid water. To actually get dry air you need to physically remove it from the systems air with a dryer of sorts.

I have had problems with my sandblaster clogging, and need to figure out what to use with that.
It is a really annoying problem.
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I am a big fan of silica beads for my plasma cutter and for HVLP but for sandblasting you probably want one of those big Mg pellet units that habour freight sells, or you will be changing the beads in the dryer all the time.

Somthing like this:
https://www.princessauto.com/en/in-line-desiccant-air-dryer/product/PA0008055757

The refrigerated-type dryers seem to be extraordinarily expensive.

Looks like the dessicant-type is the only reasonable solution for hobby use.

I guess a water separator is only good for capturing condensate that is flowing in the piping ?

Edit:
The PA dryer has some really bad reviews.

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The PAL one sorta gives you what you give it. If you drain it regularly it works well. It's continuously miss labled as a dessicant dryer which it is not, it is a deliquesant dryer that uses pellets of magnesium chloride and salt. So if left sitting full of water between uses, it will rust.

You can buy big jugs of blue bead silica dessicant on amazon for pretty cheap and use those in it, bonus you can reuse the silica.

Yes, the water traps only get condensate which leaves you with 100% humid air.

In your nozzle the air cools so quickly that you get water.


This effect actually sank at least one usa submarine as the blow tank valves iced up under emergency blows.

GreenTwin said:

The refrigerated-type dryers seem to be extraordinarily expensive.

Looks like the dessicant-type is the only reasonable solution for hobby use.

I guess a water separator is only good for capturing condensate that is flowing in the piping ?

Edit:
The PA dryer has some really bad reviews.

.

Click to expand...

I bought a Dayton Speed-Aire refrigerated air dryer (RAD) from Zoro. I think I used a 20% (or more?) discount coupon, and it came in around $750. It was well worth it to me to protect my CNC mill, which needs air to function. I have heard of folks DIYing a RAD based on an old mini-fridge.

I've seen people go as simple as a coil of copper in a cooler with bags of ice and then a water seperator, it must work well because you can literally buy such units. Just bulkhead fittings a cooler and coils of copper iirc for over a grand usd.

Pretty humid where i am; water just pours out of the tank. So I disconnected the compressor plumbing from the tank and instead ran it to the "Franzinator" I built on the wall. It is amazing how much water it separates out by cooling the air and reversing the direction of flow. After going through additional black iron cooling runs and finally a desiccant dryer, the air goes into the tank.

My Franzinator is a "no-weld" Franzinator. I tapped the inside of the 3/4" nipple to accept 1/2" pipe as well. Here are a few pictures that would help if one is making one. These work so well that I am surprised they are not more common.

Ken

SpringHollow,

I'm intrigued by your Frazinator. I would like to see a drawing of the complete unit with some dimensions and specs, and/or a picture of it installed where it "lives" in your shop.

--ShopShoe

That is an interesting device.
I did a quick search and found a number of variants of it.
No time to really look at it closely.

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I'd never heard of a "Frazinator" before but my seperator works on a similar principle. The top Tee connects to the tube and piston in the lower pictures.

Air enters and lifts the piston while flowing out through swirl vanes which aspirates the water and blows it up along the sides of the tank.

The brass knurled valve in the picture causes the piston to drop and suck out water discharging it into a pail. It's a sliding piston so that the gap at the bottom is always minimized.

The clear braided hose, that attaches by the safety valve, is air out to the system.

This works very well at capturing oil vapour As well as water. I use full synthetic which is not compatible with almost all air system gear... so I try to catch it early on.

I also use the port on the compressor meant for draining water as the air supply. This means the apperatus is at tank pressure, so it adds to the total available air volume.

ShopShoe said:

SpringHollow,

I'm intrigued by your Frazinator. I would like to see a drawing of the complete unit with some dimensions and specs, and/or a picture of it installed where it "lives" in your shop.

--ShopShoe

Click to expand...

Hi ShopShoe,

If you open the pictures, you will see the dimensions. It is made out of black iron pipe and fittings, along with some brass or bronze. Basically whatever I had available.

I am in the process of relocating my shop. The best I can do is below. The pipe to the very left of the Franzinator is the inlet. The other black iron pipes are "U's" in series after the franzinator with drains on the bottom. The franzinator and black iron drains are connected together and run to an automatic drain valve. On the platform above, is a desiccant dryer and an additional storage tank. The compressor's tank and the additional storage tank are hooked up after the dryer. I regenerate the desiccant dryer once a year or two even though I do not need to since it will still indicate a dew point below 0° F.

Ken

Interesting, but I get hardly any condensate , or maybe I just don't make much compressed air to worry about the condensate. I Just blow-down after use and dump the spray on the concrete floor. Not a flood.
K 2

I never get any water out of my tanks when i test the drains.

SpringHollow,

Thank You for the additional information. I understand it better now.

Steamchick,

In most cases what you describe is also my experience. I have all my air piping installed following all the usual things like sloping the lines and putting drains in low spots and regular checking and maintenance. However, in my part of the USA we sometimes get damp weather and high humidity. ( My shop has no permanent or constant heating or cooling.)

I may consider experimenting with the Frazinator concept in the future.

May everyone experience moderate weather and pure air in their lines.

--ShopShoe

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Steamchick said:

Interesting, but I get hardly any condensate , or maybe I just don't make much compressed air to worry about the condensate. I Just blow-down after use and dump the spray on the concrete floor. Not a flood.
K 2

Click to expand...

I have a "full" tank all the time. It is very warm and humid most of the time. The compressor at work has an automatic tank drain valve that keeps draining the tank for 1-2 seconds ever so often. I assume 20 l per day is our average water amount. We used to pour it out in the morning, but now I pump it into the drainage.

GreenTwin said:

The refrigerated-type dryers seem to be extraordinarily expensive.

Looks like the dessicant-type is the only reasonable solution for hobby use.

I guess a water separator is only good for capturing condensate that is flowing in the piping ?

Edit:
The PA dryer has some really bad reviews.

.

Click to expand...

I was looking into that and as far as I recall it was about 500 - 800 USD. This example is for a rel. large compressor. Maybe some peoples workshop would be full already.
800 USD Internet offer (you know where from) plus shipping ( another 800).

Found another offer for 140 l/ min, the smaller units might be either horrible or more expensive, because not that common.

This type goes for approx. 200 USD plus shipping (100 $).
Maybe you can get one from H..........ht, or G....ly, or A...z.n?

Greetings Timo

p.s. I am reasonable sure that I would not come far with the dessicant dryer, as my condensate amounts are really quite visible.

Just an automatic drain on your tank with no dryer will mean you have air at 100% humidity at that room temperature/pressure. That assumes you have used things to cool your air and remove moisture. You will likely get condensate at your tool etc because the air will cool as it expands. For some applications, that does not matter. For others, it is the kiss of death.

I store dry air vs drying post storage because that increases the tank life and limits the amount of rusting on black iron. The negative is that I dry air that might be used for a process that does not need dry air. However, the large regenerative desiccant dryer I have far exceeds the capacity of a normal 10 HP or less air compressor and costs almost nothing to run. The negative is that it takes up the floor space of a vertical air compressor by the time you include the regenerative parts, piping, etc. Most companies would not look at a used system and most hobbyists can not afford the space. I mount mine on a stand above the compressor. If you can, it pays to look for deals on Ebay, Craigslist, etc because they can go pretty cheap. Mine was significantly less than a cheap refrigerated dryer and like already mentioned, costs almost nothing to run.

I've been responsible for procuring dry air systems. -40C. Dewpoint, etc. For power station enclosed electrical equipment, industrial laboratory use, etc.
Just never realised "hobby" workshops used such quantities of dry compressed air.
Fascinating!
K2