Dry air before compressor pump?

[TaperPin]

Once upon a time an industrial engineer made a comment about one of the best times to dry air is before it enters the air compressor, but that idea has never seemed to make it down to smaller compressors.
Has anyone seen a small refrigerated pre-drier, or built one? I have heard of one guy who runs the air inlet of a small home compressor through a deep freezer and said it works great, but that’s not a real elegant solution. Lol

I have experimented with a 15 amp window air conditioner running constantly, with smallish round duct added, but the evaporator ices up until no air is flowing. A small dehumidifier has the same problem with low airflow unless it’s allowed to cycle on and off. A heating/AC guy said the same will happen even with a 5 ton residential evaporator - low air flow ices them up.

I own enough refrigeration tools to cobble something together, but I need a decent starting point.

I have an insulated tank that an evaporator coil could cool to near freezing and a second coil for the air supply to cool off and condense the moisture, but thats not very elegant either.

I like the design of the commercially available driers with a larger copper coil carrying air and smaller refrigerant line wrapped alongside the larger coils - hell, maybe I should just buy one and replace the air tubing with larger diameter. Having a drier larger than my compressor isn’t the best use of space.

I probably need to research evaporator controls that accurately keep the temps near freezing, but not actually frozen, IDK

 

[Cole2534]

You'll need a MUCH larger dryer if you do that. The higher pressure helps the water condense and drop out in liquid form in the tank, a benefit you don't get at atmospheric pressure.

You have to also look at the heat transfer numbers. 1scf at 0psig is ~10x the volume of 1scf at 165psig. This will affect the sizing of your hx because you now need a lot more surface area, and your vessels get a lot larger.

I'm no compression or refrigeration wizard, but I have never seen drying processes done at anything but the highest available pressure. Be it desiccant, mole sieve, refrigeration, gylcol adsorption, amine adsorption...all done at the highest pressure available in the process.

I gotta think if this was such a good idea I'd have seen it in my industry, we dry billions of feet of gas every day.

 

[EPAIII]

I have designed/re-designed and installed/modified a number of compressed air systems where even a single drop of water meant disaster (in air bearings). Here is the general form those systems took.

Yes, drawing dry air into the compressor is a positive step. But I never purchased any refrigeration equipment for that purpose. Where possible I simply located the air intake (via piping) in a climate controlled (air conditioned) area. This often required adding some form of muffler and, if the distance was more than a few feet, I went up at least one or two pipe sizes within a foot of the compressor's air intake. This minimized the moisture in the air before the compressor but cost little or nothing except the cost of the pipe and muffler.

After the compressor and before the tank I liked using a cooling coil. About 50 feet of copper pipe is about right for the systems I had. The compressed air enters that copper pipe at the BOTTOM where a collection jar is located. And it travels UPWARD for every inch of the pipe so there are no low spots to hold puddles of water. All the water drains BACKWARDS and DOWN toward that collection jar while all the air goes forward and UP. This separates them nicely. It is best to locate this cooling pipe in a cool space (again, air conditioned).

Then, only after most of the water has been eliminated, the air was passed through a refrigeration style dryer. It had collection jars before and after. The before jar collected almost nothing. The refrigeration dryer would collect only a bit of water each day. The "after" jar stayed as clean as the day it was purchased over periods of YEARS.

From the refrigeration dryer the air flowed UP HILL to the area where it was used. Only at the points of use did the pipes come down to additional collection jars. Those additional jars also stayed clean enough to eat out of.

That is how I designed air systems that did not pass even a single drop of water for year in and year out. NEVER!

Other than an air conditioned room, I do not know of any air drier that could function ahead of the compressor. Perhaps they exist, but I suspect they would be expensive.

 

[CJ5Dave]

Not to hijack, but I got a surplus Ingersoll Rand T30 which has a heat exchanger coil in the line between second stage and tank. It was used to operate dampers in a HVAC system on a large building. This coil receives air at the top, hooks to tank at bottom, has no drain on coil. Looking at brochures this is not standard equipment. What does it do? Dry the air?

 

[Peter from Holland]

Not to hijack, but I got a surplus Ingersoll Rand T30 which has a heat exchanger coil in the line between second stage and tank. It was used to operate dampers in a HVAC system on a large building. This coil receives air at the top, hooks to tank at bottom, has no drain on coil. Looking at brochures this is not standard equipment. What does it do? Dry the air?

The cooler air after the heat exchange has less volume Remember every 27dgr Celcius cooler means 10% less volume
On a shop instalation it is best practice to get the air from the coolest place Even switch perhaps during seasons Less volume and also cool air can hold less vapor

Peter

 

[DDoug]

Not to hijack, but I got a surplus Ingersoll Rand T30 which has a heat exchanger coil in the line between second stage and tank. It was used to operate dampers in a HVAC system on a large building. This coil receives air at the top, hooks to tank at bottom, has no drain on coil. Looking at brochures this is not standard equipment. What does it do? Dry the air?

That is very much standard equipment on pretty much every 2 stage compressor I have seen & worked on.

 

[CJ5Dave]

OK thanks.

 

[rons]

Stock with 1/2 tube from 2nd stage outlet to tank.


Replaced tube with 50' of 7/8" copper.


After the change the tank has been on the wagon ever since...

 

[CarbideBob]

Air compressors are noisy and not liked.
Hence they will often get their own "air compressor room" or big box style enclosure.
Air intake in these tends to be very high in temp meaning the air will carry a lot of water which you then have to remove.
Without doing extra cooling where is your air intake? Would it be better piped to the outside building air? (thinking where in the USA makes for different)
Do you cool and dry the air in the compressor room? Really.. AC my air compressors in their sitting alone and soundproofed room?
Does this make sense and save money long term over all operating costs involved?

Way clean and super dry air at 130-140 psi and shop volume for the air bearings and yes nice for my toolchanger.
Is this too much an ask?
I have tried so many things or arrangements.

 

[Cole2534]

Stock with 1/2 tube from 2nd stage outlet to tank.
View attachment 397886

Replaced tube with 50' of 7/8" copper.
View attachment 397887 View attachment 397888 View attachment 397889 View attachment 397890

After the change the tank has been on the wagon ever since...

Well done, sir!

 

[TaperPin]

I’m sure it’s the most cost efficient to remove air after the compressor, but I still find it interesting that 100% of the moisture goes into the inlet before compression and nobody spends any effort on removing even some of it. Even if it were only 25% as efficient as removing moisture from the compressed side, you would think someone would have at least heard of this number.

Having said that, in areas with high humidity, pulling air from within an air conditioned room is doing exactly that - yet nobody knows how much of a difference compressing air conditioned air makes. Heck, pulling air from within a freezer may not be as silly as it sounded at first, but nobody knows.

If the inlet air is 50-70 degrees cooler, that reduces compressor heat, reduces the amount of cooling required to condense moisture in the compressed air, and slightly increases volume of air entering the compressor.

 

[metalmagpie]

The OP is correct in that an ideal installation will draw air from the outside. If you were located in a cold place, the air would generally be dry and also more dense and the process of compressing it will become more efficient. What you do in Houston is completely beyond me.

metalmagpie

 

[sfriedberg]

What you do in Houston is completely beyond me.

Or Washington DC in August. Man, is that muggy!

 

[friesen]

I have a 5hp rotary compressor with built in dryer. (Atlas copco GX4FF) I run dehumidifiers in my shop set around 40-45. The dryer ends up really not putting that much out the drain pipe, so I have come to the conclusion that intake air humidity really does make some significant difference.

 

[Cole2534]

I have a 5hp rotary compressor with built in dryer. (Atlas copco GX4FF) I run dehumidifiers in my shop set around 40-45. The dryer ends up really not putting that much out the drain pipe, so I have come to the conclusion that intake air humidity really does make some significant difference.

Of course it makes a difference, but you're not drying air going directly into the compressor, you're drying the whole room. You don't size for the instantaneous demand of the compressor in that manner and that's a big difference.

I’m sure it’s the most cost efficient to remove air after the compressor, but I still find it interesting that 100% of the moisture goes into the inlet before compression and nobody spends any effort on removing even some of it. Even if it were only 25% as efficient as removing moisture from the compressed side, you would think someone would have at least heard of this number.

Having said that, in areas with high humidity, pulling air from within an air conditioned room is doing exactly that - yet nobody knows how much of a difference compressing air conditioned air makes. Heck, pulling air from within a freezer may not be as silly as it sounded at first, but nobody knows.

If the inlet air is 50-70 degrees cooler, that reduces compressor heat, reduces the amount of cooling required to condense moisture in the compressed air, and slightly increases volume of air entering the compressor.

If you're looking for some easy to use RoT, good luck with that. If you want to get kinda close I bet a stopwatch and amp clamp could put you in the ballpark. If you want to be able to predict behavior over a variety of conditions get with an engineer that specializes in refrigeration or gas compression. They will absolutely tell you how temp and humidity will affect your compression process.

As for drawing from a freezer....you need to put some numbers to this to realize how ridiculous it sounds. A quick look at Frigidaire's website suggests that the largest they make is 25 cu ft. My little Eastwood scroll compressor does about 13cfm @ 90psig or about 91scfm. You're going to turn over the air in that unit about every 17s; the freezer will never keep up with the cooling duty required.


Here's an interesting snippet on this, from the Engineer's Toolbox- https://www.engineeringtoolbox.com/water-content-compressed-air-d_1275.html

 

[johansen]

I have thought to build a passive desiccant air dryer system

3 tubes of desiccant and a handful of valves that rotate where the tube is in the system.

So one tube is after the compressor but before the coils and the tank. The heat pushes the moisture out, which gets condensed in the coil and drains into the separator before it reaches the tank.

Another tube is located before the compressor.

A third tube is located after the tank, regulator, etc

Now you just need 18 valves, 3 for each end of the 3 tubes.

When the desiccant in the outlet tube gets wet, it gets transfered to the output of the compressor. The tube which was there, gets transfered to the air inlet of the compressor...the air flowing through it cools it down, and it begins to absorb moisture. The tube which was previously there, gets moved to the final output.

My prior thoughts before reading this thread was to use 3 tubes but let the third tube cool off to ambient before inserting it back into the que, not connecting it to the inlet of the compressor at all.

 

[Cole2534]

I have thought to build a passive desiccant air dryer system

3 tubes of desiccant and a handful of valves that rotate where the tube is in the system.

So one tube is after the compressor but before the coils and the tank. The heat pushes the moisture out, which gets condensed in the coil and drains into the separator before it reaches the tank.

Another tube is located before the compressor.

A third tube is located after the tank, regulator, etc

Now you just need 18 valves, 3 for each end of the 3 tubes.

When the desiccant in the outlet tube gets wet, it gets transfered to the output of the compressor. The tube which was there, gets transfered to the air inlet of the compressor...the air flowing through it cools it down, and it begins to absorb moisture. The tube which was previously there, gets moved to the final output.

My prior thoughts before reading this thread was to use 3 tubes but let the third tube cool off to ambient before inserting it back into the que, not connecting it to the inlet of the compressor at all.

Heat doesnt push moisture out, quite the opposite. Air at 100deg F holds 10x more water than air at 30deg F, for reference.

The optimal point to remove water from compressed air is when the air is simultaneously at the highest pressure, lowest temp and lowest velocity. The first 2 criteria contribute physically to air's ability to retain moisture and the 3rd lends to the moisture's ability to coalesce.

 

[Bill D]

Not to hijack, but I got a surplus Ingersoll Rand T30 which has a heat exchanger coil in the line between second stage and tank. It was used to operate dampers in a HVAC system on a large building. This coil receives air at the top, hooks to tank at bottom, has no drain on coil. Looking at brochures this is not standard equipment. What does it do? Dry the air?

I believe on my Quincy that coil is between first and second stage. Simply a cooler. Or, I guess, an intercooler.
Bill D

 

[Bill D]

If it freezes in your climate shift the intake to outside air during winter. Free air pre cooling. In my climate winter air is much wetter.
Bill D

 

[CJ5Dave]

Mine has both an inter cooler between stages and this second coil after the second stage and before the tank. No drain on this coil.