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Compressor recomendation

#41
I just completed installing my air compressor system so I could spray paint. I installed an Ingersoll Rand 80 gallon 175psi, 5hp dual stage continuous duty vertical compressor. $950 at Tractor Supply. The only downside to this compressor is the electric motor. It turns 3,450rpms which will produce heat. The compressor pump has 1,375 rpm. This also produces heat. A compressor pump that is in the 600 to 800rpm range produces less heat, which mean less water in the compressor tank and your air lines.

To combat any water issues, I fabricated a copper aftercooler, installed a 3/4" watertrap at the compressor and a 3 system 3/4" air filtration system at the end of the line. So far, no moisture to speak of past the first aftercooler riser.
 

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#42
The amount of heat generated by the motor or by compression, has absolutely nothing to do with the amount of moisture that is in the tank. This a common misconception. The amount of a moisture in the tank is only a function of what the ambient moisture is, how much air is compressed, and how high a pressure the air is compressed to. If there is 10 grains/Kg of moisture in the ambient air, and you are compressing the air to 150 PSI, then you are going to have about 10 times as much moisture in the tank as you do outside it.

Of course, some of this moisture condenses out as the temp of the air in the tank drops, but compressing air will always raise the amount of moisture in a direct ratio that matches the ratio of uncompressed air to compressed air pressure. This is a law of physics and can not be changed. The best we can do is to try and remove the moisture from the compressed air after it is compressed.

This can be accomplished by only 1 of three methods, Temperature drop, which will cause condensation, but will still leave the air at 100% humidity at the lowest temp that you can get it to. A refrigerated dryer will get the air stream down to about 38 degrees at it's best, and that is pretty close to being low enough that no air will condense out from pressure drops at nozzles or air tools. Most paint booths will follow this with desiccant dryers to lower that dew point even lower (usually about -40 F), to assure there is absolutely no chance of moisture condensing in the air stream. There is also a fairly new technology for drying air and that is a membrane dryer. These work on the principal of a molecular sieve. These dryers do not require regeneration like a desiccant dryer, and require almost no maintenance. More paint shops are starting to use them because of this low maintenance, but they also have to bleed air whenever in use, so they are not exactly energy efficient.

Your aftercooler loop is a very good idea.These can remove a fair amount of moisture, but remember, that being in a 75 degree shop, the best dewpoint you can get the air to is 75 degrees, and the air will be 100% saturated with moisture at 75 degrees F, and that any drop in temp below this will condense some additional moisture. Unfortunately you will usually not see the moisture condensing out, as the velocity of the air is so hight coming out of the spray gun nozzle, that the moisture will exist as microfine droplets, but they can affect the finished paint job. A disposable desiccant filter inline before the gun is still a good idea if you only spraying paint infrequently. They are fairly inexpensive and require no maintenance. When they change color from blue to pink, just throw it away and replace with a new one. A permanent desiccant dryer will be several hundred dollars and will require periodic desiccant regeneration. you have to dump the desiccant out of the filter, and bake it on a cookie sheet in the oven until it truns from pink back to blue. Then you dump it back in the filter body and you are good to go until the color changes again.
 
#43
John,

Thank you for the reply and your expertise. I learned something new today, that's always a good thing.

I was planning on installing one more item on my air compressor (a heat sink) on the 1/2" copper tubing between the compressor pump and the tank to help lower hot air entering the tank. If you feel this is a waste of money ($30.00 dollar), I'll skip it. I've attached a picture of the heat sink as reference.

My 3 stage filtration system has a 11oz desiccant dryer (very right side of attached picture). If this is not going to handle a moisture issue at the gun, I would also install a 2' in-line desiccant snake hose before the spray gun. The only thing I can come up with to see if any moisture is coming out of the gun is to do a dry run with no paint in the gun,into a paper napkin or tissue?.
 

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#44
John,

Thank you for the reply and your expertise. I learned something new today, that's always a good thing.

I was planning on installing one more item on my air compressor (a heat sink) on the 1/2" copper tubing between the compressor pump and the tank to help lower hot air entering the tank. If you feel this is a waste of money ($30.00 dollar), I'll skip it. I've attached a picture of the heat sink as reference.

My 3 stage filtration system has a 11oz desiccant dryer (very right side of attached picture). In this is not going to handle a moisture issue at the gun, I would also install a 2' in-line desiccant snake hose before the spray gun. The only thing I can come up with to see if any moisture is coming out of the gun is to do a dry run with no paint in the gun,into a paper napkin or tissue?.[/QUOTE

I would doubt that that heat sink will help enough to make it worth the trouble installing. A lot of compressors have a finned heat exchanger between the first and second stage of the compressor, but that is mainly to make the second stage more efficient. The tank makes a pretty big heat sink and with all the copper you have on that aftercooler, you are probably doing about the best you can. I would imagine that the desiccant dryer you have at the filter will be fine, but it might not last very long in the hot humid time if the year. Just be sure that you check the color before every use. When the blue goes away, get a new charge in it. I keep a fresh change in the shop at all times. That way, I do not have to stop what I am doing and bake out the desiccant. I can do it at my leisure, and don't have to interrupt my work flow. The problem with a lot of the disposable desiccant dryers like the snake, it that a large percentage, do not have any indication when they are used up.
 
#46
What if you found a way to dry the air before it entered the compressor, like a dehumidifier type of setup for the compressor to draw from? Just a thought.
 
#47
What if you found a way to dry the air before it entered the compressor, like a dehumidifier type of setup for the compressor to draw from? Just a thought.
That's above my pay grade, tex.......lol.

Nice idea though. I have seen guys install a B&M transmission cooler between the compressor pump and inlet to the compressor tank to help cool the air before entering the compressor tank. It does have some benefits, however, I believe it adds additional stress on the compressor pump due to flow restriction from the 3/8" trans-cooler line size, which adds more heat to the compressor pump.

Here's a set up using the B&M transmission cooler.

 
#49
John, your way more techical than I am. Not remotely interested in the science. I'm 100% sure your correct but my experience differs.

Over the past fifteen years I've had numerous compressors and in my experience when I slow the pump down it has less water in the tank when I drain it. My little Quincy 5hp runs around 450 rpm. I built it that way. It has far less water in the tank than the 5hp Kellogg that ran at 900 rpm for at least 7 years. The little Quincy has been running for 5 years so I have a pretty good baseline. On paper I'm probably full of BS but in my world it makes a difference.
 
#50
John, your way more techical than I am. Not remotely interested in the science. I'm 100% sure your correct but my experience differs.

Over the past fifteen years I've had numerous compressors and in my experience when I slow the pump down it has less water in the tank when I drain it. My little Quincy 5hp runs around 450 rpm. I built it that way. It has far less water in the tank than the 5hp Kellogg that ran at 900 rpm for at least 7 years. The little Quincy has been running for 5 years so I have a pretty good baseline. On paper I'm probably full of BS but in my world it makes a difference.
Of course you're right.
Slower is cooler.
Bottom line, less heat equals less water.
 
#51
there is a set amount water in the air. you cant change it with speed or size of pump . now you can change it with compressing the air . more air more water . we got into this when i had the van company . i had 2 big compressors running all day . i kept the tank pressure at 80 lbs . the guy helping us would get into it and make me go cross eyed . back then a refrigerated dryer was an arm and a leg . but we ran a butt load of filters and at least kept it workable . learning about air density when racing drove me nuts . if the water is not in your tank it is still in the air .
 
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dhutton01

Active Member
#52
A compressor cannot create moisture. It’s really that simple. If you take in the same amount of air then you will have the same amount of moisture regardless of how it is compressed. It’s a matter of where the moisture is released. In the tank or in the lines etc.

Don
 

elwood

Registered Users
#53
Friend of mine used to do mold remediation work. Has a meter that measures what's called water grains per pound of air. He fixed my air dryer and measured the gpp. Air before going into dryer was 135-140. Air coming out of air dryer was 66. He said that is like desert air. So an air dryer cuts alot of the moisture out of the air that you are spraying with.
 
#54
By the time you buy a transmission cooler big enough to handle a large amount of air without restriction, all that copper tubing, fittings and ball valves, not counting any labor, you can probably buy a HF dryer and have a superior system. I like building my own stuff, but sometimes it is just better to buy it.
 
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#55
There is another issue that has not been mentioned here. When you run a pump hard and heat the air, the compressor does generate moisture because the tank always contains a lot of moisture even with an automatic purge valve. If your tank is full of hot air that moisture in the tank will be absorbed into the air. That same air will condense as the air cools whether it is from a refrigerated drier, long run of air line or on your painted surface as the air expands and cools coming out of the gun. Obviously, the refrigerated air drier is the best and most efficient method and on your painted surface is the nightmare scenario.

Cool air going into the tank will pick up less moisture but nothing is going to replace a quality drier. Also, as stated, warm air will not give up it's moisture until it cools except for desiccant filters which require regular replacement and for me have been cost prohibitive. The moisture needs to condense in order to be removed.

John
 
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#56
By the time you buy a transmission cooler big enough to handle a large amount of air without restriction, all that copper tubing, fittings and ball valves, not counting any labor, you can probably buy a HF dryer and have a much superior system. I like building my own stuff, but sometimes it is just better to buy it.
I agree. I have about $300 in copper pipe and ball valves. Harbor Freights dryer is about $400 dollars. Guess I should read some review on it cause I have a hard time laying down $400 dollars to HF unless it's a tool box.
 

elwood

Registered Users
#57
A friend I did some restoration work has a hf dryer in his garage. Worked really well. Only problem I had with it was the drain would get clogged up. Had to take it apart to fix it. But it took a while to clog up. It had no other problems tho.
If memory is correct they shipped it to his residence.
 
#58
There is another issue that has not been mentioned here. When you run a pump hard and heat the air, the compressor does generate moisture because the tank always contains a lot of moisture even with an automatic purge valve. If your tank is full of hot air that moisture in the tank will be absorbed into the air. That same air will condense as the air cools whether it is from a refrigerated drier, long run of air line or on your painted surface as the air expands and cools coming out of the gun. Obviously, the refrigerated air drier is the best and most efficient method and on your painted surface is the nightmare scenario.

Cool air going into the tank will pick up less moisture but nothing is going to replace a quality drier. Also, as stated, warm air will not give up it's moisture until it cools except for desiccant filters which require regular replacement and for me have been cost prohibitive. The moisture needs to condense in order to be removed.

John
John, there is one big hole in your logic. The air in your tank is already at 100% relative humidity and can not absorb any more moisture. This is pretty much true at almost any point in the system. There is never cool air going into your tank, as compressed air will always be hot. When you compress air, the temperature goes up in direct relation to the amount the air is compressed. The air only cools after it goes into the tank. That is what causes the moisture to condense out in the tank.

As the temperature of the air drops, the moisture condenses just enough to maintain 100% Relative humidity at that point. Keep in mind that Relative Humidity is just that, relative. It is the ratio of the amount of moisture in the air to the amount is is capable of holding, and can never exceed 100%. If it tries to exceed 100%, it condenses out.

Even air coming out of a refrigerated dryer is leaving at 100% humidity, but it is at 36-38 degrees. By the time it warms back up to room temp of 75 degrees, the humidity has dropped to about 20%. This is what makes a refrigerated dryer work so well.

The hot air coming out of a compressor will condense out in the tank under almost any set of conditions you can imagine. If you have 75 degree ambient air at only 20% humidity, the air will be 100% humidity once it goes into the system can cools off. The average compressor compresses air about 10 times atmospheric pressure, so the moisture will go up 10 times in the compressed air. At 75 degrees system temperature. The air will be at 100% humidity and will have already condensed approx 50% of the moisture that was in it when it compressed into the tank.

You can plot any set of conditions on a physcrometric chart (available on the web), to tell you exactly what the grains of moisture, the dewpoint, and the relative humidity is for any given set of conditions.

Regards, John McGraw
 
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#59
Still trying to wrap my head around this. So from the video I posted with the gentleman installing the B&M transmission cooler between the compressor pump and the compressor tank, he is trapping some water before it goes into the tank. Less water in the tank equals less water in the air?

Would this be correct?

Also, if I'm understanding you, a compressor pump that produces more heat than another pump, has no effect on how much water is generated? I'm trying to see if the transmission cooler really has any benifit?

How to read a physcrometric chart
https://www.brighthubengineering.com/hvac/41264-how-to-use-psychrometric-chart/

Physcrometric chart
https://www.engineeringtoolbox.com/docs/documents/816/psychrometric_chart_29inHg.pdf
 
#60
Sounds like you really want to build something - I get it - BUT that HF dryer works ! I have one. Does a great job ! Took care of all my moisture problems. As far as the drain goes, I put an automatic drain on and it works great- scares the crap out of you when it kicks on though and you’re standing near it :)- they have sales all the time . They just had one for $50 off a bigger item - can get it for $350 pretty easy - I think awhile back someone said they looked at the parts inside and they were all top name/quality parts- not Chinese junk
 
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