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Too low a humidity will have the paint drying to quickly .
just like from spraying at too great a distance .

the next page of this thread
may be devoted to diy paint booth humidifiers .

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That dip tank and enamelling oven must be looking better all the time.

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Originally Posted by quinten
add a holding tank
5 minutes at 5 cfm = 25 cu.ft. At ~100 psi that's 0.25 cu.ft., which would be a cube ~7½" on a side. While not impossible, that would be inconveniently large to deal with since it would require being held at ~32 ℉ for quite a while to cool the air, and also would need a valve to drain the water while not releasing too much pressure.

For reasons discussed below, I've abandoned for now my plan to make an in-line chiller for the air, and only will return to that idea if the lifetime of the desiccant I'm waiting to be delivered turns out to be too short. The result of an experiment today is the reason for giving up on the chiller.

The first photograph shows a 50-ft. coil[*] of ¼" Cu immersed in a container of ice water.

[Linked Image]

[*]I said it was 25 ft. in my previous post, but it's twice that long. Since the measured air flow in 25 ft. of line is ~11 cfm, it should be no more than 6 cfm in this coil.

The coil is sitting on a plastic tray so it is completely surrounded by the water. As can be seen, the water is at 32 ℉, and the Cu was in it more than long enough to reach that temperature. I re-ran the experiment 60 minutes later, with the water still at 32 ℉, using a thinner thermocouple wire and got the same result. Also seen in the water is a shorter coil of ⅜" Cu that I didn't bother measuring, for reasons to be discussed shortly.

As can be seen in the next photograph, I taped the thermocouple wire to one end of the coil that was sticking out of the water and bent the wire to ensure it was fully in the stream of the emerging air.

[Linked Image]

In the first photograph it can be seen the thermocouple was at 21 ℃. It actually started at 23 ℃ but had cooled slightly by the time I took the photograph because the insulation hadn't completely protected it from the cold of the Cu.

I then used the air hose to blow air into the other end of the coil which, from previous measurements, would have limited flow to no more than ~6 cfm. I did this for over a minute to give the end of the thermocouple wire plenty of time to adjust its temperature, but it only dropped to 20 ℃ during that time. It stayed at that temperature after I stopped the air flow, but dropped further to 19 ℃ after a few minutes. This showed that all measured temperature changes, small that they were, were due to thermal conduction from the exposed Cu tubing.

Since this experiment showed 50 ft. of ¼" copper tubing is useless for cooling ~80 ℉ air that is flowing at ~6 cfm, this eliminates the possibility of basing a simple heat exchanger on it to reduce the water content of the air.

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Originally Posted by quinten
Too low a humidity will have the paint drying to quickly .
This sounds authoritative and plausible, but seems to be wrong.

Searching for 'humidity' in the House of Kolor 149-page technical manual finds it mentioned only four times, always in the context of the influence of high, not low, humidity on the results. For example, High heat or high humidity conditions will accelerate cure time. They recommend using their KU151 Hi-Temp Flo-Catalyst "where high humidity and temperatures are present ... this is a slower curing version of our KU150," but otherwise to use their KU150 medium curing activator. If low humidity caused their paint to dry too quickly, presumably they'd mention using their slower curing activator under those conditions as well.

Originally Posted by Shane in Oz
That dip tank and enamelling oven must be looking better all the time.
At ~$300/gallon of quality automotive paint, the ~6 gallons of it the ~$1700 I've spent on my spray booth would buy wouldn't be enough to immerse even a single fork leg. So, not really.

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Paint may have been comparatively cheaper in 1950.

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Originally Posted by Shane in Oz
Paint may have been comparatively cheaper in 1950.
If nothing else, the last month or so of this thread may help people decide a rattle can is good enough.

[Linked Image]

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Or to locate the project in the Atacama from the get go.


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Originally Posted by Magnetoman
Originally Posted by Shane in Oz
That dip tank and enamelling oven must be looking better all the time.
At ~$300/gallon of quality automotive paint, the ~6 gallons of it the ~$1700 I've spent on my spray booth would buy wouldn't be enough to immerse even a single fork leg. So, not really.
If you're going to be such a cheapskate, you probably won't go with the following approach, either smile

Rather than mucking about with air compressors, filters, moisture traps, multiple pressure regulators and desiccants, use the bottled nitrogen from your plasma arc cutter. Shandy in 20% oxygen if you want a more realistic atmospheric composition, and add argon and carbon dioxide to taste if desired...

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Originally Posted by Hugh Jörgen
Or to locate the project in the Atacama from the get go.
It would be interesting to compare that to the Sonoran.

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Originally Posted by Shane in Oz
It would be interesting to compare that to the Sonoran.
The Atacama is at 11,000 ft. with annual rainfall of 0.1 inches. The Sonoran is at 2000 ft with 10 inches. Other than that, they're essentially indistinguishable.

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Originally Posted by Magnetoman
Originally Posted by quinten
Too low a humidity will have the paint drying to quickly .
This sounds authoritative and plausible, but seems to be wrong.


dry air is more thermally conductive
Humid air is less thermally conductive .

The phase transition of solvents ( evaporation , paint drying) is affected by
the humidity of the atmosphere
into which the solvents are attempting to equalize .

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Originally Posted by quinten
dry air is more thermally conductive
Humid air is less thermally conductive . .
Although there is very little difference in thermal conductivity at room temperature, it's just the opposite. At all temperatures moist air has a higher thermal conductivity than dry air.

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Originally Posted by Magnetoman
Originally Posted by Shane in Oz
It would be interesting to compare that to the Sonoran.
The Atacama is at 11,000 ft. with annual rainfall of 0.1 inches. The Sonoran is at 2000 ft with 10 inches. Other than that, they're essentially indistinguishable.
10 inches barely rates as arid; hardly drier the Stevenage's 30 inches.

An average of 10 points for the Atacama is impressively dry.

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Originally Posted by Magnetoman
Originally Posted by quinten
dry air is more thermally conductive
Humid air is less thermally conductive . .
Although there is very little difference in thermal conductivity at room temperature, it's just the opposite. At all temperatures moist air has a higher thermal conductivity than dry air.


Water vapor in the atmosphere displaces nitrogen and oxygen
Humid air is less dense .

atmosphere at higher relative humidity
impairs heat exchange efficiency
by reducing the rate of evaporation .

why people feel uncomfortable
when it's humid is related to the same phenomen that slows paint solvent evaporation .

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I was under the impression that, in very simple terms, materials have similar thermal conductivity and electrical conductivity properties.

Most people would be happy holding a plugged in toaster in air. Most people wouldn't be happy under a mist of water. If moist air is more electrically conductive then why wouldn't it be more thermally conductive?

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will an air cooled engine running at a stable temperature get hotter if moisture is added to the airstream?


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Originally Posted by kevin
will an air cooled engine running at a stable temperature get hotter if moisture is added to the airstream?

Is the moisture in the form of Scotch mist?


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Originally Posted by triton thrasher
Is the moisture in the form of Scotch mist?
Aye. Things are much clearer with the aid of a glass.


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the reason im asking is because theres more things going on than i can keep straight in my head. if the thermal conductivity goes up with temperature in the range we re interested in, then heat transfer will presumably be more efficient in moist air

but the extra mosture in the air increases the overall heat in the airstream because of the greater heat capacity of the water. does the moist air absorb more heat, faster, as it gets hotter? if so, the motor will get cooler. or does the increased heat in the moister air slow the absorption of heat by the air from the fins and permit less heat to cross the boundary? or is there a crossover? i dont understand this stuff.

its like a radiator. even if warm water transfers heat faster than cold water, a cooling system running hot water wont work as well as one running cold water. what happens with dry air versus moist air?

the owners manual fo rmy old hudson recommended using alcolhol in the cooling system, but i dont think they had scotch mist in mind


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As ambient air temperature goes up, the temperature difference between your hot engine and the air becomes less, so don’t run away with the (slightly mad) idea that hot air will cool your engine better than cold air.

Water vapour is over 1000 times less dense than liquid water. It does not have the heat conduction properties of liquid water. It is less dense than air. As already said above, in the atmosphere it is displacing air.


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The figure in triton thrasher's post is from a magazine article, but the following one is from an engineering publication.

[Linked Image]

Although at least one of the figures has to be wrong, both show humidity only has a 1% effect even at 40 ℃, which would mean it's negligible in the context of painting, which is the context I care about at the moment. If someone wants to look deeper into this to figure out which one is correct (or, maybe both are wrong), be my guest.

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Magneto man's illustration is incorrect

the sweat on your forehead evaporates off
at lower humidity
because lower humidity has better heat conductivity .
( the system is working so well you don't even know it's a system and working )
...( its hot , but its a dry heat )

The sweat on your forehead pools and drips down into your eyes ... at higher humidity
because the atmosphere is less thermally conductive
( it takes heat transfer for water to phase transition into water vapor )
... and heat transfer , in humid air , is happening at a lesser rate
You feel hotter because water vapor in the air hampers evaporative conductivity )
( its the same hot , but now a wet heat)
... humans experience humid air as being heavier ... but it is actually lighter .

The same phenomenon is happening with the solvent in paint .
Spraying aerosoled paint through dry air and some solvent will evaporate ... on the way to the Target .
at the worse ,
This can affect how the paint wets out and lays down . (the surface may orange peel or even prill little spheres of dry paint onto the surface )
some effects are started before the the paint even hits the target
and others are after the aerosol lands .
If the evaporation rate is too high the paint may not have enough time to wet out and level
and Humidity is a key factor in evaporation rates .

if it didn't matter
no one would ever say
"man it's humid today"
or " I'm moving to Arizona ... because it's less humid "

some of today's catalyzed paints even require humidity to properly cure .

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Sweat evaporating to cool your fevered brow by latent heat of evaporation and conductivity of air are two different things.

If the atmosphere is saturated with water vapour, sweat will be reluctant to evaporate. Air will still conduct heat away.

Last edited by triton thrasher; 05/14/22 5:59 pm.

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I couldn't stop myself from looking into the thermal conductivity conflict. The magazine article that has the curve triton thrasher included in his post explains:

-------------------------
Some time ago, one of my colleagues (Nelis Mies, Philips Lighting) approached me with the same question, and I sent him an old graph proving more or less my standard answer. However, he was not completely satisfied and found software that could create the desired graphs1. The software employs kinetic theory describing a mixture of two gases; in this case, atmospheric air and water vapor. Figure 1 shows some interesting results.
[graph]
The strange shapes of the curves were rather surprising. With increasing moisture content the thermal conductivity decreases, contrary to what I expected.
-----------------------

As the above quote shows, the magazine writer included someone else's software-generated curve even though he, the writer, found the result "rather surprising."

Submerging myself deeper, I have access to the following recent article through my university's library (but you, too, can download a pdf of it from the publisher for a mere $45):

[Linked Image]

I highlighted an interesting section from the Conclusions:

[Linked Image]

Surprisingly, even though both air and water have been around for quite a while (unverified reports suggest maybe for even more than 500 years...), what we know about the thermal properties of humid air is based on theoretical models that have yet to be experimentally verified.

Unwanted moisture that might be added to paint would be "chemically bad," so the humidity inside the compressed air line has to be low enough that droplets can't form when the air emerges from the gun. However, even 100% relative humidity changes the thermal conductivity of air by at most 1%, one way or the other, in the temperature range where we paint so the humidity of air is "thermally irrelevant" to the task of painting.

Everyone I know, myself included, hates warm humid air, so now I'm really done with the topic.

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