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Author Topic: What temp does water boil in vacuum?  (Read 854 times)

E Yoder

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What temp does water boil in vacuum?
« on: January 25, 2019, 03:08:07 AM »

Got a question for smarter guys than me.

If the outdoor furnace is down a pretty steep hill from the house about 60' what temp will the water at the top of the loop flash to steam? Discussing a possible install with someone and I'm not sure if I've ever pushed it quite that far. Maybe once that I can think of long ago.
I've found tables with inches of mercury, but only went to 29 I think.
I'm thinking separating with a flat plate is a better option, but now I'm curious.
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heat550

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Re: What temp does water boil in vacuum?
« Reply #1 on: January 25, 2019, 03:52:13 AM »

If the pump was at the stove . Pulling from bottom of the tank with check valve wouldn't it work just fine. Let me know.  Head pressure be kinda high.
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E Yoder

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Re: What temp does water boil in vacuum?
« Reply #2 on: January 25, 2019, 05:01:06 AM »

Yes. Pump would be at the stove. If there's no leaks up high and air is all purged I'm thinking it's like a 60' tall finger on a straw.
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BoilerHouse

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Re: What temp does water boil in vacuum?
« Reply #3 on: January 25, 2019, 06:24:10 AM »

Not one of the smarter guys, but I will still chime in

These are ball park numbers

minus 5 psi  boils at 194 F
minus 10 psi boils at 158 F
minus 14.7 psi boils at 72 F (perfect vacuum)

Here is a helpful link that I got from a quick google search

https://www.quora.com/What-happens-to-boiling-point-of-water-in-vacuum

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greasemonkoid

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Re: What temp does water boil in vacuum?
« Reply #4 on: January 25, 2019, 07:22:53 AM »

It's going take a mean pump. A checkvalve is pointless in my opinion.

At 176 degrees water boils at about 7psia which is about a 17' reverse water column. Adjust for the additional pressure created when the fluid is moving and you will be a little higher. Fortunately the column gets higher with velocity or restrictive pressure.

I'm not worth a crap at math, but my belief is that your success will lie in cramming as many gpm through that pipe as possible to get the head pressure up, or put a simple valve after the heat HX (and all of the HX's placed on the uphill side as expected).

Put a clear line in the house right after the valve so the residents can watch the water fall.

I would definitely put a valve after the last HX in case you need to create some more loop pressure on the top side, or use the valve at the return at the boiler, but someone would probably open it and wonder why they don't have heat.

Here are some handy resources:

https://www.engineeringtoolbox.com/hydrostatic-pressure-water-d_1632.html

http://www.jbind.com/pdf/Cross-Reference-of-Boiling-Temps.pdf

https://www.engineeringtoolbox.com/water-evacuation-pressure-temperature-d_1686.html

« Last Edit: January 25, 2019, 07:28:26 AM by greasemonkoid »
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RSI

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Re: What temp does water boil in vacuum?
« Reply #5 on: January 25, 2019, 07:44:58 AM »

I wouldn't even bother trying it. Just too much to go wrong. If you have that much vacuum, the pex or heat exchangers might implode. Even if nothing like that happens, you still probably will keep getting air in it and air locking.

I would just put plate heat exchanger in the back of the stove and pressurize it to 30 PSI.
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mlappin

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Re: What temp does water boil in vacuum?
« Reply #6 on: January 25, 2019, 10:11:24 AM »

Good question on that. I recall reading an old service manual for AC on combines. They actually wanted the service tech to pull a vacuum then run a LP torch over any metal lines in the system to help get all the moisture out. Low pressure gauge on my R134/R22 fill hoses reads down to -30 lbs. I routinely see a minimum of -28 recommended to fully remove any moisture from a system that blew a hose or was left open, thats at room temperature as well.
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RSI

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Re: What temp does water boil in vacuum?
« Reply #7 on: January 25, 2019, 10:32:10 AM »

A few years ago, I was thinking about trying to build a vacuum wood kiln but decided it would be too much trouble.
If you can get the vacuum high enough to get the water to boil in the wood, it should dry super quick.
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mlappin

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Re: What temp does water boil in vacuum?
« Reply #8 on: January 25, 2019, 03:41:02 PM »

A few years ago, I was thinking about trying to build a vacuum wood kiln but decided it would be too much trouble.
If you can get the vacuum high enough to get the water to boil in the wood, it should dry super quick.

Ive thought the same for hay.
.
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Stihl 023
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E Yoder

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Re: What temp does water boil in vacuum?
« Reply #9 on: February 01, 2019, 01:09:12 AM »

Thanks for the responses. This is not one I'm actually installing so I don't know if I'll know what they decided to do. I'll post if I hear anything.
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