False pressure on start up

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cnadon

#1: Post by cnadon »

I've got a heat exchange machine. On start up in the morning it come up to pressure at a little over one bar. Open the steam valve and, poof, it's down to zero. But after this bleeding, it does come up to steam and maintains it just fine. And water temps go up accordingly. So it's not a major problem, just a bit annoying. But I'm curious as to 1) the cause; and 2) possible fixes. Thanks.

Best, Chris

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HB
Admin

#2: Post by HB »

Dan Kehn

bogiesan

#3: Post by bogiesan »

cnadon wrote:I've got a heat exchange machine. On start up in the morning it come up to pressure at a little over one bar. Open the steam valve and, poof, it's down to zero. But after this bleeding, it does come up to steam and maintains it just fine. And water temps go up accordingly. So it's not a major problem, just a bit annoying. But I'm curious as to 1) the cause; and 2) possible fixes. Thanks. Best, Chris
I have never considered the actual cause—must be simple physics—but bleeding off the startup pressure has been the recommended standard procedure for all machines, not just heat exchangers, for as long as I can recall. I've been doing it automatically on all of my machines since 1992 from Krups and Solis though Rancilio and Salvatore.

Interesting question. Wonder what the real answer is.

david boise ID

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mhoy

#4: Post by mhoy »

Here is my guess on the physics.

1. valve sticks while cooling down, pulls a vacuum in the boiler.
2. water boiler at a lower pressure/temp in a vacuum and quickly pressurizes and causes the p-stat to turn off the heat.

Do I win the prize?

Mark

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cbrucecampbell

#5: Post by cbrucecampbell »

That's sort of the way I thought. (The little HX machine I have doesn't even have a vacuum breaker valve afaict.) So, I reasoned if I waited for the machine to cool down and then open the steam valve and let the pressure equalize, then the next time it fired up the false pressure or vapor lock or whatever the heck it is would be gone.

Didn't work.

I, too, would like to know exactly why this happens. Salvatore documents clearly that the bleed is necessary at startup - every time - at least on this machine. I suppose I could ask him.
Bruce Campbell

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cafeIKE

#6: Post by cafeIKE »

When the boiler cools, it sucks in air.
Total gas pressure is the sum of the pressure of all the gasses. wiki Dalton's Law
When the boiler heats, the pressure is held at ~1 bar, which is the sum of the air pressure and the steam pressure.
When you bleed the boiler, you remove [some of] the air.
Because the pressure is lower, the boiler heats to ~1bar again.
Once all the air is removed, the water is at ~1bar steam temp and the pressure is maintained.

It's SOP to bleed a boiler a cupla times before using the steam.

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cbrucecampbell

#7: Post by cbrucecampbell »

Thanks Ian.

That makes sense but I'm still a little confused. If I follow, then putting a machine that relies on a pressurestat on a timer to heat up an hour or so before you get out of bed isn't practical. You will always have to bleed it before the group will really get up to temp...

Is that right? If so, then that's a bit of a negative for home HX machines that seems to get little attention.

(To perhaps belabor the obvious. My machine will get up to a "false" 1 bar in four minutes. But it needs the standard half hour or so after the bleed to really get to temp. Until it's bled, it will sit at the false pressure - not getting hot enough - indefinitely.)
Bruce Campbell

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cafeIKE

#8: Post by cafeIKE »

If the HX machine has been on an hour, it's possible that the group is close to ideal temperature without a steam bleed. [It's been too long since I did partial gas pressure calculations to recall how much lower the boiler temperature could be.] On an HX, the group is usually too hot, requiring a cooling flush.

With a vacuum breaker, some of the air is automatically purged at start up. There's no need to bleed the boiler if the group is at temperature and you are not making steam. Recovery will be slower, but that's not always an issue at home.

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cbrucecampbell

#9: Post by cbrucecampbell »

OK. Thanks much Ian. I think I get it now.
Bruce Campbell

VS_DoubleShot

#10: Post by VS_DoubleShot »

It's strange that the One Black doesn't have a vacuum breaker because the photos of the inside of the machine, on Salvatore's website, show a vacuum breaker sticking out of the boiler. I'm hoping to add one with a brass T to another fitting that leads to the BP gauge. I guess this is good info for all those people out there trying to save a few bucks by getting the One Black.
Regards,

Vince