Forward Motion

PMJ

I finally needed some heat this season and could take some photos of the gauges I installed inside the pigtail to illustrate what I have been saying about steam staying in forward motion to the rads when you use natural vacuum between cycles as opposed to open venting.

The Magnehelic is showing the pressure difference between the boiler and the dry return where it drops to the waterseal and wet return which is at a remote corner from the boiler. The vacuum gauge shows the ever increasing vacuum of the whole system - the photo names show minutes from when the burner shut off. Next fire occurs 10 min after the previous shutoff. The 5 inches difference occurs right after burner shutdown and is much higher than during the previous burn which is barely measurable or even negative which I attribute to the high speed of the steam.

As with the vacuum demonstrator post recently some gentle boiling continues through the shutdown period as the vacuum deepens. Because there remains a slightly higher pressure at the header than the dry return, steam continues to flow slowly to the rads from the mains throughout the shutdown. Rad supply pipes stay too hot to touch.

Contrast this with the operation of the conventional 2 pipe system with an open vented dry return. I have left mine open to see the difference and it is quite dramatic. With the dry return always open vented to the atmosphere the Mag gauge drops to and sits at 0 during the off period. When the burner goes off the space created by the condensing steam in the rads is filled by air from the dry return(which obviously came from the living space at 70 degrees) instead of by steam from the mains and things start flowing backwards. The supply lines to the rads cool quickly. No noticeable boiling is taking place at the boiler after a minute. At next fire the Mag gauge goes to 2-3 inches of water as the new steam has to push out a lot of air it didn't have to before. New steam to the rads is on each fire is obviously slower.

While I have enjoyed this even heat for some years now it is interesting to watch on the gauges how the vacuum generated on the dry return side of the rads is coaxing more steam along into them even when the burner is off.

SWEI

Vacuum steam got the short shrift for sure.

Really great to see some data on this.

PMJ

Hatterasguy said:

That is beautiful.

What's the reading on the vacuum gauge when the boiler restarts?

It is approaching -60 inches of water and still sinking at that point.

ChrisJ

Hat,
I think he does have venting, it's electric and doesn't open unless there's a positive pressure.

ChrisJ

Personally, I would start to question the accuracy of the vacuum gauge because of the extreme you're using it under.

Off the top of my head I don't believe any gauge is recommended to be used past 50% or so of it's range repeatedly.

I'm not saying you're not developing a decent vacuum, I'm saying you should get a different gauge that goes down lower.


Perhaps one of these two from Dwyer.

LPG4-D7722N
Low pressure gage, range -100-0" w.c. (-25-0 kPa).

LPG4-D7822N
Low pressure gage, range -160-0" w.c. (-40-0 kPa).

My next concern would be none of those vacuum gauges should ever see much positive pressure at all, but the dual range gauges I see don't go very far.


Another option could be a 2150 Magnehelic as I think they're very durable when it comes to reverse pressure and over pressuring.

2150 specs
Differential pressure gage, range 0-150" w.c., minor divisions 5.0.

ChrisJ

The interesting thing is, if the radiators were big enough vs the boiler size there's no way the boiler could increase the pressure.

I.E. it could run full output at 120F and as long as the radiators kept up, it would be impossible to increase the pressure or temperature.

Steam modcon as I keep saying.

PMJ

Hatterasguy said:

PMJ said:

Hatterasguy said:

That is beautiful.

What's the reading on the vacuum gauge when the boiler restarts?

It is approaching -60 inches of water and still sinking at that point.

OK..........so the system is at about 12.7 psi absolute pressure at the point of restart. Yes, there is a slight vacuum in the system but there is still a significant air presence.

Without any venting, how does the steam move at all on a restart? With a lower absolute pressure at the rads, steam will certainly go to them but at the point of restart is there still a pressure differential (absolute) between the boiler and the rads allowing steam to move despite the presence of a significant quantity of air?

The reason for the pointed question is that my original thoughts were that you'd need to extract most of the air via vacuum to enable operation without any venting. But, it appears that your work shows that only a slight difference in absolute pressure is required if you can maintain it.

Is there any risk of losing that differential if one increased the off time? Would that not be a killer for the system in terms of being able to move steam? How do you restart it after a significant off time (say a day or two)?

Excellent work, BTW.

The one thing I know for sure is that the steam leaves the boiler much faster under these conditions than it does without the vacuum. The physics of why I'm not sure of yet and could use some help explaining.

When everything is at atmospheric pressure the new steam must push air out of the system somewhere in order to occupy new space itself. This is not the case when the burner starts with the system below atmospheric pressure. For three minutes or so into the new burn the steam is just refilling the same space it already occupied when the burner shut off and the system was closed off to the atmosphere. I do not open the vent until the vacuum is totally gone which takes 3+ minutes. Opening it any sooner would flow air in not out - the opposite of what we want.

Somehow there is much less resistance to the flow of new steam into a pipe that has even a slight vacuum ahead of it. Perhaps someone can explain.

ChrisJ

Is the majority of the system filled with air, or steam?
Just because it's 12 PSIA, does this necessarily mean there's a lot of air in it?

I'm sure there's quite a bit, but maybe 70% of it is steam.

Paul48

I don't know the answer, but it is along the same lines as, why can a spaceship travel at 17000 mph in the vacuum of space?
On further reflection, it's the differential pressure.

PMJ

ChrisJ said:

Is the majority of the system filled with air, or steam?
Just because it's 12 PSIA, does this necessarily mean there's a lot of air in it?

I'm sure there's quite a bit, but maybe 70% of it is steam.

In a two pipe system on an average day I'm guessing 1/2 the total volume of the system is still air - (1/2 the rads and the dry return). And even if all the steam in there condensed in the 10 min (which obviously it does not), around 7 psi vacuum is all that could be theoretically achieved naturally. I am seeing around 2 psi which seems reasonable to me. This does point out however that the potential is far greater in one pipe systems to achieve even more vacuum this way as a far greater percentage of the total system is filled with steam during operation.

And also @ChrisJ, I'm pretty confident in the vacuum gage - I have 3 going at various places and they all show 0 when nothing is happening and seem to agree with each other after hundreds of cycles. I have tightened up a few things recently though and may need to increase the capacity of the gauges just to keep up.

SWEI

What are you using on the radiators, @PMJ ?

PMJ

SWEI said:

What are you using on the radiators, @PMJ ?

Mouat elbows on most - especially ones that fill too quickly anyway. Standard traps with seat removed on several big ones that I wanted to fill more. Basically I run the system with rads never full enough to need a trap so the dry return is always wide open to all of them.

SWEI

Ah, OK. Somehow I got the impression this was a one pipe.

SteamCoffee

PMJ, bravo! This is great stuff. Real life, not books. The naturally occurring vacuum in steam theory needs updated, for sure. Many folks think it's success was coal based only, when most period Dunham handbooks clearly show a gas option was available. Your posts have caused a bunch of us to revisit the subject. Love the analogies. Even better knowing it's a Mouat-based system. Now, if you could just get the whole system airtight! Keep the info coming!

PMJ

SteamCoffee said:

PMJ, bravo! This is great stuff. Real life, not books. The naturally occurring vacuum in steam theory needs updated, for sure. Many folks think it's success was coal based only, when most period Dunham handbooks clearly show a gas option was available. Your posts have caused a bunch of us to revisit the subject. Love the analogies. Even better knowing it's a Mouat-based system. Now, if you could just get the whole system airtight! Keep the info coming!

Thanks for this @SteamCoffee. There is a lot of skepticism about it out there for reasons I don't quite understand. I have discovered in my own system that steam is definitely a lot happier when it doesn't have to fight with air for the space! No mystery trapped air pockets etc. Just must faster speeds and quiet even operation. I'll never go back to life without vacuum.

ChrisJ

PMJ said:

SteamCoffee said:

PMJ, bravo! This is great stuff. Real life, not books. The naturally occurring vacuum in steam theory needs updated, for sure. Many folks think it's success was coal based only, when most period Dunham handbooks clearly show a gas option was available. Your posts have caused a bunch of us to revisit the subject. Love the analogies. Even better knowing it's a Mouat-based system. Now, if you could just get the whole system airtight! Keep the info coming!

Thanks for this @SteamCoffee. There is a lot of skepticism about it out there for reasons I don't quite understand. I have discovered in my own system that steam is definitely a lot happier when it doesn't have to fight with air for the space! No mystery trapped air pockets etc. Just must faster speeds and quiet even operation. I'll never go back to life without vacuum.

Same as everyone against using a 10-20% pickup factor instead of the (in my opinion) out dated 33%.

Also same for insisting on people use plain water instead of proper water treatment that has zero impact on performance.



Anyone that is trying out and proving non-conventional things is benefiting everyone in my opinion no matter how small. Just because someone or some book says it won't work doesn't mean it's true.