How vacuum can cause steam problems

HeatingHelp

How vacuum can cause steam problems

Back in the old days when most folks burned coal, vacuum was just about everyone’s friend. But when the oil burners arrived, the system problems arrived with them. Today vacuum still causes problems in steam systems, but in different ways.

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Peter Rozano

Great article. Always fascinated with Vapor Vacuum systems. However, you have a significant typo. In the 2nd paragraph, it should read "at temperatures much LOWER than 212° F."

Handyman 242

Great article Dan. It is great one to keep and save. By the way you have one typo, in the 2nd paragraph with a vacuum steam will boil at a lower temperature not a higher temperature.

Jamie Hall

Excellent, Dan! The only comment I would make is that you don't always have to have an F&T at the end of a steam main with associated dry returns -- a combination of a crossover trap and a water seal loop works too (provided you have enough venting on the dry returns). If the mains are parallel flow, you're going to really want a wet return there, too -- the drips from the steam main and dry return go into it. This arrangement works even when the whole system goes into vacuum...

Erin Holohan Haskell

Got the typo. Thank you!

Steamfighter49

Good job. (I knew it had to be a typo)
Reminds me of a job where the water kept flying out of the boiler. It wasn’t really a vacuum problem so I won’t put it here. But someday I’ll tell you my lettuce story.

DanHolohan

Sorry about the typo! Mea culpa. 

PMJ

@DanHolohan ,

When you say the following: "But then when the oil burners arrived, the system problems arrived with them", I'm going to ask if I am safe in assuming you mean some systems may encounter problems, but surely you didn't mean that all will. As presented it is possible to read this that you are suggesting vacuum's usefulness in residential heating ended with the arrival of intermittent fire.

I do understand that vacuum does cause many issues, and most of the equipment you refer to here I am not familiar with. I do run a process steam system with some of these devices, and vacuum is a problem in some areas just as you say and I employ vacuum breakers in several locations.

That said, in many simple residential applications that have no need of any of these devices like my Mouat system, natural vacuum between firings is very simple to achieve and can provide very significant performance improvement; my results being very much just as Hoffman described quite clearly in their literature on vacuum vents for use in oil and gas fired systems.

I bring this up as I think it would be unfortunate if some readers who might be inclined to try vacuum operation were inadvertently discouraged from doing so by this article.

DanHolohan

Thanks. I wasn’t talking about those systems specifically designed to produce vacuum. 

PMJ

DanHolohan said:

Thanks. I wasn’t talking about those systems specifically designed to produce vacuum. 

I'm not really sure if Mouat originally had vacuum vent(s) or not. Perhaps you could answer that. I do know that when I bought the house it was open vented on both main and dry return - like what your article suggested often needed to be done.

I removed them both. For years now the only vent has been one single 1/2 NPT solenoid valve at a remote location in my garage. That is how little venting is actually needed to run these things. On an average day that small opening is open to the atmosphere about 10% of the total time.

DanHolohan

Mouat was a vapor system, not intended to run on vacuum. More here:

https://heatinghelp.com/assets/documents/541.pdf

PMJ

Thanks. Attached is another.

At least one two pipe system not designed for vacuum that I can report does very nicely with it.

Jamie Hall

It is a common misconception that vapour systems were intended to run with a vacuum -- and it is quite true that some were. But the distinguishing feature is not the possibility of vacuum, but that they were intended to run on very low pressure differentials between the steam mains and the dry returns. It makes absolutely no difference -- except in the temperature of the transition from water to steam and back -- whether the system is operating at a 4 ounce differential with the dry return at atmospheric, or whether it is operating at a 4 ounce differential with the dry return at say 5 inches vacuum.

jumper

>> intended to run on very low pressure differentials between the steam mains and the dry returns<<

Seems to me that the way to achieve the above is to expel air and limit its re-entry? Didn't Mouat utilize a wimpy ejector to help move steam? It's a waste of steam to use it to compress air. Wonder if Mouat contractors sealed joints somehow?

Jamie Hall

We tend to think in terms of pushing air out somehow -- or sucking it out. 4 ounces total differential isn't going to push much, and there's no real advantage to getting the returns down to 4 ounces of vacuum vs. getting the boiler up to 4 ounces of pressure (consider: the barometric pressure (psia) varies by a good deal more than that, and that -- not the pressure gauge reading (psig) -- is what counts for temperature). That said, some -- but not all -- systems were set up to operate the whole thing under vacuum as the coal fires died down -- that's the origin of the Hoffman 10, 11, 15, 16 and 76 vents, for instance (the 15 was particularly for differential loops -- wish I had one. But always maintaining that low pressure differential. On the other hand, it wasn't that uncommon to find no vent at all -- just an open pipe (sometimes to the chimney!) to serve as the air vent.

Another thing to remember is that these systems all used crossover traps (well, almost all, anyway); the use of those (which have at least the capacity of the modern Big Mouth) meant that the progress of steam in the mains was limited by how fast the mains can heat up, not how fast the air could get out.

It's really fascinating to observe one of these things in action -- the boiler fires up at full song, and you expect the vent(s) to begin letting air out in great quantity -- but if you go and put your finger (or tissue or whatever!) up there... maybe a gentle wisp, and that's it.

PMJ

Jamie Hall said:

It is a common misconception that vapour systems were intended to run with a vacuum -- and it is quite true that some were. But the distinguishing feature is not the possibility of vacuum, but that they were intended to run on very low pressure differentials between the steam mains and the dry returns. It makes absolutely no difference -- except in the temperature of the transition from water to steam and back -- whether the system is operating at a 4 ounce differential with the dry return at atmospheric, or whether it is operating at a 4 ounce differential with the dry return at say 5 inches vacuum.

Quite true @Jamie Hall and an extremely important point. There seems to be the idea going that some dramatic changes happen at negative system pressure when just as you say, they don't. Relatively relationships inside the system really stay quite the same.

There is one important difference however. When the dry return is open to the atmosphere, header pressure at least slightly over atmospheric pressure is required to push steam into the radiators. Not much, but something. The pressure condition at the exit of every radiator is the same...atmospheric. When the burner goes off the dry return then almost instantly becomes the highest pressure place in the system so flow from the mains stops right then. The void created from steam condensing in the radiators is replaced by an unlimited supply of air from the dry return. When the dry return is closed off from the atmosphere and the system pressure drops rapidly, more of the residual heat in the boiler is made into steam, much of it and what steam is still in the mains continues to flow to the radiators as the dry return now retains its status as the lowest pressure place in the system. It isn't a huge amount of steam but the flow of what there is now also favors the radiators in colder areas more than it does when the dry return is at atmospheric pressure. Small differences in condensation rates in different radiators controls the flow more now than when every radiator is facing the same atmospheric pressure at its discharge. In these conditions the steadily deepening vacuum level in the dry return is being effected separately by all radiators individually. This small percentage change in flow every burn cycle adds up to a surprising amount of balancing that does not occur open vented. And it is completely automatic changing with the conditions.

The(to me) very annoying condition of warmer upstairs/colder downstairs in the bitter cold and colder upstairs/warmer downstairs in very mild weather in my house disappeared with the switch to vacuum operation.

Jamie Hall

I wish I could get my hands on a NOS Hoffman #15... then I'd have the best of both worlds! Since I know from experience that the system Cedric powers can drop into quite a significant vacuum and stay there for hours. It's purely personal whimsy that keeps me from adding any more electrical whizbangs!