Another discussion about steam boiler sizing

Jamie Hall

How deep a vacuum are we talking about here? Not much, I daresay. And to really find out what is happening, you mustn't rely on a single sensor at a single point. You have to look at the pressure gradients throughout the piping system, and then look at the gas (steam or air) flow in those pipes.

You will have some areas -- a puddle of water, perhaps, or a cold radiator -- where the pressure will be low. Maybe a vacuum. You will have some areas where the pressure is high. Maybe an ounce or two. You will then find that the steam (or air, if the pipes have not been emptied yet) is moving from the high pressure areas to the low pressure areas (surprise!). You will find that the whole thing is very dynamic.

ChrisJ

Jamie Hall said:

How deep a vacuum are we talking about here? Not much, I daresay. And to really find out what is happening, you mustn't rely on a single sensor at a single point. You have to look at the pressure gradients throughout the piping system, and then look at the gas (steam or air) flow in those pipes.

You will have some areas -- a puddle of water, perhaps, or a cold radiator -- where the pressure will be low. Maybe a vacuum. You will have some areas where the pressure is high. Maybe an ounce or two. You will then find that the steam (or air, if the pipes have not been emptied yet) is moving from the high pressure areas to the low pressure areas (surprise!). You will find that the whole thing is very dynamic.

In my case, lower than what's at the boiler.

ethicalpaul

Thanks for the response, @EBEBRATT-Ed, I appreciate it! I too am curious about how much of a vacuum you saw...I would only expect very localized ones as described by Jamie above, not an overall system vacuum--that would require more steam to collapse than was being produced and seems impossible.

But what about when a boiler is steaming at full output and you have a sagging steam main (a very common occurrence) and the steam is collapsing due to condensate in the main and starts hammering. That also is caused by a vacuum.

I'm not sure what you mean above...I have seen you and perhaps others talk about how a vacuum can hold back condensate from returning in a pipe, and I don't follow that either (if that is related to what you are saying in this quote). Flowing water doesn't care about an overall vacuum state in a pipe.

If you are talking about the phenomenon of how an upside down container can hold water due to no air being able to be let in to take its place, that is a different thing.

You did mention a pipe having a sag in it...I am not considering that because all kinds of crazy piping can exist, but when I say "neither a condensate pipe nor a radiator's condensate flow is affected by a vacuum" (something I believe and can show), I'm referring to ones that are properly pitched and piped.

EBEBRATT-Ed

It doesn't have to be much of a vacuum as @ChrisJ mentioned it causes an erratic flow of steam as some steam condenses, the pipes heat and some more steam condenses.

But that is not really the problem. It creates an unseen problem as the water level in the boiler jumps around and drives the feed water pump nuts.

Why is it doing this you might ask?

Consider that the boiler and piping if piped correctly (and they were in my example) so when the vacuum forms out in the system the boiler pressure drops a little When this happens you get the normal steam the boiler can produce and on top of that the lowered pressure in the boiler causes some of the boiler water to flash into steam. So now you have a boiler actually exceeding it's rating for a short time or trying to

This additional steam and the added velocity suck (or try to) the water out of the boiler along with the flash steam causing an unstable water line.

As I said the engineer undersized these boilers. I have no idea what the connected EDR was we just installed what he specified. I suspect the boiler may be large enough to heat the school with probably nothing left for pick up.

It was very strange the way it acted. But as soon as the pressure reading on the electronic transducer went positive at .2 PSI positive everything was fine. The feed pumps worked properly and the boiler water level was stead. It only happened on a cold start.


And here I will start another issue everyone will chew on.

When a boiler is selected we know it will steam at 212 deg. But what return water temperature is the btu output or the firing rate based on??

If you fire the boiler at it's rated input and the condensate is cold you will not get the published rating out of the boiler.

If you fire the boiler at it's rated input and the condensate is warmer than it should be you are actually overfiring the boiler. Btu input from flame + extra BTUs in the condensate return.

Jamie Hall

You do come up with nifty little questions, @EBEBRATT-Ed ! And yes, there will be small effect on apparent firing rate -- or more accurately, perhaps, on pounds of steam produced per minute -- depending on condensate temperature. As you observe, it's due to the extra BTUs required to heat the returning condensate back to the boiling point. But as several of us -- including, if I recall correctly, you -- there is remarkably little heat, by comparison, in the condensate. If we consider the most extreme possible case -- all the condensate coming back at 32 F -- it will take 180 BTU per pound of water to heat that up, and a further 1000 BTU per pound to boil it -- about 18%, if we look strictly at the boiler. However, any heat lost from the condensate to cool it that far must have gone somewhere, so in most cases it isn't really lost. In more normal situations, with the condensate coming back at say 120 or so, we require a good deal less.

ethicalpaul

Thanks Ed! I will excuse myself from further comment on that case since it includes feed pumps, pressure transducer controls, and tanks.

I suspect the behavior of a feed pump pumping cold tank water into a hot boiler might do all kinds of things, but those are beyond the scope of this thread (but interesting!)

EBEBRATT-Ed

@ethicalpaul

Yes, pumping cold feed water into a steaming boiler will kill some steam.

Sylvain

ethicalpaul said:

I suspect the behavior of a feed pump pumping cold tank water into a hot boiler might do all kinds of things, but those are beyond the scope of this thread (but interesting!)

The OP was indeed speaking about a system without a feed pump.

Cold feed water.
This was suspected here:
https://forum.heatinghelp.com/discussion/180306/steam-boiler-seems-to-be-pulling-a-vacuum-causing-it-to-shut-down-on-low-water
Altough there was no final diagnostic.