Vent placement in home 2-pipe steam heat system

The quick summary would be:

Vents:

Balancing with vents would mean the vent speed can delay how long the radiator will take to fill with steam, likely in lower heat loss times of they year to the point the steam never reaches the vent before the thermostat is satisfied so a varying amount of the radiator will get hot depending on cycle length but a sufficiently long cycle will fill the radiator with steam and close the vent so at the end of a long cycle the entire radiator will be heating(depending on the heat loss of the building vs the radiator sizes cycles may never actually be this long in reality or only during recovery from setback.)

in summary in a long cycle the balance of the system is ultimately set by what size radiators were selected.

Orifice plates/throttling valves:

A more or less fixed amount of steam is emitted in to the radiator once the system reaches steady state if the pressure at the boiler is limited with a vaporstat, the orifice plate should be sized so that amount of steam is less than the amount of steam that the radiator can condense so it never reaches the outlet of the radiator. no matter how long the cycle is, only the amount of the radiator set to heat by the orifice plate will heat.

in summary in a long cycle the balance set by the orifice plates continues indefinitely.

The radiators are likely oversized on the coldest day and definitely are oversized on mild days so heating the entire radiator probably isn't desirable.

in reality setting the basic balance with the radiator vents then using the radiator valves to throttle areas that overheat is probably a good strategy to use here.

Short answer. No.

only as much steam gets in as air can get out. if the orifice is infinitely small only an infinitely small amount of steam could get in to that runout before the cycle ended unless the cycle was infinitely long.

Steam will condense any time it is in a situation to lose enthalpy to something else. Next to a cool surface? It will condense to heat the surface. Sp if some steam could get to the infinitely small orifice location, yes some would condense on the pipe walls near the orifice. Exactly enough, in fact, to compensate for the heat loss of the pipe into the surrounding space.

But… this has nothing to do with whether the orifice is blocking steam or not. Exactly the same amount of steam would condense in that location and pipe if there were no orifice at all.

Or let's look at it in a more direct answer to your question as stated, which was "if the orifice were infinitely small (no hole), would the steam just stay hot for the duration of the heating cycle? Or would it condense, at least small amount, in the valve/piping," If the steam were present, and the supply piping were cooler than the steam, heat would transfer from the steam to the supply piping and the steam would condense.

Or take a third whack. Saturated steam is, by definition, water vapour in equilibrium with liquid water at the stated temperature and pressure. If any energy is extracted from the water vapour, that energy will come from condensation of some of the vapour in the form of liquid water.

I suggested it earlier, I will again: look up the definition of enthalpy, and then find a good thermodynamics textbook and study it to understand it, and then you will be on the way to understanding how the physics of heat transfer using combined vapour/liquid phase systems (of which steam heat is one — and so are heat pumps and refrigerators) works.

Hmm… at some risk of appearing contrary, what is it really we are trying to achieve here? It would seem that perhaps the thread title isn't really what's at issue?

Can we — hopefully @pacoit , our OP, restate and perhaps narrow the objective down some?

I'm not going to go all the way back in all this, but I seem to recall a spread sheet or other computer simulation in there somewhere… and I am still a little concerned that in going the calculating direction rather than the empirical direction you may, honestly, be a bit over your head — unless I am misreading your background and experience in thermodynamics, which I may be.

"Third point, a vacuum can be employed to eliminate venting time on heatup cycle. It allows steam to travel everywhere very fast (still limited a bit if cold pipes); thus providing balance;"

This is only partly true. It is convenient to consider that the steam front velocity (what you are looking at as "travel everywhere very fast") can be increased by using a vacuum to "eliminate venting time on a heatup cycle". However, that is not the case. You are still thinking in terms of "limited a bit if cold pipes". It's more than a little bit. In fact purely experimentally — never mind theoretically here — provided adequate venting is in place, additional venting — or applying a vacuum — will make no difference at all. Steam moves (like any well-behaved gas, which it isn't) in response to pressure differences. However, the pressure at the point in a main or runout where the steam is condensing is governed by the vapourization temperature of the steam at that point. And that latter is governed by the temperature of the pipe at that point.

The actual dynamics at and near that point are interesting. If we make the assumption that there is at least enough steam being produced by the boiler, the pressure at the point on the pipe where the temperature has reached the local boiling point — we assume 212 F, but it's actually variable — the pressure at that point — and, by extension, in the air beyond that point — will be atmospheric. If there is enough steam being produced, enough condensation will take place at that location to heat the pipe and the location on the pipe where condensation is taking place will move along. If that steam front is to move, of necessity the air must also move, and thus the pressure at that point will be, of necessity, slightly higher — but only enough to cause the air to move slowly out of the way.

This can be demonstrated on a vapour system with crossovers, which has only one vent located at the boiler (if indeed the dry return is not simply open to the atmosphere at that point, which many old systems were). There is virtually no flow out of that vent until all the radiation and all the pipes are up to the condensation temperature at whatever the system pressure is. I emphasize: virtually NO FLOW. How can this be? Simply because all of the steam being produced by the boiler is being condensed somewhere in the system.

Now if we take another extreme case, consider a sealed system. In such a system the dynamics are a little trickier to visualise, but if we suppose that the initial case is the system is at a high vacuum — most of the non-condensable gas pulled out in some way — as heat is applied to the boiler water will vapourise at that low temperature and warm up some of the piping. Depending on the balance between heat input at the boiler and heat loss in the now warmer parts of the system, the pressure may rise and more of the piping will become warm. At some point — which may or may not, probably not, be around atmospheric pressure — the heat input to the boiler and the heat output from the warmer parts of the system will equalize, and the system will run indefinitely at that pressure.

This assumes that there are no non-condensables trapped in the system. If there are, they must be removed, or the areas where they are trapped will not heat.

Now it is quite true that in an inadequately vented two pipe system adding vent capacity will speed things up — up the point where the boiler heating capacity limits. In a one pipe system with radiator vents, it is also true that slowing certain radiators at the expense of others will reduce the average heat output of those radiators. It should be evident from the above, however, that speeding up already fast radiators will not increase their average heat output. Where this can get confusing is in the case of long, unvented runouts where adding venting on the runout (NOT on the radiator) and allow air — non-condensables — to escape at a lower pressure, and thus allow steam into those runouts more rapidly than others (but at the expense of speed in the others…).

One other thought: it should also be evident from this that if one wants to reduce the time from steam generation at the boiler to steam appearing at the radiator, reduce the amount of heat it takes to warm the pipes and the amount of heat they lose to the environment. Insulate the pipes!

On your last paragraph. Yes and no. The air flow is not exactly zero, of course. Indeed, by the time the system is completely heated — which may take an hour or more in a decently sized system — all the air in it will have been forced out of that one main vent.

In an hour or so. That is such a small rate of flow — a few cfm at most — as to be almost (but, granted, not quite!) negligible (to put a number on it — suppose that we have the equivalent of 400 feet of 3 inch pipe to deal with — that will be about 0.3 cfm to vent, which a single Gorton #1 can handle at 1 ounce pressure differential. Indeed, this is why the "go to" vacuum tight vent for vapour systems was (and is) the Hoffman #76 (if you can afford one…). It would handle a system of that size quite happily at a two ounce steady state differential pressure. Even if we consider only the pipes, and suppose that the condensation/steam front moves at 15 feet per minute (a pretty good clip), that's still only about 0.8 cfm and a single Gorton #2 will handle that at 1 ounce.

It's also important to note that once the power demand from the installed radiation rises — as the radiators fill — to approach the power output of the boiler, the venting rate will fall, and the pressure registered at the boiler is only that which is required to overcome friction loss in the pipes — again, a few ounces.

This is also why true two pipe, and particularly vapour, systems, which use crossovers, can and do function brilliantly with remarkably little venting (by conventional thinking) and startlingly low pressures.

In principle, what will be observed as steam rises in the boiler is an initial rise to perhaps one or two ounces; at first this will indeed be the resistance of the main vent, but as time progresses the demand on the main vent will drop, and thus its pressure drop, while the distance that steam must travel will increase — and hence a pressure rise due to friction. In practice this results in that initial small pressure rise — and then a very constant pressure for the remainder of the run.

If the boiler is oversized, then when all the radiators are full the pressure will start to rise again — which is when the boiler should cycle off long enough for the system to catch up, but that's another story.

I'm not at all sure that a system starting under a deep vacuum will result in a shorter travel time to the radiators. Indeed, it might be longer — since the system will start at a much lower temperature, and thus take longer for the pipes to warm up. I'll have to work on that one.

I might add, however, that supposing that one can indeed maintain a deep vacuum at will, then the power output of the radiators can be modulated as required by allowing the pressure to change which, in turn, can be done by adjusting — modulating — the power output of the boiler. This is not novel technology, however: a coal fired boiler, the power of which can be modulated by the dampers, coupled with a vapour system with a vacuum vent can accomplish just this trick — and did. Even more remarkable was that the modulation was accomplished by a thermostat, but all with no electricity involved! The efficiency was horrendous, but it worked. A century ago…

Slightly, but the specific heat involved in a transition of a delta t of say 110 f degrees vs 130 f degrees isn't much. The main reason vacuum systems were used was to increase the time the boiler would steam as the coal fire died out or some mechanical vacuum systems would use the vacuum to lift condensate from emitters below the water line.

Yes to the first paragraph. None — so far as I know the Emprise State Building is a pretty plain vanilla two pipe system. Not even vapour…

Didn't realiuse that.

However… may I humbly point out that steam systems don't know anything about the gauge pressure? Or vacuum? All they know is the pressure differential. So, to take the above comments on the Empire State Building as an example, lets' suppose that they have a steam feed pressure of 2 psi GAUGE and are operating at 10 inches vacuum. Now 10 inches vacuum is almost exactly - 5 psi GAIGE so that the building is operating on a pressure differential of 7 psi. It will in fact — except for temperature of the radiation — operate in exactly the same way with a boiler pressure of 7 psi GAUGE and open vents or, for that matter if you could get there, - 5 psi GAUGE at the boiler (10 inches of vacuum at the boiler) and 20 inches of vacuum on the returns…

Bottom line. If you really want to understand what is going on in a steam system, always work in absolute pressure and differential pressure. Gauge pressure and inches vacuum with trip you every time.

I think @ethicalpaul went on this quest a couple years ago, but you're going to have trouble finding a valve with low enough cracking pressure.

plus you need to save up to deal with that asbestos first.

Natural Vacuum with a true 2 pipe system, read his postings and/or talk with @PMJ he is doing it and would not go back.

You may find this interesting too.

Coal boilers also had about 10x the mass of iron and water compared to a modern boiler.

To be clear, @PMJ , if I had the money I would replace the Hoffman #75 and Gorton #2 which Cedric has with a three Hoffman #76s (to get the same venting capacity).

I don't.

So I won't.