Add vacuum system to multifamily building

andersonj56

We have a 19 unit apartment building with a two-pipe steam system. The two steam mains are 180’ and 230’. But, the time from steam production to arriving at the ends of the mains is 14 minutes. The time doesn’t change even when the vents are removed and two 1” holes are left in the condensate drains. The system is fairly tight and will produce 5” hg vacuum between firing cycles. But even that vacuum only reduces the distribution time by a couple minutes.

I’d like to add a mechanical vacuum boost just to get steam to the end of the mains quicker. My question is how do I wire the controls? The boiler has a Tekmar 279 controller. My thought was to take 24vac off the Tekmar to start both the boiler and vacuum pump. The 24vac for the vacuum pump would go through a pump controller then 110vac would go through an aquastat (set to shut off when steam arrived) and finally to the vacuum pump.

Jamie Hall

Your scheme will work. Question is, will it make enough difference to warrant the complexity and maintenance of the vacuum pump? It is critically important to remember with steam distribution that it isn't just the velocity of the air or steam in the pipe that is important. In fact, in most systems if you actually do the numbers you will find that the steam should get to the end much faster than it does, if you assume that it's just a matter of filling a pipe. Problem is, it isn't. The steam cannot go any faster than it can heat the pipe to near steam temperature. Before that, it is condensing at the front between the steam (in a hot pipe) and air (in the cold pipe). So the very first thing to do, especially with long distribution pipes such as you have, is to make sure they are thoroughly insulated. For example, let us suppose that your steam velocity in the mains in on the order of 10 feet per second (Cedric's mains run at 20 feet per second, more or less). Not unreasonable. If you assumed that the steam front moved at that speed -- that the mains could vent at that rate, which they won't -- it would only take 18 seconds for the steam to get to the end of your shorter pipe. Obviously not true. The difference is that time it takes to heat the pipe, and imposing a vacuum won't make that much difference.

So begin with the insulation.

andersonj56

Thanks for the confirmation for wiring and recommendations.

The entire supply system has the original insulation except for in the boiler room. Those lines have fiberglass insulation that was installed with the new boiler.

During mild weather, the boiler fires longer to get steam to the ends of the mains than the actual heating time. Any recommendations for more efficient operation would be greatly appreciated.

ethicalpaul

To me, that is the nature of the shoulder months. It is possible that even with the insulation, since there is so much time between calls for heat, the mains cool to at or near room temperature.

And with mains as long as yours, it's just going to take some time to heat them up again. My main is only maybe 50 feet or so of 2" and 1-1/2" and it takes 8 minutes to get steam to the end from room temperature.

And I'm really doubtful about a vacuum pump helping you in this situation. It's not that the steam needs help getting to the end of the main, it's that the steam isn't there, it's condensing to heat up the main which is what? 80 to 120 degrees? And massive!

Now I'm speaking from serious ignorance here. I don't have a vacuum system, nor a two pipe system but I just can't see how you're going to avoid heating up that main in order to get heat to the radiators.

Curious, how often was it calling for heat when you measured the 14 minutes?

PMJ

It is true there is no avoiding heating up the main to distribute the steam vacuum or not.

What is not appreciated here anywhere near enough is that vacuum operation keeps the mains and rads much warmer between cycles than open vented systems do, so time to steam is less always. Vacuum does this by continuing to produce steam after the burner goes off and continuing to distribute steam from the mains to the rads during this time also. This is as opposed to an open vented system which begins immediately back-filling everything with room temperature air the instant the burner goes off. Also, on each new call for heat steam production begins sooner due to the lower boiling point. The result is a significant reduction in time to steam at the rads every cycle. The more the vacuum the more the reduction. The net delivery of steam to the rads per burn minute is significantly higher.

@andersonj56 , you will hear arguments that all this is marginal improvement and not worth the effort. Don't believe that. I've been doing it for years. The dead men really knew their stuff.
I think you have a really cool project to work on.

ethicalpaul

Honest question, not trying to make a point here @PMJ (in fact I’ve been wanting to do some vacuum experiments but I can’t find modern vents with a low enough cracking pressure)

So the question is: how long does your system hold its vacuum between calls for heat? Like will it hold for an hour or 3?

PMJ

ethicalpaul said:

Honest question, not trying to make a point here @PMJ (in fact I’ve been wanting to do some vacuum experiments but I can’t find modern vents with a low enough cracking pressure)



So the question is: how long does your system hold its vacuum between calls for heat? Like will it hold for an hour or 3?

All the vacuum in my system will be gone 3 hours after the last burn ended. But mild days are a non-issue anyway because no heat is needed! Let's talk about operation when you actually need heat. On average days I will do about 2+ cycles per hour with waits of 25 minutes or so between. Vacuum will still be deepening at the end of the 25 minute wait when the next burn starts.

Do you have 2 pipe? I use one electric solenoid so there is no cracking pressure. But even 1/2psi cracking pressure wouldn't be an issue in a 2 pipe as the system spends so little time above atmospheric anyway.

ethicalpaul

Thanks! Yes I was sure that it didn't matter during the cold seasons, I was just asking after having thought about these months since that's the OPs current concern.

No, I have one-pipe so this is really just wild experimentation I have been entertaining

Jamie Hall

May I just sort of remind folks to remember some basic physics..

Steam systems operate on pressure differentials, not absolute pressure, to move the steam. In most cases, we don't think much about that, since the default is that somewhere there is a vent or vents open to the atmosphere (until steam gets there!) and we think in terms of gauge pressure. But if we apply a vacuum to the system at some point, such as where a vent might be, then we can no longer think in terms of gauge pressure -- we have to think in terms of absolute pressures and pressure differentials. Which wouldn't matter, except in many cases, such as water traps to separate mains and returns, or Hoffman Differential Loops, or even the impact on traps or flow through orifices, it is the absolute pressure differential which is significant, not that gauge pressure.

What operating under a vacuum does do is affect the volume of a pound of steam and the heat content of that pound. The volume will be greater at a given temperature, but the heat content will be less.

EdTheHeaterMan

@Jamie Hall I absolutely agree

PMJ

Jamie Hall said:

May I just sort of remind folks to remember some basic physics..

Steam systems operate on pressure differentials, not absolute pressure, to move the steam. In most cases, we don't think much about that, since the default is that somewhere there is a vent or vents open to the atmosphere (until steam gets there!) and we think in terms of gauge pressure. But if we apply a vacuum to the system at some point, such as where a vent might be, then we can no longer think in terms of gauge pressure -- we have to think in terms of absolute pressures and pressure differentials. Which wouldn't matter, except in many cases, such as water traps to separate mains and returns, or Hoffman Differential Loops, or even the impact on traps or flow through orifices, it is the absolute pressure differential which is significant, not that gauge pressure.

What operating under a vacuum does do is affect the volume of a pound of steam and the heat content of that pound. The volume will be greater at a given temperature, but the heat content will be less.

I agree and I think I have always understood these things. All I watch in my system is the relative pressure difference between header and the dry return. I have a permanent differential gauge at the boiler showing this. I have made the point before that vacuum merely lowers the absolute pressure of the entire system and that relationships inside remain the same.

But speaking about relative pressure there is one big difference about vacuum operation in this regard however that I don't think is well understood. When you cycle in vacuum the lowest pressure in the system is always in the rads, burner on or off. This means that steam flow is always from boiler to rads...always. In open vented systems when the burner goes off the rads go very quickly from being the lowest pressure point in the system to being the highest. Obviously this works directly against the overall project which is as I understand it to deliver steam to the rads. Surprisingly (to me anyway) this rather unfortunate aspect of open vented/intermittent operation is generally regarded as a non-issue. In my experience it is actually a big issue. I have found that there is a whole lot to be gained in both efficiency and comfort by just not stopping the flow of steam into the rads instantly every time the burner goes off.