vacuum steam questions

David Laidlaw

I am intrigued by the possibility of modifying my own residential single-pipe steam system to use vacuum steam. I have a few questions.

I would like to fake out my existing system so that it "thinks" that atmospheric pressure is much lower than it is. To do so, I am imagining presenting a vacuum to the vents and to the pressuretrol.

If the output of radiator vents are at a vacuum, will they operate appropriately for cooler vacuum steam? In other words, will the lower pressure lead to an appropriately lower closing temperature?

Are there Pressuretrols or some equivalent that will maintain a pressure difference relative to the amount of vacuum I am using? If I could put a vacuum chamber around the pressuretrol and keep it at the vacuum level I want, I think that should work, but that seems very clunky.

I fear that I am missing something here. Any thoughts appreciated!

EBEBRATT-Ed

for a one pipe system the only kind of vacuum system is a naturally induced vacuum system. The air vents you need to run that way are no longer made as far as I know.

SeanBeans

Unless you do what @AMservices had mentioned in another post.

Something about running tubing from the air vent locations on the radiators to a central “vacuum main”

Hap_Hazzard

That other post was https://forum.heatinghelp.com/discussion/177777/autumn-saturday-radiator-repipe

Jamie Hall

As you have said, the pressuretrol is a gauge pressure instrument, not an absolute pressure instrument. For that reason, if you want to maintain the same differential pressure on your system while it is under a vacuum, you will indeed need to have the pressuretrol in a sealed enclosure connected to your system vacuum.

And no, the vents will not function properly at the low temperature vapour in a vacuum system. They are, after all, temperature sensitive. But you won't be using vents if you run lines to the radiators to pull the vacuum -- and if you leave the vents on, they will be open when cool and you will be unable to pull a vacuum.

However, may I enquire as to the objective of this exercise?

AMservices

@DavidLaidlaw What problems are you having with your steam system that makes you want to modify it to a vacuum system?

Does it take a long time to heat up?
Are some radiators not heating at all?

Or on the other hand, Are you overheating your space and hope lowering the steam temperature will fix that?

Pumpguy

@DavidLaidlaw , It sounds like you might be thinking converting to a Paul air line system.

The attached file illustrates the concept. To the best of my knowledge, Paul Vent valves for the radiators are still available.

If I understand this system correctly, all air removal is provided through the air lines by a mechanical vacuum pump.

David Laidlaw

Thanks for the responses.

My motivation is saving energy. With a lower steam temperature, I'm guessing that I might save 20-30% on operating costs. Beyond that, I'm an aficionado of steam heat and a tinkerer, so the concept of addressing the "low efficiency" of steam heat is appealing.

My system works well. All the radiators work. Because it is a single zone, some parts of the house are warmer or colder depending on the outside weather. Rainy, windy weather cools the master bedroom more than other parts of the house, for example. Depending on how a vacuum system is implemented and controlled, I might also be able to get some zoning to work, which would be a plus. That could increase comfort and reduce costs a bit more, since I could drop temps in different parts of the house at different times.

@Jaime, it is my understanding that float-type vents operate by having a liquid (alcohol+water) flash to steam to close them. That transition temperature would be different at different pressures, so if the vent has a lower pressure on both its input and output, I think that the flash temperature should also be lower. For some range of pressures, that should mean that the vents will work as expected -- they will close when steam reaches them and open when there is no steam, even if the steam is cooler than at atmospheric pressure. So, I think that if I pull the vacuum *though* the vents, then the system should behave as if the entire world is at a lower pressure, letting air out when the radiators are cooler, blocking steam from leaving, and keeping the overall system at the right pressure. Maybe?

The variable that seems most problematic is system leakage. Under normal (atmospheric pressure) operation, the pressure difference between the system and the outside is usually small, less than 1 PSI. Even if vacuum builds up to higher (negative) pressures, leakage would only help. Leaks with a vacuum system depending on vacuum pressures would likely disrupt operation. Having dealt with a shallow-well pump, I can attest that vacuum leaks can be insidious. My concern is that if the vacuum is pulled through vents, then system leaks could gradually add air back in, creating imbalances that would confuse a pressurtrol that was measuring relative to a reference vacuum level.

I read through the AMservices post. Very interesting and certainly related. No vents though, which seem like a very useful element, since they keep steam within the system.

David Laidlaw

Oops, missed @Pumpguy's post. That system looks related. It does pump vacuum through what look like normal vents. The condensate return looks different, though, with a pump and valves that I don't understand the need for. And I'm not sure how the boiler is controlled to keep the steam pressure appropriate.

Pumpguy

@David Laidlaw, Paul vents, as I understand them, are thermostatic air vents; opening when cool to allow the vacuum pump to remove air, and close when hot and steam is present.

Other than that, the system is the same as any other one pipe system; steam enters and condensate leaves the radiation through the same pipe.

Jamie Hall

On vents. And traps. You are correct that there is a thermostatic element -- sometimes a liquid, sometimes, a bimetal. Unfortunately, they are sealed and won't change their operating temperature to respond to outside pressure. Oops.

Second, if you pull a vacuum in one part of a system, you will pull a vacuum in all of it, unless you have sufficiently large vacuum pumps to create a head loss in the piping -- which is unlikely.

If you can pull a vacuum throughout the system, you will reduce the temperature at which the radiators are operating, and the temperature at which steam is forming in the boiler. However, this will have at best a minor effect on the efficiency of the boiler, which is the main determinant of system efficiency.

One of the most common fallacies regarding steam is that it is inefficient. It isn't. A properly set up and controlled modern steam boiler runs at about 85% efficiency for residential applications, which is the same as a hot water boiler (a condensing boiler -- in condensing mode -- can go higher). You do lose efficiency in overall operation by having to heat the system and fill it with steam at the beginning of a cycle -- and that is where some improvement can be made. However, the amount is small (see some of the work @PMJ has been doing on this).

PMJ

I have found that the best way to think about the efficiency of a steam system is to note the boiler run time vs the net time steam is actually being delivered at the rads. Vacuum systems actually do deliver significantly more steam per unit of boiler run time for two reasons. First there is far less cooling of the rads and supply piping between cycles so time to steam at the rads on each new cycle is much less. Secondly, a very significant amount of steam is both generated and delivered every cycle after the burner goes off. I have attached Hoffman literature which contains a section about one pipe vacuum gravity systems both coal fired and gas/oil fired. I have seen their claims on this dismissed around here as nothing more than sales propaganda. I find this unfortunate because from my own personal experience over many years I think they were right.

The benefits of vacuum operation don't stop with improved efficiency. I have posted about them elsewhere. In sum these improvements are not a small difference. It is a real shame that the technology has largely been abandoned. I have a two pipe system so for me it is easy. But I can say that from what I have learned about vacuum, if I had a one pipe system I would surely looking at finding or making a vacuum vent as @acwagner is now as a first step. Air lines as @AMservices suggests would likely follow.

izhadano

Dear  David Laidlaw,

that's intriguing how similar questions (and solutions!!!) arrived in different heads independently.

I did tinkered with "pure (leak free)" vacuum heating system and learn a lot from transparent plastic lines for supplies and condensate returns. Dan H. kindly agreed to visit my house and described this "crazy" system . I postponed these ideas so far (not gave up yet!)  because of common biases against vacuum heating system and problems with steam traps.  No new vacuum heating system was installed recently, but hopefully the tide will reverse.  "Good ideas get accepted with time like grass making it's path through asphalt..." Anonymous 
But  steam heating system "light retrofit" into vacuum heating can be done with old piping and radiators. Works without problem since 2015 in my 100+ years old system, and couple more systems of 1880s - Ed Infantino was behind these retrofit and described them in details. 
Your idea of "tricking" steam system can be accomplished by connecting each radiator to vacuum pump and adding independent control circuit for vacuum pump. The circuit includes in sequence normally open vacuum relay (to start vacuum pump/open valve on vacuum pump entrance at say below 14"Hg and stop vacuum pump/close valve on vacuum pump entrance at say 20"Hg ) and normally closed temperature relay at vacuum separator entrance (relay opens above 40 C). The settings are indicated as an example, can be optimized for your particular system. 
So, when boiler is cold started, vacuum pump quickly remove air to 20"Hg and stop. Boiler starts producing vapor which quickly and evenly get into radiators, heat them, push the rest of the air into back of radiators and return lines and drop the vacuum to 14"Hg level. Vacuum pump starts again, sucks the rest of air from the system and is stopped when hot vapor heats separator entrance above 40 C. Boiler meanwhile continues to supply vapor to the system till pressure reaches say 2 psi pressuretrol setting, stops  till pressure drops to 1 psi (1 psi differential), starts again and osculates between 1-2 psi till room temperature is matched.
This boiler/vacuum pump control algorithm has advantages as follows:

• boiler starts in vacuum (quicker and even heat distribution through radiators)
  
• in warmer days low temperature vapor in vacuum usually provide enough heat and system does not operate at positive pressure at all
  
• in colder days boiler is employed for longer time and system works under positive pressure - higher vapor temperature and no air leakage into the system
  
• small capacity vacuum pump is employed for short time intervals - 5-7 minutes, total time -1-1.5 hours/day
  
• vacuum  pump is never exposed to high temperature vapor
  
• no water loss 
  
• when boiler stops, additional heat is extracted from boiler by naturally induced vacuum
  
• minimal air/corrosion in a system
  
• no steam traps to fail
  

The results of 7 years study of steam system retrofit into vacuum system confirm 13-30% fuel savings (depends on vacuum level in a system) and improve comfort in a house. 
Hope this info helps.
Igor