“Vacuum Boost” for steam heating system

Erin Holohan Haskell

Here's a great paper from the brilliant mind of our friend, Igor Zhadanovsky. Thanks for sharing, Igor!

Henry

There is a large luxury apartment building in downtown Montreal that has a one pipe vacuum system. When there is a call for heat a vacuum pump goes on. Each radiator has a 3/8 pipe to where the air vent should be. There is a bimetal contraption there. When steam reaches the bimetal, it cuts out the vacuum. When most radiators have steam, the vacuum pump cuts out. Steam reaches the radiators very quickly.

DanHolohan

How old is that building, Henry?

Pumpguy

@Henry,
This sounds like a Paul air line system. The vent valves with air lines are called, oddly enough, "Paul valves".

Would you happen to know what type of controller starts and stops the vacuum pump?

What vacuum levels are maintained on the 3/8" air lines?

And finally, what type of vacuum pump is used on this system?

Henry

I will have to check this week by stopping by.

Henry

It has been played with. It has a Dunham-Bush vacuum pump now. I have not been to the building in 10 years.

izhadano

Paul system is a great inspiration for me. Actually, Frank "Steamhead" suggested to look at vacuum systems more closely - I did not expect that it will turn out into such adventure.
In my first setup the air was pushed through bimetal valve - check valve combo on each radiator and later run in naturally created vacuum. With time those valves started to fail and deteriorated system performance, - it's a reason why I started tinkering with vacuum pump.
It will be interesting to have a look at the one pipe vacuum system in Montreal and check for simplification.

AMservices

I can understand how a leaking air check could put a stop to performance in a vacuum system. Maybe there needs to be a more reliable check valves invented and the piping can be charged with a vacuum before starting up from the main air vents lines. Similar to the way refrigeration system piping is done.
When working on refrigeration piping I pull a vacuum from a single location (sometimes through 6-8 ductless coils), after reaching 500 microns I close the line to the vacuum pump (before turning it off) and fill the system with refrigerant.
If the vacuum is is first mechanically induced before the burner fires, then the air vents will only be venting oxygen that separates from the water and if air finds a way in through leaks during operation.
Might be simpler not to run separate vacuum lines, only use the pump for charging the radiators and let the steam maintain the vacuum. Or maybe I'm missing something.
Whatever the best way is, there's no doubt in my mind that I'll be installing Central steam heating systems.

Pumpguy

IME, leaking air check valves is one of the most common problems with vacuum systems; steam heating or other applications.

After trying various soft elastomer coated seats and clappers, along with Nylon and Teflon, including valves specially made for vacuum service, I'm coming to the conclusion that old fashioned metal-to-metal construction is the best choice today, if it's air tight to begin with.

There's no question that something better is needed, but being a mechanical device with moving parts, it's going to wear out eventually.

jumper

Don't need check valves or vacuum return lines in one pipe. Plug the vents and suck anywhere in supply. If you want to get fancy you can include closing king valve,sucking,opening king valve,firing in control sequence.

AMservices

jumper said:

Don't need check valves or vacuum return lines in one pipe. Plug the vents and suck anywhere in supply. If you want to get fancy you can include closing king valve,sucking,opening king valve,firing in control sequence.

So have you seen or operated a steam system installed in this manner? I understand how it works and have to ask, why don't people plug all the holes, put a port to connect a vacuum pump (only for commissioning the boiler), put a vacuum gauge next to a vaporstat pressure troll, fill the boiler 1 time only because where is the water going to go? Run the boiler at 6 ounces or less and never have a problem other then a vacuum break or a fuel shortage?
"If something sounds to good to be true it probably is"
But then again, I think this has a shot.
With what sounds like a few hundred dollars in parts, I think I'm just going to go try it and surprise my next steam customer.

izhadano

Jumper, as it said - the devil is in details
There are some subtleties in seemingly simple vacuum heating system . Would you mind letting me know more detail of your project?

AMservices

I will let you know of any steam work that comes my way.
I'm still waiting to hear from the home owner I last talked to that had Mickey Mouse the shoe maker working on his steam system for 30 years.
I talk to people every where I go trying to find steam systems.

Paul48

@jumper

Trying to think that through, makes my head hurt. How do you prevent the vacuum source from becoming the highest vacuum in the system? I understand the Paul system, as it puts the source where you want it, at each radiator, but generalized vacuum, I don't get. Can you explain the sequence of operation, so I might grasp it?

jumper

izhadano said:

Jumper, as it said - the devil is in details

There are some subtleties in seemingly simple vacuum heating system . Would you mind letting me know more detail of your project?

I think you've worked out the details quite well,Igor. Especially using a separator/accumulator to protect vacuum generator.

The way I understand/remember some steam heated buildings in Toronto fifty years ago is that they were no vents anywhere but a single vacuum generator. With no traps and no vents differences between one pipe and two pipe loses significance. In boiler room was a city water powered eductor. Periodically operator isolated boiler (butterfly valves I think) and opened valves from steam pipes to eductor. Then he turned on city water -in winter Toronto water is cold- and sucked down to over 28" vacuum. Finally he isolated eductor; re-opened boiler valves; energized burner.

I'm not certain.....I think evacuation was done only when boiler water temperature was deemed too high.

The point is that if there are no traps,whole heating system is connected and air can be removed from anywhere.Traditional two pipe vapor systems eventually eliminate air and then work beautifully for decades.Why not one pipe?

ttekushan_3

I've had a lot of problems with check valve longevity. Even where those valves are placed in such a way that they are immersed in a low spot in piping, e.g. at condensate pump outlets.
"Where's the vacuum?" Or "Why's the condensate pump to vented receiver running all the time?" The pumps lose prime in the latter circumstance. [it's a big system with boiler feed for multiple boilers-- no passive return here]

It's getting to where I specify unions at checks so they can be replaced every 2 years. I don't remember this being quite so epidemic in the past.
Maybe it's what @Pumpguy is getting at: the very old check valves were metal to metal seals so would last a long time. I'll seek out the old fashioned kind before they became "new and improved."

AMservices

With 28 inches of mercury steam would still be moving at 100°F.
Probably need a butterfly valve to stop the steam when the house is satisfied.

Paul48

If you can get water to boil when not adding any btus to it......How many btus can you get out of it?

AMservices

Paul48 said:

If you can get water to boil when not adding any btus to it......How many btus can you get out of it?

Not sure of the exact btu output of the radiator with 100° steam, but apparently from this chart I found it has more Latent heat with it.
Please someone explain how the Latent heat goes up with the deeper vacuum.

Paul48

At over 28" of vacuum, the water boils at 80*. I contend that the radiators aren't seeing it until you've added enough btus to heat the pipes anyway.

jumper

Maybe not. Rads will be pretty cool so steam will condense in them slowly. I think control was commonly a White-Rogers timer of some sort. Above 50° burner was on maybe 20% of time. Below 10° 100% or until cut out.

AMservices

If there's 28" of vacuum, does the system build pressure if you bring the temp up to 180°?

AMservices

I don't know. Can you leave the burner on till it gets there? You can change the temperature of steam at atmospheric pressure can't you? Why not under a vacuum

AMservices

I was thinking that the system starts with the piping under a vacuum of 28", then the steam at 110 will equalize the system back to zero. At that point by adding more heat and with enough water in the evaporator you will never build pressure because you started in a vacuum?

izhadano

It seems to me that the vacuum heating system operation is a dynamic process and should be considered accordingly.
Sorry, have to repeat this
Some thoughts about heat distribution in a vacuum system. In regular steam system local vacuum spots are created (by steam condensation) and moved from the boiler to the radiators at a total pressure of 1‐2 psig within a regular steam system.  From one side of this local vacuum spot, steam is supplied by the boiler‐ from the other side, air from the air vent. Thanks to this, the pressure throughout the system is uniform, but in
some places there are vacuum spots.
The concept of the vacuum system is quite different: the vapor
from the boiler is moving along the tubes, condensing and creating local vacuum spots. Unlike in a steam system, no air is sucked in through the air vents, so the radiators are still under a vacuum. When vapor gets into and heats some radiators, the temperature and vapor pressure in these radiators increases,
so less vapor is sucked in. Correspondingly, more vapor is sucked into other colder radiators so the heat distribution is naturally balanced by the system.

jumper

The 28" vacuum is to evacuate air. It is not maintained after the vacuum generator is isolated from the heating system. Then the pressure will depend on boiler temperature. Generally that will be about zero gauge. So system works at about 212° and load is matched to time. Latency may be improved somewhat depending on water volume of boiler. Warm up will be much faster when there's no air to be removed.

Some commercial vacuum systems are supposed to operate at lower pressure to avoid temperature fluctuations. In the seventies boilers were being replaced with multiple smaller boilers. So usually one boiler ran continuously,second boiler came on before morning and before late afternoon,third boiler came on for cold weather. LAOSH says that is wasteful strategy but building owners were please with results.

Pumpguy

@ttekushan , This is off topic to the thread, but I believe the "loosing prime" issue you describe only occurs when the condensate pump is Discharging from a vacuum into a vented i.e. boiler feed receiver, causing the pump to become air bound.

In this scenario, when the condensate pump's check valve starts to leak, the first fluid to back flow through the check valve is the condensate water that's in the pipe between the check valve and the vented receiver.

If the condensate pump doesn't start before the vacuum pulls all the water out of the pipe, then the next fluid the vacuum pulls past the check valve is air. This back flow of air pulls all the prime out of the condensate pump, so when it does start, there is no water in the volute and the impeller is spinning in air, so it doesn't pump any water.

A temporary fix for this problem is to change the check valve, but the problem will return when the check valve starts leaking again.

A permanent solution to this problem is to change the piping so there is always a water seal present.

This can be accomplished in any one of 3 ways:

1) Change the piping so you are filling the boiler feed tank FROM THE BOTTOM.

2) Install an internal submerged water outlet pipe inside the boiler feed tank. The open end should always be below the tank's lowest water level.

3) Revise the boiler feed tank's external inlet piping so it first drops down to the lowest level of the tank, then using a 90 elbow and a tee, pipe back up to the boiler feed tank's normal inlet connection. Now run a horizontal pipe from the remaining tapping on the tee to the tank's drain tapping.

Any one of these 3 arrangements will provide a permanent water seal, so when the check valve starts leaking again, the only fluid that will back flow will be water.

You may notice the condensate pump short cycling and come to the conclusion that the check valve is leaking again, but it now won't become air bound and stop pumping altogether.

AMservices

For my understanding (FYI I'm not an engineer, just a technician trying to learn about the systems I plan on installing)
I can still raise the temperature in the system passed the saturation starting point without first "cooking the block" , but I won't see a zero gauge reading until I get 212°?

Paul48

Could you repeat that....lol

Paul48

I looked and saw 13 new posts, and said to myself ...wow, that's getting a lot of attention.

izhadano

Jumper said:
Don't need check valves or vacuum return lines in one pipe. Plug the vents and suck anywhere in supply. If you want to get fancy you can include closing king valve,sucking,opening king valve,firing in control sequence.
====================================
That simple recipe may work for ideal leak tight system but for existing 50+ year old single pipe system you'll get problems:
- the modified system would not heat up quicker - heavy steel piping have to be preheated before any heat get into radiators regardless if it's run in vacuum or at positive pressure.
- vacuum pump will catch a lot hot condensate from the system unless it's cool down completely.

Actually, I had tested such a lab pilot (3 radiators, copper lines, Propress fittings, some leaks in a boiler) - dropped the idea.

It would be more reasonable to add a small diameter vapor supply line to the top of each radiator and use original piping for condensate return and air evacuation via a vacuum
pump.

Adding vacuum lines to radiator has benefits. By serendipity (if you're lucky you don't have to be smart) I found that additional air can be removed from the system when vapor from boiler is pushing air into radiators and vacuum pump sucks air from air vents. When system cools, 25-26"Hg vacuum is achieved while vacuum pump switch is set to only 14-18"Hg. This can't be done in a closed single pipe vacuum system.

DanHolohan

My take on what Igor has done:

https://heatinghelp.com/systems-help-center/a-new-look-at-vacuum-heating/

Pumpguy

@Hatterasguy Exactly!

Sorry about all the multiple posts. For some reason, my post was "waiting for approval", and still showed in the Leave a Comment box every time I checked in. I now see it got approved multiple times!

Never had that happen before, so not sure how that procedure works. I'll have to look more closely next time.

DanHolohan

@Pumpguy I just cleaned that up. It all got caught in the Spam filter.

AMservices

At 32 degrees you need 180 BTUs to bring the water temperature 212. You need to inject another 970 Btu's to make the jump to 215 degrees with 1 psi pressure.
I would think no longer needing to build pressure with the steam is where his massive savings are coming from.

DanHolohan

That 970 additional Btu gives you steam at 0 psi.

ttekushan_3

Sometimes vacuum steam confusion arises because some vacuum steam systems use the vacuum to merely suck on the return system to actively vent the system and give a healthy boost to the pressure differential that allows flow.

"High pressure flows to low
pressure- always" -DH [some world
famous steam/hydronic guru]

The aforementioned vacuum system's primary [but not only] function addresses this. Of course, vacuum communicates all the way back to radiator valves be they fixed or thermostatically controlled.

But then there's the Other vacuum system, subatmospheric vacuum where the whole system goes negative including the boiler. The idea is to:
1) expand the steam
2) lower the boiling temperature of water
3) increase the latent heat content of that steam per unit of WEIGHT. [it takes up a lot more space so that there are fewer pounds of steam in a given radiator, this less total heat delivered]

This is the magic of Igor's system. It becomes easy to put a little heat into the system without a complex of equipment and active controllers.

The heating medium simultaneously becomes:
Cooler
Lighter
Faster
More heat per pound (not temperature!) but at a greater volume.

It's still critical to get and keep
air out since air will expand under vacuum too. This is why I believe Igor wants such a deep vacuum while the system is cool. And also why he introduced a small, simple vacuum pump to assure this critical feature is always met.

At that critical starting point, the pressure differential is generated solely by both the vast expansion of water to steam at the boiler and the equal and opposite localized vacuum created by the vast contraction of steam back into water. Think of a pushmepullyou.

The power required for air purging is removed and the thermal transfer is at its most efficient.

And, as Igor has pointed out, the room temperature itself sets up larger temperature differential at the radiator or convector that translates directly into a greater pressure differential wherever it's needed. No external controls necessary.

I might add, that I think some of the benefit is brought about by lowering the mass of the piping supply components. This allows for lower condensation-derived pressure differential within the piping and maximizing it at the radiators. The expansion effect at the radiator inlet also increases the steam's latent heat and dryness right at its point of use. We also see this effect in Gerry Gill's Improved Mini Tube steam heating system ( aka SelecTemp without the problematic unit heaters). Remarkably low fuel consumption is seen here too.
Igor's idea excels since the boiler doesn't have to fully commit to full temp and building a head of steam to force the preheating and air purging. A little dab of heat will do ya. But without a plethora of pumps and controllers.

@Pumpguy that's a good idea to feed the vented receiver from a low point. The check valves are well immersed as it is. But we haven't gotten to cutting and welding a threadolet in the bottom of the feed tank since we need a healthy size there owing to the duplex condensate unit's ability to run both pumps at the same time. That and access there stinks. And it still doesn't answer the question of "We can send a man to the moon but we can't make a check valve last more than two years!" Okay. That wasn't a question but you know what I mean.

DanHolohan

Terry, that was a great explanation. Thanks!

izhadano

Terry,
I wish my writing was of such clarity as yours ... Thanks.

Pumpguy

@ttekushan, To have a permanent water seal, you can use my example #3, and no welded threadolet is needed.

I'm sure the tank has a plugged drain tapping. Remove the plug and run a horizontal line out from this drain tapping to the run of one side of an appropriately size. Pipe the branch tapping of this tee up to the normal inlet connection on the vented boiler feed tank. Now pipe the remaining run tapping of this tee to the condensate pump discharge pipe.

Wallah! Permanent water seal, and no more air bound condensate pump when it discharges from a vacuum.

jumper

I don't disagree. Disappointed in Propress. Did you try to seal leaks? Smear a bit of vacuum grease?

My suggestion is mainly to eliminate air,not run in vacuum.Takes less work to move steam in an empty pipe.
Has anyone measured interior temperature in steam radiator? I suspect condensation occurs significantly below 212° because air is present.

izhadano said:

Jumper said:
Don't need check valves or vacuum return lines in one pipe. Plug the vents and suck anywhere in supply. If you want to get fancy you can include closing king valve,sucking,opening king valve,firing in control sequence.
====================================
That simple recipe may work for ideal leak tight system but for existing 50+ year old single pipe system you'll get problems:
- the modified system would not heat up quicker - heavy steel piping have to be preheated before any heat get into radiators regardless if it's run in vacuum or at positive pressure.
- vacuum pump will catch a lot hot condensate from the system unless it's cool down completely.

Actually, I had tested such a lab pilot (3 radiators, copper lines, Propress fittings, some leaks in a boiler) - dropped the idea.

It would be more reasonable to add a small diameter vapor supply line to the top of each radiator and use original piping for condensate return and air evacuation via a vacuum
pump.

Adding vacuum lines to radiator has benefits. By serendipity (if you're lucky you don't have to be smart) I found that additional air can be removed from the system when vapor from boiler is pushing air into radiators and vacuum pump sucks air from air vents. When system cools, 25-26"Hg vacuum is achieved while vacuum pump switch is set to only 14-18"Hg. This can't be done in a closed single pipe vacuum system.