Warren Webster..vacuum or not

@Onepipe @Pumpguy we'd have to check the pipe sizes against the various capacity charts. In a system designed for pumped vacuum, the return lines would be smaller than for gravity return because the pump would increase the pressure differential between supply and return.

Many Webster systems would use naturally-induced vacuum to allow the old coal boilers to keep making steam as the coal fire burned down. But this would still use gravity return.

If the boiler-feed pump replaced an old Return Trap, this would still have been gravity return unless the boiler pressure got too high, then the Return Trap would kick in.

This is all covered in @DanHolohan 's book "The Lost Art of Steam Heating Revisited", available in the Store on this site.

Good place to start:

https://www.heatinghelp.com/systems-help-center/the-25-steps-in-the-repair-of-a-vacuum-steam-system

If no one has messed with the original valves on the radiators (yeah, right, I know) it's even easier. Free up the valves (patience, and don't be a gorilla) and then try closing a valve — pick a radiator, any radiator — until the radiator get hot 80 to 90 percent across. Then stop. Done. The original valves on almost all vapour systems were actually designed just for that purpose… some of them (Hoffman) actually have two elements inside — one on/off too hot/too cold, which a handle, and an inner element which you aren't supposed to mess with after initial adjustment (need to take the valve partly apart and use a wrench) which is adjusted to the size of the radiator — at the maximum 6 to 8 ounce working pressure differential of the system.

That's what I'd use, then — much less hassle than orifices…

I'd say to go for it without the pump… the worst it can do is not work!

@Onepipe, I believe what @Jamie Hall was trying to say in his earlier post is to not totally close, but throttle the radiator's steam valve to limit the amount of incoming steam so the radiator gets hot across 80 or 90% of its surface.

This has the same effect as an orifice except the flow rate of steam can be changed to more or less, where with an orifice you're stuck with just one flow rate.

The nice thing about this arrangement is the radiator condenses all the steam it receives, so there's no steam left to get into the return lines.

On the flip side, the bad thing is a set throttling valve or orifice can't modulate, so during colder weather that radiator can't put out more heat because now its not getting enough steam.

Regarding your vacuum pumps, yours are not really vacuum pumps as such, but common centrifugal pumps that pump water through an eductor venturi nozzle to produce vacuum using an entrainment principle. Repairs to these pumps are usually quite simple, requiring just a new shaft seal and motor bearings. The impeller could have cavitation damage, and if so, a new impeller would also be needed.

When fitting a new shaft seal, check what elastomer is used. Viton or Ethylene Propylene are more heat resistant than common Buna N.

I thought I read in a post above an implication that vacuum in the return line affects the flow of returning condensate. Thats a common misconception. The condensate flows back to the receiver tank by gravity. The only case where return line vacuum affects the flow of condensate is where there is uphill piping in the return lines and then the vacuum lifts condensate to a higher level return line.

Depends on how much vacuum you pull in the return line. At any reasonable level of vacuum, the effect is not relevant.

Regarding setting throttle valve; it would be nice if one can disable handle so that folks don't continuously mess you up. Regarding those DomesticPump units; ingenious fellows can improve set up by splitting system into separate vacuum receiver and pump receiver. Somebody at ITT may have schematics?

The question of return pipe mystifies me. Pumpguy is correct that condensate moves by gravity. Even a wimpy eductor sucks air plenty.

Its really quite simple to measure the air removal (CFM) capacity of a vacuum pump. Just valve off your receiving tank and then connect an orifice that is open to atmosphere. Then operate your vacuum pump and see what vacuum level is showing on the gauge. For this test to be accurate, it is important that the only air entering the receiving tank is air that is flowing through the orifice.

If you tell me the horsepower of the centrifugal pump that is used to produce the vacuum, I can give you a starting point for orifice size.

The attached file goes into more detail.

The typical air removal capacity of a water eductor type vacuum pump is in the range of 6 to 10 CFM per motor horsepower.

Another type of vacuum pump commonly used on units like yours is a positive displacement liquid ring type vacuum pump. These are more efficient, moving between 12 to as much as 17 CFM per motor horsepower.

Another issue with vacuum pumps is their sizing. In a new system that is air tight and all traps are working as they should, 1/3 of a CFM per thousand square feet EDR is adequate. As the system gets older, the load on the vacuum pump increases due to minute air leaks that develop and traps not holding as they should.

Recognizing that today's vacuum pumps are almost always working on old systems, I recommend going with one (1) CFM per thousand square feet EDR.

It is a very common problem for vacuum pumps to be sized too small for a given system and operating conditions, and when this is the case, the operator is never able to fully realize the benefits vacuum pumps can provide.

Unlike condensate pumps, there is no hard and fast rule on sizing vacuum pumps; its largely a matter of experience and judgement. One thing is for sure; within reason, you can't go too big in sizing vacuum pumps for steam heating applications.

You're fine on capacity in the returns

»you can't go too big in sizing vacuum pumps for steam heating applications.«

Dry vacuum pumps suck more air per horsepower than liquid ring. But you need a "knockout tank" to prevent liquid slugs. Another item to watch.

Those hurled water package units are inefficient on account of low NPSH as well as on account of venturi eductor. Another issue is condensate temperature. Both liquid ring pumps and eductors have more capacity with cooler water.

I wonder if anyone measured steam flow resistances through hermetically air free pipe?

I haven't had any trouble…Nearly all of the two pipe systems we see in Chicago were originally naturally induced vacuuum, so historically that didn't seem to be a problem. How long the system will hold vacuum during a firing cycle obviously depends on its tightness and the length of the steam cycle. Just basic physics would tell us that if the system is quite tight, and the load is constant, then each heating cycle would just reach 0 psi at the end of the cycle. Most of the heating cycle time over a heating season should then operate in a vacuum. You would not get the performance of having a vacuum pump ( unless you are using a steam powered air evacuation unit like on Moline Systems), But starting a system at 8 inches vacuum on a typical cycle should give incredibly good distribution like a vacuum pump. Once the system is primed the vacuum created by condensing steam in the radiators should keep things moving very nicely.

Again, in Chicago, I find extremely few straight two pipe steam systems… nearly everything was originally naturally induced vacuum or had vacuum pumps.