Modern Alternatives to a Venturi Vacuum?

FlyingScotsman

I'm starting in on a new large project in the next few weeks. I don't have a lot of detail. It's a few high rise residential buildings connecting to a local high pressure municipal steam supply point after making their own steam for 35+ years. From what I've heard the facilities team like to supply 10psi to the buildings, but also run vacuum pumps, one per building. The buildings are 1950s era residential, two pipe with traps; beyond that I have no information, but they swear the system is sized for vacuum.

My initial thinking was that if the system was sized for vacuum, then a 10psi supply pressure seemed high, but I don't know much about how systems were designed in the 1950s.
The pressure drop across a trap, depending on the vacuum, could be 12-13psi. Perhaps the facilities team are using that pressure to do mechanical work, and push condensate around, or air if the vacuum pumps aren't functioning properly.

Given the low-maintenance environment and flood risk, the vacuum pump is a point of failure, running on electrical power, but there's high pressure steam available. I know back in the old days a little high pressure steam run through a venturi was used to generate a vacuum for Paul Systems and the like; is there anything like that on the market now?

I'll keep the thread open and report back as I learn more, but your thoughts are as ever, greatly appreciated.

Steamhead

How did they control these systems? Some, such as the Dunham Vari-Vac, had a modulating valve to admit steam from a higher-pressure source to the system and a vacuum pump to exhaust the air. They could regulate the steam temperature by varying the vacuum level and admitting only a certain amount of steam.

Take a look at the controls on these systems and post pics if you can.

FlyingScotsman

Thanks for the response. I'm confident this was a bare-bones system; it would have been a low-budget 1950s installation. There's not been anything like this on site before, I was just wondering if I can save a future electric pump failure with a modern equivalent to the old venturi systems.

Jamie Hall

Why not just use a venturi vacuum pump? There are any number of them available, in a wide range of capacities and materials... most of them are designed to operate on compressed air, but I'm sure a little search would find one.

Or you could generate vacuum -- with good control -- in the original Watt steam engine way; quenching steam in a closed container with a water spray. Sound a bit Heath-Robinson, but again I imagine something could be contrived with a little ingenuity.

Pumpguy

Is there a vacuum pump on the system now? If yes, what brand and type?

The 2 most common types of vacuum pumps used on low pressure 2 pipe steam heating systems are water venturi multi jet type and positive displacement liquid ring type. Both types have been used for the past 100 years.

Of these 2 types, the liquid ring type is more efficient in terms of ACFM per horsepower. A typical 1 HP liquid ring vacuum pump has an air capacity of 12 - 14 ACFM.

Last year I sold one of these 1 HP vacuum pumps for use on a Paul one pipe system for use in a home in Milwaukee.

Back in the day, steam jets and domestic water eductors were used to produce vacuum, but today their operation would be very costly and impractical.

If there is any kind of condensate pumping equipment now installed (condensate return or boiler feed), it would be a relatively simple matter to install a liquid ring vacuum pump in conjunction with this equipment.

I am really surprised to read that the proposed operating steam pressure is 10 PSI. Typically you should be able to successfully operate with 2-3 PSI on the steam side and 7-10" Hg. vacuum on the return side.

Also curious what happens to the condensate from this system? Is it just pumped or dumped to the sewer? Is anything being done to address the temperature of the dumped condensate?

If there is a condensate cooler, it should be installed before the condensate pump, not after. Allowing the pump to handle cooler condensate will improve the pump's life and performance. Cooler condensate will improve the performance and life of a vacuum pump too. The condensate cooler can be used to preheat domestic hot water.

Feel free to contact me offline to discuss your system and vacuum pump application.

EBEBRATT-Ed

Sounds like you need a steam pressure reducing valve, Spence, or Sarco or Watson McDaniel etc. Lower the pressure to 1-2 psi. Can't see needing a 12-13 psi drop across a trap.

Harry_6

I agree, a vacuum system like this would have been designed to run at a maximum of 2 psi in the coldest weather and usually below atmospheric in moderate weather. Nash vacuum pumps were factory set at around 8-10" of vacuum. There would most likely have been a PRV (pressure regulating/reducing valve) as mentioned in the previous post (I see lots of Boylston on old installations - they are simple and indestructible - and mostly Spence on newer) to get it down into the right pressure range from the HP. In a simple system one would manually adjust the pressure at the regulator, based on need. Fancier ones, like Vari-Vac, could do outdoor reset on their own. As for a simple vacuum pump . . I've wondered that myself. Venturi types usually need higher pressure and waste steam. There were once water-driven pumps, but they trashed the water (cha-ching!). Which kinda leaves us with electric. I'm a fan of the old Nash pumps, but like all things, the new ones aren't a fraction as good as the ones built 80 years ago!

Pumpguy

The Nash vacuum pumps built 80 years ago were large, slow speed with re-greasable bearings and packed stuffing boxes. Most needed separate motors with flexible coupling drive.

The stuffing boxes required periodic maintenance and they would cost a fortune if built today.

Today's Nash or similar liquid ring vacuum pumps, in the size needed for the small systems usually described on this site, would be high speed 3450 RPM with mechanical shaft seal, close coupled to 56J or similar frame motors with shielded permanently lubricated bearings. No periodic maintenance is needed.

Efficiencies in terms of ACFM per horsepower are only very slightly better, although this can vary depending on the actual ACFM requirement.

Typical seal and bearing life is 10 years. Hot condensate or unfavorable chemistry or particulates will shorten the life.

jumper

Don't liquid ring pumps require some sort of water circulation?
I remember some that used once through when water was free. Others recirculated water through an ice bin for maximum vacuum. Add venturi ballast and you could approach high vacuum.

DanHolohan

@FlyingScotsman, what city are you in?

Pumpguy

@jumper, Yes they do. Typically this is recirculated; either from a separate seal water tank, or recirculated condensate.

If a separate seal water tank is used, this requires a city water make up arrangement.

I have also seen some systems that use straight once through city water for their seal.

Each arrangement has advantages and disadvantages. I prefer to use recirculated condensate, although this requires accessories that are a little complicated.

And also, you are correct about temperature affecting the vacuum performance. This temperature issue is equally true for other types of vacuum producers as well.

jumper

Oilless & oil consuming pumps are simpler & more efficient than liquid ring. The OP's project is probably a tight one if it was 10 psi. So probably he doesn't need an efficient vacuum generator. Steam jet ejectors are available and can operate on surprisingly low steam pressure.

Pumpguy

@jumper, Agreed. My concern with dry rotary type vacuum pumps is water and moisture carryover. Applying a vacuum pump to a steam heating system is essentially a wet application.

Wet vacuum pumps, liquid ring type as well as water or steam jet venturi types, can handle wet carryover with no problem.

I have no personal experience with a dry vacuum pump applied to a steam heating application. That said, I am following the work of @Igor to see how the pumps he is using work out long term. Always open to progress and new technology.

jumper

Definitely don't want any liquid at all in any PD pump except for liquid ring. And even then you don't want too much no matter what sales literature says. But it's not new technology to protect pumps with knockout tanks and condensers.

The OP can actually utilize heat from venturi's exhaust in his application so efficiency is moot. Providing he can find somebody who knows what he's doing. In my day a rep for JetVac or S&K would provide design for free.