Why so high? ( 2 pipe Steam/vacuum system)

LFsuper

Hi All,

Just wondering what the simple logic is behind the modern system of large steam boilers set to 5-7lbs (but regulated by large electric valve on the main controlled by outside set point) circling back to vacuum/condensate tanks. I do get the principle but want to be able explain the science simply. I was stumped by "Why not manually open valve 100% and just operate at 3lbs. New hi-rise building. System designed by much smarter guys than myself...I hope

2- 300HP Cleaver brooks (set to Lead-Lag) operating @ 6lbs- cut off 7lbs
Steam header nicely done. 30" up into drop header .approx 10" line.
20Hg vacuum. ( Hope that's a good thing- on a very cold day, may drop go 10Hg)
Small condensate tank ( over flows on warmer days when not many people using heat or home)
water comes back 125-150 degrees.

Thanks

Steamhead

Is this a Dunham Vari-Vac system?

LFsuper

@Steamhead yes it is. I have worked with these before but never grasped the high pressure settings.thanks

Steamhead

Basically, it uses that motorized valve to let in just enough steam to heat the building. The vacuum controller adjusts the level of vacuum, and thus the steam temperature, based on the outdoor temperature to meet whatever the building's heat load is at that moment. The advantage of doing it this way is there's much less loss from the steam piping than if you were running the steam at pressure throughout the system.

If there's no other use for steam in that building, you could probably turn the boiler pressure down.

LFsuper

Thanks. It's strictly for heating but a very large bundling. I may start dialing it back one pound at a time and wait for the complaints. Thanks for your help. Always been baffled about engineers setting these systems to 7lbs

Steamhead

If you get any complaints, look for bad traps or other issues. You can get OEM replacement elements for these Dunham traps from MEPCO.

vaporvac

I don't know the exact system and I'm sure you probably know @LFsuper , but the Empire Stat building runs its pressure at about 2.5lbs. Not at all familiar with your system, but steam rules even at low pressure for very tall buildings.

LFsuper

Hi @vaporvac from the building opening 2 years ago engineer/start up set to 7lbs but that seems so wasteful to me. It is a very wide building. (Half a city block) but only 31 stories high. Boilers are really good cleaver Brooks and hit 7lbs in no time.

Steamhead

Seems it would make more sense to at least partially modulate the firing rates rather than maintain that much pressure and control the steam with a motorized valve....................

SWEI

I'd love to design the controls for that...

Paul_11

Another reason i understand for why the dunham-varivac system is set up this way is the idea that the vacuum stretches out the steam. Since it is moving in a vacuum it remains steam longer because in a vacuum you will have steam at less than 212 degrees. When you have steam at 180 degrees or less you have less condensate and a very efficient system.
I have never worked on a new one, only old ones and I leave them they way they are.
I work on one 27 story vacuum systems (not dunham) where I operate at 2 pounds or less.

LFsuper

Mmmm very interesting. Thanks. Well the system runs very well. I would just love the idea of dialing back to 2lbs and see where I stand. As a bundling manager I come in long after the design process. I reached out to the engineer to ask why 7lbs and was told it needs 7lbs. My system is boiler-header- supply system- returns to vacuum tank- sent into condensate tank- back to boiler.

JUGHNE

Does the steam have to lift the condensate anywhere at all?

It needs 7 PSI "because I said so" and don't question my knowledge......so there!

LFsuper

Not that I know of, it's pretty straight forward. There is a separate header that delivers steam to a first floor retail space for perimeter heating. Separate design engineer said he needed minimum 5lbs for those. I'm hoping that there is no way in this world that this system is set so high just to supply a ground floor retail space with perimeter heating

LFsuper

Just pulled this from drawings of it helps

Dave in QCA

I cannot answer as to what pressure is required for the perimeter heat on the lower floor of the building in order for it to operate properly. I do know that control valves used on steam systems are calculated based on pressure. Sometimes, pressure can be lowered slightly minimally without effect. And of course, too much drop in pressure can disable the control valves ability to deliver the maximum require amount of steam needed during maximum design conditions. I am assuming that there are control valves on the perimeter heating, this may or may not be accurate. Of course, the steam supply valve to the VariVac system is through a control valve and its sizing may have been based on 7 or 10 PSI. Running too low of a pressure may affect the proper operation of the system.

The operation of the VariVac system is similar to a hot water reset. The VariVac system looks at the outdoor temperature and uses this to set a steam temperature. The heat disseminators on the system may be controlled in some manner, but generally, the building is controlled only by the temperature of the steam. Contrasted to a Heat Timer type of setup, where the cycle time is based on out door temperature, and thus very dependent on even distribution of steam, especially on moderate weather, the Varivac system has excellent and continuous circulation of steam, with the steam temperature varied through the coordinated control of the depth of vacuum produced by the vacuum pumps and the amount of steam let into the system by the control valve. The losses from the piping system are greatly reduced because the temperature of the piping varies with the demand for heat whereas in a conventional steam system, the piping rises to 212 and cools little during intermittent cycling. Steam piping losses into the building shell are a major component affecting overheating of buildings and/or temperature control difficulties.

I have noticed that G.S. Dunham has new products and technology available or coming soon for VariVac systems. See: http://www.gsdunham.com/vari.html

LFsuper

Thanks a lot @Dave in QCA very helpful. I'm definitely not here to criticize or say the engineer was wrong. More hoping to get an insight on his thinking process. This is very helpful, thanks.

JUGHNE

Good points Dave, however the engineer could have explained to LF a little better, (or than again.....)

I have a Hospital (1977) with the 7 PSI operation and no one is sure why. It does lift condensate and also has pneumatic controls. This setting has been passed down thru numerous maintaince personsal for 40 years.

Dave in QCA

Boiler pressure is a very complex subject. But, it cannot simply be reduced to "Pressure is bad."
Everything that I understood from my prior experience with pressure steam went out the window when I was confronted with a residential vapor steam system. I had to learn a whole new set of rules. Residential steam systems are designed around the premise that the maximum pressure is 2 psi. However, that amount of pressure is almost never required and seldom occurs in systems where the boiler is properly matched to the load, which means that the system must be able to function properly with zero pressure. Huh? (scratches head)

We all know that it is so important to keep the pressure down. Dan has so many examples of systems not working. He gets called for a consult, pulls out his screwdriver and turns the pressure down, and everything starts to work just fine. And so, it is so important to keep steam systems operating within the parameters in they were designed, which usually, but not always means turning the pressure down.

Energy is neither created nor destroyed - a basic law of physics.
But often times we get a little confused because we can't actually see what is going on inside of those pipes and boilers, although we all paint little pictures in our heads of what we think is happening . We tend to erroneously picture a boiler running under pressure as if it were an air compressor and a pressure tank. The higher the pressure builds in the tank, the more energy is required to pump the air into the tank. If we can get the air to move down a 100 ft long hose at a half a pound of pressure, why would we waste all that energy pumping it up to 60 psi? We visualize that energy used in building boiler pressure is lost or wasted, or perhaps destroyed. But... while mental pictures help us to understand, they sometimes cause us to misunderstand.

Boilers that run under significant pressure are most typically of much higher water content than a residential boiler, so lets think about a small scotch marine boiler with 200-300 gallons of water. (it makes it easier to visualize.) As the boiler is running and building pressure, something else happens at the same time, the temperature goes up---both the temperature of the steam and that of the water. At 10 PSI, the temperature will be about 240 F. If a boiler is set to cutout at 10 psi on back in at 5 PSI, the burner will shut off when the pressure reaches 10 PSI. If there is demand for steam somewhere in the connected system, steam will be flowing out of the boiler and as it does so, the water in the boiler will continue to boil. As this happens, the temperature of the water in the boiler falls in relationship to the amount of steam that has been produced in the boiler and used somewhere in the system. This boiling, and dropping of both pressure and temperature will continue until the pressure falls to 5 PSI, at which point the burner will start again. So, the amount of steam that comes out of a boiler that is under pressure is far more than an equivalent amount of air in a pressure tank. In fact, ALL of the energy that went into the boiler water will come back out in the form of steam. Its that "change of state" magic of steam.

Now, the picture I painted above assumes a perfect world, which never happens. The inefficiencies come from standby losses and boiler and piping losses, which equal wasted heat. The higher the pressure, the higher the temperature, and thus the greater the losses, which are mainly wasted heat to that HOT boiler room.

When comparing a VariVac system to a conventional steam system in which the building temperature is controlled by intermittent firing, the added efficiencies of the VariVac system more than offset any inefficiencies in the boiler room. It gives a much more even and controllable means of getting the right amount of heat to all parts of a building. At the same time, the moderate temperature of the piping reduces losses to areas that do not need to be heated and are overheated by intermittent steam systems.

My descriptions above are dependent on well designed boilers and piping that are properly controlled and insulated, also dependent on good engineering. I have seen many examples of both good and bad engineering, as well as outright engineering errors. They can be nightmares to the people charged with operating a building.

Dave in QCA

JUGHNE said:

Good points Dave, however the engineer could have explained to LF a little better, (or than again.....)

I have a Hospital (1977) with the 7 PSI operation and no one is sure why. It does lift condensate and also has pneumatic controls. This setting has been passed down thru numerous maintaince personsal for 40 years.

Thanks JUGHNE! I agree with you completely, the engineer could have provided a much better answer.

I am closely acquainted with quite a few engineers! What I have come to learn is that they usually accurately answer the question that is asked, but being people of few words, they don't usually offer more information unless it is requested. Often times the answers are not helpful because the question was unclear (to the engineer) although it would have been perfectly clear to a person of normal mental processes. Depending on the engineers social skills, a broadly worded question, which can make the person asking feel dumb, is the best way to go.

Example,
"Please explain to me why the pressure is set for 7 PSI and why would it not be better and more efficient at a lower pressure?"
If you're lucky, you might get a good explanation.

However, if you ask, "wouldn't it be better to turn the pressure down to 2 PSI?"
You'll likely get a one word answer, "No."

We gotta love our engineers! They are the ones that figure out hard stuff the first time. Then the rest of us just copy it.

LFsuper

So The engineer finally got back to me and told me to contact installer. Installer told me to contact engineer... I reached out to the good folk at G.S Dunham and they confirmed their system works optimally between 5-7lbs. It appears in this case that the Vari-Vac system is calling the shots and just needs the boiler to deliver it's load to that point.

Dave in QCA

There you have it! excellent detective work. When I was in charge operations at a hospital complex, I HAD to know "why" too.

LFsuper

Thanks @Dave in QCA As much as most may not agree, In some cases there are exceptions to the 2lb optimal setting I suppose.

SWEI

I suspect with a thorough review of the design you could probably drop the pressure -- perhaps even do so dynamically based on OAT.