Subatmospheric steam systems

tkrasniqi

I recently started working in an 18 story commercial building. The building has a 2-pipe vacuum pump system. I dropped the operating pressure on the boiler to 1/2 psig from 3 psig. The system seems to be fine. I am wondering if I can drop the pressure even more, whether I should install a vaporstat and compound pressure gauge. Is sub atmospheric operation possible? I don't know much about sub atmospheric systems. If anyone has some literature they can share or a book they can recommend it would be much appreciated.

nicholas bonham-carter

Do a search here for vacuum, and you will find a number of people who are sub atmospheric devotees, with great success!
The advantages of such a system could be the modulation of the steam temperature in the shoulder seasons, as water boils at lower temperatures in a vacuum. Additionally, the air removal is no longer an issue, as the system starts to heat.
Definitely get a 0-3 psi gauge from valworxs.com, and see how low you can drop the pressure, even if you do not immediately go vacuum. A boiler that size probably has a staged burner, which could be controlled with a vaporstat, to go to low fire as the pressure rises to maybe 6 ounces.
Some pictures of the boiler, main air vent, and radiators would help in the identification of the system.--NBC

vaporvac

Here is a bit of info on pumped systems. The library also has some good reading under two-pipe natural vacuum such as I have. You have an advantage because the air is removed almost immediately. After that, you should be able to continue in a vacuum.
https://heatinghelp.com/systems-help-center/category/steam#steam-vacuum-systems

nicholas bonham-carter

An additional benefit of these systems may be the subtraction of the pickup factor, using the heatloss for sizing, resulting in a smaller, less expensive boiler, or a lower firing rate.
I can imagine a pair of boilers, the lead of which will fire continuously to maintain enough steam for vacuum operation, with the lag ready to fire as the temperature drops during the coldest days of the winter.
These could be controlled by a combination of outdoor reset, and pressure. When the temperature drops, the lead boiler fires, and if there is not enough heat in X minutes, the lag will fire until the pressure goes to a couple of ounces, or when the control is satisfied.
Since the vacuum pump is taking care of the air removal, the traps could be replaced by orifices, or metering valves (handles removed)--NBC

vaporvac

Here was another long and fascinating discussion of vapor vacuum from a while back: http://forum.heatinghelp.com/discussion/139813/another-look-at-vapor-vacuum/p1

tkrasniqi

Very interesting feedback. Thanks all!

nicholas bonham-carter

Keep us informed as you reduce their fuel consumption 15-30%--NBC