Steam/water's boiling point??

Charles G.

You're right--it's crazy. By lowering the temp of your "steam", you're reducing the available Btus the steam provides. And if the radiation isn't sized to heat w/reduced temp., the space will never get warm. Now how much are you saving?

Toolfreak_2

steam

Here's a crazy question.

I'm no expert (just a homeowner), but it seems to me that if there was a way to lower the boiling point of water, my boiler wouldn't have to work as hard to make steam, thereby saving me money on my oil bill. Does this make any sense?

If so, is there some type of chemical additive that I could add to my boiler's water to lower the boiling point?

TF

Mike T., Swampeast MO

Actually not so crazy. Vapor systems did just that by reducing the pressure below normal atmosphere thus lowering the boiling point. Unfortunately they don't work this way with modern steam boilers as this requires a modulating heat source. Modern vacuum systems do this as well, but they're typically only used in large buildings due to the extra maintenance.

Additives don't work that way however. For instance, think of alcohol. It has a slightly lower boiling point than water. During distillation the temperature is carefully controlled so that only the alcohol (not the water) boils. Even though the water and alcohol are mixed in solution, they both retain their individual boiling points. Liquids with boiling points lower than water tend to be both volatile and energetic--put them into vapor form and they're explosive!!! Fuel burns, vapor explodes...

Brad White_105

Yes we are not crazy...

Not vapor so much as vacuum systems can achieve fast heating albeit at a lower temperature. I do not know if varying the vacuum to make it a form of outdoor reset has ever been tried.... If Steamhead says yeah or nay, then it would be so....

The vacuum systems I have worked on (often courthouses and post offices built in the 1930's) had issues maintaining normal vacuum range, perhaps 4 to 8 inches of mercury ("Inches Hg.") We were lucky to maintain three inches...

At the "ideal" standard of 5.5 inches, this was a reduction of about 2.7 psig, so the steam would be at 12 lbs. absolute and would have a temperature of about 202 degrees, F. However, each pound at that pressure would have a heat of evaporation of 976.6 BTU's compared to 970.3 at atmospheric pressure and 968 or so at 1 psig. Not much difference we agree but properly run, those systems heated very quickly. Of course the radiators had to be sized for lower temperatures.

EDIT: Useless but fun trivia: Never say "vacuum" to a deaf person. :^O>

ttekushan_3

vacuum heating

The rationale for vacuum steam heating was (is) that the typical steam system was sized on absolute worst case cold outdoor temps. You can look at some of Dan's notes on the turn of the (last) century desire for fresh night air during the winter. Also, many steam systems were built during the coal era and of course, coal often goes out. So fast recovery time was important too.

My point is (and i do have one) is that these large systems are difficult to control during mild temperatures. Overheating and balance problems could crop up under low demand conditions. Enter: Vacuum heating. The vacuum operation lowered the boiling point of the water, increasing the steam volume. This lower volume steam could travel effortlessly far and wide. It eliminated overheating and balance problems. Without these troubles greater economy was obtained. Though fewer pounds of steam would be in a radiator, the latent heat held by each pound is actually somewhat higher at lower pressures, allowing for gentle yet effective heating.

The most sophisticated of these systems were true subatmospheric systems, whereby the vacuum induced upon the system largely controlled room temperature. Systems like this could be operated at levels approaching or exceeding atmospheric under extreme conditions so that -20 pedal to the metal operation was still possible. Really a very civilized set-up.

See the LIBRARY under "Steam Vapor/Vacuum Heating"

and also of interest (to me, anyway):