latent heat question in steam

Jamie Hall

are our explanations and thoughts that bad?

Adam_13

latent heat technical question

Hi all, I have a latent heat question relating to steam radiator output. Bear with me here, I'm somewhat wordy...

Downstairs

O.K., I understand that latent heat is the heat required for the phase change in water (enthalpy of vaporization or something) (from Dan's books, the pot of water is on the stove with a thermometer. I light the burner, the temperature rises until 212 at which point I haven't touched the fire so the heat's going into making steam, etc.). Thus a new boiler producing good dry steam is going to deliver the maximum amount of latent heat upstairs to the radiators, right? (assuming piping is correct and properly covered, etc, etc)

Upstairs

According to my 1925 Audel's book, a cast iron radiator is supposed to transfer ~1.6 BTU per hour per sqaure foot of surface per degree difference in tempurature for EITHER hot water or steam. Hot water is figured on 150 BTU per sq. ft. (1.6*(165-70)) and steam 250 BTU ~(1.6*(215-70)). Thus, it seems that the output of steam radiators for sizing purposes only takes into consideration the heat given off from the radiator based on the temperature difference alone.

My question is- shouldn't a steam radiator that's being supplied with good dry steam (from a new boiler properly installed by a knowlegeable pro) put out much more heat than 250 BTU per sq. ft.?
In other words, wouldn't you be getting most all that latent heat back at the radiator as well, in addition to the heat from the temp. difference?
If so, is the 250 BTU rating only a safety factor to figure on worst case scenario (wet steam)?

I feel that I am missing something here, any thoughts or comments would be greatly welcomed.

Thank you for your time, -Adam

PS- this came about because I have a multi-family home with three independent (and identical) one-pipe steam systems and apartments. One apartment has a 20 year old replacement boiler piped totally wrong (wet steam) and another has a brand new boiler installed within the past year (by a true steam pro). It seems to me that between the two apartments, for the one with the new boiler the radiators just feel like they're putting out so much more heat than the identical sized ones in the other apartment.

Jamie Hall

You're not really missing anything...

the problem is that you are confusing the characteristics of the heat transfer mechanism from the boiler to the radiator with the heat transfer mechanism from the radiator to the room (or, at the other end, from the flame to the water). The output of the radiator is determined by its surface temperature; how the radiator surface reaches that temperature makes no difference at all. The advantage of steam (oh no, I'm not biased!) is that it carries the energy as latent heat -- and thus carries a lot more energy per pound than hot water does. It also happens to carry it at a usually higher temperature -- but there is no reason from physics why you couldn't have a hot water (pressurized!!!) loop at 215 F; in fact, your car's engine does just exactly that.

That help?

Adam_13

latent heat

Thanks Jamie, so in other words the latent heat is the vehicle by which the heat gets from the flame's surface so to speak to the radiator's surface? Physics is next semester for me (I can't wait!), -Adam

ttekushan_3

One more interesting thing

about steam is that steam is a gas lighter than air--and yet the energy it carries in such a small mass is remarkable. A measly pound of water (.016 cu. feet) and expands nearly 1,700 times when 970 btu's or so are applied. So the steam exits the boiler fast and furious.

But most interesting to me is that the reverse happens at the radiator. Heat transfer is extremely efficient here because condensation only really occurs at the interior surface. As you might imagine, the condensing involves the 1,700 times collapse in volume when the latent heat is released to the iron of the radiator. The vacuum thus created pulls the steam as much as the boiler pushes it.

This mechanism is what drives the self equalizing characteristic of a well balanced steam system. Example: I once rented a storefront in a 1926 vintage building with a two pipe steam heating system, storefronts at grade, two more floors of apartments above. One building thermostat. If we had the front door open, of course the temp would drop. The next cycle would come along and equalize the temperature. Why? Because cool radiators condense more steam than hot ones. Presto! The cooler radiator delivers more BTU's until the room temperatures equalize. All without zone controls.

Steam heat rules!

-Terry

Brad White

Reading this somewhere in Boston

an engineer weeps openly.





Thanks Terry...

Beautifully said.

Brad White

Not at all, Jamie

I LOVED it!!

Sorry you took it any other way... Terry's description was brilliantly simple and eloquent, so I edited it- thank you. You know me, I would never publicly disparage anyone.