Increase in steam temp vs edr

aircooled81

Does an increase in temperature effect the edr of a radiator?
Basically, if 1 psig steam at 215*F in a room at 70*F produces 240btu's, then would 2 psig steam (about 219*F) decrease the edr of the same square foot because as the psig increase the btu's decrease?
Topic came up, where hotter radiator would output more btu's, but the saturation chart for water/steam doesnt seem to agree.

Jamie Hall

Eh? No -- the effective BTUh output of a radiator -- or anything else, for that matter -- is purely a function of the temperature of the radiator. Therefore higher psi saturated steam, condensing at a slightly higher temperature, will produce slightly more BTUh than lower, all else being equal.

Which it never is. The actual BTUh of a radiator is also affected -- sometimes very greatly -- by the installation and by the surface condition and finish. That 240 figure is for an average radiator in an average installation with an average finish ya da ya da ya da...

All that said -- that is not an excuse to crank up the pressure! The increase in output is very very small.

Dave in QCA

You're both right. Answering the original post, no, a change in conditions does not change the EDR. But remember, EDR, or rather, Equivalent Direct Radiation, is measured in square feet. So, a changed in conditions, i.e., a change in room temperature or steam pressure does not make the radiator bigger or smaller. But, I think you were eluding to the effective output, which in normal conditions is EDRx240 BTU/Hr. The heating rate DOES vary with a change in conditions, as Jamie stated. Change in room temperature or change in steam pressure/temperature will effect the heating rate per EDR.

ChrisJ

Jamie Hall said:

Eh? No -- the effective BTUh output of a radiator -- or anything else, for that matter -- is purely a function of the temperature of the radiator. Therefore higher psi saturated steam, condensing at a slightly higher temperature, will produce slightly more BTUh than lower, all else being equal.

Which it never is. The actual BTUh of a radiator is also affected -- sometimes very greatly -- by the installation and by the surface condition and finish. That 240 figure is for an average radiator in an average installation with an average finish ya da ya da ya da...

All that said -- that is not an excuse to crank up the pressure! The increase in output is very very small.

Here's the ya da ya da ya da.....

One square foot of EDR is able to liberate 240 BTU per hour when surrounded by 70 °F (21 °C) air and filled with steam of approximately 215 °F (102 °C) temperature and 1 psi of pressure.

But as had already been said, even turning a ceiling fan on low, effects this.

Higher steam pressures will produce more heat, slightly, yes.
My radiators run at, or extremely close to 212F.

Dave in QCA

An increase to 6 psi would increase output by a factor of 1.14 and 15 psi would be 1.32. Conversely, when steam is at 6.5" Hg vacuum output is reduced by factor 0.87 and 10.9" Hg vacuum by a factor of 0.78.

Abracadabra

aircooled81 said:

Does an increase in temperature effect the edr of a radiator?
Basically, if 1 psig steam at 215*F in a room at 70*F produces 240btu's, then would 2 psig steam (about 219*F) decrease the edr of the same square foot because as the psig increase the btu's decrease?
Topic came up, where hotter radiator would output more btu's, but the saturation chart for water/steam doesnt seem to agree.

If you are thinking about trying to get more heat out of an existing radiator, try adding a fan, something like this http://www.amazon.com/gp/product/B00CC2WDFO

It'll give you plenty more heat. Add a disc thermostat as a switch attached to the first section to turn the fan on and off.

aircooled81

Thank you all so very much for your great responces...
but i am not satisfied yet
So, wheres the math? What equation is used to determine the btu/hr change when the pressure is not 1psig. Dave in qca through out a few numbers, is that from a chart, or is there an actual equation?
Just looking at a p/t chart....
Isn't there a drop in latent heat, as the pressure increases? Does that relate to a drop in btu per sq ft if all other factors stay the same? I see there is an increase in sensible heat, but latent heat goes down. Does edr not dervice itself from one or the other, but rather some combination of the two?

So, if the measurement of quantity of heat is from a baseline 215*F surface temp, but of 240btu's, what increases or decrease the btu's?

Thank you all again, this one is just not sinking into my thick skull.
Btw, im not working on a radiator here, im working on my theroy of heat, and how it applies to btu's.

Dave in QCA

My information was from a chart from ASHRAE. It looks at varying steam pressure as well as varying room temperatures. I just pulled a couple samples. If it is of interest, I will scan and post.

Jamie Hall

As I believe I mentioned, the heating capacity of a radiator -- or any other partly radiant partly convective heat source -- is a function only of the temperature of the surface of the emitter and its emissivity. It has nothing to do with the characteristics of the heat transfer medium inside the emitting device -- could be hot water under pressure. Could be steam. Could be pure glycol. Could be oil. Doesn't matter.

Your steam tables give you some interesting information about how much heat is available from a pound of steam condensing at various pressures. They also give you the temperature at which it will condense at that pressure. The latter number is the only one which is relevant to how much heat the radiator will produce. The former number will allow you to determine how many pounds of steam per hour you need to supply that emitter to its capacity -- but not what its capacity is.

aircooled81

Thats ok dave in Qca, im going to do a bit more reading. I'm currently looking for a chart for any radiator that shows the btu increase change based on pressure, do you know of a manufacture that provides that?
Is it fare to say, as the temperature increase, it is only increasing about 1btu per 1*F, because the effective 970btu's available from steam at atmospheric pressure is traveling down, but the sensible heat has increased by a degree which inturn only adds 1 additional btu? That might not make sense because it's still saturated steam, but on a p/t chart, latent goes down, sensible goes up, and total steam goes up only 1?

Jamie Hall

Let me try this again.

There is NO change in BTU output from an emitter, such as a radiator, with any change in pressure of the medium inside the emitter. None. Nada. Nothing.

What there is is a change in the BTU output from an emitter if you change the surface temperature of the emitter.

Now if your medium is saturated steam, the pressure inside the emitter will control the temperature at which the steam is condensing, and therefore the temperature at which the emitter surface is running.

That temperature, however, has absolutely nothing to do with the latent heat content released when the steam condenses.

I fear that you are badly confused between how much heat can be released by an emitter at a particular temperature and how much medium (steam, water, whatever) needs to be supplied to that emitter to supply that heat output. I can think of several places where the confusion might arise, but without knowing a good deal more about how much thermodynamics you know -- or don't know -- I'm not sure where to start.

Jamie Hall

Reading through your most recent post again, I see you are looking for a chart relating the BTU output of an emitter to the pressure of the medium inside the emitter.

Let me be quite clear: there isn't one, because there is no such relationship.

There is a relationship between temperature of the surface of the emitter and the heat emitted, and there are tables for that under ideal conditions (or at least tightly specified conditions). I might point out, further, that the relationship is not simple, since in the real world part of the heat from the emitter is transferred by radiation but most of it by convection (assuming a typical radiator -- get up to higher surface temperatures, say over 900 Fahrenheit, and the pure radiatve component becomes more important). Both, however, are governed by the surface temperature. Not pressure of the heating medium.

aircooled81

Ok, now im seeing this a bit more clearly... I hope.
So what i think your saying is, because steam is saturated, no matter what the pressure (1psi/215*f, 2psi/219*f) one square foot of 'emitter' will not release more btu's because the relationship is only due to heat exchange through the medium, not the content of the heat in the medium.
Regarding thermodynamics, i have no knowledge.
But, i think my confusion was because i was picturing higher pressure saturated steam holding a higher heat content. Why not, it's hotter as far as sensible heat goes, and it appears that more 'heat' has been crammed behind 1sqft. Or, what it gains in sensible heat, it must loose in latent heat.
In reality, Higher pressure just means the boiling point has moved, not changed the btu's it gains or release when it changes state.
So i had assumed that i was giving up more heat because the steam would be 'hotter' at the higher pressure. But that is obviously wrong because the 'heat content' of saturated steam is irrelevant to the EDR relationship, and because it is still saturated, so when it changes state from vapor to liquid it's still only giving off the same btu's.
Hmm, maybe i'll read your post two more times.

Thanks for putting up with my lack of knowledge in thermodynamics, and for your efforts explaining.
i'm just going to wrap my head around this till its crystal clear.

Dave in QCA

Yes, Jamie is correct. I didn't mean to add to the confusion. The answers that I previously posted are based only on the temperature of the steam and subsequently, the temperature of the radiator surface. The temperature of steam varies with pressure. All other characteristics of steam that may vary with the pressure have no effect on the capacity of the radiator. Only temperature.

Jason_13

Let's not forget that the higher the pressure there is an exponential increase in fuel and the steam will move slower.