Engineering Question

[Deleted User]

Would a steam coil and hot water coil be the same size if 212 degree water were used in the hot water coil? I would think the steam coil would be smaller as the latent heat (960 Btu's) would be a factor. Our engineer tells me that is not the case. He says 212 degrees is 212 degrees and that is all that counts. He also says that a steam coil with a trap that has failed open will put out the same amount of heat as one with a properly operating trap.

What is more confusing is why on a 90 plus condensing boiler do you have to measure the output condesate to get a accurate measument of the heat output in order to account fot the 960 btu extra output that cannot be measured with a stack temp reading.

EBEBRATT-Ed

This sounds like a good Brad White question and maybe he will reply after finishing his egg nog.

I will take a guess. Steam at 212 will give off about 970 btu/lb of steam (latent heat) plus tep difference between condensate and steam temp. For example a coil that uses 100 lb of steam with 212 steam and 180 condensate the btu out put would be 100 X 970 plus 212-180=32 or 1002 btu/lb of steam. That why 1000 btu/lb rule of thumb usually works.
1002 X 100=100,200 btu/hour.

The equal in hot water would be 100.200 btuh devided by 10,000 or 10.02 gpm at a 20 deg. td/gpm.

I guess the real answer would have to come from a coil manufacturer. My guess is the steam coil could have a smaller physical size.

ED

Brad White_163

Ed is Right

And Eggnog has nothing to do with it

It is all about temperature, the approach or initial temperature of your hottest incoming fluid and your coldest entering air at the coil.

In consideration of steam, the latent heat (heat of evaporation) is what is considered for quantity of steam (BTUH required divided by the latent quality of the steam to give you how many lbs. per hour.

What is a little misleading is that, once steam has condensed there is obviously still some capacity left because the condensate is still hotter, one assumes, than the air you are trying to heat. The reason the latent capacity of steam is used is, that is the "useful" heat available while the steam is in it's gasseous state. After that, it is assumed to have passed out through the trap. When Ed added that little bit of sub-cooling (down to 180F or so), that is what he is refering to. It is really a bonus, it does occur while waiting for an F&T trap float to release, but is not counted in the coil net capacity.

When a trap fails, your engineer is correct, the heat output per hour is the same so long as the temperatures remain the same. It is just uncontrolled.

The condensing boiler is not really comparable for the condensation you are measuring is on the fuel side not the fluid side; it is a combustion value and represents the latent value in the fuel, given that the sensible portion of flue gas heat (that which is above the flue gas dewpoint) has already been absorbed.

Does that make sense?

Cheers and Eggnog,

Merry Christmas!

Brad