Steam coil condensate load 3

[MikeONeill]

Wow, my first question on here...
I'm trying to calculate the condensate load of a steam coil (steam is taken from the cold reheat line off a boiler) used to warm inlet air to the boiler. Conditions are as follows:

65psig superheated steam @ 610F inlet pressure, heating 176,000 cfm air from 0F to 123F. Steam velocity is roughly 750FPM. friction through the coil is less than .3 inches of water.

I can calculate the condensate load based on saturated steam, but I can't find a formula or factor to apply for superheat. The load based on saturated would be 27,060 lb/hr. I can't imagine a load that high taking into consideration the time needed to subcool the steam, and the heat conduction of upstream steam.

 

[TD2K]

Just look up the enthalphy for superheated steam using steam tables.

65 psig at 610F is about 1336 BTU/lb. Saturated condensate at 65 psig is 282 BTU/lb (saturated 65 psig steam is 1183 BTU/lb).

That amount of superheat might affect your heat transfer coefficient inside the coil. That's pretty hot steam for this type of an application.

 

[MikeONeill]

Yes it is, but it's 'free' (haha). The coils are 40 years old and have worked well up until 4 of the 24 coils froze and burst (yep, trap failures).
Thanks TD2K.

 

[abeltio]

the sensible heat of steam (from superheated to the condensation line) is very low compared with the heat capacity during condensation (latent heat).
due to the extremely low delta p it can be considered a constant press application.
The mollier diagram should show you that the temperature change from superheated to saturated is very low so the sensible heat (the heat that can be used for heating) in that region Q = W (Hsh - Hsat) = W (1336 - 1183) = W*153

The latent heat of vaporization is...
Q = W (Hsat-Hcond)= W*(1183 - 282) = W * 901
For subcooling:
Q = W * c * (Tsat - Tcool) = W * 1 * (312 - 150) = W * 162
(assuming 150 outlet temp to have a good differential with the outlet air)

As you can see the lion's share of the available heat is in the condensation...
except for high efficiency/high volume calculations of heat exchange... most designers take the superheat as a "margin"... one of the reasons being that the superheated steam has a lousy heat transfer coefficient... so to extract the sensible heat from the steam requires a lot of surface ($$$) while the heat transfer of the condensation can have very high values. the trick is in designing the flow area and heat transfer surface in such a way that droplet condensation is achieved (against laminar condensation).

HTH


saludos.
a.

 

[MikeONeill]

Thanks abeltio. I'm going with thermostatic traps for that exact reason (12 degree subcool).

 

[rmw]

If you know the physical characteristics of the coil, such as tube count, metallurgy, tube pitch, etc., and can determine the velocities of the superheated steam inside the tubing, while it is giving up its sensible heat, you can use

http://www.processassociates.com

website to determine the inside and outside "h", and then use the site to give you an overall "U" value for the sensible heat transfer portion of the process, which will give you the heat transfer rate on a btu/hr/ft^2 basis (for those of us who still think in english units).

You should have enough physical data available to determine how much surface area is required to remove the sensible portion of the steam's heat, and how much surface, then would be left over for condensation.

I recently worked out a similar problem for superheated steam heating a fluid flowing inside a tube bank (at very low reynolds numbers) using this site, and by also doing some "modelling" to make it fit the limitations of the site. It was a great help for me, and solved my problem. I hope it works for you.

 

[25362]

Although condensation, as albetio says, provides a higher HTC than desuperheating, it is mainly the resistance on the air side that will determine the overall HTC. Thus the OHTC for steam desuperheating and condensing would be about 0.15 and 0.2 kW/(m[sup]2[/sup]*[sup]o[/sup]C), respectively. Both are indeed quite low values.

 

[quark]

What steam flowrate we are talking here? Have you ever tried to check the steam(/condensate?) temperature at the exit of the coil?

Can you regulate steam flow to extract latent heat? You may have to go for a parallel flow heat exchanger for this.

Regards,

 

[lilliput1]

Use the same size since they have worked well. Usually traps will have a safety factor of 3. Make sure each coil gets a dedicated trap.