How to calculate rate of steam condensation on steel? 3

[sshakkour]

The steam is to be used as a blanket over the top of a large hot water storage tank. It will be in contact with internal wall and roof of the storage tank. Thanks

 

[zekeman]

Need the temperature of the water, dimensions of tank, the method of administering the steam (e.g. in an enclosure?), the temperature of the steam, and if it is in an enclosure, the dimensions of the enclosure.
Looks like a not so simple heat transfer problem that is easily solvable using conservative assumptions.

 

[waross]

The presence, thickness and quality of insulation may be important.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[chicopee]

You need to calculate the inside temperature of the steel tank (wall and roof) in the steam space to determine if condensation will occur first; the figure out the heat transfer rate(btu/hr) thru wall and roof. From steam chart determine enthalpy change(btu/lbm) between steam temp and wall/roof temps. Divide (btu/hr) by (btu/lbm) to determine
condensation rate in (lbm/hr).

 

[sshakkour]

Thank you all for your valuable input.
Zekeman, here are the details, the cylindrical steel tank is 9.3m dia. and 18.65 m high with a knuckle roof. Insulation is double layer mineral wool, 5" thick with 26 gauge metal cladding on the outside. Min water temperature is 55°C. Minimum ambient temperature is -25°C. Source of steam is a small electric steam generator (low pressure) that is solely used for this purpose(i.e. no condensate back to the boiler), and I need to size it, but let's assume the steam temperature is 120°C for now. Water level is at 1.2 m from the top of the cylinder.
Chicopee, thanks for the reply (which now I'm thinking should have been obvious to me!). I think my difficulty now becomes determining the btu/hr heat tranfer rate.

 

[sshakkour]

Sorry I just realized that I may have made a mistake by assuming that the steam inside the tank is superheated. It most probably should be at saturated state since condensing is occuring, right? The blanket pressure is 2" w.c.

 

[zekeman]

The problem boils down to heat transferred to the outside -25 deg C as the outside load and the heat transferred to the tank as the inside load, both transferred from the steam.
However, if your intent is to maintain tank water at 55 deg you would need to control the steam flow; otherwise the tank water would eventually reach the steam temperature.
Now, it seems that effectively, you have a jacketed condition where the steam fills the void between the tank and the steam condenses on the inside of the outside wall and the outside of the tank tank.
If this true, then all that is needed is the cross section of the outside walls and the materials and their overall areas. You have already given the tank dimensions, so that the information still needed are the dimensions of the housing and all their cross sections.
I then may be able to estimate of the heat transfer coefficients to the respective surfaces under the condensing conditions .
Correct or comment on my observations, especially if the intent of the steam jacket is to prevent freezing or to control tank temperature.

 

[waross]

As a quick check on your calculations, the condensation rate will be equal to the evaporation rate of the boiler.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sshakkour]

I'm sorry if I haven't stated my problem correctly. The steam is not the only source of heat into the tank. The tank stores hot water generated by boilers at 95°C with a return temperature of 55°C. So in effect water is constantly moving in and out of the tank (at the same rate) and that's where the major source of heat to the tank is. The purspose of the steam blanket is to have no air inside the tank. So the steam space is above the water level inside the tank. I understand that there will be heat (and mass) transfer (condensation) between the steam and the water at the contact point. Not sure how to calculate that even. Thanks again!

 

[waross]

I suspect that the largest mass of condensation will be into the 55 deg to 95 deg water.
Given the large ratio between the specific heat of water and the latent heat of vaporization and the efficiency of steam heat, you may end up by heating the contents of the entire tank to 100 deg before you can maintain a positive pressure. How about filling the tank completely and using a smaller expansion tank with an inert gas blanket.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zekeman]

The light cmae on. So you are putting steam in the tank above the water level.
Well, the heat flow out of the tank above the water is relatively easy to estimate. Since the condensing heat conductance to the inside wall is very much larger than the
the conductance thru the insulation and thru the outside film.
An estimate is
1 Btu/hr/ft^2/deg F for the outside film and roughly the same 1 for your insulation ( you could get a better number for this).
So this combination gives an estimated overall U of
1/2 BTU/hr/ft^2/deg F
Multiplying this by the area not wetted by water yields the outside heat transferred.
The heat transferred to the water at 55 degrees is more difficult to obtain but so far the literature suggests that it is smaller than the outside but I haven't confirmed that.
If you make that assumption all you do is take the heat transferred, namely
Q=U*A* delta T=0.5*A*(55-(-2%)*1.8
where the 1.8 converts C to F degrees
You can supply the "A", unwetted area
The steam usage is then
Q/hfg
hfg= heat of vaporization.
I will pursue the second mode of heat transfer if possible.
I have one final question. How do you maintain the steam space, since if left alone it looks like it would vanish.

 

[sshakkour]

"I have one final question. How do you maintain the steam space, since if left alone it looks like it would vanish."

By constantly operating the electric steam generator for this purpose. I'd control its output through pressure sensing. Mind you, it's very small pressure in the tank that is needed. I'll have a Pressure / vaccuum relief valve at the top.

My other option is to use N2 as waross suggested, but in the same space (not in a separate expansion tank). So far N2 is looking more feasible.

Thank you all for your input.