Calculation for Temperature of wall insolation and Heat from steam

[Saver2008]

Hi everybody!

I need your comments about it:
I need to find out the heat generated from superheated steam that runs into a 8"Ø pipe. I know all conditions but I´m not sure if with law of cooling I can solve it...is it possible to do it with that law? And the other thing is to calculate the temperature of outside wall of insulation. (T2). But I´m not sure if with the same equation and but I do not have the heat....(Please see attached file)
What do you recommend me?

thanks!!!

 

[ione]

There are superheated steam tables available, just google to find them. For your stated conditions the superheated steam specific enthalpy is approx 3057 kJ/kg.
Furhter to the outer surface temperature you need to know the ambient temperature, the insulation thermal conductivity, pipe wall thickness and pipe thermal conductivity. You can assume pipe inner surface is at superheated steam temperature. You have then a series of thermal resistances. In steady state conditions, heat through each layer is the same. Go for iterative approach.

 

[TD2K]

You are trying to calculate the skin temperature of the pipe?

I don't know what you mean by "law of cooling".

Yes, it's solvable. You can download a free copy of 3E plus, it gives you a skin temperature for insulated pipes.

You have a series of resistances, superheated steam to the pipe wall, across the pipe wall (minor), pipe wall to insulation, across the insulation, insulation to jacket and then jacket to ambient air plus the thermal insulation properties of any mastics used. Basically, you need to come up with a heat transfer coefficient (steam to pipe surface) or thermal conductivity (eg. across the insulation) for each layer. You solve for all the various temperatures until the heat loss across each layer is the same (law of conservation of energy). 3E plus will do this all to you.

The resistance across the pipe wall is insignificant and the heat transfer coefficient from the steam to the pipewall is likely also minor compared to the thermal resistance of the insulation.

The heat loss to atmosphere is a combination of radiation and convection.

 

[Saver2008]

Thanks ione and TD2K!!!
According to the problem, we have CONVECTION CASE dont we? Or is a combination of convection and radiation according to TD2K? I mean, Law of cooling is the general equation for convection (Newton`s law of cooling?) What equation do I need to solve it?
I`going to try with 3E plus.
thanks again
R

 

[IRstuff]

Newton's Law of Cooling is ostensibly for a linear process, which convection more or less is. Nonetheless, a full solution requires radiation as well.

TTFN
faq731-376

 

[Saver2008]

First of all, thank you very much for your answers.

@Ione,

I´m searching a Formula that with X kg/hr of steam, I can find the Q heat that this steam generates. I have found this formula:
Q = MS Hfg where Q is the generated heat, MS is the mass steam and HFG is the enthalpy in evaporation...but I still do not know if I can use this formula.....what do you think?

@TD2K
you are right. I had to calcualate it for pipe wall and isolation wall by conduction formulas and air by radiation - convection formula.

q = 2Pi KP (T1-T2) / 2.3 log (D2/D1)
q = 2Pi KS (T2-T3) / 2.3 log (D3/D2)
q = HA Pi D3 (T3-TA)

Thanks