Temperature drop along steam pipes 4

[GSWR]

Hello,

I have searched without much luck. I am curious as to what sort of rough numbers you would be looking at in terms of a temperature drop of the pipe surface along a pipe run. Say at one location the pipe surface temperature was measured at roughly 315 deg F, it has steam inside at roughly 100 psig and flowing constantly at maybe 100 ft/s, and the pipe is 1" nominal size Schedule 40 rusty steel pipe. Also this would be inside with a fairly warm ambient temperature of 85 deg F or so.

100 feet down the pipe run, what is a reasonable temperature to assume for the pipe surface? I am guessing that it will only drop by 5 deg F or less per 100 ft, given the warm ambient temperature.

Thanks.

 

[GSWR]

Oh, I forgot to add that it is not possible to go back to the site and take more temperature readings along the pipe run. Unfortunate, because that would be ideal.

 

[IRstuff]

Q=area*htc*deltaT

It'll probably drop about the amount you say, but not because of the warm ambient; it's because the steam is exposed to that section of pipe for only 1 second.

TTFN

FAQ731-376

 

[GSWR]

Isn't that delta T for the difference from the surface to the ambient? So if you have a section of pipe at a certain temperature and the surroundings at a certain temperature, I can see how you get the Q, but how does that get me to a new temperature of the pipe surface further down the pipe run?

 

[Compositepro]

Is the steam saturated or superheated.The temperature of saturated steam inside the pipe is is determined by its pressure. Heat losses cause condensation - not a decrease in temperature. Calculate the pressure drop at 100 fps and that will give you the temperature.

 

[IRstuff]

The Q is then subtracted from the steam flow to determine the change in energy, which would determine change in temperature, at least, to the first order. If condensation, etc., occurs, then the calculations are more complex.

TTFN

FAQ731-376

 

[GSWR]

Thanks for the responses. I am not sure if the steam is saturated or superheated. I was going to guess that it was superheated but looking at steam tables it actually looks like it would probably only be saturated.

 

[GSWR]

So I think I am just going to assume it is saturated. We do not have the temperature of the steam, only the pressure of the steam and the temperature of the surface of the pipe.

So, Compositepro, I can just calculate the pressure drop and then that gives me the temperature drop? I am assuming this is using the ideal gas law, but I thought steam was not an ideal gas? But assuming it is we have delta P = delta T, is that correct?

 

[Compositepro]

The temperature is determined by the vapor pressure of water, which is tabulated in steam tables. Pressure drop is due to pipe friction at 100 fps, which is a fairly high velocity.

 

[TBP]

Vertical lines lose less heat than horizontal lines. Any heat loss warms the nearby air, which then rises. As the warmer air rises, the delta-T for the pipe above falls.

I know I have a tables of heat loss for steam lines someplace, but it's awkward to look for just now. Try Spirax-Sarco's literature and/or website.

 

[speco]

Compositepro has it right, if the steam is saturated, which it probably is. Let's also assume that the pipe wall temp is the same as the steam temperature. If you put a few numbers on it, it works out like this:

At 315 F the steam is saturated at 83.5 psia.

If there is a 5 psi drop in the line, the end pressure is 78.5 and the saturation temperature is 311 F.

If there is a 10 psi drop in the line, the end pressure is 73.5 and the saturation temperature is 306 F.

You can get this from a steam table. You need to calculate the pressure drop through the line to get the real answer.

Regards,

Speco

 

[GSWR]

Thanks everyone.