Steam Tracing Heat Transfer

[sgulick]

I have been requested to estimate how many feet of steam tracing will be required to maintain the vessel walls (to avoid the process vapor condensing on the sides) at 110F with 140psig steam at saturation temperature (353F). The vessel is a vertical cylinder with 1.5" mineral wool insulation.

I have already calculated the heat output of the vessel through the insulation on a worst case scenario day (-20F with a 25 mph wind). My question is how to acurately calculate the heat output from the steam tracing per unit lenth ( 1 foot) and how much of that heat can be credited towards keeping the vessel at 110F?

 

[racookpe1978]

Start with your known and assumed steam conditions: 140 psig at 353F, right?
Fine, get that energy (enthalpy/entropy) from the steam tables.

Now, you know the desired minimum conditions for the vertical hollow tube (110 F with 1.5 inch insulation 9which isn't enough!) against heat losses into -20 F at 25 mph.) (per foot, eh?)

figure out your heat losses (per foot of length for this problem).
Figure out your potential maximum heat "loss" from the steam conditions_in - conditions_out. Which are NOT stated.

Subtract. Figure out how much steam mass flow will yield the missing energy.

 

[TD2K]

I'm not sure about how accurate any methods are to calculate/estimate the heat output from steam tracers that you are likely to find. For something like this, most design tools you'll find on the Internet are unlikely to be designed to accurately model the heat transfer in the wall of the vessel to ensure that all sections are kept above the dewpoint temperature while minimizing the margin above dewpoint you have to maintain much of the vessel at.

These folks have put on a couple of presentations at our office.

http://www.csiheat.com/

If you want to avoid vapor condensing, you need a good distribution of heat to keep the wall above dewpoint temperature. Now, you could simply wrap the hell out of the vessel but at a cost of additional steam consumption.

Although you didn't ask this question, what is the problem with the condensation?

 

[sgulick]

The probelm with condesation is that the vessel exists before a compressor.

 

[TD2K]

I've seen a lot of suction KO drums before a compressor. I've never seen one that is heat traced to prevent liquids from condensing on the shell. Why won't it just drain to the bottom? Is this a suction pulsation bottle on a recip?

I've seen some clients insist on heat tracing the suction line to the compressor to avoid any condensation, especially recips but also one recycle gas compressor in a hydrocracker, not sure I buy the last one but it wasn't my design.