jacketing used like HX

[gelsi]

Hi
anyone can help me with steam jacketing length calculation for heating a gas flowing into a pipe?

I know duty, in and out temperature of gas, saturation temperature of steam
and have to calculate minimum lenght of jacketed pipe (how many 6 meters long bares of jacketed pipe it needs to rise temperature of gas?)

it seems a double tube HX calculation with condensation, but I need a short cut method or detailed criteria HELP

http://www.eng-tips.com/viewthread.cfm?qid=380033

 

[marty007]

This really isn't too difficult of a problem.

Internally, you will have regular gas flow convection heat transfer. You know your gas volume and properties, so can calculate a Reynolds and Prandtl Number. From this you can calculate a Nusselt Number using a typical laminar or turbulent heat transfer correlation (any fluids textbook). That solves the ID of your pipe.

On the OD, you can probably assume you have a heat transfer coefficient many times higher than your gas flow, as condensation heat transfer is very good. As such, your inner metal temperature will be very nearly at 150C (based on your other post). You could also track down a good correlation to be more accurate.

If you're doing this in excel, then just break a model into say 20 sections. Calculate a Reynolds number and convection coefficient at each step, then calculate the heat transferred into the gas. Based on the surface area of each step in your excel model, you now have the heat input to the gas. Calculate the temperature rise based on a heat capacity, and now you know the output temperature of your first step, which is now the inlet temperature to your second step. Now just copy/paste columns in your spreadsheet for the total length of your jacketed pipe and you'll calculate an exit temperature.

Throw on safety and fouling factors as needed, and you're off to the races.

Cheers,
Marty

 

[gelsi]

TX for your
"On the OD, you can probably assume you have a heat transfer coefficient many times higher than your gas flow"

this is an interesting simplification, I can fix 149°C and use a safety factor of 130% on lenght, do you agree?

is the razionale that h for gas is low and for condensing, expecially steam that has an high latent heat, is high?

 

[marty007]

Generally that is the case with gas vs. condensing steam. That said, to be certain you should track down a condensing heat transfer rate somewhere to have a defensible design.

The method you've spelled out will get you pretty close, particularly if you are just comparing options (jacket vs. shell/tube).

Another thing to consider is non-condensibles. You will need some sort of method for these to escape, or they will slowly build-up, reducing the effectiveness of the exchanger.

Cheers,
Marty

 

[gelsi]

neglecting h0 (annular side) i find that:
nr 3 jacket pipes 3"/4" of 6 m give about 12000 W, that are needed to rise air temperature from 25 to 145°C
Anyone can say if this is an abnormal result?

 

[LittleInch]

This is the danger of having two threads - I've answered your other thread post.....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[gelsi]

tx

 

[marty007]

gelsi,

As LittleInch pointed out, this is why we avoid cross-posting. I will respond from now on in your other thread.

Marty