Steam heat transfer question

[elliottr]

Been working out a problem I have to figure out at work. I have a large pipe (~6") that needs to be steam heat traced but i dont know much about the fluid properties within the pipe. This fluid will be heateded in a separate tank and pumped across a plant site. the steam heat trace just needs to maintain temperature. Is it ok to assume - if I keep the inner pipe temperature the same as I need the fluid temp (mid 100 F) i can use that temperature to figure out how much steam i need to supply?

 

[EdStainless]

If the fluid coming in is at the desired temp then yes you can make that assumption.
You are really only making up for external heat loss.

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P.E. Metallurgy, Plymouth Tube

 

[chicopee]

I envision the plant piping to be very long and the plant to be relatively cool so it is highly unlikely that you'll be able to maintain the desired temperature throughout the plant piping even if the piping is insulated. You'll be better off to have a higher temperature within the heated tank.

 

[EdStainless]

This is one reason that electric tracing is popular.
While it is more expensive to install it can be designed to hold temp over very long runs.
It also does not waste steam, and in many plants the O&M cost is less for electric than steam tracing.

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P.E. Metallurgy, Plymouth Tube

 

[racookpe1978]

Electrical trace heating can be controlled very simply with a sensor + controller + power supply, it is a fairly robust system compared to the details and many problems forced by a steam trace heat system. Further, if each section of each pipe that needs trace heating is monitored with a thermometer, then each separate length can be energized only when needed. That long pipe from the hot tank can be broken into three, four or five trace heat control sections, and so each separate length is heated only as long as it needs, and only in the length that is needing heat.

 

[EdStainless]

They make electrical systems that are self regulating as well, with the max temp limited.
Look into electric.
Steam systems tend to waste a lot of steam and can require a lot of maintenance.

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P.E. Metallurgy, Plymouth Tube

 

[elliottr]

Thanks guys,

Ill try to convince them but the client wants steam due to having it already on site, and probably some other things. Electric makes much more sense to me, and it is what I have done at previous jobs.

i believe its only going roughly 500ft, but this is still proving to be a challenge. a couple calculations ive used have it a bit all over the place - ranging around 30 btu/hr/ft (most including my calc sheet i made give me this - even electric trace design programs) lost up to around 100.

im assuming 10 degrees F out with a moderate wind. I also mistakenly said 6" pipe when i meant to write 3" with 4" foam glass insulation and a metal jacket.

 

[racookpe1978]

OK. But at that small a diameter of the pipe, you will be very close (if not right at) the limiting diameter for minimum heat loss after insulating the pipe. If the diameter of the insulated pipe gets larger, the increased area of the OD of the insulation increases heat loss to the environment, even though the insulation is thicker. Now, ADDING a steam heat trace pipe INSIDE the primary pipe insulation is only going to make those losses greater. Further, just claiming "steam is on-site" does NOT route that steam heat trace pipe over to either end of the new 500-800 foot pipe, adding the increased steam traps (and their losses) to the feed pipe, heat trace pipe, and return pipe(s). And all of that heat trace pipe needs to be welded, NDE, and itself insulated and supported.

Electric makes much more sense: The new electric bands go inside the new pipe's insulation. No other new pipe supports, new pipe support structural steel and foundations, no new cuts and taps into existing steam lines ......

 

[elliottr]

oh don't get me wrong, I think its the wrong route to go too. My first question when this was brought to me was... "why steam?". But since I have to give them the numbers from this before pushing electric some more, I wanted to make sure I was getting them good info.

 

[chicopee]

Just curious about the purpose of the steam; is it to tap off at pipe intervals in order to draw steam for curing concrete within forms throughout the plant?

 

[elliottr]

The steam is being used for maintenance temperature purposes. Its a petro-chem site, and they already produce a lot of steam.

I ended up making a sheet that covers all the radiation, conduction, and convections (Free and forced) heat losses per foot.

what i get from that, Ill just divide by 1000 to get the lbs/hr i need.

 

[georgeverghese]

See if the table 11-22 on economic thickness of insulation in Perry Chem Engg Handbook 7th edn. matches up with the insulation thickness you've selected for the corresponding cost of energy used to produce this steam at this site. The narrative indicates there is little difference to the thickness selection for the type of insulation (ie foamglass or calcium silicates, mineral wool etc). At only 100degF maintain temperature, looks like 3inch pipe with 4inch insulation seems over the top. Unless steam generation heating cost is well beyond USD8/mmbtu.

 

[Rspain]

Hi All,

If the process is using steam, they most likely have a high pressure condensate recovery, with hot condensate being either re-fed directly into the boiler or heat exchanged before arrival to the hot well.

How about using that hot condensate in a closed loop with the HP condensate recovery tank to trace the pipe?

That might result in very cheap energy (specially if their condensate recovery system is not very efficient) and at the end its just pumping condensate.

It could be done counterflow with HP condensate tracing flow rate linked to end fluid target temperature.

BR,
Rto