Heat loss from buried pipe in "concrete conduit"

[JVKS]

looking to figure the heat loss from steam and condensate piping in a buried "concrete conduit." the conduit is a concrete tunnel about 1' tall by about 1' wide that is buried at a depth from 3' to 10' - hilly terrain. the conduit is open at both ends to the steam pits or buildings. the steam piping is 3" at 100 psi with 2" of mineral wool, the condensate is 2" with 1" mineral wool.

if it was a buried insulated pipe i use the simple method of assuming the earth is at a relativley constant temperature and cipher it from that. i try to be conservative and the number are good enough.

any thought on the best way to approach this? i am thinking of including the space between the outside of the insulation and the inside of the concrete and the concrete as more insulation and figure the heat loss from there. among other things this basically ignores any convection in the conduit space.

the actual situation is much more complicated and messier than i describe.

 

[trashcanman]

Yes, it is complicated and messy. You don't give the length of the piping. For a conservative answer, assume the air around the piping is still, especially if the length is long (?). Heat loss from piping is from conduction, convection and radiation. I would calculate the outside temp of the insulation. Heat transfer by conduction through the insulation = heat loss from convection + radiation. It is a trial & error solution. Assume air temp is somewhere between steam temp and ground temp (big range there) and it varies as the outside air temp varies. Worst case is in winter. Process Heat Transfer by Kerns or Heat Transfer chap in ASHRAE Fundamentals handbook will provide equations. The concrete + earth is a huge heat sink, so the air temp should stabilize eventually.

 

[JVKS]

oops! really meant to put this in the HVAC thread but this will do.
the length is hundreds of feet and in reality varies between 8" to 2" and to make it more complicated the conduit in areas is crushed, missing insulation, leaky steam pits, steam pits fill with water and run down the conduits ruiing insulation. Just wanted to keep it simple for now.

i can get the outside temp of insulation thru calculations.

i agree with surrounding air temp between steam or outer insulaiton temp and ground temp.
i agree about still air - for all practical purposes it is.

yes trial and error - will probably use a high number or "average" to give all involved a conservative number.

i do not have my ASHRAE with me but will look it up. lots on the web.

i agree about the air temp stabilizing around the conduit.

Thank you

 

[IRstuff]

" i am thinking of including the space between the outside of the insulation and the inside of the concrete and the concrete as more insulation and figure the heat loss from there. among other things this basically ignores any convection in the conduit space."

Actually, you didn't, ostensibly, since you are including the air as a layer of insulation.

It does not necessarily need to be iterative, the solution solves a bunch of simultaneous equations, so programs that have systems of equations solvers can do it directly. You just have to balance the heat flows, since the energy is conserved.

TTFN
faq731-376

 

[dvd]

Search for "heat loss" and utilidors

Here's one:

http://www.crrel.usace.army.mil/library/crrelreports/CR99_07.pdf