Client in trouble 1

[Carlsberg75]

I need a bit of help calculating something for a client who has gotten into a bit of bother. He has installed heating into a site of 60 houses. Each house has an 8 port manifold with each port distributing to an individual radiator. The problem lies with the pipe running from the manifold to the radiator. This pipe runs below the insulating layer and therefore according to building regulations (Ireland) should be insulated, but instead he has simply run the pipe through a corrugated plastic sleeve. I need to calculate what heat is being lost into the ground through this 'pipe-in-pipe'. The information i have is:
-water inlet temperature
-radiator size in KW
-pipe inner and outer diameter
-thermal conductivity value of PEX pipe and plastic sleeve

I'm not certain what temperature i will assume for the surrounding concrete but i also need to know:
-Mass Flow Rate (for formula- Q=M*A*(T1-T2)
-U-value for the pipe and plastic sleeve (it's been 25 years since i've studied this and am struggling to remember the correct formulae and also the diameter of the corrugated plastic sleeve has a max and min- should i take an average)

Any advice would be greatly appreciated as it looks like this client will be getting sued by the developer.

Thanks

 

[zdas04]

It is a tough problem, with a bunch of assumptions that must be documented, to me the key question is "did he use centralizers on the pipe-in-pipe?". If he did, and if the ones he used are temperature insulating, then there is a chance that the air gap will be adequately insulating. If he didn't, then you have to look at heat transfer directly from the pipe to earth.

David

 

[IRstuff]

What's delta between OD inner and ID outer? It'll affect whether there's convection or not.

TTFN

FAQ731-376

 

[Carlsberg75]

The plastic sleeving is corrugated therefore has a maximum and minimum internal and external diameter. The minimum internal diameter is 18mm while the OD of the pipe is 15mm. There is an air gap there but because the pipe is not suspended, it will be in contact with the plastic sleeving. The maximum OD of the sleeving is 22mm. I know the information i have is pretty scarce but thanks for the help.

 

[IRstuff]

Your could somehow measure the effective heat transfer coefficient. This could be done with a surrogate section of the pipe filled with water at some initial temperature and placing the sealed assembly in a circulating water bath. By measuring the temperature dropping and knowing the amount of water, you could determine the net heat transfer efficiency.

The other option is to enter the structure into a thermal analysis program.

I doubt that any ad hoc hand calculation will adequately capture the behavior of this structure.

TTFN

FAQ731-376

 

[timbones]

Sounds like it doesn't meet the building code, so who cares what the heat loss is? Doesn't he have to fix it so it meets regulations?

Why do it right, when you can do it twice.

 

[magicme]

hi there

i cannot help with the building code issues or with the details of the required input values for this particular problem.
and i'll spare your client my tedious lecture on "do it right or don't do it".

but the basic equations for addressing this type of problem are here:

http://myweb.wit.edu/leod1/MECH594/Texit.html

you no doubt will have to make assumptions and tweak input values to fit your problem.

i hope this is helpful

regards

magicme



------------------------------------
there's no place like gnome.

 

[trashcanman]

Measure the temp before and after the piping goes under the slab. Measure the flow rate. Heat loss is simply
500*(T1-T2)*gpm = BTUH, where T1 is temp before, and T2 is temp after.

Flow rate can be accurately measured by a strap on magnetic flow meter.