Heat Loss of Cold, Clean water in CPVC pipe (uninsulated) 5

[Tris4K]

Hi All,

I've had a look through the forums and there are 2 similar threads, but both have variables that mean I'm personally unable to apply the answers to my particular problem.

The situation is that I have a pipe that passes from a heated space into a (potentially) unheated space. The problem is that the 'heated' and 'unheated' space temperatures are considerably variable, but for the purposes of this should be assumed to be:

Heated area: 17°C (62.6°F)
Unheated Area: -4°C (24.8°F) or 0°C (32°F)

The pipe length in the heated are is to be assumed to be 15M in length.
The pipe in question is 22.199mm ID, 26.670mm OD (so 2.2355mm wall). Material is CPVC with thermal conductivity (BTU/hr./ft.(squared)/°F/in): 0.95
The water is stagnant, with no additives.
The pipe into the unheated space is a dead-end.

Heat will conduct within the pipe (from heated space, through pipe wall, and through stagnant water)

What i need to know is the length of pipe within the unheated space before the water sill reach <4°C due to heat loss (i.e. the maximum pipe length before i need to consider lagging and/or trace heating to prevent freezing).

Thereafter I need to know to apply the same but with the pipe in the unheated space being insulated with 20mm insulation foil-faced, mineral wool insulation(Rockwool Rocklap: 0.84 kJ/kgK (nom.) at 20°C, and 0.033W/mK @ 10°C). Average air gap between pipe and inside of insulation is 0.165mm.

Rightly or wrongly, I've typically assumed <500mm of uninsulated pipe is acceptable, and <1000mm of insulated pipe (insulation as per spec noted above) before trace heating is needed. this was based on advice from a superior some years ago, but now i'm responsible for the design and implementation of these systems I need to have a better understanding of how this conclusion was reached, and actually if it's even correct.

I'll hold my hands up and say that the formulae and methodology I've found to solve this is beyond my capabilities. So any help anyone could offer to dumb this down to my level would be greatly appreciated.

Thank you in advance, please let me know if you need more information.

 

[IRstuff]

In residential installations, the hot water distribution often has an added return line that's used with an auxiliary pump to "pre-load" the hot water piping with hot water before the user actually demands water. This minimizes wasted water that the user might dump until they get the hot water they're looking for.

Similarly, you could possibly add a return line with an auxiliary pump to circulate the water at low flow rates when the temperature drops below some threshold. This might be a more power efficient approach, and relatively inexpensive, when amortized over time.

https://gadgetsgo.com/Rinnai-GTK15-Grundfos-tankless-recirculation-loop-Pump-Timer.html

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[Tris4K]

IRSTUFF: I've applied that exact approach in a big loft in a care home before. Avoided trace heating. We measured water temp on end of 'return' leg to prove minimum temp. It worked surprisingly well, but was a ball-ache to install pipework in a perfect loop (to guarantee flow) whilst picking up all head positions, so hugely increased frictional losses, meaning bigger pump, so ended up similar to trace heating in terms of initial cost. Running costs were less though!

 

[IRstuff]

In my old house, the recirculation loop is not operational during actual usage, since the recirculation pump itself is off, but that might be an issue if the recirculation isn't running often enough and there's stagnant water in the return loop

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[Lukezo]

Hi,

in my opinion no insulation helps. I'd suggest making a double pipe stangnant 'heat exchanger' (inner tube: cool water and outer tube: warm air).

For sure there will be heat loss but security matters more than that.

Regards,
Lukezo

 

[Tris4K]

Lukezo, Now THAT is a fantastic, and absolutely achievable solution!!
Incidentally the pipe is being boxed in, so all we need to do is make a second penetration adjacent to the pipe penetration, we already have an un-sealed hole at the head position (which will encourage flow of air), and hey presto, warm air flow along the length of the pipe.

 

[LittleInch]

Sounds a bit simplistic to me I'm afraid.

what will cause the warm air flow?
Where will it go?
what happens when someone stuffs the hole to prevent cold air coming back?

You never said it was boxed in. Makes a big difference to heat loss not to have moving air past the pipe.

why not just stuff the annulus with insulation (simple rockwool type) and be done with it??

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

 

[Lukezo]

Please take note; condensation might occur.

 

[Tris4K]

@Littelinch

I wanted worst-case scenario assumption. Pipe is lagged and boxed in all cases BUT air flow would always be zero across pipe whether boxed or not (we are talking about a room that's well sealed to atmosphere and usually around 3M2-5M2 floor area).
Stuffing with insulation is a no-go as we need somewhere for hot gases to go in case of fire (so they enter boxed-space and activate the concealed head).

Lukezo: Condensation should not be an issue as water contents will always be warmer than outside-pipe temp unless water is flowing (in which case there's a fire on the balcony, and who cares about condensation?).

 

[georgeverghese]

Water vapor in the warm air flowing through the outer pipe walls will condense and freeze, blocking any further flow of warm air?