Thermodynamics? number crunching thermal conductivity

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Discussion Overview

The discussion revolves around calculations related to heat transfer from polyethylene (PE) pipe, specifically focusing on thermal conductivity and the conversion of units for heat transfer calculations. Participants are seeking clarification on how to properly compute BTU/hr based on given parameters such as thermal conductivity, wall thickness, and temperature difference.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant states that the thermal conductivity (TC) of PE pipe is approximately 0.46 W/(m·C) and converts this to BTU/(hr·ft·F) using a factor of 0.5779, resulting in 0.266 BTU/(hr·ft·F).
  • Another participant mentions the need to calculate the heat load (Q) using the formula Q = (k/d)ΔTA, where k is the thermal conductivity, d is the wall thickness, and ΔT is the temperature difference.
  • There is a query about whether the wall thickness affects the calculations, with a suggestion that it may not matter.
  • One participant expresses uncertainty about the units and requests assistance in applying the math to the provided figures, specifically questioning if k/d results in 2.22 BTU.
  • A later reply emphasizes the importance of expressing the wall thickness in feet for accurate calculations, prompting a consideration of unit conversion.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the calculations, as there is uncertainty regarding the impact of wall thickness and the correct unit conversions. Multiple viewpoints on the relevance of these factors remain present.

Contextual Notes

There are unresolved issues regarding the proper expression of wall thickness in feet and how this affects the calculations. The discussion also highlights potential confusion over unit conversions and their implications for the heat transfer calculations.

fastline
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I am working on some basic calcs for heat transfer from polyethylene pipe. My numbers are not working out right so I need a little refresher.

The PE pipe would have a TC of about .46 W/(m.*C). to get to BTU/(hr.ft.*F), I mult by .5779 to get .266.

Assuming 10sf of PE pipe, and let's say a dT of 10*F, how do I arrive at my BTU/hr? Wall thickness of piping is .120" but I am told that does not matter. IIRC, the unit is actualy per sf PER ft so I might actually divide by my thickness which gets me closer at around 2.22 BTU/hr/sf*F of pipe?
 
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fastline said:
I am working on some basic calcs for heat transfer from polyethylene pipe. My numbers are not working out right so I need a little refresher.

The PE pipe would have a TC of about .46 W/(m.*C). to get to BTU/(hr.ft.*F), I mult by .5779 to get .266.

Assuming 10sf of PE pipe, and let's say a dT of 10*F, how do I arrive at my BTU/hr? Wall thickness of piping is .120" but I am told that does not matter. IIRC, the unit is actualy per sf PER ft so I might actually divide by my thickness which gets me closer at around 2.22 BTU/hr/sf*F of pipe?

The formula for the heat load Q (BTU/hr) is:
Q=\frac{k}{d}ΔTA
where d is the wall thickness.
 
I guess I am second guessing the units here. Would you mind applying the math to my above figures? Would this indeed be

k=.266
d=.120

k/d = 2.22BTU?
 
fastline said:
I guess I am second guessing the units here. Would you mind applying the math to my above figures? Would this indeed be

k=.266
d=.120

k/d = 2.22BTU?
You need to use d expressed in feet. How many inches are there in 1 ft?
 

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