New pex question
New pex question
(OP)
Hello,
I have a (another) question about heat transfer properties of Pex. I have looked high and low for a heat transfer coefficient for standard 5/8" pex tubing. I was hoping this value would be in terms of BTU/(hr x ft^2 x F). But instead I found a coefficient of thermal conductivity in terms of (3.2 BTU in) / (hr x ft2 x F). Can someone explain how I would apply this coefficient? Where does the "in" come into play? Does this mean a 1" pex would have a transfer coefficient of 3.2 BTU / (hr x ft^2 x F)? Please advise...
thanks,
EEPROM
I have a (another) question about heat transfer properties of Pex. I have looked high and low for a heat transfer coefficient for standard 5/8" pex tubing. I was hoping this value would be in terms of BTU/(hr x ft^2 x F). But instead I found a coefficient of thermal conductivity in terms of (3.2 BTU in) / (hr x ft2 x F). Can someone explain how I would apply this coefficient? Where does the "in" come into play? Does this mean a 1" pex would have a transfer coefficient of 3.2 BTU / (hr x ft^2 x F)? Please advise...
thanks,
EEPROM





RE: New pex question
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RE: New pex question
Thermal conductance [Btu/(hr ft^2 F)] = Thermal conductivity/ thickness
RE: New pex question
The electrical analogy works here, which might be helpful to you in particular. Think of DC voltage as temperature and thermal resistance as DC resistance, and heat flow as DC current. You have a bunch of resistances in series for which the total resistance determines the flow. If one resistance dominates, the others matter much less. Thermal conductivity (k) and film coefficients (h) are conductances (i.e. inverse resistances).
RE: New pex question
So if I have a known thermal conductivity of pex tubing (thermoplastic) as 3.2 (BTU in)/(hr x ft^2 x F), this is a unit of measure per inch of wall thickness of tubing. Is this correct? As a sample calculation: suppose a 5/8" pex tubing has a wall thickness of 0.0625". Then Uo is equal to 3.2/0.0625 = 51.2 BTU/(hr x ft^2 x F). Can that be right? That's a higher heat transfer rate than steel pipe.
RE: New pex question
1. heat transfer from the fluid film to the inner surface (convection)
2. heat transfer from the inner pipe outer surface through the pipe wall thickness (conduction)
3. heat transfer from the pipe outer surface to the (convection and radiation)
In order to apply the formula: Q = U*A*deltaT
You need to know the overall heat transfer coefficient U (Btu/hr/ft^2/F), that you'll hardly find in any standard and that has to be calculated for your specific field conditions.
RE: New pex question
RE: New pex question
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