Heat and Mass Transfer Problem

  • #1
dzj633

Homework Statement


A plastic pipe (k=0.092 W/m.K) of inner and outer radius of 1 and 1.25 cm, respectively. If the inner and outer surfaces are at 60 °C and 30 °C, respectively. The rate of heat transfer per unit length is close to:
a) 98 W/m b) 89 W/m c) 78 W/m d) 168 W/m e) 112 W/m

Homework Equations


Qk = ( Ti - To )/( ln| ro / ri | / 2π * k * l )

The Attempt at a Solution


I set up all of my given values into the heat transfer equation above but i am not given a length of the pipe.. Am i missing key information to finish this problem, or do i not need the length to solve? Also, I converted temps to Kelvin and the radii to meters.[/B]
 

Answers and Replies

  • #2
The key phrase here is heat transfer per unit length. You do not need the total length of pipe because you are not looking for the total heat transferred.
 
  • #3
dzj633
I see.. Would it be a safe assumption to assume L = 1 meter? Per "1" unit length?
 
  • #5
dzj633
W/m means Amount of heat transfer per unit length to me and a watt is the rate at which work is done. [J/s]
 
  • #6
dzj633
W/m means Amount of heat transfer per unit length to me and a watt is the rate at which work is done. [J/s]
With that being said, my thought of "a unit" would be 1 unit of length.
 
  • #8
dzj633
Correct, after my calculations I got 78 W/m.
 

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