# Heat and Mass Transfer Problem

1. Jul 9, 2017

### dzj633

1. The problem statement, all variables and given/known data
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

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

3. 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.

2. Jul 9, 2017

### NFuller

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. Jul 9, 2017

### dzj633

I see.. Would it be a safe assumption to assume L = 1 meter? Per "1" unit length?

4. Jul 9, 2017

### Staff: Mentor

What do the units W/m mean to you?

5. Jul 9, 2017

### 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. Jul 9, 2017

### dzj633

With that being said, my thought of "a unit" would be 1 unit of length.

7. Jul 9, 2017

### Staff: Mentor

Yes, it's Watts per meter.

8. Jul 9, 2017

### dzj633

Correct, after my calculations I got 78 W/m.