ChE 306: HEAT TRANSFER
FALL 2016
Homework #3
 Chapters 3, 4 & 5
(10 points each; 80 points)
DUE: MONDAY, September 12, 2016 in class
Office hours for HW help on this set: Thursday 9/8 1:30 – 3:00 in HM Comer 339A with Dr. Brazel; HW Help Session Thursday 9/8 5:00 PM in HM
Comer 337; Friday 8/26 11:00 – 12:00 in SEC 3438 with Katie Baggett {I’ll also be available Friday 45 in HM Comer 339A}
TEAMS
: You are encouraged to work in teams. A single assignment can be turned in by up to 4 team members. Include all team
members’ names on the HW set when turning in. Each team member should understand how to solve ALL problems.
HOMEWORK SETS
: Turn in all problems, in the order they were assigned. Staple, if possible, and fold the set in half lengthwise.
Write your name (all team member names) on the outside fold, so that it will be easy to return.
FORMATTING:
Work neatly, draw a diagram of the problem, where appropriate, keep problems in the correct order; if the grader
can’t find a problem, then it may be scored a zero. Write on only one side of each sheet of paper. Engineering paper is NOT required.
LATE HOMEWORK
: Homework will not be accepted late. Turn it in during class, send it with a classmate, or drop it in a RED folder
outside my office door BEFORE the solutions post on Blackboard (~ Noon on class days). If you have an excused absence for a
homework set, I will drop that set, and award a score of 0 out of 0.
CHAPTER 3
1. A stainless steel pipe carries steam across a chemical plant. The outside surface of the pipe is 250
o
F, and the pipe has
an outside diameter of 3.0 inches and a length of 40 feet. The bulk air (T
∞
) is 88
o
F, and the air has a convective
heat transfer coefficient of 31.7 Btu/hft
2

o
F.
A. What is
the rate of heat transfer from the pipe to the air by convection
?
B. Fins are added to the pipe in the shapes of discs, surrounding the
pipe such that there are 24 fins per linear foot of pipe, and
each fin has a thickness of 0.10 inches and an outside diameter
of 4.0 inches (the inside diameter of each discshaped fin is
equal to the pipe diameter, or 3.0 inches). Note that when a fin
is added, it reduces the exposed surface area of the original
pipe since 0.10 inchthick sections are no longer exposed to air,
but adds significantly more surface area by having the disc
shaped fins exposed to air. Assuming h does not change when
the fins are added (in reality, h drops a little) and that the
temperature of the outside surface of the pipe (including the fins) is still 88
o
F (the fins are metal and
have a good thermal diffusivity, making this assumption valid),
what is the new heat transfer rate for
the finned pipe
, and
what is the overall surface eff
ectiveness, ε
f
(= q with fins / q without fins)?
A. By convection q = h A (Ts – Tinf) (no fins)
q = 31.7 * pi * (3 in/12 in/ft) * (40 ft) * (250 – 88 F) =
161,000 Btu/h
The surface area is 31.4 ft2
B. for the fins, the surface area of the straight pipe is reduced a bit since each fin takes up 0.1 inches (thick) around the
circumference.