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Unformatted text preview: Stevens Institute of Technology
E234  Thermodynamics
Assignment 3
Due Date:
Sections A and B : Tuesday, September 22, 2009
Section C: Thursday, September 24, 2009
Study material for Exam 1 on Wednesday:
1. Chapter 1; Module 1
2. Chapter 2 up to and including section 3 and including the section on heat
transfer; Module 2 to slide 28
3. "A Really Nice Way to Solve Heat Transfer Problems" under "Additional
Notes" on the web site. There will be a problem in the exam similar to the
one studied in this text.
Learning Objectives
You should be able to:
a) Describe the various contributions to the internal energy
b) Define the three modes of heat transfer
c) Calculate rates of heat transfer by these mechanisms including situations when
more than one mechanism is important
Reading and Study Assignment:
1. Chapter 2 including the section on heat transfer. Read section 27 so that you can
explain and use the three equations therein.
2. Lecture notes – Module 1 and Module 2 up to slide 28.
Homework
There are tables of data at the back of the book.
Homework which consists of a succession of numbers with no explanations and no
literal equations WILL NOT be graded.
Please write a structured solution to this problem that your parents could read and
understand.
A typical Hoboken house has the following dimensions: 18 ft width x 27 ft height x 40 ft
length.
The walls consist of a 1 cm thick plaster plate (k = 0.79 W/m.K), a 6 cm insulation (k =
0.04 W/m.K) and an outside brick layer 10 cm thick. Assume the roof has the same
thermal resistance as the wall. Ignore losses through the basement.
It has 8 windows and a door on the front and 8 windows and a door on the back. Assume
doors and windows have the same size of 3 ft x 7 ft and the same thermal properties.
They are double pane (each 4 mm thick) glass windows with an air gap of 1 cm. Fall 2009 The house is to be kept at 20oC. When the outside air is at –10oC in the winter, the
outside convection heat transfer coefficient is 175 W/m2.K. When the outside air is at 35
o
C in the summer, the outside convection heat transfer coefficient is 20 W/m2.K. Assume
that the inside glass surface temperature is 20oC.
a. Determine the thermal resistance of the walls and the windows.
b. Find the rate of heat transfer through the windows and the lowest temperature
in the air gap of the window in winter and summer.
c. Find the rate of heat transfer through the walls
d. If energy costs $0.1/kWh,
i. Determine the cost of cooling the house in summer (assume 3 months
of operation with the temperatures given)
ii. Determine the cost of cooling the house in summer (assume 3 months
of operation with the temperatures given)
iii. What is the annual cost of cooling and heating the house? Fall 2009 ...
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 Spring '09
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