Page 50 - Chemical engineering design

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INTRODUCTION TO DESIGN
1.6. (US version) Estimate the optimum thickness of insulation for the roof of a house
given the following information. The insulation will be installed flat on the attic
floor.
Overall heat transfer coefficient for the insulation as a function of thickness, U
values (see Chapter 12):
thickness, mm 0 25 50 100 150 200 250
U, Wm 2 Ž C 1 20 0.9 0.7 0.3 0.25 0.20 0.15
Ž
Average temperature difference between inside and outside of house 12 C; heating
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period 250 days in a year. Cost of insulation, including installation, $120/m .
Capital charges (see chapter 6) 20 per cent per year. Cost of fuel, allowing for the
efficiency of the heating system, 9c/MJ.
2
Note: the rate at which heat is being lost is given by U ð T, W/m ,where U
is the overall coefficient and T the temperature difference; see Chapter 12.
1.7. What is the optimum practical shape for a dwelling, to minimise the heat losses
through the building fabric?
Why is this optimum shape seldom used?
What people do use the optimum shape for their winter dwellings? Is heat retention
their only consideration in their selection of this shape?
1.8. You are part of the design team working on a project for the manufacture of
cyclohexane.
The chief engineer calls you into his office and asks you to prepare an outline
design for an inert gas purging and blanketing system for the reactors and other
equipment, on shutdown. This request arises from a report into an explosion and
fire at another site manufacturing a similar product.
Following the steps given in Figure 1.2, find what you consider the best solution
to this design problem.