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Sample%20Midterm%202A04 - CHEMICAL ENGINEERING 2A4 DAY...

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Page 1 / 5 CHEMICAL ENGINEERING 2A4 DAY CLASS Duration 2 hours Dr M. Thompson McMaster University Mid-Term (Deferred Soln) March 17, 2003 This test includes 4 problems on 2 pages. You are responsible for ensuring that your copy of the paper is complete. Bring any discrepancy to the attention of your invigilator. SPECIAL INSTRUCTIONS 1. Any calculator is permitted. 2. Open books and notes. 3. Only solutions written in PEN will be accepted. 4. Marks for the problems are given (Total of 50 marks) 5. State clearly the assumptions you make in solving a problem 6. Ensure name and ID number are included on all answer booklets. Question 1. (15 Marks) A thermocouple is a common sensor used to measure temperature and is made up of two wires each of a different metal joined together by a bead weld. It is at this junction point where temperature is measured by a thermocouple. The bead weld resembles a sphere and its diameter will determine how fast the thermocouple will react to a change in temperature. A copper-constantan thermocouple is exposed to a convective environment where the air temperature is 0 o C and its convection coefficient is 45 W/m 2 -K. How fast will the thermocouple respond to temperature changes in the air (i.e. what is the thermal time constant) provided the convection coefficient remains constant, for a bead diameter of (a) 0.127 mm and (b) 0.250 mm? Use average metal properties for the bead weld; relevant metal properties are determined at 300 K c o p e r n s t a Figure 1.
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Page 2 / 5 Solution: Properties: Table A-1 p.819 k([email protected] K) = 401 W/m-K k([email protected] K) = 23 W/m-K Cp ([email protected] K) = 385 J/kg-K Cp ([email protected] K) = 384 J/kg-K ρ ([email protected] K) = 8933 kg/m
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Sample%20Midterm%202A04 - CHEMICAL ENGINEERING 2A4 DAY...

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