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Semester test 2 MTX 311 2015 memo
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Thermodynamics MTX 311Semester test 2: May 2015
Examiner
Dr AS Lexmond (University of Pretoria)
Instructions
1 Time: 90 min.
2 Answer all the questions
3 Full marks: 100
4 Neatness saves you points, especially when only part of the question is correct.
5 Give the equations before you start filling in numbers; for the questions marked with an
*, also give a short but clear solution strategy
6 You are allowed to take fundamentals of thermodynamics (7th or 8th edition), non-ideal
gas hand-out, one page, A4, with your own notes (both sides) and all the material from
the click-up page “lectures”
7 For all questions: use a reference temperature and pressure of T0=25°C, P0=1 bar
All questions in this test have to do with a system to cool a chemical reactor that is being operated at
125K. Overall efficiency of this process is unacceptably low; that’s why a thermodynamic analysis is
required. Fig. 1 gives an overview of the combined power-cooling cycles, table 2 gives the process
stream characteristics and table 2 gives equipment description.
630 kW D
1800K
11 surroundings
I
4 14 15
H N
3 8 9
K
A E F G J
2
B 16
1 5
O L
7 10 13
C
6 M
15 kW
Question 5 Question 2 125K 12
Fig. 1 Overview of combined cooling-power cycle
, Table 1 process flow conditions
m T P h s H S Ψ
kg/s K kPa kJ/kg kJ/kgK kW kW kW
1 0.7 298.15 100 298.62 6.8631 209.0 4.80417 0.0
2 0.7 610 1000 617.9 6.9328 432.5 4.85296 208.9
3 0.7 967.3 1000 1108.3 7.5412 775.8 5.27884 425.3
4 0.7 1800 1000 2003 8.174358 1402.1 5.722051 919.4
5 0.7 1100 100 1161.2 8.2445 812.8 5.77115 315.5
6 0.7 640 100 670.78 7.6772 469.7 5.37404 90.8
7 5.75 121 300 118.1 5.6428 679.1 32.4461 1054.0
8 5.75 280 5000 280 5.7125 1610.0 32.84688 1865.5
9 5.75 251 5000 251.3 5.568249 1445.0 32.01743 1885.0
10 5.75 120 300 120.7 5.63 694.0 32.3725 1090.9
11 7.53 260 1000 520 0.72617 3915.6 5.468059 40.2
12 7.53 250 1000 500 0.647728 3765.0 4.877395 65.7
13 1.02 246.85 100 381.98 1.7456 389.6 1.780512 4.3
14 1.02 303.15 666.3 428.56 1.7609 437.1 1.796118 47.1
15 1.02 298.15 666.3 234.6 1.1201 239.3 1.142502 44.2
16 1.02 246.85 100 234.6 1.1477 239.3 1.170654 35.8
Table 2 process equipment
work
description kW
A compressor -208
B&C regenerator
D heater
E turbine 409.2
F non- adiabatic compressor -900.9
G turbine 751
H&I gas cycle heat exchanger
O reactor cooler
J R134A cycle compressor -47.5
K expansion nozzle
L&M R134A heat exchanger
N condenser
shaft friction -3.8
1) What are the second law efficiency and the irreversibility of the entire process (the combined
power-refrigeration process)?
2) What are the second law efficiency and the irreversibility of the low-temperature (air based)
compression expansion cooling cycle (containing process flows 7-10)
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