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MTX311 2014 -Exam
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Sick test MTX311 2019Question 1
A combustion process (fig. 1) produces 142g/s of a mixture at 7.5 bar and 245°C, consisting of
carbon dioxide (40% by mole) and water (60% by mole). The mixture is diluted with 605g/s of R12,
also at 7.5 bar and 245°C.
The resulting 3-component mixture is cooled down at constant pressure until the first component
(water) just starts to condense.
1A (5) What is this temperature called?
1B (10) At what temperature will condensation of water start?
1C (10) How much heat will have to be removed in this cool-down process?
142 g/s mixture mixture
P=7.5 bar P=7.5 bar P=7.5 bar
T=245°C T=245°C
40% (mol) CO2
60% (mol) H2O adiabatic mixer heat exchanger
fuel
oxygen
combustion
Q out
605 g/s
P=7.5 bar
T=245°C
R-12
fig. 1 Overview of gas mixing process
Question 2
A heater is designed to generate 100kW of heat at 677°C by combustion of 2.5g/s of liquid butane,
which is stored at room temperature. To run the process, 5kW in electricity is needed to run the fuel
pump, the air fan and the control system.
2A (10) What are the second law efficiency and the irreversibility of this system?
liquid butane (2.5 g/s)
Hot waste air
157.5 g/s air at ambient conditions 160 g/s
360K, 1 bar
5 kW electricity
100kW at 677°C
fig. 2 Heater using butane
To find ways to improve the efficiency, we want to estimate how much free energy is leaving the
process with the waste gas.
2B (10) What is the exergy of the waste gas?
, 2C (5) Is this a significant contribution to the total irreversibility of the heater?
Question 3
(This question is also based on the butane-based heater from Q2 and fig. 2.) It is important to use
excess air in combustion processes. However, it seems that 157.5g/s of air is a bit much for the
combustion of 2.5 g/s of butane.
3A (5) Give one good reason to always use excess air in a combustion process
3B (15) Calculate the stoichiometric and the actual air-fuel ratio by mass.
3C (5) How much excess air (in %) is used in this process?
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