Thermodynamics MTX 311
Sick test May 2017
Examiner
Dr AS Lexmond (University of Pretoria),
Dr WG Le Roux (University of Pretoria)
Instructions
1 Time: 90 min.
2 Answer all the questions
3 Full marks: 100
4 Neatness saves you marks, 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 any book and all notes with you
7 T0=25°C, P0=1 bar
Q1)* 20pts GHB (gamma hydroxy butyric acid, C 4H8O3, M=104 kg/kmol)) is a bit of a
nasty substance with mostly illegal applications. We want to destroy a shipment of
400kg of this stuff by combusting it with 150% excess air.
What is the required air-fuel ratio (by mass)?
Q2)* 20pts We want to cool down a process stream of 20g/s consisting of a mixture of carbon
dioxide(10% by mole), nitrogen (20% by mole) and water vapour (70% by mole). Initial conditions are
120°C, 110kPa. After cooling down, the temperature is -10°C, the pressure is 100kPa.
How much heat has to be extracted?
Q3 10pts An air-water vapour mixture at 1 bar contains 400kg of air. The temperature is 28°C, the
relative humidity is 35%. How much water vapour is present?
You might remember this process from the assignment.
A fancy furnace is used to power a dual air-steam power cycle, see fig. 1 A light oil with a heat of
combustion of 41,625 kJ/kg is combusted with excess air, forming a very hot gas mixture that is fed
to a plate heat exchanger. In this heat exchanger, compressed air is heated up to 1600K and
expanded in a turbine to generate work.
, The combustion gases coming out of the heat exchanger are still quite hot and used to generate
superheated steam in a high pressure boiler. The steam from the boiler is also used to generate
work.
We have no use for the hot gases coming out of boiler. However, on the plot next to ours is a
recycling plant that needs hot air for drying of their waste material, so we are able to sell the hot air.
The power production and consumption of the pumps, turbines and compressors are given in Table
1. Process flow properties are given in table 2.
Q4 20pts What are the second law efficiency and the irreversibility of the air-based power cycle
(with the red circle around it?)
Table 1 Energy consumption and generation of process units
Sick test May 2017
Examiner
Dr AS Lexmond (University of Pretoria),
Dr WG Le Roux (University of Pretoria)
Instructions
1 Time: 90 min.
2 Answer all the questions
3 Full marks: 100
4 Neatness saves you marks, 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 any book and all notes with you
7 T0=25°C, P0=1 bar
Q1)* 20pts GHB (gamma hydroxy butyric acid, C 4H8O3, M=104 kg/kmol)) is a bit of a
nasty substance with mostly illegal applications. We want to destroy a shipment of
400kg of this stuff by combusting it with 150% excess air.
What is the required air-fuel ratio (by mass)?
Q2)* 20pts We want to cool down a process stream of 20g/s consisting of a mixture of carbon
dioxide(10% by mole), nitrogen (20% by mole) and water vapour (70% by mole). Initial conditions are
120°C, 110kPa. After cooling down, the temperature is -10°C, the pressure is 100kPa.
How much heat has to be extracted?
Q3 10pts An air-water vapour mixture at 1 bar contains 400kg of air. The temperature is 28°C, the
relative humidity is 35%. How much water vapour is present?
You might remember this process from the assignment.
A fancy furnace is used to power a dual air-steam power cycle, see fig. 1 A light oil with a heat of
combustion of 41,625 kJ/kg is combusted with excess air, forming a very hot gas mixture that is fed
to a plate heat exchanger. In this heat exchanger, compressed air is heated up to 1600K and
expanded in a turbine to generate work.
, The combustion gases coming out of the heat exchanger are still quite hot and used to generate
superheated steam in a high pressure boiler. The steam from the boiler is also used to generate
work.
We have no use for the hot gases coming out of boiler. However, on the plot next to ours is a
recycling plant that needs hot air for drying of their waste material, so we are able to sell the hot air.
The power production and consumption of the pumps, turbines and compressors are given in Table
1. Process flow properties are given in table 2.
Q4 20pts What are the second law efficiency and the irreversibility of the air-based power cycle
(with the red circle around it?)
Table 1 Energy consumption and generation of process units
