Aspekte van vibrasies in romp- en buis hitteruilers
- Jordaan, Christiaan Cornelius
- Authors: Jordaan, Christiaan Cornelius
- Date: 2015-02-12
- Subjects: Heat - Transmission , Turbulence , Eddies
- Type: Thesis
- Identifier: uj:13303 , http://hdl.handle.net/10210/13320
- Description: M.Ing. (Mechanical Engineering) , Shell and tube heat exchangers are commonly used in industrial processes. In the heat exchangers the tubes incur flow induced vibrations. These vibrations lead to fretting on the perimeter of the tube at the baffle plates and to fatigue failure at the tube sheets. Various studies were and are done to qualify and quantify these vibrations. This is then used to develop design criteria and methods for lowering the vibration amplitudes of the tubes. In the thesis the response of a tube bundle in cross flow is shown. The effect of increasing cross flow velocity on the vibration amplitude of the tubes is determined in a low speed wind tunnel. The results show similarity to the work done by Gorman [27]. By raising the natural frequency of the tubes in the first row upstream it is shown that the vibration peaks exhibited by the unmodified model.at cross flow velocities lower than the critical flow speed do not materialize in the modified tube bundle. This modification can be implemented by decreasing the unsupported length of the tubes in the first row. The inclusion of fluid damping when calculating the total damping of the tubes according to Blevins [24] is shown in the experiment. From experimental results it is shown that the prediction of the critical velocity according to Blevins [24] results in a better prediction of this velocity. Numerical solutions of two dimensional flow over a single cylinder and a pair of cylinders are obtained by simulating the flow on the STAR CD flow simulation package. The effect of neighbouring tubes on one another is determined by qualifying and quantifying the flow over and the forces on the tubes. The vortex shedding frequency determined from the simulation coincides with experimental results. This shows great potential in future simulation of flow over a tube bundle. A computerised design model is also developed. Themodel determines the vibration potential of the tube bundle in a shell and tube heat exchanger. If needed, modifications to new designs and built heat exchangers can be done and the vibration potential is easily determined with the model. The results of the experimental work are used to further develop the design criteria preventing detrimental vibrations in heat exchangers. Promising results to decrease vibration amplitudes of tubes in tube bundles are obtained in the experiments done. This and the results obtained in the simulation of the vortex shedding frequencies that coincide with the results in the experimental work, show great potential for flow simulation and development of methods to decrease flow induced vibrations of tubes in shell and tube heat exchangers.
- Full Text:
- Authors: Jordaan, Christiaan Cornelius
- Date: 2015-02-12
- Subjects: Heat - Transmission , Turbulence , Eddies
- Type: Thesis
- Identifier: uj:13303 , http://hdl.handle.net/10210/13320
- Description: M.Ing. (Mechanical Engineering) , Shell and tube heat exchangers are commonly used in industrial processes. In the heat exchangers the tubes incur flow induced vibrations. These vibrations lead to fretting on the perimeter of the tube at the baffle plates and to fatigue failure at the tube sheets. Various studies were and are done to qualify and quantify these vibrations. This is then used to develop design criteria and methods for lowering the vibration amplitudes of the tubes. In the thesis the response of a tube bundle in cross flow is shown. The effect of increasing cross flow velocity on the vibration amplitude of the tubes is determined in a low speed wind tunnel. The results show similarity to the work done by Gorman [27]. By raising the natural frequency of the tubes in the first row upstream it is shown that the vibration peaks exhibited by the unmodified model.at cross flow velocities lower than the critical flow speed do not materialize in the modified tube bundle. This modification can be implemented by decreasing the unsupported length of the tubes in the first row. The inclusion of fluid damping when calculating the total damping of the tubes according to Blevins [24] is shown in the experiment. From experimental results it is shown that the prediction of the critical velocity according to Blevins [24] results in a better prediction of this velocity. Numerical solutions of two dimensional flow over a single cylinder and a pair of cylinders are obtained by simulating the flow on the STAR CD flow simulation package. The effect of neighbouring tubes on one another is determined by qualifying and quantifying the flow over and the forces on the tubes. The vortex shedding frequency determined from the simulation coincides with experimental results. This shows great potential in future simulation of flow over a tube bundle. A computerised design model is also developed. Themodel determines the vibration potential of the tube bundle in a shell and tube heat exchanger. If needed, modifications to new designs and built heat exchangers can be done and the vibration potential is easily determined with the model. The results of the experimental work are used to further develop the design criteria preventing detrimental vibrations in heat exchangers. Promising results to decrease vibration amplitudes of tubes in tube bundles are obtained in the experiments done. This and the results obtained in the simulation of the vortex shedding frequencies that coincide with the results in the experimental work, show great potential for flow simulation and development of methods to decrease flow induced vibrations of tubes in shell and tube heat exchangers.
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Heat transfer coefficient of a snow bag
- Authors: Da Veiga, Willem Richter
- Date: 2014-09-10
- Subjects: Heat - Transmission , Dry ice , Refrigerated transport , Snow bag
- Type: Thesis
- Identifier: uj:12226 , http://hdl.handle.net/10210/11993
- Description: M.Ing. (Mechanical Engineering) , In snow shooting, pressurised liquid carbon dioxide is injected via a lance into a permeable snow bag mounted near the ceiling of an insulated transport container. The decrease in pressure causes the liquid carbon dioxide to convert to "snow" and vapour inside the snow bag. The snow bag acts as a phase separator, allowing the sublimated snow to cool down the products inside a container. In this thesis the heat transfer coefficients of such a snow bag were determined experimentally and theoretically. It was found, that on average the measured heat transfer coefficient was 31% lower than the theoretical prediction. The theoretical model was used to correlate the experimental heat transfer coefficient as function of snow height. With this correlation it was possible to predict the time of the snow life to within 24%.
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- Authors: Da Veiga, Willem Richter
- Date: 2014-09-10
- Subjects: Heat - Transmission , Dry ice , Refrigerated transport , Snow bag
- Type: Thesis
- Identifier: uj:12226 , http://hdl.handle.net/10210/11993
- Description: M.Ing. (Mechanical Engineering) , In snow shooting, pressurised liquid carbon dioxide is injected via a lance into a permeable snow bag mounted near the ceiling of an insulated transport container. The decrease in pressure causes the liquid carbon dioxide to convert to "snow" and vapour inside the snow bag. The snow bag acts as a phase separator, allowing the sublimated snow to cool down the products inside a container. In this thesis the heat transfer coefficients of such a snow bag were determined experimentally and theoretically. It was found, that on average the measured heat transfer coefficient was 31% lower than the theoretical prediction. The theoretical model was used to correlate the experimental heat transfer coefficient as function of snow height. With this correlation it was possible to predict the time of the snow life to within 24%.
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Effects of condenser performance on thermal heat rate
- Authors: Narainsingh, Soomesh
- Date: 2014-03-17
- Subjects: Coal-fired power plants , Heat - Transmission , Condensers (Steam)
- Type: Thesis
- Identifier: uj:4346 , http://hdl.handle.net/10210/9695
- Description: M.Tech. (Mechanical Engineering) , Eskom fossil fuel power stations operate under various conditions and the ageing fleet of fossil fuel power stations are under strain due to the rising demand in electricity. The quest for cheaper electricity in growing businesses and bringing foreign investment into South Africa is significant for economic growth. Therefore, minimizing operation costs and improving availability, reliability and thermal heat rate are key objectives for the operation of the business. Thermal heat rate calculation is a method used to assess the performance of fossil fuel power stations. It provides an indication of the thermal performance which reflects the condition and operation of the plant. This dissertation refers to the tools used to measure the condenser performance and to identify the reason for unhealthy condenser performance and an increase in thermal heat rate. Problem: Poor condenser performance causes significant losses in generation and thermal heat rate in fossil fuel plants. Loss in generation and thermal heat rate is caused by condenser backpressure. This has an adverse effect on turbine thermal heat rate and henceturbine efficiency. The performance of the condenser affects the turbine performance to a large extent. Decreased condenser performance will increase the turbine thermal heat rate, i.e. kJ/kWh, because of the increased steam consumption per kWh of energy generated and therefore lowering the overall turbine efficiency. This will result in poor thermal heat rate efficiency resulting in an increased coal burn rate, i.e. kg/kWh, and an increased fuel bill and carbon foot print for the fossil fuel power station. The aim of this dissertation is to determine the effects of condenser performance on thermal heat rate and to provide the engineering and operating personnel with a guide for monitoring condenser back pressure in order to improve turbine thermal heat rate and turbine efficiency. Method and Procedure: Chapter 4 describes the method and procedure used for the pre and post outage evaluation to cost-effectively determine the current condition of the condenser and to quantify the increase in thermal heat rate due to condenser degradation. The pre and post outage evaluation offered an opportunity to evaluate and quantify the effectiveness of the maintenance program and the value of the capital layout undertaken during the scheduled Unit outage. Findings: Chapter 5 discusses the findings of the pre and post outage data analysis of the condenser using the EtaPRO system as the tool. Re-tubing 50% of the condenser inner loop tubes and cleaning the other 50% of the outer loop tubes demonstrated a decrease in condenser backpressure and therefore an improvement in thermal heat rate. This will also result in an improvement in turbine cycle efficiency and reduce coal cost.
- Full Text:
- Authors: Narainsingh, Soomesh
- Date: 2014-03-17
- Subjects: Coal-fired power plants , Heat - Transmission , Condensers (Steam)
- Type: Thesis
- Identifier: uj:4346 , http://hdl.handle.net/10210/9695
- Description: M.Tech. (Mechanical Engineering) , Eskom fossil fuel power stations operate under various conditions and the ageing fleet of fossil fuel power stations are under strain due to the rising demand in electricity. The quest for cheaper electricity in growing businesses and bringing foreign investment into South Africa is significant for economic growth. Therefore, minimizing operation costs and improving availability, reliability and thermal heat rate are key objectives for the operation of the business. Thermal heat rate calculation is a method used to assess the performance of fossil fuel power stations. It provides an indication of the thermal performance which reflects the condition and operation of the plant. This dissertation refers to the tools used to measure the condenser performance and to identify the reason for unhealthy condenser performance and an increase in thermal heat rate. Problem: Poor condenser performance causes significant losses in generation and thermal heat rate in fossil fuel plants. Loss in generation and thermal heat rate is caused by condenser backpressure. This has an adverse effect on turbine thermal heat rate and henceturbine efficiency. The performance of the condenser affects the turbine performance to a large extent. Decreased condenser performance will increase the turbine thermal heat rate, i.e. kJ/kWh, because of the increased steam consumption per kWh of energy generated and therefore lowering the overall turbine efficiency. This will result in poor thermal heat rate efficiency resulting in an increased coal burn rate, i.e. kg/kWh, and an increased fuel bill and carbon foot print for the fossil fuel power station. The aim of this dissertation is to determine the effects of condenser performance on thermal heat rate and to provide the engineering and operating personnel with a guide for monitoring condenser back pressure in order to improve turbine thermal heat rate and turbine efficiency. Method and Procedure: Chapter 4 describes the method and procedure used for the pre and post outage evaluation to cost-effectively determine the current condition of the condenser and to quantify the increase in thermal heat rate due to condenser degradation. The pre and post outage evaluation offered an opportunity to evaluate and quantify the effectiveness of the maintenance program and the value of the capital layout undertaken during the scheduled Unit outage. Findings: Chapter 5 discusses the findings of the pre and post outage data analysis of the condenser using the EtaPRO system as the tool. Re-tubing 50% of the condenser inner loop tubes and cleaning the other 50% of the outer loop tubes demonstrated a decrease in condenser backpressure and therefore an improvement in thermal heat rate. This will also result in an improvement in turbine cycle efficiency and reduce coal cost.
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Condensation heat transfer and pressure drop coefficients of R22/R142b in a water cooled helicaly coiled tube-in-tube heat exchanger.
- Authors: Kebonte, Shiko A.
- Date: 2012-08-20
- Subjects: Heat exchangers - Fluid dynamics , Heat - Transmission , Refrigerants , Condensation
- Type: Thesis
- Identifier: uj:2741 , http://hdl.handle.net/10210/6181
- Description: M.Ing. , Heat transfer and pressure drop characteristics during in-tube condensation of nonazeotropic mixtures of R22/R142b in a smooth helically coiled copper tube with an inside diameter of 8.11 mm are investigated. The experimental results are compared with prediction from correlation. The coefficient of performance of.the heat pump built and used for experiments has been studied. The mass flux of the refrigerant was varied during the course of the experiments. At similar mass flow rate of fluids, the average heat transfer coefficients for mixtures were lower than those for pure refrigerant R22 used as reference for comparison. Also, the heat transfer coefficients of all the refrigerants increased with increasing mass flux.
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- Authors: Kebonte, Shiko A.
- Date: 2012-08-20
- Subjects: Heat exchangers - Fluid dynamics , Heat - Transmission , Refrigerants , Condensation
- Type: Thesis
- Identifier: uj:2741 , http://hdl.handle.net/10210/6181
- Description: M.Ing. , Heat transfer and pressure drop characteristics during in-tube condensation of nonazeotropic mixtures of R22/R142b in a smooth helically coiled copper tube with an inside diameter of 8.11 mm are investigated. The experimental results are compared with prediction from correlation. The coefficient of performance of.the heat pump built and used for experiments has been studied. The mass flux of the refrigerant was varied during the course of the experiments. At similar mass flow rate of fluids, the average heat transfer coefficients for mixtures were lower than those for pure refrigerant R22 used as reference for comparison. Also, the heat transfer coefficients of all the refrigerants increased with increasing mass flux.
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Roasting of sulphide concentrates and acid leaching of calcines emanating from conventional and microwave assisted roasting
- Authors: Phiri, Thulane Raymond
- Date: 2012-06-05
- Subjects: Leaching , Gold - Metallurgy , Heat - Transmission , Microwave heating
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/380427 , uj:2395 , http://hdl.handle.net/10210/4849
- Description: M.Tech. , The study presented, investigated the acid leaching behavior of calcines emanating from conventional roasting and microwave assisted roasting of sulphide ores. The aim was to investigate a possible change in the mineral composition of the calcine produced and to investigate the leaching behavior of the calcine produced by conventional and microwave assisted roasting. Two sulphide concentrates were received, pyrite concentrate from OTJIHASE concentrator and sphalerite concentrate from ZINCOR. The mineralogical study was done using the XRD (PANalytical X-PertPro X-Ray diffractometer) to determine the mineral composition of the concentrate and the head grade of each concentrate was determined with an Atomic Absorption Spectrometry (AAS) machine for copper, iron and zinc. The concentrates were wet screened at 500, 355, 212, 150, 106 and 75 μm screens for size distribution and for the purpose of the study it was required that 80% of concentrate should pass the 75 μm screen since after roasting, the calcine obtained were to be acid leached. Conventional roasting was carried out at temperatures of 500oC, 700oC, 850oC and 1000oC at durations of 5min, 10min, 20min, 30min, 1hour and 2hours, while microwave assisted roasting carried out at powers of 200W, 400W, 600W, 800W and time intervals of 3s, 10s, 30s, 30min and 1hour
- Full Text:
- Authors: Phiri, Thulane Raymond
- Date: 2012-06-05
- Subjects: Leaching , Gold - Metallurgy , Heat - Transmission , Microwave heating
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/380427 , uj:2395 , http://hdl.handle.net/10210/4849
- Description: M.Tech. , The study presented, investigated the acid leaching behavior of calcines emanating from conventional roasting and microwave assisted roasting of sulphide ores. The aim was to investigate a possible change in the mineral composition of the calcine produced and to investigate the leaching behavior of the calcine produced by conventional and microwave assisted roasting. Two sulphide concentrates were received, pyrite concentrate from OTJIHASE concentrator and sphalerite concentrate from ZINCOR. The mineralogical study was done using the XRD (PANalytical X-PertPro X-Ray diffractometer) to determine the mineral composition of the concentrate and the head grade of each concentrate was determined with an Atomic Absorption Spectrometry (AAS) machine for copper, iron and zinc. The concentrates were wet screened at 500, 355, 212, 150, 106 and 75 μm screens for size distribution and for the purpose of the study it was required that 80% of concentrate should pass the 75 μm screen since after roasting, the calcine obtained were to be acid leached. Conventional roasting was carried out at temperatures of 500oC, 700oC, 850oC and 1000oC at durations of 5min, 10min, 20min, 30min, 1hour and 2hours, while microwave assisted roasting carried out at powers of 200W, 400W, 600W, 800W and time intervals of 3s, 10s, 30s, 30min and 1hour
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