D.Ing.

Cooling water systems are generally designed with a set of heat exchangers arranged in parallel. This arrangement results in higher cooling water flowrate and low cooling water return temperature thus reducing cooling tower efficiency. Previous research on cooling water systems has focused mainly on heat exchanger network thus excluding the interaction between heat exchanger network and the cooling towers. The studies completed on cooling water system in which the interaction between the cooling tower and the heat exchanger network was taken into consideration were limited to systems with single cooling tower. The main aim of this study was to develop a design methodology for synthesis and optimization of cooling water systems with multiple cooling towers. The design intends to debottleneck the cooling towers by reducing the circulating water flowrate. The study focuses mainly on cooling systems consisting of multiple cooling towers that supply a common set of heat exchangers. In this work the mathematical optimization technique was developed for optimization and synthesis of cooling water system. The heat exchanger network was synthesized using the mathematical optimization technique. This technique is based on superstructure in which all opportunities for cooling water reuse are explored. The cooling tower model was used to predict the thermal performance of the cooling towers while taking the thermal conditions of the associated heat exchanger network into account. The propose technique debotlleneck the cooling towers by decreasing the circulating water flowrate. This implies that a given set of cooling towers can manage an increased heat load. From the case studies, 22% decrease in circulating water flowrate was realized. The blowdown and makeup were also decreased by 7%. Furthermore, the cooling tower effectiveness was also improved by 4%. A decrease in the overall circulation water has an added benefit of decreasing the overall power consumption of the circulating pumps. There is also a potential for the reduction of makeup and blowdown water flowrate. / Dissertation (MEng)--University of Pretoria, 2011. / Chemical Engineering / unrestricted

An experiment was designed and carried out on the fundamental, but poorly understood problem of oscillating flow past a single, transverse, circular cylinder. This is an approximation of the flow about a single element in a matrix-type regenerator used in Stirling-cycle engines. The experimental rig was designed and built to allow tests to be carried out for the wide range of fluid flow parameters characteristic of various Stirling engines. The influence of these parameters on convective heat transfer rates was measured so the approximate effects of these same parameters on a Stirling engine regenerator could be determined. The main conclusion from the experiment was that average Nusselt numbers, based on test-cylinder diameter and subject to flow conditions similar to those found in Stirling engine regenerators, were 40 to 80% higher than those predicted by a steady flow correlation, for a given Reynolds number. This may be due to the high levels of turbulence generated near the test-cylinder. A secondary conclusion is that the compression and expansion of the working fluid due to a 90 degree phase angle difference between the motion of the pistons raises convective heat transfer rates from the test-cylinder substantially over the 180 degree phase angle, or "sloshing" motion case. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

M.Ing. / Dry air cooled heat exchangers form a vital part of industrial heat transfer systems, especially in countries where the supply and availability of clean cooling water is limited. Headerboxes are rectangular pressure vessels that act as the inlet distribution and outlet collection devices. As rectangular pressure vessels, headerboxes are subject to design codes such as ASME (ASME Section VIII, Division 1, 2007). Unfortunately ASME (ASME Section VIII, Division 1, 2007) offers no guidance on how to allow for the effect of external loads applied to the headerbox through the vessel’s nozzles. This creates a difficult situation, since vessel designers are mandated by ASME to consider the effects of nozzle loads by American Petroleum Institute standard 661 (API, 2006). The aim of this project was therefore to develop a closed form design methodology that accurately predicts the stresses inside a headerbox that is subject to external loadings as well as internal pressure. After extensive research it was decided that the only viable approach would be to extend ASME’s rigid frame theory. This was done, and a new set of equations describing the stress distribution inside a headerbox were derived. These equations were then tested using 2D Finite Element Analysis (FEA) to determine whether they represented the reality of the 2D model they described. It was found that the equations were accurate enough in 2D and the next step was to test the model experimentally and using full 3D FEA. A local manufacturer of air cooled heat exchangers was approached and they helped design an experimental specimen and agreed to fund its construction. Unfortunately, due to time constraints, it was not possible to build and test the specimen experimentally. The specimen geometry was then analysed using the Abaqus (Dassault Systѐmes Simulia Corp., 2010) FEA package. The 3D FEA analysis considered several different load cases. After carefully analysing the results it was seen that the rigid frame model could make useful qualitative statements about the effects of the nozzle loads, but it performed poorly as a quantitative prediction method. However, since the effects of the nozzle loads are generally quite small it is possible that, with appropriate safety factors, the rigid frame model could be used as a conservative design methodology. The usefulness of a commonly used empirical guideline was also examined. This project is far from conclusive and much more work is required to fully examine the usefulness of rigid frame theory. That being said, this project has made important steps towards a more complete understanding of rectangular pressure vessels and has shown possible ways forward.

M.Ing. / The research in this thesis focuses on the theoretical thermal modelling and design of a Latent Heat Storage system (LHS) for an absorption refrigeration machine. A shell-and-tube latent heat storage exchanger retaining any excess solar thermal energy is selected. Here, solar thermal energy supplied by a collector is transferred to and stored by the LHS. During low insolation, stored thermal energy is transferred by a Heat Transfer Fluid (HTF) into the generator, a component of an Ammonia Absorption Refrigerator (AAAR), to ensure efficiency of the cooling cycle. The shell-and-tube LHS contains Phase Change Material (PCM) which fills space outside the tube heat exchangers. The HTF flowing through the tubes exchanges thermal energy with the PCM. The selection of a suitable PCM for a LHS is based on several factors. A primary criterion for an efficient, reliable storage unit is the correct melting point of the PCM at a desired operating temperature of the heating application. An analytical model describing both the freezing process in the PCM and increased HTF temperature in the tube heat exchangers is investigated. The model is developed using energy balance equations. It is solved in terms of dimensionless parameters. The thermal resistance of the tube heat exchangers is considered for this model. From the result of the analytical model, the design approach to size the LHS is provided and the different steps are given in order to determine the volume, mass, number of tube heat exchangers, inner and outer radius of the tube heat exchangers and other parameters of the LHS. The dimensions of LHS are given as a function of a storage period, PCM properties, HTF properties, inner and outer radius of the tube heat exchangers, material of construction of the tube heat exchangers and the nature of load on the heating process. Simulations from the analytical model developed are provided for the output thermal parameters of the storage system. These thermal parameters of the shell-and-tube latent exchanger are given in terms of the HTF outlet temperature, the front solidification of the PCM and the heat transfer rate during the solidification process of the PCM. A case study to demonstrate the application of the design approach with respect to the size shell-and-tube latent heat exchanger is provided.The integration of the tube heat exchangers thermal conductivity in the modelling of the LHS resulted in an increase of 2% in mass of the storage material compared to an analytical model neglecting the thermal conductivity of the tube heat exchangers. The results of the model developed compared well with the results obtained from other analytical models at similar operating conditions.

In recent years much attention has been devoted to the development of efficient heat exchangers. The regenerative or moving matrix heat exchanger is one type which has been considered. Most regenerative heat exchangers use one of the conventional types of high area matrices mounted in a drum or disk in such a manner that it can be moved in a continuous cycle from a cold fluid stream to a hot fluid stream and back again. A rather different approach to the design of a regenerative heat exchanger which has been patented in this country is a set of flexible moving bands which carry heat energy from one fluid stream to another in a continuous manner. Such a regenerator is illustrated in Figure 1. This is essentially a parallel plate heat exchanger with a variation in the temperature of the plates and the fluid in the direction of the length of the plate.

No description available.

A computer model of a fin and tube air-to-air heat exchanger is presented. The model incorporates a computational scheme to account for latent effects due to small amounts of moisture in one or both fluid streams. A testing program is described which was performed in order to mathematically characterize the heat transfer and pressure drop relationships of the tube with turbulator used in the heat exchanger. These relationships are included in the computer model. A comparison of the computer model to heat exchanger test data indicates that the computer model may be relied upon to provide design and analysis information. Finally, a parametric study is performed using the computer model in order to explore the characteristics of the heat exchanger and to demonstrate its usefulness to the heat exchanger designer. It is concluded that, in addition to presenting an analysis tool for heat exchanger design, there are several important secondary results. These include: verification of the modelling techniques, the analytical description of the tube with turbulator and, identification of a problem area in the header design.

Yes / Fouling in heat exchangers (HE) is a major problem in industry and accurate prediction of the onset or degree of fouling would be of a huge benefit to the operators. Modelling of the fouling phenomenon however, remains a challenging field of study. Cleaning of heat exchangers, coulpled with the down time, is a financial burden and for industrialized nations and costs can reach to almost 0.25 % of the country’s Gross National Product (Pritchard, 1988).This work presents the development of a dynamic fouling model based on experimental data collected using a laboratory concentric tube heat exchanger handling a saline system. Heat transfer coefficients were obtained from first principles as well as from either the Sieder-Tate or Petukhov-Kirillov correlations modified by Gnielinski depending on the flow regime. The outlet temperatures were calculated using the Effectiveness-NTU method. The dynamic fouling factor was based on the Kern and Seaton fouling model and validation was completed by comparing the experimental outlet temperatures with those predicted by the model. The model predicts the outlet temperatures with an average discrepancy of 1.6 °C and 0.4 °C for the cold and hot streams respectively.

No description available.