Master of Science

The design, construction, speculation, and observation of a small wooden tower. / Master of Architecture

Typescript (photocopy). / Digitized by Kansas Correctional Industries

published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

Cooling water systems are used to remove excess heat from a chemical process to the atmosphere. The primary components of these systems are the cooling tower and the heat exchanger network. There is a strong interaction between these individual components, thus their performances are interrelated. Most published research in this area has focused mainly on optimization of the individual components i.e. optimization of heat exchanger network or optimization of the cooling towers. This approach does not optimize the cooling water system as a whole. Previous research work in which a holistic approach was used is limited to cooling water systems with single cooling water source. This work presents a technique for integrated optimization of complex cooling water systems. The system under consideration consists of multiple cooling towers each supplying a set of heat exchangers. A superstructural approach is employed to explore all possible combinations between the heat exchangers and the cooling towers. The cooling water reuse opportunities within the heat exchanger networks are also explored. A detailed mathematical model consisting of the cooling towers and the heat exchanger networks model is developed. Two practical scenarios are considered and the mathematical formulations for Case I and II yield nonlinear programing (NLP) and mixed integer nonlinear programming (MINLP) structure respectively. Although the reuse/recycle philosophy offers a good debottlenecking opportunity, the topology of the associated cooling water network is more complex, hence prone to higher pressure drop than the conventional parallel design. This is due to an increased network pressure drop associated with additional reuse/recycle streams. Therefore, it is essential to consider pressure drop during the synthesis of cooling water networks where the reuse/recycle philosophy is employed. The on-going research in this area is only limited to cooling water networks consisting of a single cooling water source. The common technique used is mathematical optimization using either superstructural or non superstructural approach. This work further presents a mathematical technique for pressure drop optimization in cooling water systems consisting of multiple cooling towers. The proposed technique is based on the Critical Path Algorithm and the superstructural approach. The Critical Path Algorithm is used to select the cooling water network with minimum pressure drop whilst the superstructural approach allows for cooling water reuse. The technique which was previously used in a cooling water network with single source is modified and applied in a cooling water network with multiple sources. The mathematical formulation is developed considering two cases. Both cases yield mixed integer nonlinear programming (MINLP) models. The cooling tower model is also used to predict the exit condition of the cooling tower given the inlet conditions from the cooling water network model. The results show up to 29% decrease in total circulating cooling water flowrate when the cooling water system is debottlenecked without considering pressure drop. Consequently, the overall cooling towers effectiveness was improved by up to 5%. When considering pressure drop the results showed up to 26% decrease in total circulating water flowrate. / Thesis (PhD)--University of Pretoria, 2013. / gm2013 / Chemical Engineering / unrestricted

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The transmission line (TL) tower is an important component in electrical network system. These towers consist of members (angle sections) and connections (bolted connections) plus foundation, which act together to resist externally applied loads. The latticed towers are used to support conductors in transmission network for transmission and distribution of electricity. These towers are constructed in large numbers all over the world. The connections in electric TL classical latticed towers are peculiar compared to other types of bolted connections in buildings and bridges because (i) the angle members are connected directly or through gusset plates with bolts, (ii) the eccentric application of load due to the non-coincidence of centroid axes of angle members near the connection and (iii) members are designed as beam column element to sustain tensile or compressive forces. Bearing type bolts are used in TL towers in preference to friction type bolts, because they (i) connect thin walled angle members, (ii) are easy to use for erection at all heights, (iii) can be galvanized, (iv)erosion of galvanizing can be remedied and (v) do not require skilled personnel for installation. However, these connections are subjected to reversal of stresses due to wind load. Damage in the bolted connections generally occur due to loosening of bolts due to stress reversals (Feenstra et al. (2005) [23). The damage induced after extreme wind and earthquake may lead to collapse of the whole tower. The failure of a TL tower results in power shut down, which has huge impact on national economy. Hence, the structural safety and reliable performance of these towers are extremely important. The design of TL tower is based on minimum weight philosophy. The TL towers are highly repetitive and therefore, their designs need to be commercially competitive. The TL tower design has the following deficiencies such as misappropriate design assumptions, deficit detailing, defects in material, errors in fabrication, force fitting of members during erection, variation in grade of bolts, improper gusset plate detailing, notch cutting of member, vocalization of bolt holes, etc. Hence, to check the design and detailing aspects of members along with bolted connections and to study the behavior of tower under complex loading conditions, the prototype testing of tower is made mandatory requirement in many countries throughout the world. The structural behavior of TL tower is determined from its deflection response. Thus, the full scale testing of the towers is the only way that one can counteract the un conservatism due to structural analysis. The premature failure of TL towers occurs during prototype testing due to deficiencies in joint detailing, uncertainties in framing eccentricity, force fitting of members, unequal force distribution in bolts and gusset plate connections, etc. To have better structural response of TL tower to be tested, there is need to develop reliable model for bolted connections in TL towers. The bolted connection model plays an important role in determining the deflection response and predicting the premature member buckling failure of TL towers. The issues related to prototype testing of full scale TL towers such as fabrication errors, force fitting and notch cutting of members, application of loads, joint and crossarm detailing are discussed. The need to develop bolt slip model to simulate the actual behaviour of bolted connection in TL towers is also discussed. The bolted connections in TL towers play an important role in determining its structural behavior. The angle members used in TL towers are subjected to bi-axial bending in addition to axial load. The slip will occur in the bolted connections, due to the provision 1.5 mm bolt hole clearance. In the conventional Finite Element Analysis (FEA), the bolted connections are modeled as pin joint assuming the axial load transfer. The deflection predicted from pin joint analysis in TL towers generally does not match with experimental results. The analytical and experimental deflection value varies in the range of 30 to 50%. Hence, there is need to develop model to account bolt slip for accurate deflection and dynamic characteristic prediction of TL towers. Experimental and analytical investigations have been carried out to develop and validate bolt slip model for bolted connections in TL towers. All six degrees of freedom (both translational and rotational) have been considered to simulate the exact behaviour of bolted connections in TL towers. The model is developed based on experimental results of Ungkurapinan’s bolt slip model for axial stiffness. The rotational stiffness is formulated based on the component level experiment conducted on lap joint made of steel angle with single and double bolt subjected to tensile loading. The axial and rotational stiffness for different stages of bolt tightening is also formulated based on component level experimental investigation on lap joint. The proposed model is validated by comparing with experimental results at sub-structural level on full scale king post truss subjected to tensile loading. Further the bolt slip model is validated for different bolt tightening and failure prediction of TL tower sub panel subjected to tensile loading. Finally the proposed model is also validated for full scale TL tower for deflection prediction. NE NASTRAN, a nonlinear finite element analysis (FEA) software is used for analytical simulation and the load-deflection predictions, which are compared with the corresponding experimental results. The experimental and analytical results are in good agreement with each other. The steel pole structures are replacing the conventional lattice towers, because they have smaller plan dimension and occupy less space, when compared to lattice towers. The steel pole structures are dynamically sensitive structures and the determination of their natural frequency is extremely important. For the calculation of wind load through gust factor method, the preliminary estimation of natural frequency is required. Hence, the primary step involved in dynamic analysis is the evaluation of its natural frequency. Hence, a simplified model is proposed based on model order reduction technique for the evaluation of natural frequency of TL towers and steel pole structures. For the development of base line model to detect damage in TL towers, the natural frequency has to be updated. A semi empirical approach is proposed based on the deflection by using the proposed bolt slip model. The proposed approach of updating natural frequency is validated for different cases of member damage in TL tower sub panel, such as removal of tension, compression and hip bracing members. The transmission pole structures accumulate damage during their service life. Damage in these structures will cause a change in stiffness of the system and the physical properties of these structures, such as modal frequencies and mode shapes. Hence in the present study, the damage localization study based on modified modal strain energy approach is carried out for steel pole structures and the location of damage is identified correctly. To prevent premature failure of towers during its service life testing and failure analysis of TL towers is a mandatory requirement. In the present study, forensic failure investigation of a full scale TL tower due to deficient design of a redundant member is emphasized and the remedial measures are explained in detail. The stub failure of TL tower due to reduction in cross sectional area due to unfilled bolt hole is also discussed. To investigate the effect of unfilled bolt holes on the compression capacity of leg member, detailed FEA is carried out and compared with experimental results. The reason for failure of 9 m roof top communication tower due to redundant member deficiency is also discussed. The importance of guyed tower accessories in the guy rope design of 7 m roof top guyed pole structure is also investigated. Finally, failure investigation of compression bracing member, which has failed during testing of TL tower sub panel has been investigated. The failure load is predicted by using the proposed bolt slip model in the analysis. Thus the overall research contributions emerging from this thesis are, i) development of bolt slip model accounting for rotational stiffness, ii) development of direct method of damage detection for steel pole structures based on modified modal strain energy approach, iii) development of simplified model for prediction of natural frequency of TL tower and steel pole structures, iv) development of model updating technique through natural frequency based on semi-empirical approach and v) prediction of failure load for TL tower panel using the proposed bolt slip model.

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Thesis (MScEng)--University of Stellenbosch, 2008. / ENGLISH ABSTRACT: The performance of a natural draught wet-cooling tower can be improved by reducing the average drop size in the rain zone. In this thesis, the effect of installing different horizontal grids below the fill on drop size in the rain zone is investigated experimentally and theoretically. A specially designed horizontal grid consisting of evenly spaced slats and a grid made from expanded metal sheeting are tested. Drop size distribution measurements are taken below different cooling tower fills to determine the respective Sauter mean drop sizes and also below different configurations of splash grids to determine the reduction in drop size. Drop break-up through a grid of horizontally placed slats is modelled and compared to measured data to determine the optimum configuration in terms of spacing between the grid and fill, slat width and slat spacing. A cross flow rain zone is modelled under different air and water flow combinations with CFD for two distributions that represent the rain with and without splash grids and the results are compared. The Merkel transfer characteristic for all the flow conditions using both distributions are determined using a Lagrangian, Merkel, Poppe and e- NTU method in order to quantify the increase in rain zone Merkel number. Pressure drop over the cross flow rain zone is also determined and compared for the two distributions under considerations. / AFRIKAANSE OPSOMMING: Die verkoelingsvermoë van ‘n reënsone van ‘n natuurlike trek nat koeltoring kan verbeter word deur die verkleining van die gemiddelde druppelgrootte. In hierdie tesis word die effek wat horisontale roosters op die druppelgrootte het, wanneer dit onder die pakking geïnstalleer is, eksperimenteel en teoreties ondersoek. ‘n Spesiaal ontwerpte rooster bestaande uit horisontaal gepakte latte en ‘n gerolde metaal rooster word onderskeidelik vir hierdie doel gebruik. Druppelgrootte metings word geneem onder verskillende koeltoring pakkingsmateriaal om die Sauter gemiddelde diameter te bepaal, asook onder die verskillende rooster opstellings om die verkleinde druppelgrootte te bepaal wat die rooster veroorsaak. Druppelopbreking deur ‘n laag horisontaal gepakte latte word gemodelleer en vergelyk met gemete data om sodoende die beste kombinasie tussen die afstand onder die pakkingsmateriaal, latwydte en latspasiëring te bepaal. ‘n Kruisvloei reënsone word gemodelleer met CFD onder verkillende lug- en watervoeikombinasies vir twee druppelverdelings wat die reënsone met en sonder roosters verteenwoordig. Die Merkel oordragskoëffisiënt vir die twee verdelings word bereken en vergelyk deur van ‘n Lagrange- , Merkel- , Poppe- en e-NTU metode gebruik te maak om sodoende die verbetering in reënsone Merkelgetal te kwantifiseer. Drukvalle oor die reënsone word ook bereken en vergelyk vir die twee verdelings wat beskou is.

Thesis (MScEng)--Stellenbosch University, 2007. / ENGLISH ABSTRACT: A natural draught wet-cooling tower (NDWCT) was modelled using the Merkel method with an improved energy equation as recommended by Kloppers and Kroger (2005a) - referred to as the Improved Merkel method. The improved energy equation is used for calculating the heat rejection rate of the tower and includes the energy associated with water evaporation. The sensitivity indexes of a NDWCT were calculated numerically with the Improved Merkel method model. It was found that the perfonnance of a NDWCT is most sensitive to the fill Merkel number. The "Natklos" fill test facility at Stellenbosch University was used to estimate typical uncertainties found in fill performance characteristics. The zeroth order uncertainty for the Merkel number and loss coefficient was calculated to be 0.2100 m-1 and 0.4248 m- 1 , respectively, while the first order uncertainty for the Merkel number and loss coefficient was calculated to be 0.1933 m- 1 and 0.2008 m-1 , respectively. ASME requires that the uncertainty in tower capability has to be less than 6 % for a NDWCT perfonnance test to be deemed ASME approved. Propagating typical measurement uncertainties found in NDWCT test standards and experimental data into the tower capability showed that the 6 % uncertainty limit imposed by ASME is unrealistic and too stringent. Performance curve generator (PCG) is a software package developed that generates NDWCT perfonnance curves. With these performance curves it is possible to easily and effectively adjust the off-design test results in order to detennine whether the NDWCT has met its guarantee or not. / AFRIKAANSE OPSOMMING: Die werksverrigting van 'n natuurlike trek nat koeltoring (NTNT) is gemodelleer deur gebruik te maak van die Merkel metode met 'n verbeterde energie vergelyking, soos aanbeveel deur Kloppers en Kroger (2005a) - Verbeterde Merkel metode. Die energie vergelyking word gebruik om die toring se tempo van warmteoordrag te bereken en sluit die energieverlies as gevolg van verdamping in. Die Verbeterde Merkel metode model was gebruik om die sensitiwiteits-indekse van 'n NTNT te bepaal. Die analise toon dat die toring se werksverrigting die sensitiefste is vir die pakking se Merkel getal. Die Natklos pakkingstoetsfasiliteit aan die Universiteit van Stellenbosch was gebruik om tipiese onsekerheid in die pakkingsprestasiekarakteristieke te bepaal. Die zero-orde onsekerheid in die Merkel getal en verlieskoeffisient was bereken as 0.2100 m· 1 en 0.4248 m· 1 , onderskeidelik, terwyl die eerste-orde onsekerhede bereken was as 0.1933 m·1 en 0.2008 m· 1 , onderskeidelik. Die toelaatbare onsekerheid in toringvennoe vir 'n NTNT aanvaardingstoes volgens ASME is 6 %. Deur tipes meetonsekerhede, soos gegee deur NTNT aanvaardings-toesstandaarde sowel as eksperimentele data, deur te propageer, word 'n onsekerheid veel groter as die toelaatbare 6 % gegenereer. 'n Renekaarpakket, genaamd Performance Curve Generator (PCG), is ontwikkel om werksverrigtinskurwes vir 'n NTNT te genereer. PCG se werksverrigtinskurwes maak dit moonltik om maklik te bepaal of a NTNT sy ontwerpskriterea bereik het of nie.