Investigation of performance enhancing devices for the rain zones of wet-cooling towers

Terblanche, Riaan (2008-03)

Thesis (MScEng)--University of Stellenbosch, 2008.

Thesis

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.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/21441
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