Browsing by Author "Graaff, Andre Hendri"
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- ItemPerformance evaluation of a hybrid (dry/wet) cooling system(Stellenbosch : Stellenbosch University, 2017-03) Graaff, Andre Hendri; Reuter, Hanno Carl Rudolf; Owen, Michael; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: The performance of a hybrid (dry/wet) cooling system (HDWCS) is investigated. The HDWCS aims to compete with dry cooling systems with added performance stability during hot ambient periods. The system consists of two stages – the first, finned tube bundles operating dry, and the second, a bare tube bundle operating dry or wet. A numerical performance prediction model is developed to predict the performance of the HDWCS in terms of outlet process medium temperature, outlet air temperature and air-side pressure loss. For the finned tube bundles, the heat transfer coefficient correlation and pressure loss correlation proposed by Ganguli et al. (1985) are used to predict the air-side heat transfer coefficient and air-side pressure drop over the bundle. For the bare tube bundle under dry operation, the air-side heat transfer coefficient of Khan et al. (2006) and the air-side pressure drop correlation of Reuter and Anderson (2016) is used. For the bare tube bundle under wet operation, the mass and heat transfer coefficients proposed by Mizushina et al. (1967) are used together with the air-side pressure drop correlation from Reuter and Anderson (2016) to predict the thermal performance. Analysis of both the finned tube bundles and the bare tube bundle are based on a so-called integral method, where the outlet conditions were determined across the bundles from inlet conditions. Experimental work investigates air flow rate, to avoid high air pressure drop across the bare tube bundle, and the minimum water flow rate to avoid dry-out inside the bare tube bundle are determined. A maximum air mass velocity of Gav = 3 kg/sm2 and a minimum water mass velocity of Гdw/do = 1.89 kg/sm2 is suggested. Thermocouple attachment methods are investigated for the bare tube bundle under wet operation. It is found that by attaching the thermocouples with 4 zip ties instead of 3, the certainty for measuring water temperature is significantly improved under the suggested air and water flow rates. The air-side pressure drop across the bare tube bundle is influenced by the number of tube rows present, together with air and water flow rates. The pressure drop under wet operation is slightly higher than under dry operation. The bare tube bundle’s dimensions are determined from a parametric study, delivering a bare bundle, 800 mm in both width and height which can boost the cooling systems’ performance by between 35 % and 140 % relative to a conventional system, depending on air relative humidity at an ambient dry bulb temperature of 32 ºC. Inlet process medium temperature remains at 38 ºC. The HDWCS can serve as a strong competitor, delivering a significant increase in performance compared to dry coolers especially during hot periods, when the performance of dry only cooling systems are low.