Air-cooled heat exchanger systems (ACHE) are used widespread in the Oil&Gas, Petrochemical and Power industry where large amounts of heat have to be removed and discharged into the atmosphere. In LNG liquefaction plants they are used for condensing refrigerant gases and the plant productivity are strictly dependent on their performance [2]. Due to the interaction of the environmental conditions (ambient temperature, wind speed and direction) with the ACHE configurations, the heat removal efficiency and, subsequently, the Plant production rate can be severely lowered.
Three mechanisms are deemed responsible for the reduced performance of ACHE systems under windy conditions [2]:
1) Reduction of fan performance.
2) Recirculation of the hot exit air into the air inlet of the Air Cooler.
3) Circulation of hot gases coming from others process units/equipments.
Many international institutions/organizations are developing Test Codes to address the performance of Air Cooled Condensers due to its crucial importance in LNG and Power Plants [3].
Due to the peculiarity of site conditions and the complexity of plant layout, Computational Fluid Dynamic (CFD) modelling appears to be the only tool capable to take into account simultaneously the plant physical details, the heat source emissions and the site-specific ambient conditions [3]. SAIPEM, B.U. Onshore, has recently focused its business on LNG Plants and has planned to face the afore mentioned issues by developing CFD models to represent the whole LNG Plant area subjected to site-specific wind and temperature conditions. The aim is to predict and improve, the effectiveness of the heat removal as a function of different plant configurations and arrangement of ACHE. The model is intended to accomplish this by simulating the wind fluiddynamic all over the plant and, more specifically, how the air circulation is affected by the relative positioning of obstruction structures and by the presence of hot sources (flue-gas emission). This paper describes the used CFD model for a project currently under development. Specific submodels that have been developed to describe the wind induced flow over the plant, the heat exchange through the Air Cooler, etc. are detailed. Particular emphasis is devoted in describing the many hot sources (characterising the real Plant configuration) that have been implemented in the Boundary Conditions of the CFD model. Intermediate results of the application of the model to the project under development and main plant improvements are discussed. Moreover a preliminary evaluation of the model results reliability is provided by comparison with other CFD results available in the literature.
References
[1] V. Mehrotra, J. Berkoe, D. Messersmith, Evaluating Performance of Air-Cooled Heat Exchangers in LNG Plants, Prepared for presentation at the AIChE Spring National Meeting, New Orleans, 2003.
[2] Kröger, D.G., Air Cooled Heat Exchangers and Cooling Towers, PennWell Corporation, Tulsa, OK., U.S.A., 2004.
[3] Zammit K., Air-Cooled Condenser Design, Specification, and Operation Guidelines, Electric Power Research Institute, Technical Report, 2005. |
