Experimental study to improve chimney performance by using a natural draft air-cooled heat exchanger laboratory model

Md. Mizanur Rahman (2011) Experimental study to improve chimney performance by using a natural draft air-cooled heat exchanger laboratory model. Doctoral thesis, Universiti Malaysia Sabah.

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Abstract

Natural draft air-cooled heat exchangers (NDACHE) are well known for low energy consumption. However, the main identified problems for NDACHE are strong crosswind, cold inflow and flow recirculation that result in significant reduction in its efficiency. The cold air inflow at chimney exits have been observed in cooling devices such as cooling towers, solar chimneys to electronic vertical channels. The loss of draft in chimneys due to cold inflow has not yet been measured. Therefore, the aim of this research was to improve the chimney performance of a NDACHE model and minimize the losses. The main objective of this study was to investigate the performance of natural draft air-cooled heat exchanger and find out the existence of flow reversal in NDACHE models. To carry out the study, an experimental model was designed and fabricated. An empirical model was also developed to explain the behavior of chimney. Experiments were conducted in NDACHE models at three different face areas 0.56 m2, 1.00 m2 and 2.25 m2 with different solid wall chimney heights varied from 0.3 m to 1.2 m and heat loads varied from 1.00 kW to 2.5 kW. A total of 96 runs were carried out at triplicate readings for wire-meshed and non-wire meshed chimney configurations. The experimental results showed that the average airflow velocities through the NDACHE models were from 0.01 ms-1 to 0.1 ms-1 for different heat loads. Multiple factors are believed to cause cold inflow, which was discussed. The cold inflow weakens the chimney efficiency by increasing loss of air flow velocity and differential pressure. The experimental results show that the measured entrance air velocity and differential pressure of chimney with wire mesh configuration were higher than that without wire mesh. The wire mesh minimized the cold inflow and restored the draft loss in NDACHE models. Critical velocity and Froude number calculations indicated that the existence of cold air inflow in the improved NDACHE model, where as the smoke test results showed significant defense against cold air inflow. For the plume chimney height to be more than zero, cold air inflow would need to be absent in the system. The ratio between total effective height and solid wall chimney height was more than unity, when the apparent heat gain ratio was 0.80 and higher. The standard deviation of temperature inside the solid wall chimney of NDACHE models varied from 0 to 6 K that indicated solid wall chimney were free from the cold wall problem as well as cold inflow at exit point. Effective plume-chimney heights were also calculated from the measured differential pressure by performing an aerodynamic balance. The magnitude of effective plume chimney height was significantly larger than experimental error providing good indication of the existence of the cold inflow. Detailed investigation was carried out by determining the plume axial and radial temperature profiles, where results show that the exit air temperature at the center line axis of flow above the hot screens do not indicate the existence of a zone of flow establishment. The wire-meshed chimney configurations significantly reduced the effect of cold inflow in NDACHE models. In this configuration, the chimney efficiency increased by decreasing the losses of airflow velocity and differential pressure. Therefore, the wire meshed chimney configuration could be used in the design of NDACHE when the apparent heat gain ratio was more than 0.8 to enhance its performance.

Item Type:	Thesis (Doctoral)
Keyword:	Natural Draft Air-Cooled Heat Exchangers, Cold air inflow, Chimney performance, Flow reversal, Airflow velocity, Differential pressure, Wire mesh configuration
Subjects:	T Technology > TH Building construction > TH1-9745 Building construction > TH2025-3000 Details in building design and construction Including walls, roofs
Department:	SCHOOL > School of Engineering and Information Technology
Depositing User:	DG MASNIAH AHMAD -
Date Deposited:	11 Apr 2025 10:02
Last Modified:	11 Apr 2025 10:02
URI:	https://eprints.ums.edu.my/id/eprint/43454

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