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A novel "partial bypass" concept to augment the performance of air-cooled heat exchangers
(2012-09-01)
Wang, Chi-Chuan
;
Chen, Kuan-Yu
;
Liaw, Jane-Sunn
;
Tseng, Chih-Yung
;
機械工程學系
This study proposes a novel "partial bypass" concept which facilitates the heat transfer augmentation without the pumping power penalty. The basic idea is to introduce bypass cold stream which bypasses the front part of the heat exchanger and mixes with the hot stream across the front part of the heat exchanger, and the mixed stream flows through the rear part of the heat exchanger. Through this concept, heat transfer performance at the front part is slightly reduced but the rear part is substantially increased. The concept can be implemented with either maintaining the same total flowrate, heat transfer rate but accompanies with a much lower pressure drop or it can be applied with the same pumping power but accompanies with a much higher heat transfer rate. It is found that the "partial bypass" concept is especially fruitful for a heat exchanger with a comparatively large number of transfer unit (NTU). The concept is especially helpful in the design of a deep tube bank operated at lower frontal velocity. For validating the proposed concept, the calculation is also compared with the experimental measurements, and good agreement amid the calculations and measurements are reported. (C) 2012 Elsevier Ltd. All rights reserved.
Performance of bare-tube bundle having small diameter tube: With and without partial bypass
(2015-10-01)
Chang, Neng-Chieh
;
Li, Chao-Jen
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
The present study experimentally investigates the air-side performance of the staggered bare tube bundle having a small diameter tube in association with the partial bypass design. A total of ten arrangements are made and tested with corresponding surface reduction of 5%, 10%, and 20%, respectively. The measured heat transfer and pressure drop results of the conventional bare tube bundle agree well with the existing predictions. For the typical tube bundle, both heat transfer rate and heat transfer coefficient (HTC) first increase with the tube row and peaks at the fourth row. The HTC then remains almost unchanged when the tube row is further increased. Through the partial bypass designs, the front part of the tube bundle suffers some heat losses for having some fewer available surfaces but the rear part of the tube bundle will achieve a higher heat transfer rate for a higher temperature difference. The phenomenon of losing heat transfer in the front part but gaining performance in the rear part becomes more pronounced with the surface area reduction. For the partial bypass design with the same area reduction, it appears that the vertical reduction designs are marginally better than those lateral reduction designs. For comparisons of the effective thermal resistance subject to the same pumping power. It is clearly seen that the convectional design still outperforms all partial designs. As the pumping power becomes larger, the performance of the "partial bypass" designs are slowly and gradually improved to close to the traditional tube bundle. The difference is comparative small with the 5% and 10% "partial bypass" design. This trend indicates that the "partial bypass" is more beneficial at the operating condition of higher pumping power especially for the 5% surface reduction design. (C) 2015 Elsevier Ltd. All rights reserved.
Effect of partial bypass on the heat transfer performance of dehumidifying coils
(2014-11-01)
Wang, Chi-Chuan
;
Cheng, Yu-Cheng
;
Liaw, Jane-Sunn
;
Tseng, Chih-Yung
;
機械工程學系
This study extends a so called "partial bypass" concept to dehumidifying fin-and-tube heat exchangers. Tests are performed for a 2-row and 4-row fin-and-tube heat exchangers having plain fin configuration. The inlet dry bulb temperature is fixed at 25 degrees C while the inlet relative humidities are 50% and 80%, respectively. Test results showed that for a 2-row coil under RH = 50%, the corresponding heat transfer augmentation ratio (Q(R)) and pressure penalty ratio (P-R) decrease with the rise of bypass ratio (BR). At a smaller frontal velocity, the regime of appropriate bypass ratio where Q(R) > 1 and P-R < 1 is more apparent. The effect of partial bypass decreases as the velocity increases. As the velocity gets close to 2 m s(-1), it shows that the performance at high bypass ratio becomes more efficient. At the same time, when bypass ratio becomes lager, the tendency of the decreasing of Delta P is more drastic than the decreasing of Q. Nevertheless, not for all the cases that the performances get better as the bypass ratio increases. For V-fr = 4 m s(-1), the experiment results show that the overall effect is less efficient\' than at V-fr = 2 m s(-1). The major reason is caused by the flow pattern. The rise of bypass airflow is quite large which may act as an air curtain to distort the main airflow, and result in higher pressure drop. This implies that the design of the bypass design is quite imperative in the practice of real applications. For a 4-row heat exchanger, the experiment data of QR and PR for the 4-row coil performs better than a 2-row coil. (C) 2014 Elsevier Ltd. All rights reserved.

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