Augmentation of heat transfer performance in coiled flow inverter vis-à-vis conventional heat exchanger
Introduction
Coiled tubes are commonly used in industries due to their compact structure and high heat and mass transfer coefficient. They are frequently encountered in practical applications, such as heat exchangers, chemical reactors and piping systems. Numerous studies (Adler, 1934; Ito, 1959; Kubair and Kuloor, 1966; Mori and Nakayama, 1967; Shchukin, 1969; Dravid et al., 1971; Kalb and Seader, 1972; Mishra and Gupta, 1979; Nigam and Saxena, 1986) have been carried out to understand the transport phenomenon in coiled tubes after the pioneering work of Dean, 1927, Dean, 1928. Several review papers have been published in order to understand the hydrodynamics in curved tube for fluid flowing under laminar and turbulent flow regime. Recently, Vashisth et al. (2008) presented an exhaustive review paper dealing with fluid flow, heat transfer and mass transfer in curved tubes for wide range of process conditions. They also reported limited studies on new class of chaotic configuration introduced by Saxena and Nigam (1984). The innovative CFI device developed by Saxena and Nigam (1984) was based on the concept of multiple flow inversions which were achieved by changing the direction of centrifugal force in helically coiled tubes. They suggested an optimal design configuration which consisted of 90° bends at equal intervals of length in coiled tube geometry. They showed that the device behaves as a plug flow reactor. Kumar et al. (2007) carried out experimental study in coiled flow inverter (CFI) heat exchanger at pilot plant scale to investigate heat transfer and pressure drop at low Reynolds number. The heat exchanger consisted of CFI tube in form of eight banks of helical coil connected in series. Each bank consisted of four arms with equal lengths and each arm consisted of four turns of helical coil. The CFI tube was placed inside a cylindrical shell. Experiments were conducted with hot water flowing with Reynolds numbers ranging from 1×103 to 1.6×104 in the tube side. The hot water was being cooled by either cooling water or ambient air in the shell side. They observed that the Nusselt number values in CFI were 12–25% higher as compared to the coiled tube data previously reported in the literature. Recently, Mridha and Nigam (2008a) carried out numerical study using Fluent 6.2 to investigate the pressure drop and heat transfer in the CFI for fluid with Reynolds number ranging from 1×104 to 3×104. They reported that even at high Reynolds number, CFI shows 4–13% enhancement in the heat transfer as compared to the coiled tube while relative increase in pressure drop was 2–9%.
The present work constitute of mainly two studies at different process conditions. In the first study, the performance of CFI heat exchanger has been compared with conventional heat exchangers in order to bring out clearly the concept of CFI as efficient heat exchanger. Therefore, experiments were carried out in conventional shell and tube heat exchanger (SHE) and plate type heat exchanger (PHE) at pilot plant scale and their performance were compared with experimental data reported by Kumar et al. (2007). The heat transfer area and process conditions were identical as reported for CFI. In this part of study, the fluid flows under laminar flow regime. Experiments were conducted with hot water in tube side which was cooled by cooling water counter currently in shell side of heat exchangers. Friction factor and Nusselt number were calculated for SHE and PHE at various process conditions. Number of transfer units were calculated and compared with CFI heat exchanger.
Understanding of heat transfer and pressure drop characteristics of fluid flowing under turbulent flow is of significant importance for the design of heat exchangers used in the industries. Therefore, in the second study, performance of CFI for fluid flowing under turbulent flow condition was experimentally investigated at the pilot plant scale. The experiments with hot, compressed air in tube side of CFI heat exchanger with Reynolds number varying from 3×104 to 1.4×105 is being reported for the first time. The pressure of air was varied from 10–30 kgf/cm2. Cooling water or ambient air was used to cool hot fluid in counter current mode at the shell side of heat exchanger. The experimental results were compared with the data available in open literature for coiled and straight tubes.
Section snippets
Laminar flow
The detailed description of pilot plant setup and experimental procedure with water system was reported by Kumar et al. (2007). The conventional heat exchangers of SHE and PHE with identical heat transfer area as CFI heat exchanger were installed in the pilot plant. All the three heat exchangers had heat transfer area of 1.76 m2. The designs of conventional heat exchangers are described below.
Pressure drop studies
Pressure drop studies were carried out in SHE and PHE and compared with CFI heat exchanger at pilot plant scale. All the three heat exchangers had identical heat transfer area. Fig. 3 shows the variation of pressure drop at different flow rates of water in tube side of SHE, PHE and CFI heat exchangers, respectively. The figure shows that pressure drop increases with increase in flow rates of water. The pressure drop in CFI heat exchanger is higher than SHE and PHE. The increment of pressure
Conclusions
In the present study, experiments were carried out in conventional shell and tube as well as plate type heat exchanger at pilot plant scale and their performance was compared with results reported for CFI heat exchanger by Kumar et al. (2007). It was found that the enhancement of heat transfer in CFI heat exchanger as compared to SHE and PHE is higher than the increase in pressure drop. Number of transfer units for CFI heat exchanger is higher as compared to SHE and PHE for identical process
Notation
Cp specific heat, kJ/(kg K) d diameter of tube, m Dc diameter of coil, m f friction factor g acceleration due to gravity, m/s2 H pitch, m L length, m m mass flow rate, kg/h NDe Dean number (=NRe/√λ) NHe Helical number NNu Nusselt number NPr Prandtl number NRe Reynolds number (=ρvd/μ) P pressure, Kg/cm2 Rc radius of the coil, m t shell side temperature T tube side temperature U overall heat transfer coefficient, W/m2 K v velocity, m/s Greek letters λ curvature ratio, d/Dc μ dynamic viscosity, kg/m s ρ density of fluid, kg/m3 Subscripts c curved
Acknowledgment
The authors gratefully acknowledge the Ministry of Chemical and Fertilizers, GOI, India for funding the project.
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