Thermal performance and pressure drop of the helical-coil heat exchangers with and without helically crimped fins

doi:10.1016/j.icheatmasstransfer.2006.11.009

International Communications in Heat and Mass Transfer

Volume 34, Issue 3, March 2007, Pages 321-330

https://doi.org/10.1016/j.icheatmasstransfer.2006.11.009 Get rights and content

Abstract

In the present study, the thermal performance and pressure drop of the helical-coil heat exchanger with and without helical crimped fins are studied. The heat exchanger consists of a shell and helically coiled tube unit with two different coil diameters. Each coil is fabricated by bending a 9.50 mm diameter straight copper tube into a helical-coil tube of thirteen turns. Cold and hot water are used as working fluids in shell side and tube side, respectively. The experiments are done at the cold and hot water mass flow rates ranging between 0.10 and 0.22 kg/s, and between 0.02 and 0.12 kg/s, respectively. The inlet temperatures of cold and hot water are between 15 and 25 °C, and between 35 and 45 °C, respectively. The cold water entering the heat exchanger at the outer channel flows across the helical tube and flows out at the inner channel. The hot water enters the heat exchanger at the inner helical-coil tube and flows along the helical tube. The effects of the inlet conditions of both working fluids flowing through the test section on the heat transfer characteristics are discussed.

Introduction

Due to their high heat transfer coefficient and smaller space requirement compared with straight tubes, curved-tubes are the most widely used tubes in several heat transfer applications, for example, heat recovery processes, air conditioning and refrigeration systems, chemical reactors, food and dairy processes. Helical and spiral coils are well known types of curved-tubes which have been used in a wide variety of applications. For helically coiled tubes, numerous theoretical and experimental works have been reported on heat transfer and flow characteristics. Dravid et al. [1] investigated numerically the effect of secondary flow on laminar flow heat transfer in helically coiled tubes. Numerical results were compared with the measured data in the range in which they overlap. Austen and Soliman [2] studied the influence of pitch on the pressure drop and heat transfer characteristics of helical coils under uniform wall heat flux. Two coils with different pitch ratios and the same diameter ratio were tested. Water was used as working fluid. Prasad et al. [3] experimentally studied the pressure drop and heat transfer coefficient in the tube and shell sides of a helical-coil heat exchanger. Yang and Ebadian [4] solved the k–ε model to analyze the fully developed turbulent convective heat transfer in a circular cross-section helicoidal pipe with finite pitch. Xin et al. [5] studied the effects of the Prandtl numbers and geometric parameters on the local and average convective heat transfer characteristics in helical-pipes. The experiments were performed with three different working fluids: air, water, and ethylene-glycol. In their second paper, Xin et al. [6] experimentally investigated the single-phase and two-phase flow pressure drop in annular helicoidal pipes. Inagaki et al. [7] used a full-size partial model to predict the flow-induced vibration, heat transfer and pressure drop of helically coiled tubes of an intermediate heat exchanger (IHX) for the HTTR. Chen et al. [8] studied the conjugate heat transfer of a finned oval tube. Acharya et al. [9] numerically studied the phenomenon of steady heat transfer enhancement in coiled-tube heat exchangers due to chaotic particle paths in steady, laminar flow with two different mixings. Ju et al. [10] investigated the performance of small bending radius helical-coil pipe. The formulas for the Reynolds number of single-phase flow structure transition, and single-phase and two-phase flow friction factor were obtained. Ali [11] proposed the pressure drop correlations for fluid flows through regular helical-coil tubes. Generalized pressure drop correlations were developed in terms of the Euler number, Reynolds number, and the geometrical group. Prabhanjan et al. [12] illustrated the advantage of using a helically coiled heat exchanger versus a straight tube heat exchanger for heating liquids. The experiments were done in the transitional and turbulent flow regions. In their second paper, Prabhanjan et al. [13] experimentally investigated the natural convection heat transfer from helically coiled tubes. The predicted outlet temperature was validated by comparing with the measured data. Chen and Zhang [14] studied the combined effects of rotation, curvature, and heating/cooling on the flow pattern, friction factor, temperature distribution, and Nusselt number. Ko and Ting [15] studied the entropy generation of the fully developed laminar convection in a helical coil with constant wall heat flux. Effect of Reynolds number, coil-to-tube radius ratio and nondimensional coil pitch were discussed.

Although some information is currently available on the method to calculate the performance of the helical-coil heat exchanger, there is still room to discuss whether it gives a reliable prediction of the heat transfer characteristics of the helical-coil heat exchanger. The objective of this paper is to experimentally study the thermal performance and pressure drop of a helical-coil heat exchanger with and without helically crimped fin. Effects of relevant parameters on the heat transfer characteristics of the helical-coil heat exchanger are discussed.

Section snippets

Experimental apparatus and method

A schematic diagram of the experimental apparatus is shown in Fig. 1. The test loop consists of a test section, refrigerant loop, hot water loop, cold water loop and data acquisition system. The test section is a helical-coil heat exchanger consisting of a shell and helically coiled finned tube unit (Fig. 2). The test section and the connections of the piping system are designed such that parts can be changed or repaired easily. In addition to the loop component, a full set of instruments for

Data reduction

The data reduction of the measured results is summarized in the following procedures:

Heat transferred to the cold water in the test section, Q_w,c, can be calculated from $Q_{w, c} = m_{w, c} C_{p, w} (T_{w, c, out} - T_{w, c, in})$ where m_w,c is the mass flow rate of cold water, C_p,w is the specific heat of water, T_w,c,in and T_w,c,out are the inlet and outlet cold water temperatures, respectively.

Heat transferred from the hot water, Q_w,h, can be calculated from $Q_{w, h} = m_{w, h} C_{p, w} (T_{w, h, in} - T_{w, h, out})$ where m_w,h is the hot water mass

Results and discussion

Fig. 3 illustrates the variation of the outlet temperature of cold water with hot water mass flow rate for helically coiled finned tube heat exchanger. It is found that when the inlet hot and cold water temperatures, and cold water mass flow rate are kept constant, the outlet cold water temperature increases with increasing hot water mass flow rate. This is because the heat transferred from the hot water to cold water increases with increasing hot water mass flow rate. Therefore, the outlet

Conclusion

This paper presents new experimental data from the measurement of the average in-tube convective heat transfer characteristics and thermal performance of helical-coil heat exchanger. The heat exchanger consists of thirteen turns concentric helically coiled tubes with and without helically crimped fins. The conclusion can be summarized as follows;

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Outlet cold water temperature increases with increasing hot water mass flow rate.
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An average heat transfer rate increases as hot and cold water mass

Nomenclatures

Area

C_p

Specific heat, kJ/ (kg °C)

Tube diameter, m

Coil diameter, m

Friction factor

Correction factor

Heat transfer coefficient, kW/(m² °C)

Thermal conductivity, kW/(m °C)

Mass flow rate, kg/s

Nusselt number

Prandtl number

Heat transfer rate, kW

Tube radius, m

Coil radius, m

Reynolds number

Temperature, °C

Overall heat transfer coefficient, kW/(m² °C)

Thickness, m

η_o

Overall surface efficiency

Fin efficiency

Effectiveness

Subscripts

ave

Average

Cold

Fin

Hot

Inside

Inlet

LMTD

Long mean temperature

Acknowledgements

The author would like to express their appreciation to the Srinakharinwirot University (SWU) for providing financial support for this study. The author also wishes to acknowledge Mr. Jumras Tritasanon, Mr. Montree Fenner and Mr. Monton Moungthong, for their assistance in some of the experimental work.

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Pressure drop, heat transfer and performance of a helically coiled tubular exchanger
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G. Yang et al.
Turbulent forced convection in a helicoidal pipe with substantial pitch
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R.C. Xin et al.
An experimental study of single-phase and two-phase flow pressure drop in annular helicoidal pipes
International Journal of Heat and Fluid Flow
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Thermal hydraulic study on a high-temperature gas–gas heat exchanger with helically coiled tube bundles
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Analysis of heat transfer enhancement in coiled-tube heat exchangers
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Pressure drop correlations for flow through regular helical coil tubes
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There are more references available in the full text version of this article.

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^☆: Communicated by W.J. Minkowycz.

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Thermal performance and pressure drop of the helical-coil heat exchangers with and without helically crimped fins☆

Abstract

Introduction

Section snippets

Experimental apparatus and method

Data reduction

Results and discussion

Conclusion

Nomenclatures

Acknowledgements

Experimental Thermal and Fluid Science

Heat Recovery Systems & CHP

International Journal of Heat and Mass Transfer

International Journal of Heat and Fluid Flow

Nuclear Engineering and Design

International Journal of Heat and Mass Transfer

Fluid Dynamics Research