There are specific applications in which helical coil heat exchangers are advantageous over straight tube ones. The foremost of these is for high temperature applications where long straight tubes may pose severe mechanical problems due to thermal expansion , which can be minimized in coiled tubes. Another advantage of coiled tubes is that they can be used to pack more surface area in a small volume and are therefore used for applications with small temperature difference or high volumetric heat rating. Coiled tubes also have better heat transfer coefficient and residence time distributions and are therefore used in compact heat exchangers. The main disadvantages of coiled tube heat exchangers are that they are more costly than the straight tube type to manufacture, they cannot be cleaned easily and are therefore not suitable for concentration or crystallization type of applications. Figure 1 shows a coiled tube evaporator with five coils with boiling occurring on the shell-side.
A variant of the helical coil boilers is the Spiral Plate Heat Exchanger shown in Figure 2. Here, the spiral surfaces are separated by raised bosses and sealed with two end plates. The hot fluid enters at the center of the unit and flows towards the periphery in a spiral path. The cold fluid enters at the periphery and flows towards the center setting up a countercurrent flow configuration. One of the advantages claimed for this design is that the swirling path of the fluids produces a scrubbing effect which reduces fouling. Also, the end plates can be removed to give full access to the flow passages for cleaning. However, this limits it to low pressure applications.
Figure 1. A helical coil evaporator. From Hewitt, Shires and Bott (1994). Process Heat Transfer, CRC Press.
Figure 2. A spiral tube heat exchanger. From Hewitt, Shires and Bott (1994). Process Heat Transfer, CRC Press.
The swirling motion produced by coiling a tube increases the frictional pressure drop and the average heat transfer coefficient compared with those in a straight pipe of the same total (uncoiled) length. There are several empirical correlations to calculate these for single phase flow. These are usually in the form of an enhancement factor, over the straight tube value, correlated in terms of the flow Reynolds number and the tube-to-coil diameter ratio. The correlations of Ito (1959, 1969) and those of Schmidt (1967) are widely used to calculate the friction factor and the heat transfer coefficient, respectively, over a wide range of flow conditions. The calculation of these parameters for two-phase flow through coils is much less well-established, although some specific effects of coiling on the flow patterns (such as "film inversion") have been studied. An upper limit for these may be obtained by multiplying the two-phase straight tube values by the single phase enhancement factors implicit in the correlations of Ito and Schmidt mentioned above.
A special feature of coiled tubes is that their critical heat flux (CHF) is less than that of straight tubes under subcooled flow conditions whereas it is significantly higher in the high-quality region (Figure 3). This led to the use of helical coil steam generators in the nuclear industry. The correlation of Berthoud and Jayanti (1990) can be used to calculate the CHF vs. quality relation in the high quality region.
Some other applications of helical coil heat exchangers are discussed in Hewitt et al. (1994).
Figure 3. Schematic relation between critical heat flux and quality in straight tubes and coiled tubes.
Berthoud, G. and Jayanti, S. (1990) Characterization of dryout in helical coils, Int. J. Heat Mass Transfer, 33(7), 1451-1463. DOI: 10.1016/0017-9310(90)90042-S
Hewitt, G. R, Shires, G. L., and Bott, T. R. (1994) Process Heat Transfer, CRC Press.
Ito, H. (1959) Friction factors for turbulent flow in curved pipes, J. Basic Eng., 81, 123-134.
Ito, H. (1969) Laminar flow in curved pipes, Z. Agnew. Math. Mech., 11, 653-663.
Schmidt, E. F. (1967) Wärmeübergang und Druckverlust in Rohrschlangen, Chem. Ing. Tech., 39, 781-789 .