Non-typical designs of Polypropylene Capillary Heat Exchangers
Abstract
The present article describes a heat exchanger with transparent (smooth) fibres with an atypical body shape and an atypical arrangement of polypropylene capillaries inside. The exchanger cross-section was of a square shape. This type of exchanger was subjected to the investigation of the impact of the fibre arrangement on the overall heat transfer coefficient and behaviour of fibres during the experiment. The exchanger was examined in the counter flow arrangement. The exchanger with 1,400 transparent fibres with the outer diameter of 0.275 mm was examined at the secondary fluid flow rate of 150 l·h-1 to identify the overall heat transfer coefficient k which amounted to 520 W·m-2 ·K-1 . When compared to an exchanger with identical parameters of fibres placed inside a cylindrical exchanger body, a decrease in the overall heat transfer coefficient represented 14%. At the flow rate of 200 l·h-1 , the value of the overall heat transfer coefficient identified experimentally was 632 W∙m-2 ·K-1 . When compared to cylindrical exchanger with comparable fibre parameters, the value was 33% lower.
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Introduction
The use of polypropylene fibres in heat exchangers, the benefits of the used material, the production of fibres (capillaries) and sealing them in a potting were described by several authors, for example in papers [1-3]. It is a novel approach to designing exchangers with the heat-transfer surface consisting of polypropylene fibres with either an absolutely smooth and gas-proof surface, or a porous surface. The number of capillaries in a bundle placed inside a polyurethane tube with a diameter of ca 30 mm may range from several hundreds to thousands of pieces. It depends on a fibre diameter and a potting size, but primarily on the desired exchanger performance.
Conclusion
An analysis of two different types of heat exchangers comprising polypropylene fibres with identical parameters showed that the heat transfer was more intensive in an exchanger with a circular body. This may be attributable to the fact that the fibres were forced against, and attached to, the wall (in a square-shaped exchanger body). At such places, the fibres were not sufficiently flown around by water (see Fig. 1), and such insufficient flow around the fibres was also observed in the entrance part of the exchanger - next to the potting. It is therefore possible to state that in this exchanger type a certain part of the fibres within their length practically did not contribute to the heat transfer. Removing this drawback in a square-shaped exchanger design might result in approximately a 20–25% increase in the value of the overall heat transfer coefficient. This analysis was based on a fictitious removal of the part of fibres which did not contribute to the heat transfer. As the fibres were forced against the wall, the heat-transfer surface area was in fact reduced.
A comparison of the results of the experimental identification of the value of the overall heat transfer coefficient in exchangers with circular or square external surface clearly indicated that higher intensity of the heat transfer may be achieved by using exchangers with cylindrical surfaces. As stated in the annotation of this article, at the maximum flow rate inside the fibres of 200 l·h-1 , the square-shaped exchangers exhibited the value of the overall heat transfer coefficient which was lower in as much as 33%.