New Low-Potential Heat Exchangers Design
Abstract
This article deals with the effects of different structural arrangements inside a heat exchanger made of polypropylene tubes on the overall heat transfer coefficient. The experiments indicated that overall heat transfer coefficient k with stretched tubes was lower than the value observed when the tubes were slightly loosened. Tubes should not be loosened by more than 5 % of their length. If this value is exceeded, tubes may accumulate near the wall and their contact with water is insufficient; this results in reduced heat transfer. Equally important is to prevent tubes from attaching to each other. This may be achieved with a variety of turbulators. A turbulator may be any small object, metal or plastic, which is inserted among the tubes. Laboratory investigation indicated that turbulators can increase overall heat transfer coefficient by as much as 54 %compared to heat exchangers without turbulators, in identical operating conditions.
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Introduction
Heat exchangers which are primarily intended for the use with low-potential heat sources have been described in paper [1]. Heat-transfer surfaces in such heat exchangers are on polypropylene tubes and they may be either absolutely smooth and gasproof or porous. Polypropylene tubes in these exchanges are hollow. A required amount of tube ends are sealed into a polyurethane tube approximately 35 mm long to form a potting. A fluid (water) which absorbs heat from the surrounding environment (a low-potential heat source) flows through these tubes sealed in the potting. An identical potting is on the outflow end of the heat exchanger. A length of tubes between the pottings ranges from approximately 400 mm to max. 1,000 mm. A heat-transfer surface area S is calculated using the amount and dimensions of the used tubes and it determines the heat output of a heat exchanger. An inner diameter of a capillary tube usually ranges between approximately 0.15 and0.30 mm. Heat exchangers of this type, with simple designs, have been subjected to experimental research aimed at obtaining information on the overall heat transfer coefficient k, as presented in the quoted literature.
The heat exchangers presented in this article consisted of the same tubes as described above, but with a special feature, i.e. the arrangement of tubes between the two pottings. The article describes an analysis of three different designs of a heat exchanger.
Conclusion
The article describes three different designs of heat exchangers made of propylene capillary tubes. Results of this experimental laboratory investigation indicated that the most optimal design of a heat exchanger for achieving maximum values of overall heat transfer coefficient is the heat exchanger with tubes that are prevented from transverse motions and only slightly loosened (max 0.5 %). This type of heat exchanger meets the requirement of providing sufficient heat transfer from a heat-transfer fluid. However, it is necessary to bear in mind that these experiments were carried out with pure water. With polluted water, it is necessary to count with reduced amounts of transferred heat. In this investigation, with pure water, a maximum heat output of the heat exchanger was 4,450 W.