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In desalinization devices and some heat exchangers making use of low-quality heat energy, both wall temperatures and wall beat fluxes of the heated tubes are generally quite low, hence they can not cause boiling in flooded tube-bundle evaporators with common large tube spacing. However, when the tube spacing is very small, the incipient boiling in restricted spaces can occur and results in higher heat transfer than that of pool boiling at the same heat flux. This paper investigated experimentally the effects of tube spacing, positions of tubes and test pressures on the boiling heat transfer of water in restricted spaces of the compact in-line bundles consisting of smooth horizontal tubes. The experimental results show that the tube spacing and tube position have significant effect on the boiling heat transfer in a compact tube bundle. There is an optimum tube spacing, which provides the largest heat transfer coefficient at the same heat flux. [ABSTRACT FROM AUTHOR]