  There is a scarcity of available data on boiling process in vertically
oriented tube bundles in accessible sources. Lack of systematic studies is
limiting further expansion of this highly efficient process of heat transfer
into heat recovery field. In this paper boiling process of three triangular
pitched and vertically oriented tubes has been studied in ethanol at
78$^{\circ}$C. The main focus of this work was to study the effect of tube
spacings on heat transfer coefficient (HTC) and bubbles behavior (bubble
departure diameter in particular) that were visualised with the help of a high
speed camera. Experiments were performed in a wide range of tube spacings (from
10.75 to 0.25 mm) and heat flux densities (from 3 to 70 kW/m$^2$).
  The obtained results show that, long spacings i.e., much longer than bubble
departure diameter, have no influence on HTC as well as on bubbles behavior. On
the contrary, the spacings on the order of the bubble departure diameter tend
to create slug flow in the bundle, that is very beneficial for the heat
exchange at low heat fluxes. Finally, narrow spacings that are much shorter
than the bubble departure diameter have shown the potential to enhance the HTC
in tube bundles with low length to diameter ratios.
