Abstract: For common flooded type evaporators, since heat fluxes and wall temperatures of heated tubes are generally quite low, the nucleate boiling heat transfer cannot generate on the heated tubes. However, when the distance between tubes is very small，the nucleate boiling in restricted spaces can generate at low heat flux or low wall temperature conditions easily and it can effectively enhance the heat transfer performance of the evapo-rators. The boiling heat transfer enhancement of water on plane tubes in compact, staggered tube-bundle evaporators under atmospheric and reduced-pressure conditions is investigates experimentally. The experiment investigates the effects of the tube spacing sizes, the positions of tubes and the test pressures on the boiling heat transfer characteristics in restricted spaces of compact tube bundles. The experimental results indicate that for the compact tube bundles, the effect of the tube spacing is very significant on the boiling heat transfer. The boiling heat transfer has a maximum enhancement when the tube spacing corre-sponds to an optimum value. The optimum tube spacing equals approximately to the departure diameter of the bubbles during a pool boiling. The enhanced heat transfer effect from the com-pact bundles would gradually decease with the decrease in the test pressures.

Key words: Boiling, Bundle, Compact heat exchanger, Enhanced heat transfer