Abstract: Studies on horizontal in-tube condensation flow and heattransfer in different gravity conditions are reviewed, with focus concentrated on thecase of small scale tube, for low flow rate of ammonia, which is a routine practice inspace applications. Gravity influence on in-tube condensation in partialgravity conditions are discussed in detail. Condensate film on the innerperimeter in horizontal in-tube condensation leads to the differences of itsflow configuration from forced boiling and adiabatic two-phase gas-liquidflow, which enhances the effects of surface tension and increases the valueof the critical Bond number with regards to the transition to wavy andstratified flow regimes. Thus, the influence of gravity on flow and heattransfer in horizontal in-tube condensation with small scale tube and lowflow rate will be weakened, even be come negligible. It was found that themeasured frictional pressure drops in these cases are close to the predictionsgiven by smooth annular flow model, which are smaller than the predictions byempirical correlations based on the experimental data of two-phasegas-liquid flow at conventional conditions. Better predictions are obtainedonly by using a method based on a coupling of the void fraction with theinterfacial shear rate.