Bends are extensively used in industrial heat transfer equipment such as U-tube heat exchangers, serpentine tubes in air coolers, elbows in thermosiphon pipes, turnarounds in heat recovery bundles, and fired heaters. The presence of bends and properly predicting pressure drop through them can have a significant impact on accurately estimating the thermal and hydraulic performance of the overall unit. Bend pressure drop not only affects the overall pressure drop in the heat exchanger but also influences phase-change heat transfer downstream of the bend because of its effect on the two-phase equilibrium temperature and redistribution of the phases.
HTRI recently collected extensive single-phase and two-phase pressure drop data using a vertical serpentine test section with a 180-degree return in its Multipurpose Boiling Unit. These data were used to develop an improved two-phase pressure drop model in bends that accounts for bend geometry and flow regimes. Pressure drop in bends is higher than for straight tubes due to the additional momentum loss caused by phase separation and turbulence generated by the bend.
A homogeneous two-phase pressure drop model usually underpredicts measured pressure drop because the bend tends to separate the phases, reduce entrainment, and increase phase slip, thus making invalid the assumption of homogeneous flow. A separated flow pressure drop model, on which the improved model is based, is more suitable for bends because it can take into account the additional momentum loss due to phase separation caused by the centrifugal force generated as the flow travels through the bend. The improved pressure drop model accounts for smooth transitions from all liquid to two-phase flow and from two-phase flow to all vapor.
HTRI continues to investigate the effect of phase separation on downstream boiling heat transfer, which can result on local dry wall due to the redistribution of the liquid phase after the bend. Accurate prediction of localized dry wall is important to prevent local burnout, coking, or tube failures in fired heaters.