27-03-2014, 12:42 PM
Two-Phase Pressure Drops
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Summary:
Accurate prediction of two-phase pressure drops in direct-expansion and flooded evaporators,
in tube-side and shell-side condensers, and in two-phase transfer lines is of paramount importance to the
design and optimization of refrigeration, air-conditioning and heat pump systems. Taking direct-
expansion evaporators as an example, the optimal use of the two-phase pressure drop to obtain the
maximum flow boiling heat transfer performance is one of the primary design goals. In these evaporators,
typically a two-phase pressure drop equivalent to a loss of 1.4°C (2.5°F) in saturation temperature from
inlet to outlet is set as the design limit. Yet, pressure drops predicted using leading methods differ by up
to 100%. Putting this into perspective, if an evaporator is inaccurately designed with a two-phase pressure
drop only one-half the real value, then the system efficiency will suffer accordingly from the larger than
expected fall in saturation temperature and pressure through the evaporator. On the other hand, if the
predicted pressure drop is too large by a factor of two, then fewer tubes of longer length could have been
utilized to obtain a more compact unit. Hence, accurate prediction of two-phase pressure drops is a key
aspect in the first law and second law optimization of these systems.
In this chapter, methods for predicting two-phase pressure drops for flows inside tubes (horizontal and
vertical) and for flows over tube bundles (horizontal) will be presented. In addition, two-phase pressure
drop data for microfin tubes and corrugated tubes will be presented and a prediction method for microfin
tubes described.