Pressure Drop in Two-Phase Flow: HVAC Design Explained

▶ Overview of Two-Phase Pressure Drop

In two-phase flows the total pressure drop (dp/dz) is typically divided into: • Frictional losses arising from viscous forces at the walls and between phases. • Momentum losses that occur when the phases accelerate or decelerate due to evaporation or condensation. • Hydrostatic pressure changes, especially in inclined or vertical flows, which depend on the relative densities of the liquid and vapor and the system orientation.

For example, the 2017 ASHRAE Handbook details these components in Equation (29a) and explains how void fraction—the fraction of the flow that is vapor—affects these calculations ​.

▶ Impact on HVAC System Design

In HVAC applications such as flooded evaporators, direct-expansion coils, and plate heat exchangers, designers must balance high heat transfer rates with acceptable pressure drops. A high pressure drop can lead to:

• Increased Compressor or Pump Requirements – More energy is needed to overcome the loss, affecting the operating cost. • Maldistribution of Refrigerant – Uneven pressure drop may result in poor refrigerant flow distribution, reducing heat exchanger effectiveness. • Design Constraints on Channel Size and Enhancements – Many modern designs incorporate microchannels or enhanced surfaces to boost heat transfer; however, these features can also increase the pressure drop if not carefully optimized.

For instance, in microchannels—which are increasingly used for compact heat exchangers—the friction multiplier and the overall two-phase pressure drop are influenced not only by the channel size but also by the mass flux and flow regime. Correlations such as those of Lee and Lee (2001) and Mishima and Hibiki (1996) are employed to predict these effects, as highlighted in the document ​ and ​. The challenges are even more pronounced in plate heat exchangers, where the complex geometry (including chevron angles, port holes, and narrow passages) introduces additional pressure drop components that must be carefully accounted for ​.

▶ Empirical Correlations and Their Role

Because the hydrodynamic and heat transfer aspects of two-phase flow are not as straightforward as those for single-phase flow, engineers rely on empirical correlations like the Friedel correlation, Lockhart-Martinelli model, Grönnerud correlation, and the Müller-Steinhagen and Heck correlation. These correlations provide designers with a means to estimate pressure drops under various operating conditions, though they can sometimes vary by as much as 30–50% from measured values. For example, the Friedel correlation starts with a single-phase pressure drop and applies a multiplier to account for the additional effects in two-phase flow ​.

Designers use these models as approximations, adding safety margins and verifying with experimental or in-service data to ensure that the system will operate reliably. It’s a classic design trade-off: increasing the heat transfer area (often through enhanced surfaces) can improve performance but might also result in a higher pressure drop, necessitating a careful optimization balance.

▶ Practical Implications for HVAC Design

When designing HVAC systems that utilize two-phase flow, engineers must: • Evaluate the appropriate channel or tube sizes to minimize undue pressure losses. • Consider the installation of flow control devices (like valves or baffles) that can further affect the pressure distribution. • Use available correlations—not as absolute predictors but as tools to guide the selection of components (compressors, pumps, heat exchanger geometries) while understanding the inherent uncertainties. • Optimize enhanced heat transfer surfaces such that they improve condensation or boiling efficiency without causing excessive pressure drop, which in turn could reduce the overall system performance ​.

In summary, pressure drop in two-phase flows is a critical factor in HVAC design because it determines the energy efficiency, refrigerant charge, and reliability of the heat exchanger components. A comprehensive understanding—bolstered by empirical models and correlation data from sources like the ASHRAE Handbook—is essential for developing systems that are both high-performing and cost-effective.