Effect of noncondensable gases on flow boiling

The presence of noncondensable gases in boiling fluids is not uncommon and sometimes can benefit the performance of the process.

By LiDong Huang

The presence of noncondensable gases in boiling fluids is not uncommon and sometimes can benefit the performance of the process. One example results from the addition of a sparge gas (small amount of inert gas or superheated steam) to a boiling fluid to increase the circulation in a thermosiphon reboiler or to decrease the bubble point of a very high-boiling point liquid. Other benefits of having some noncondensable gas in a boiling fluid include

  • improvement in fluid circulation due to an increase in the buoyancy force
  • elimination or reduction of the subcooling zone in a thermosiphon reboiler
  • enhancement of two-phase convection

On the other hand, the presence of noncondensable gases in a boiling stream can make it more difficult to predict the boiling process and determine the vapor-liquid equilibrium condition of the mixture; it also increases the vapor-phase resistance.

Accurate predictions of the equilibrium two-phase temperature, heat transfer coefficient, and pressure drop in heat exchangers with noncondensable gases are critical for designing such equipment. HTRI has conducted intube flow boiling experimental and theoretical research for boiling hydrocarbons mixed with a noncondensable gas, resulting in improvements to our multi-mechanism flow boiling model. Flow boiling data were collected using n-pentane/nitrogen, p-xylene/nitrogen, and n-pentane/p-xylene/nitrogen as test fluids over a wide range of operating conditions and concentrations.

For equilibrium two-phase temperature, our data indicate that an ideal mixture model works well for pure components mixed with a noncondensable gas. In the case of multicomponent hydrocarbon mixtures with noncondensable gases, the recommendation is to generate heat release curves and two-phase physical properties based on a reliable vapor-liquid equilibrium model, or use measured values if available; this is important to obtain accurate temperature profiles for the mean temperature difference calculation and local two-phase properties for heat transfer and pressure drop calculations. HTRI data also indicate that the vapor phase resistance, which increases with noncondensable concentration, can be as high as half the total boiling-side resistance at high concentration of noncondensable gases.

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