Updated RPM for condensation that eliminates the requirement to specify mole fraction inert

Previously, the Resistance Proration Method (RPM) for mixture condensation used the calculated or specified inlet mole fraction noncondensables (i.e., YINERT) as a correlating parameter to determine the need for an additional correction for the effect of mass transfer resistance. CS-5, CS-9, and CT-10 document research data that were mass transfer limited, for a heavy hydrocarbon condensed from a mixture with light gases. Proper identification and interpretation of YINERT has caused many issues, both from a programmatic and a user-specification standpoint, in cases submitted from industry.

To address these issues and improve predictions, the revised RPM replaces YINERT, used to derate the vapor-phase coefficient, with a parameter based on the change of the molecular weights of the vapor mixture. This change mainly affects cases involving condensation of a heavy vapor from a high inlet concentration of a lighter noncondensing gas. In general, the modified method produces similar results to the previous method and has a similar match with HTRI experimental data. The modified method uses heat release curve data to attempt to restrict the impact of the mass-transfer correction to cases in which a noncondensing gas is present.

Compared to previous versions, users may see a variation in the condensing heat transfer coefficient for the following cases:

• Water was not correctly identified as the only condensing component. In these cases, the previous method unjustly penalized the vapor-phase coefficient. While it is unlikely that the new method will apply the additional mass-transfer derating when water is the only condensing component, some cases may show an increase in the condensing heat transfer coefficient.
• The user did not specify inlet mole fraction noncondensables and Property Tables excluded molar compositions. In these cases, the software was unable to apply the YINERT correction. The new correction may cause a decrease in the condensing heat transfer coefficient in some cases with condensation of a heavy vapor from a lighter noncondensing gas.

Additional reading: CS-5, CS-9, and CT-10; Design manual section B4.6

 

Shellside/tubeside radiation heat transfer in Xist® for sensible vapor streams

Previously, only Xace^® and Xfh^® considered radiation calculations. Xist now includes the option to calculate a radiation heat transfer coefficient for the shell side and the tube side in gas streams. The coefficient can be calculated only if the stream contains gray gases (e.g., water and CO2) and the stream composition is known and specified as the Composition based Property option. For those cases, the heat transfer predicted by Xist will be higher due to the contribution of radiation. Radiative heat transfer increases with operating temperature and gray gas fraction.

Additional reading: Design manual section B7

 

Indication of unstable flow regime on the Fair flow regime map for vertical tubeside thermosiphons

For tubeside thermosiphons, an industry rule of thumb recommended avoiding bubble flow at the exit of the tube to ensure vapor could lift the liquid from the tubes. Bubble flow at the tube exit could lead to loss of circulation in a thermosiphon. HTRI research has corroborated this rule of thumb.

• Tubeside boiling research, documented in BT-37, highlighted a region where two-phase flow of pure fluids was unstable (the outlet conditions varied periodically). The forced flow boiling data indicated that stable flow boiling is achievable at higher mass velocities in the bubble flow regime.

• Further research, documented in BG1-18,focused on turndown limits of mixtures in vertical thermosiphon reboilers. The onset of unit shutdown was correlated to the presence of bubble flow at the exit, and the Fair map in Xist now includes an Unstable Bubble flow and Stable Bubble flow area on the Tubeside Flow Regime Map, providing guidance on the operating point of the reboiler. BG1-18 also indicates that the boundary of the unstable regime depends on the liquid level in the column and the boiling range of mixtures.

Future research may improve this guidance.

Additional reading: BT-37 and BG1-18

 

Improved calculation and reporting of tubesheet temperatures

Design of an appropriate tubesheet thickness requires knowledge of the temperature to determine the allowable stresses and modulus of elasticity for the necessary thickness calculations. HTRI previously defaulted to the greater of the tubeside or shellside design temperature. If users did not specify this value, our software defaulted to a value of 500 °F or increasing the inlet temperature of the hot stream by 50 °F. However, the overly conservative temperature did not realistically evaluate the tubesheet, and designs which require improved accuracy to determine tubesheet design conditions will benefit from a more fundamental tubesheet metal temperature calculation.

Xist now predicts the tubesheet metal temperatures based on the specified process conditions. Local temperatures and film heat transfer coefficients are used to calculate metal temperatures on tubesheet surfaces. Xist calculates metal temperatures for each pass. Subsequently, Xist improves the mean metal temperature summary in the Final Results, listing average, maximum, and minimum mean metal temperatures in various tubed and untubed tubesheet regions. The enhanced summary enables users to perform improved tubesheet thickness calculations.

Additional reading: q13-2 and q14-4