Two-phase pressure drop calculation

TWO-PHASE PRESSURE DROP CALCULATION IN A PIPE LINE PIPE INTERNAL DIAMETER, inch 3.070... 

The two-phase pressure drop calculations were performed assuming the vapor void fraction (a) is given by the Lockhart and Martinelli correlation (as given in [6]),... 

The aim of the mathematical analysis is to predict the pressure drop for annular, two-phase flow. An expression is derived which predicts the pressure drop for two-component flow. This differential equation combined with energy and momentum equations, which include mass transfer from the liquid to the vapor phase, are used to predict two-phase pressure drop. The actual two-phase pressure drop calculation is carried out using a finite difference approach. The mathematical analysis is supported by the Freon flow experiment. 

Cyclones Separate solids from hydrocarbon and effluent vapors Two-phase, pressure drop calculation Pressure drop is a combination of pressure drop due to solid and vapor phases... 

Two-phase pressure drop calculations are not applicable for all flow regimes. 

A computer program (TWOPHASE) was developed that uses the Lockliart-Matinelli correlation and determines the total pressure drop based on the vapor phase pressure drop. The total length of the unit depends on the nature of the Reynolds numher. The program also calculates the gas-liquid phase regime employing a modified Baker s map [33]. Table 7-13 gives the results of the two-phase pressure drop. 

Calculate two-phase pressure drop, horizontal portions of lines. For all types of flow, except wave and fog or spray ... 

Total two-phase pressure drop, including horizontal and vertical sections of line. Use calculated... 

CHENOWETH and Martin 20,21 1 have presented an alternative method for calculating the drop in pressure, which is empirical and based on experiments with pipes of 75 mm and pressures up to 0.7 MN/m2. They have plotted the volume fraction of the inlet stream that is liquid as abscissa against the ratio of the two-phase pressure drop to that for liquid flowing at the same volumetric rate as the mixture. An alternative technique has been described by Baroczy 22). If heat transfer gives rise to evaporation then reference should be made to work by Dukler et al 23). 

An illustration of the method of calculation of two-phase pressure drop is included here as Example 5.1. 

This method uses simple, unsophisticated, methods to estimate the two-phase pressure drop through the exchanger and piping, and the convective boiling heat transfer coefficient. The calculation procedure is set out below and illustrated in Example 12.11... 

The tube inside heat transfer coefficients and pressure drop can be calculated using the conventional methods for flow inside tubes see Section 12.8, and Volume 1, Chapter 9. If the unit is being used as a vaporiser the existence of two-phase flow in some of the tubes must be taken into account. Bergman (1978b) gives a quick method for estimating two-phase pressure drop in the tubes of fired heaters. 

In summary, the calculation of pressure drops by the Lockhart-Marti-nelli method appears to be reasonably useful only for the turbulent-turbulent regions. Although it can be applied to all flow patterns, accuracy of prediction will be poor for other cases. Perhaps it is best considered as a partial correlation which requires modification in individual cases to achieve good accuracy. Certainly there seems to be no clear reason why there should be a simple general relationship between the two-phase frictional pressure-drop and fictitious single-phase drops. As already pointed out, at the same value of X in the same system, it is possible to have two different flow patterns with two-phase pressure-drops which differ by over 100%. The Loekhart-Martinelli correlation is a rather gross smoothing of the actual relationships. 

The two-phase pressure drop term (APgl/Z) can be calculated using the correlation of Huntington (Ramachandran and Chaudhari, 1984) ... 

Calculate the two-phase pressure drop. The liquid phase is in laminar flow the gas phase is in turbulent flow. Therefore,... 

Calculate total two-phase pressure drop. The total is the sum of the pressure drops for cross flow, window flow, and nozzle flow. Therefore APlp = 7.592 + 4.616 + 1.464 = 13.672 lb/in2 (94.27 kPa). 

Related Calculations. For situations where AP,p/APgo is much less than 1 /K2 and 1 /K2 is much less than 1, the equation for two-phase pressure drop can be written as... 

Typically, the calculations for two-phase pressure drop are too complicated for hand calculations. It is recommended that the design engineer use one of the available programs specifically designed for such calculations. In addition, an excellent review of two-phase flow is presented in Govier and Aziz (1972). 

For vapor-liquid mixtures, the pressure drop in horizontal pipes can be found using the correlation of Lockhart and Martinelli (1949), which relates the two-phase pressure drop to the pressure drop that would be calculated if each phase was flowing separately in the pipe. Details of the correlation and methods for two-phase flow in vertical pipes are given in Perry and Green (1997). 

It is proposed to reduce the two-phase pressure drop by 50% by introducing air into the pipeline at an upstream point. Calculate the superficial velocity of air required to achieve this if the air at 293 K enters the pipeline at a pressure of 0.35 MPa. Assume isothermal expansion of gas. Use aU three methods mentioned in problem 4.1 to obtain the superficial velocity of the air. Using an appropriate model, determine the maximum reduction in two-phase pressure drop achievable for this slurry. What is the air velocity under these conditions What proportion of the volume of the pipe is filled with liquid and what flow pattern is likely to occur in the pipe under these flow conditions Neglect the effect of air expansion. 

Are these data consistent with the predictions of the simple plug flow model Compare these values of the two-phase pressure drop with those calculated using equations (4.22), (4.24) and (4.26). 

Johanessen, T., A Theoretical Solution of the Lockhart and Martinelli Flow Model for Calculating Two-Phase Pressure Drop and Holdup, Int. J. Heat Mass Transfer, Vol. 15, pp. 1,443-1,449 (1972). 

The optimum liquid level depends upon the system. The optimum can be predicted with confidence, but the final point must be established by observation since two-phase pressure drops, product composition, and piping arrangements cannot always be precisely calculated. For most systems the optimum point results when the evaporator liquid level is approximately half the distance between the two tubesheets. 

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