with the aid of the Eckert GPDC. Similarly, for structured packings (Charts 10.5001 to 10.6504), the charts are plots of the Ester and Gill GPDC (SP) (Fig. 8.19c), with experimental data superimposed on them. The principles of the method were described in Sees. 8.2.6 to 8.2.9. A detailed procedure is given in Sec. 10.1, while Sec. 10.2 provides a guide for chart numbers. Table 10.1 lists the sources of all data plotted on the charts.

10.1 Application Guidelines for Using the GPDC Interpolation Charts

Estimating flood and pressure drop using the GPDC interpolation charts involves data interpolation and extrapolation within the framework of the generalized pressure drop correlation (GPDC) chart (Fig. 8.196 or c). This technique is expected to yield reliable estimates when appropriate data are available in the vicinity of the operating point. The reliability of the estimates will diminish with the extent to which extrapolation is required. Whenever extensive extrapolation is required, the estimates are unreliable, and the calculation is best abandoned.

1. Select the appropriate GPDC interpolation chart for the packing considered.

2. Using the operating conditions or design conditions, and the packing factor listed on the chart, determine the operating point on the interpolation chart.

3. Check if any data are plotted in the vicinity of the operating point. If not, look for the closest region of the diagram that contains data. If this region is remote from the operating point, the calculation is unreliable and is best abandoned (proceed to step 9 below). If the region is close, judiciously extrapolate data near the operating point. Items 4 and 5 below give some guidelines. Bear in mind that you are in an uncertain area. Allow for this uncertainty.

4. Check if data points in the vicinity of the operating point match the correlation. If so, obtain the pressure drop directly from the correlation curves, and go to 6 below. If not, proceed with 5.

5. Draw a curve fitting data points in the vicinity of the operating point. If the data points show a clear trend in this region, it may best be to follow the data points. If no clear trend is shown, it may best be to draw the curve parallel to the correlation curve. Often, a compromise is best. Some engineering judgment may be needed here; a conservative estimate may be best.

6. Compare the nature of the operating system (aqueous or nonaqueous) to the nature of the data points used above. Keep in mind that the aqueous data on the charts are almost entirely air-water data. If the operating system contains less than 50 percent water, it may behave more like a nonaqueous system than an aqueous system. If the nature of the operating system is not the same as that for which the data points are plotted, or if using the extrapolation procedure in step 3 above, proceed to 7 below. Otherwise, proceed to 8 below.

7. Look for a GPDC interpolation chart for a reference packing which has data for a system of the same nature as the operating system at the same operating point. The reference packing must belong to the same packing class (random, structured, or grid) as the packing considered. Attempt to select a reference packing with as similar a packing factor and as similar a geometry as possible. Using data for the reference packing, obtain an estimate of the effect of system nature on pressure drop. If this effect is small, say less than 20 percent, correct the pressure drop estimate for your packing. If the effect is large, say more than 20 percent, the calculation is unreliable and is best abandoned (proceed to step 9 below).

Keep in mind that if you reached this step, you are in an uncertain region. The uncertainty is even greater if the system nature has a considerable effect on packing pressure drop. Proceed with extreme caution, recognizing that your calculation in this case will at best be only an educated guess. It may pay to consider using the reference packing instead of the one you had in mind because it offers more confidence in the reliability of the performance prediction.

8. Check the range of column diameters and packing depths used to develop the data points in the chart (Table 10.1). Study Sees. 8.2.3 to 8.2.5 that address the inherent limitations of flood and pressure drop data. Make a judicious estimate as to the extent to which scaleup, packing technique, or other factors may influence extension of the data from Table 10.1 to your specific case. The scatter of data around the curves in Fig. 8.19c? can give some guidelines as to the magnitude of these factors. All data on this diagram were obtained using a single packing (1-in Pall® ring) and almost all using the air-water system, so the scatter does reflect the inherent data limitations. Adjust your estimate accordingly. Engineering judgment is needed here; a conservative estimate may be best. Your calculation is complete; skip step 9.

9. You would only reach this step if the calculation is abandoned. In this case, it would be best to request the manufacturer to provide data on the packing in the vicinity of the operating point, or to consider some packing for which there is more confidence in the performance prediction.

10.2 A Guide to the GPDC Interpolation Charts

The GPDC (generalized pressure drop correlation) interpolation charts are all numbered in the chapter number (10) followed by a 4-digit suffix. The four digits stand for the following a First two digits give the packing number. The numbers were allocated as follows:

10-13 second generation random packings 20-23 common third generation random packings 30-35 other third generation random packings 50 wire-mesh structured packings 60-65 corrugated sheet structured packings 80-82 grids a Third digit gives the packing material. 0 = metal; 1 = plastic; 2 = ceramic.

a Fourth digit is an indicator of the nominal size of the packing. For j random packings, it is roughly twice the nominal packing size. For ! structured packings, it is roughly 8 times the crimp height. \

All the charts contain data for pressure drop, flood, and MOC. Small j symbols represent data for aqueous systems, while large symbols rep- j resent data for nonaqueous systems. In the case of 1- and 2-in metal i Pall® rings, due to the large number of data points, each chart was j split into three diagrams: a flood data chart, an aqueous pressure drop ' data chart, and a nonaqueous pressure drop data chart. Similarly, far j Mellapak® 250Y, the chart was split into an aqueous chart and a non- ) aqueous chart each containing flood, MOC, and pressure drop data. -All random packing charts (10.1002 to 10.3517) and grid packing -charts (10.8005 to 10.8205) contain the Strigle version of the Eckert GPDC curves (Fig. 8.196). All structured packing charts (10.5001 to 10.6504) contain the Kister and Gill GPDC (SP) curves (Fig. 8.19c).

Some unique considerations associated with flood-point definition apply to the grid charts only (10.8005 to 10.8108 only). These are described in Sec. 8.2.6 and should be consulted before interpolating flood data on those charts.

Was this article helpful?

0 0

Post a comment