Zj Co

Tricalcium phosphate Precipitation temperature

Average surface temperature 145°F (63°C)

Outlet

Figure 7. Surface temperature profile with indicated calcium phosphate precipitation.

three exchangers on the SFRU operate at the same midpoint surface temperature. Since the ratio of flow rate to surface area for the shell side of the shell and tube exchangers is twice that for the PHE, the cooling water had a longer residence time in the PHE than in the tubular exchanger. As a result, the lower flow rate to surface area ratio yields a larger temperature rise of the cooling water. The result is the steeper surface temperature profile shown in Figure 7. Although cocurrent operation was investigated as a correction for this problem, the required higher hot-water flow rate was not available. Consequently, the PHE data in some of these tests was slightly penalized because part of the surface is above the critical temperature for tricalcium phosphate precipitation, a condition that normally would not be encountered in industrial operations.

1. The loss of ignition (LOI) is performed by drying the sample to constant weight at 220°F (105°C) and then combusting at 1470°F (800°C). LOI represents bound water and organics present in the deposit. An organic test on samples of test series 5 showed that about 1/3 of the LOI was organic.

2. Aluminum, silicon, and iron come from the suspended solids. Aluminum comes from operation of the clarifler. The silicon is not from magnesium silicate since little magnesium is in the analyses.

3. Chrome, zinc, and phosphorus are derived from the water treatment.

4. Low values of calcium indicate little or no tricalcium phosphate. Most of the calcium comes from association with zinc and polyphosphates in the treatment.

It is difficult to make meaningful comparisons between the fouling tendencies of different types of heat exchangers. Unless carefully done, the results are misleading. This is particularly true when attempting to compare the fouling experience in the PHE to that of tubeside operation as in the PFRU. The geometries, surface area, and operational characteristics are very different even though the cooling water may be the same. Such comparisons often are attempted using TEMA recommended fouling resistances.

TEMA recommends a fouling resistance of 0.002 h • ft2 • °F/Btu for the water system used. However, the maximum fouling resistance measured in the PHE was less than 0.0005 h • ft2 • °F/Btu-only 25% of the TEMA recommendation. This confirms the earlier assumption that applying TEMA-recommended fouling resistances for a shell and tube exchangers to the PHE seriously handicaps the performance ratings. Because of the inherently high heat-transfer coefficients, the effects of fouling resistances are more pronounced. Consider a PHE operating at 1.5 ft/ s. From Table 5, the fouled overall coefficient using a TEMA fouling resistance of 0.002 h • ft2 • °F/Btu is about one half of the overall coefficient using the measured fouling resistance of this investigation. This example illustrates the need for caution in using TEMA fouling recommendations for equipment other than shell-and-tube exchangers.

Figure 8 compares the performance of the PHE with typical tube-side data and with the TEMA-recommended fouling resistance.

A direct comparison of the PHE and the shell-side exchangers can be made through the deposit analyses. A sample analysis is given in Table 6. The amounts of phosphates, calcium, chrome, and zinc are somewhat higher for

Table 5. Effects of Fouling Resistance on PHE Performance

Overall heat-transfer coefficient

Fouling coefficient

Operating velocity (both sides)

Clean

Present study 0.0005 h • ft2 • °F/Btu (0.00009 m2oC/W)

600 1200 1800 2400

Fouling resistance (h)

Table 6. Comparison of Deposit Analyses from Plate Exchanger and Shell-Side Test Exchanger for Series 5

Fouling deposit analysis, %

Plate Shell-side

Table 6. Comparison of Deposit Analyses from Plate Exchanger and Shell-Side Test Exchanger for Series 5

Fouling deposit analysis, %

Plate Shell-side

Loss on ignition

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